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MUSCULOSKELETAL IMAGING
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High-Resolution 3-T MR Neurography of the Lumbosacral Plexus1 SA-CME See www.rsna .org/education /search/RG
LEARNING OBJECTIVES FOR TEST 2 After completing this journal-based SACME activity, participants will be able to: ■■Recognize
the normal appearance of the lumbosacral plexus at MR neurography. ■■List
the spectrum of conditions that affect the lumbosacral plexus. ■■Describe
the features of lumbosacral plexopathy at MR neurography.
Theodoros Soldatos, MD, PhD • Gustav Andreisek, MD • Gaurav K. Thawait, MD • Roman Guggenberger, MD • Eric H.Williams, MD • John A. Carrino, MD, MPH • Avneesh Chhabra, MD The lumbosacral plexus comprises a network of nerves that provide motor and sensory innervation to most structures of the pelvis and lower extremities. It is susceptible to various traumatic, inflammatory, metabolic, and neoplastic processes that may lead to lumbrosacral plexopathy, a serious and often disabling condition whose course and prognosis largely depend on the identification and cure of the causative condition. Whereas diagnosis of lumbrosacral plexopathy has traditionally relied on patients’ medical history, clinical examination, and electrodiagnostic tests, magnetic resonance (MR) neurography plays an increasingly prominent role in noninvasive characterization of the type, location, and extent of lumbrosacral plexus involvement and is developing into a useful diagnostic tool that substantially affects disease management. With use of 3-T MR imagers, improved coils, and advanced imaging sequences, which provide exquisite spatial resolution and soft-tissue contrast, MR neurography provides excellent depiction of the lumbrosacral plexus and its peripheral branches and may be used to confirm a diagnosis of lumbrosacral plexopathy with high accuracy or provide superior anatomic information should surgical intervention be necessary.
Introduction
The lumbosacral plexus is a series of nerve convergences and divergences that ultimately combine into larger terminal nerves that supply the pelvis and lower extremities. The lumbrosacral plexus is subject to a variety of insults that may lead to lumbrosacral plexopathy, a clinical syndrome that includes motor and sensory disturbances. Traditionally, diagnosing lumbrosacral plexopathy relied on medical history; clinical findings; and electrodiagnostic test results, such as electromyography (EMG), whereas computed tomography and conventional magnetic resonance (MR) imaging were used to evaluate mass lesions and guide biopsies (1). Often, differentiating lumbrosacral plexopathy from spine-related abnormalities is a clinical dilemma. Electrodiagnostic testing provides limited evaluation of the lumbrosacral
Abbreviations: MIP = maximum intensity projection, SPACE = sampling perfection with application of optimized contrasts using varying flip angles, SPAIR = spectral adiabatic inversion-recovery, STIR = short inversion time inversion recovery, 3D = three-dimensional, 2D = two-dimensional RadioGraphics 2013; 33:967–987 • Published online 10.1148/rg.334115761 • Content Codes: From the Russell H. Morgan Department of Radiology and Radiological Science (T.S., G.A., G.K.T., J.A.C.) and Department of Plastic Surgery (E.H.W.), Johns Hopkins Hospital, 601 N Caroline St, Baltimore, MD 21287; Department of Radiology, University Hospital of Zürich, Zürich, Switzerland (R.G.); Dellon Institute for Peripheral Nerve Surgery, Baltimore, Md (E.H.W.); and Department of Musculoskeletal Radiology, University of Texas Southwestern, Dallas, Texas (A.C.). Received November 9, 2011; revision requested April 27, 2012, and received August 10; final version accepted October 22. For this journal-bases SA-CME activity, authors G.A., J.A.C., and A.C. have disclosed financial relationships (see page 985). All other authors, the editor, and reviewers have no relevant relationships to disclose. Address correspondence to A.C. (e-mail: Avneesh.
[email protected]). 1
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Table 1 Our 3-T MR Neurographic Protocol for Evaluating the Lumbosacral Plexus Sequence
Field of view (cm)
Voxel size (mm3)
TR/TE (msec)
Turbo factor
Bilateral
33
0.64
800/12
6
Bilateral Bilateral
33 36–38
1.00 0.6
4500/80 4980/38
17 7
Bilateral
36–38
0.5
550/10
3
Bilateral Lumbar spine
36–38 28
1.45 1.45
1500/103 1000/99
61 69
Bilateral
36–38
0.58
4.39/2.01
…
Area
Axial T1-weighted turbo spin-echo Axial T2-weighted SPAIR Coronal proton-density SPAIR Coronal T1-weighted turbo spin-echo Coronal 3D STIR SPACE Sagittal T2-weighted 3D SPACE Coronal 3D VIBE*
Note.—Reprinted, with permission, from reference 2. SPACE = sampling perfection with application of optimized contrasts using varying flip angles, SPAIR = spectral adiabatic inversion recovery, STIR = short inversion time inversion recovery, TE = echo time, 3D = three dimensional, TR = repetition time, VIBE = volume interpolated breath-hold examination. *This sequence is optional.
plexus because of the deep location of nerves and the variable innervation of regional muscles. Depiction of the exact location, extent, cause, and character of plexopathy is often possible only at high-resolution MR neurography. In recent years, high-resolution MR neurography has been increasingly used to evaluate patients with suspected or established lumbrosacral plexopathy and help confirm the diagnosis or provide anatomic information should surgical intervention be necessary. Although the sciatic nerve (the largest of the lumbrosacral plexus branches) has been extensively studied at MR neurography, and mass lesions involving the lumbrosacral plexus have been described in scattered case reports, there is a relative paucity of literature describing the MR neurographic features of other abnormalities of the lumbrosacral plexus and its peripheral branches. In this article, we review the lumbrosacral plexus anatomy and the spectrum of diseases that affect the lumbrosacral plexus with their corresponding imaging findings at 3-T MR neurography.
Imaging Technique
The authors perform most MR neurographic examinations on 3-T imagers because of their high signal-to-noise ratio (SNR), which provides higher spatial resolution and contrast and al-
lows easy acquisition of isotropic spin-echo–type three-dimensional (3D) images. When metal is known to be present in the area of interest, 1.5-T imaging is preferred. Although 1.5-T MR neurography may provide high-resolution twodimensional (2D) images, spin-echo–type 3D imaging for larger fields of view is generally possible only at 3 T because of its higher SNR. At our institution, we follow a 3-T MR neurography protocol that includes a combination of high-resolution 2D and 3D spin-echo–type sequences to evaluate the lumbrosacral plexus (Table 1) (2). Axial T1- and T2-weighted spectral adiabatic inversion-recovery (SPAIR; Siemens Medical Solutions, Erlangen, Germany) 2D images provide detailed fascicular depiction of the lumbrosacral plexus nerve roots and their peripheral branches. Coronal T1-weighted and short inversion time inversion recovery (STIR) and proton density–weighted SPAIR images depict lesions along the long axis of the nerves, facilitate side-to-side comparison of nerve morphologic characteristics and signal intensity, and depict other incidental findings in relation to the hips or spine. In addition, coronal T1-weighted images are useful in depicting clear fat planes around the pelvic nerves; the longitudinal course of the obturator nerves; and the intramuscular course of the sciatic nerve, split piriformis, and split sciatic nerve. Three-dimensional imaging is performed with isotropic voxels, which enable the use of
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Table 2 Normal and Abnormal Imaging Findings of Peripheral Nerves at MR Neurography Size
Signal Intensity
Fascicular Pattern
Course
Continuous
Perineural Fat
Contrast Enhancement
Normal Findings Similar to that of the adjacent artery, gradually decreases distally
Isointense relaPresent on tive to skeletal T1- and muscle on T1T2and T2-weighted weighted images, isoinimages tense to minimally hyperintense on STIR and fat-saturated T2-weighted images, uniform symmetric high signal intensity on 3D SPACE images
Smooth with Yes no focal deviations
Clear fat planes
No enhancement, except in areas with a normally deficient blood-nerve barrier (eg, dorsal root ganglion)
Abnormal Findings Focal or diffuse enlargement, larger than the ad jacent artery
Hyperintense on T2-weighted images (similar to that of adjacent vessels), asymmetric areas of hyperintensity on 3D SPACE images
Single or multiple enlarged fascicles, loss of fascicular pattern
Focal or dif- No fuse deviations
multi- and curved-planar reformations. Among the types of 3D imaging performed, STIR sampling perfection with application of optimized contrasts using varying flip angles (SPACE; Siemens Medical Solutions, Erlangen, Germany) is used to evaluate the lumbrosacral plexus (from L2 through L3 to the lesser trochanter), whereas T2-weighted SPACE imaging (from L1 through L2 to the sacrum) is used to evaluate the lumbar spine. Three-dimensional STIR SPACE imaging depicts lesions along the long axis of the nerves and provides anatomic information that helps optimize preoperative planning. Maximum intensity projection (MIP) images generated from these 3D images may be used to highlight the different appearances of the peripheral nerves. The use of intravenous gadolinium contrast material is reserved for patients with suspected neoplasm, inflammation, diffuse polyneuropathy, or postoperative complications (3–5).
Peripheral Present in tustrand-like mors and inareas of fections (seen hypointenas disrupsity on T1tion of the and T2blood-nerve weighted barrier) images or nerve encasement
Interpretation Approach
Familiarity with peripheral nervous system anatomy and pathophysiology, as well as knowledge of clinical findings and electrodiagnostic test results, is paramount to interpreting MR neurographic results. The nerve roots and peripheral nerve branches are assessed side-by-side ar MR neurography (Table 2). Typically, normal peripheral nerves are symmetrical in terms of size, morphologic characteristics, and signal intensity, accounting for variations in fat saturation. The fascicular pattern of peripheral nerves is consistently visualized in large nerve branches (those with a diameter larger than 3 mm), such as the femoral and sciatic nerves, but may also be seen in smaller nerves, such as the ilioinguinal and genitofemoral nerves, when they are affected by neuropathy and enlarged (6). The regional
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Table 3 Appearance of Denervation Changes in Skeletal Muscles at MR Neurography Duration
Imaging Findings
Acute (3 months)
Areas of hyperintensity on T2-weighted images (indicative of edema) Areas of hyperintensity on T2- (indicative of edema) and T1-weighted images (indicative of fatty infiltration) Areas of hyperintensity on T1-weighted images (indicative of fatty infiltration) and reduced muscle volume (indicative of atrophy)
Table 4 Originating Spinal Roots and Sensory and Muscular Innervations of the Lumbosacral Plexus Nerves Innervation Nerve
Roots
Muscular
Sensory
Iliohypogastric
L1 (± T12)
Ilioinguinal
L1 (± T12)
Lower fibers of transverse abdominal and internal oblique muscles Lower fibers of transverse abdominal and internal oblique muscles
Genitofemoral
L1 and L2
Lateral gluteal region and area above the pubis Pubic symphysis, superior and medial aspect of femoral triangle, root of penis and anterior scrotum in men, mons pubis and labia majora in women Thigh adjacent to the inguinal ligament and around the femoral triangle; spermatic cord and scrotum in men, labia majora in women Medial and distal thigh
Obturator
L2–L4
Femoral
L2–L4
Lateral femoral cutaneous Sciatic nerve
L2 and L3 L4–S3
Pudendal
S2–S4
Superior gluteal
L4–S1
Inferior gluteal
L4–S1
Cremaster muscle
Adductor magnus, adductor brevis, adductor longus, obturator externus, pectineus and gracilis muscles Quadriceps, pectineus, and sartorius muscles None Biceps femoris, semitendinosus, semimembranosus, adductor magnus muscles Sphincters of urinary bladder and rectum Gluteus medius, gluteus minimus, and tensor fasciae latae muscles Gluteus maximus
skeletal muscles are evaluated for denervation changes, which are, as a rule, always distal to the level of the peripheral nerve insult (Table 3) (7).
Normal Anatomy and Imaging Appearance of the Lumbrosacral Plexus
At the level of the L2–L5 transverse processes, the ventral rami of the L1–L4 spinal nerve roots coalesce in or posterior to the psoas major muscle
Upper and anterior thigh, hip and knee joints Anterior and lateral thigh Gluteal region, posterior thigh, perineum, hip joint, popliteal fossa, lower leg (except the medial part) External genitalia in both men and women None None
to form the lumbar plexus (8). The lumbar plexus gives rise to the iliohypogastric, ilioinguinal, genitofemoral, femoral, and lateral femoral cutaneous nerves, which exit lateral to the psoas major muscle and the obturator nerve and lumbrosacral trunk (which course medial to the psoas major muscle) (Table 4). The ventral rami of the L4–S3 spinal nerve roots coalesce anterior to the piriformis muscle to form the sacral plexus, which gives rise to the sciatic, pudendal, superior gluteal, and inferior gluteal nerves. The lumbar and sacral plexuses connect in the lumbrosacral trunk to form the lumbrosacral plexus (Figs 1, 2) (9).
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Figures 1, 2. (1) Illustration shows the anatomy of the lumbar plexus from T12 to L5, with “T” denoting thoracic and “L” denoting lumbar nerve roots. Peripheral nerve branches (*) to the psoas and iliacus muscles are also illustrated. (2) Illustration shows the anatomy of the sacral plexus from L4 to S5, with “L” denoting lumbar and “S” denoting sacral nerve roots. + = peripheral nerve branches to the piriformis, * = quadratus femoris and inferior gemellus, ** = obturator internus and superior gemellus, x = levator ani, coccygeous, and sphincter ani externus.
aponeurosis of the external oblique muscle. The ilioinguinal nerve also courses over the quadratus lumborum muscle, along the inferior aspect of the iliohypogastric nerve, sending branches to the iliohypogastric nerve. Subsequently, the ilioinguinal nerve pierces the lateral abdominal wall and runs medially to the level of the inguinal ligament (10,11). The sizes of the ilioinguinal and iliohypogastric nerves are inversely proportional to one another. Because of their small size, depiction of the ilioinguinal and iliohypogastric nerves at 2D MR neurography is inconsistent unless they are abnormally enlarged; however, they are often seen at 3D MIP STIR SPACE imaging at the site where they exit the psoas muscles (Fig 3). Figure 3. Normal iliohypogastric and ilioinguinal nerves in a 33-year-old man. Coronal 3D MIP STIR SPACE MR image shows the iliohypogastric (arrowheads), left ilioinguinal (black arrow), and lateral femoral cutaneous (white arrows) nerves where they exit the psoas muscles.
Iliohypogastric and Ilioinguinal Nerves The iliohypogastric and ilioinguinal nerves are formed by the anterior division of the L1 root. Occasionally, they receive a small contribution from the T12 root. The iliohypogastric nerve courses anterior to the psoas major muscle and runs inferolaterally over the anterior aspect of the quadrates lumborum muscle and posterior to the kidney. It then pierces the transversus abdominis muscle and runs superior to the iliac crest in the lateral abdominal wall. Its terminal branch courses parallel to the inguinal ligament and exits the
Genitofemoral Nerve The genitofemoral nerve arises from the anterior divisions of the L1 and L2 roots and pierces the psoas major muscle inferior to the iliohypogastric and ilioinguinal nerves at, approximately, the level of L3–L4. Subsequently, the genitofemoral nerve divides into two branches—medial genital and lateral femoral—that run downward over the anterior aspect of the psoas major muscle. The medial genital branch enters the inguinal canal and courses parallel to the spermatic cord in men and the round ligament of the uterus in women. The lateral femoral branch travels lateral to the femoral artery, posterior to the inguinal ligament, and enters the proximal thigh, where it pierces the sartorius muscle distal to the inguinal ligament (12). At MR neurography, it has a relatively straight trajectory along the anterior margin of the psoas muscle and toward the inguinal regions (Fig 4).
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Figure 4. Normal genitofemoral nerves in the same patient as in Figure 3. Coronal 3D MIP STIR SPACE MR image shows the genitofemoral nerves (arrows) at the anterior margin of the psoas muscle. The right sciatic nerve (arrowhead) is also seen to split.
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Figure 5. Normal obturator nerves in a 59-year-old man. Axial T2-weighted SPAIR MR image shows the course of bilateral obturator nerves (arrows) along the pelvic side walls.
Obturator Nerve The obturator nerve is formed by the anterior rami of the L2–L4 roots. It emerges from the medial border of the psoas major muscle beneath the common iliac vessels, immediately lateral to the sacrum, and courses along the lateral wall of the lesser pelvis to enter the obturator foramen. Just before it enters the thigh, it is separated into anterior and posterior branches by the adductor brevis muscle. The anterior branch of the obturator nerve courses superficial to the adductor brevis muscle and terminates at the distal aspect of the adductor longus muscle, where it forms a subsartorial plexus that communicates with the anterior cutaneous branches of the femoral and saphenous nerves and innervates the distal medial thigh (13,14). The obturator nerve is consistently depicted on axial and coronal MR neurographic images because it courses longitudinally along the pelvic sidewall (Fig 5). The accessory obturator nerve, which is present in about onethird of people, arises from the ventral rami of the L3 and L4 roots, descends along the medial border of the psoas muscle, crosses the superior pubic ramus, and courses under the pectineus muscle, where it divides into numerous branches. One of these branches supplies the pectineus muscle, another is distributed to the hip joint, and a third one communicates with the anterior branch of the obturator nerve.
Figure 6. Normal femoral and ilioinguinal nerves in a 59-year-old man. Coronal 3D MIP STIR SPACE MR image shows normal femoral (arrows) and ilioinguinal (arrowheads) nerves.
Femoral Nerve The femoral nerve arises from the posterior divisions of the L2–L4 roots, descends through the psoas major muscle, and emerges from the lower part of the lateral muscle border. It courses inferiorly between the psoas major and iliacus muscles, posterior to the iliac fascia, and runs beneath the inguinal ligament and into the thigh, where it splits into anterior and posterior divisions; the saphenous nerve derives from the posterior division (15–17). The femoral nerves are consistently seen at MR neurography, which depicts areas that are symmetric in terms of hyperintensity and size at the level of the iliopsoas crotch and isointense (including their anterior and posterior divisions) at the site of the inguinal ligament (Fig 6) (9,18).
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images, the lateral femoral cutaneous nerve is consistently isointense along the anterior surface of the iliopsoas muscle (Fig 7). Minimal hyperintensity is normal where it courses across the inguinal ligament, a result of magic angle artifact.
Sciatic Plexus
Figure 7. Normal lateral femoral cutaneous nerves. Axial T2-weighted SPAIR MR image obtained at the level of the pelvis shows both lateral femoral cutaneous nerves medial to the anterior superior iliac spine.
Figure 8. Normal sciatic nerves in the same patient as in Figure 6. Coronal 3D MIP STIR SPACE MR image shows normal sciatic nerves (arrows).
Lateral Femoral Cutaneous Nerve The lateral femoral cutaneous nerve is a sensory branch that derives from the posterior divisions of the L2 and L3 roots. It pierces the lateral side of the psoas major muscle, runs obliquely over the iliacus muscle, and lies immediately medial to the anterior superior iliac spine, leaving the pelvis inferior to the lateral aspect of the inguinal ligament and over the sartorius muscle. In the thigh, it briefly courses under the fascia lata and, before breaching the fascia, divides into anterior and lateral branches that often communicate with the cutaneous branches of the femoral and saphenous nerves to form the patellar plexus (19,20). On axial T2-weighted SPAIR MR neurographic
The sciatic plexus is formed by the ventral rami of the L4–S3 nerve roots, which join to form the tibial, common peroneal, and posterior femoral cutaneous nerves. After the sciatic plexus exits the pelvis through the greater sciatic foramen (which descends anterior, above, or through the piriformis muscle), the tibial and common peroneal components become enclosed in a common nerve sheath, forming the sciatic nerve. The sciatic nerve then enters the gluteal region and courses inferiorly between the adductor magnus and gluteus maximus muscles to the distal onethird of the thigh, where it divides into the tibial and common peroneal trunks. The posterior femoral cutaneous nerve is a sensory branch of the sacral plexus that arises from the posterior divisions of the S1 and S2 roots and the anterior divisions of the S2 and S3 roots and travels immediately posterolateral to the sciatic nerve. It courses under the gluteus maximus, branches into the perineal and inferior cluneal nerves, and descends in the posterior thigh over the long head of the biceps femoris to the back of the knee, where it pierces the deep fascia and accompanies the small saphenous vein to the middle back of the leg. The terminal segments of the posterior femoral cutaneous nerve communicate with the sural nerve (9,12,21). The sciatic plexus and sciatic nerves are consistently seen at MR neurography, normally with symmetric morphologic characteristics and signal intensity, given the limitations of inconsistent fat suppression (Fig 8).
Pudendal Plexus The pudendal plexus is formed along the posterior wall of the pelvis by the anterior divisions of the S2–S4 and C1 nerve roots. It branches into the perforating cutaneous, pudendal, anococcygeal, visceral, and muscular nerves, the most important of which is the pudendal nerve (formed by the S2–S4 nerve roots), which passes between the piriformis and coccygeus muscles and exits the pelvis through the lower part of the greater sciatic foramen. The pudendal nerve then
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crosses the ischial spine between the sacrotuberous and sacrospinous ligaments and re-enters the pelvis through the lesser sciatic foramen, where the inferior hemorrhoidal nerve branches off. The pudendal nerve then courses through the pudendal (Alcock) canal with the internal pudendal vessels. The pudendal canal is located along the lateral wall of the ischiorectal fossa and medial to the obturator internus muscle, with its outer border formed by the obturator fascia. Finally, the pudendal nerve divides into the perineal nerve (the larger branch) and the dorsal nerve (the smaller branch) of the penis or clitoris. The perineal nerve communicates with the homonymous branch of the posterior femoral cutaneous nerve, which accompanies the internal pudendal artery along the ramus of the ischium and inferior pubic ramus and runs between the superior and inferior layers of the urogenital diaphragm, where it pierces the inferior layer of the diaphragm, courses anteriorly along the dorsum of the penis (with the respective dorsal artery), and terminates in the glans penis. In women, the perineal nerve is very small (22,23). On axial T1weighted and fat-suppressed T2-weighted MR neurographic images, the pudendal nerve is easily seen between the sacrotuberous and sacrospinous ligaments at the level of the ischial spine and in the pudendal canal. The distal perineal and penile branches are not consistently seen.
Superior Gluteal Nerve The superior gluteal nerve originates from the L4–S1 nerve roots and exits the pelvis through the greater sciatic foramen, above the piriformis muscle, with the superior gluteal artery and vein. It then branches into the superior and inferior nerves: The inferior gluteal nerve originates from the L5–S2 nerve roots and exits the pelvis through the greater sciatic foramen, inferior to the piriformis muscle (9). Because of their small size, the gluteal nerves are not visualized at MR neurography unless they are abnormally enlarged.
Clinical Indications for MR Neurography
The clinical indications for MR neurography of the lumbrosacral plexus are evolving. Currently, they include: (a) confirmation of lumbrosacral plexus involvement and definition of the extent
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of disease in patients with a tumor or tumorlike condition; (b) assessment of the extent of injury; (c) evaluation of the lumbrosacral plexus in patients with indeterminate results at MR imaging of the lumbar spine; (d) exclusion of a mass lesion in patients with unilateral abnormalities at EMG; (e) exclusion of lesions in patients with normal or indeterminate findings at EMG and persistent symptoms; (f) confirmation of lumbar plexitis or plexopathy in patients with clinically confusing findings and underlying known systemic conditions; (g) evaluation of peripheral branch nerve abnormalities and associated lesions, such as piriformis syndrome, pudendal neuralgia, meralgia paresthetica, and nerve entrapments after hernia repair; and (h) planning for MR imaging–guided administration of pain medication (7,24).
Pathologic Conditions and Clinical Findings
The lumbrosacral plexus may be locally involved by extrinsic compression or infiltration or associated with systemic conditions, such as metabolic, autoimmune, ischemic, and inflammatory disorders and vasculitis (9). Causes of local involvement include neoplasms, such as benign and malignant peripheral nerve sheath tumors; lymphoma; malignancies, such as cervical cancer, uterine cancer, colorectal cancer, mesenchymal tumors, and metastatic infiltration; tumor variants, such as perineuroma and amyloid; entities related to the psoas major muscle or greater sciatic notch, such as hematoma, abscess, and phlegmon; endometriosis; and aortic aneurysm. Compared with the brachial plexus, which is vulnerable to blunt injury, the lumbrosacral plexus is relatively protected by the axial skeleton; thus, direct trauma is an uncommon cause of plexopathy. However, the lumbrosacral plexus may be indirectly affected by lumbar spinal injuries, pelvic fractures, and hip fractures or dislocations and directly affected by iatrogenic injuries resulting from surgical gynecologic or anesthetic procedures, compression, traction, and vascular insults (25). Systemic and inflammatory causes of lumbrosacral involvement include diabetes mellitus (diabetic amyotrophy), inflammatory neuritis (eg, Guillain-Barré syndrome), ischemic conditions, chronic inflammatory demyelinating polyneuropathy, hereditary neuropathies (eg, Charcot-Marie-Tooth disease), radiation neurop-
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dial thigh, or pelvic area and altered sensation in the inguinal area (10). Patients may present with debilitating leg pain that radiates to the lower back and buttocks and progresses posterolaterally down the leg, followed by numbness and weakness. In some patients, the clinical picture may be further complicated by footdrop, sensory changes to the top of the foot, a loss of tendon reflexes, and, rarely, bowel and bladder incontinence, as well as sexual dysfunction (9,33,34). Typically, unilateral localization of symptoms indicates a localized entity, whereas bilateral symptoms indicate a systemic process (9).
Figure 9. Known peripheral nerve sheath tumors in a 25-year-old man with right groin pain. Coronal 3D MIP STIR SPACE MR image shows multiple peripheral nerve sheath tumors (arrows), which demonstrate the target and tail signs, involving the right ilioinguinal nerve.
athy, sarcoidosis, and connective tissue disorders (8,26–28). In addition, a primary or idiopathic form of lumbrosacral plexopathy has been reported and is probably a result of an abnormal immunologic response; it is considered analogous to idiopathic brachial plexopathy (29,30). This form of lumbrosacral neuropathy is associated with a favorable outcome and spontaneous recovery, and it is diagnosed on the basis of the exclusion of other organic causes (31). The clinical picture of lumbrosacral plexopathy varies and depends on the location and degree of involvement. Typically, patients with upper nerve root involvement present with symptoms in the femoral and obturator nerves: Femoral neuropathy causes weakness of the quadriceps and iliopsoas major muscles, with or without sensory deficits over the anterior and medial thigh and anteromedial leg, whereas obturator neuropathy causes weakness in the hip adductors and sensory changes in the upper medial thigh (32). Sensory symptoms may include a loss of sensation (dysesthesia) to the anterolateral thigh, a result of lateral femoral cutaneous nerve involvement, and decreased sensation (paresthesia) in the mons and labia majora, a result of genitofemoral nerve involvement. Damage to the ilioinguinal or iliohypogastric nerves manifests with burning pain in the lower abdomen, upper me-
MR Neurographic Findings of Lumbrosacral Plexopathy
Similar to other peripheral areas in the body, evaluation of the lumbrosacral plexus at MR neurography relies on direct imaging features, such as changes in nerve size, fascicular morphologic characteristics, signal intensity, and nerve course, and indirect features, such as effacement of perineural fat planes, a result of focal fibrosis or mass lesions, and changes in regional muscle denervation (5). Similar to the brachial plexus, the signal intensity of the lumbrosacral nerves at T2weighted imaging is considered abnormal when it approaches that of adjacent vessels and is asymmetric to that in the contralateral side. Minimally increased signal intensity at T2-weighted MR imaging should be approached with caution because “magic angle” artifact is a well-recognized occurrence at MR imaging of the lumbrosacral plexus (9,35). If a nerve really is abnormal, changes in signal intensity will persist for a longer duration or other findings will be present.
Neoplasms and Tumorlike Conditions Peripheral nerve sheath tumors manifest with areas of focal or fusiform nerve enlargement that variably demonstrate classic signs, such as the tail, target, fascicular, bag-of-worms, and split-fat signs (Figs 9, 10). In general, differentiation between benign and malignant variants is not reliable at imaging, although a large size (>5 mm in its largest diameter), ill-defined margins, peripheral contrast enhancement, intratumoral cystic or necrotic changes, substantial interval growth, infiltrative borders, and a perilesional edema-like rim are suspicious for malignancy (Fig 11) (36,37).
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Figure 10. Neurofibromatosis type 1 in a 32-year-old man with bilateral leg pain and weakness. (a) Coronal 3D STIR SPACE MR image at the level of the lower abdomen and pelvis shows numerous hyperintense peripheral nerve sheath tumors, some of which demonstrate the target sign (arrows). (b) Coronal 3D MIP STIR SPACE MR image at the level of the lower abdomen and pelvis shows diffuse enlargement of the sciatic nerves and multifocal lesions (arrows). Various segments of the lumbrosacral plexus are involved, a finding in keeping with the clinical diagnosis of neurofibromatosis type 1.
Figure 11. Malignant peripheral nerve sheath tumor in a 35-year-old man with left leg pain and swelling. Coronal T2-weighted 3D STIR SPACE (a) and contrast-enhanced T1-weighted fatsaturated 3D volume-interpolated breath-hold examination (VIBE) (b) MR images at the level of the lower abdomen and pelvis show a heterogeneous enhancing mass that is predominantly hyperintense at T2-weighted imaging. Internal areas of necrosis that involve the left lumbrosacral plexus (arrow) and extend along the left femoral nerve (arrowhead) are seen. The presence of a malignant peripheral nerve sheath tumor was confirmed at surgery.
Perineurioma, a rare benign neoplasm that occurs in children and young adults, manifests with a functional loss of slow motor skills. At MR imaging, enlargement of a short (isointense relative to muscle on T1-weighted images) or long (hy-
perintense relative to muscle on T2-weighted images) nerve segment, which features a prominent fascicular pattern and intense contrast enhancement and is associated with muscle denervation changes, is seen. Neurolymphoma manifests with diffuse, predominantly homogeneous thickening and multifocal nodularity of the involved nerve.
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Figure 12. Lumbrosacral plexopathy and right sciatic neuropathy caused by a desmoid tumor in a 31-year-old woman with pain and swelling in the right leg. Axial T2-weighted fat-saturated (a) and coronal contrast-enhanced T1-weighted fat-saturated 3D VIBE (b) MR images at the level of the pelvis show a heterogeneous enhancing mass that is predominantly hypointense at T2-weighted imaging (arrows), involves the right lumbrosacral plexus, and extends along the right sciatic nerve. The sciatic nerve (arrowhead in a) is anteriorly displaced and demonstrates abnormally high signal intensity. Desmoid tumor was confirmed at histologic analysis.
Figure 13. Sciatic plexopathy caused by endometriosis in a 39-year-old woman with left leg pain and intermittent footdrop. (a) Axial T1-weighted MR image shows a masslike lesion (arrow) in the left greater sciatic notch and surrounding the sciatic plexus. (b) Axial T1-weighted fat-suppressed MR image shows the left sciatic nerve fascicles (arrowhead), which are abnormally hyperintense, enlarged, and hemorrhagic. Additional hemorrhagic foci (arrows) are also seen. Endometriosis was confirmed at histologic analysis.
Fibrolipomatous hamartoma demonstrates diffuse enlargement of the involved nerve, which has a coaxial cable–like appearance on axial images and a spaghetti-like appearance on longitudinal images (37). Neuroma is a tumorlike lesion that may form a few months after nerve trauma and is characterized by fusiform enlargement of the nerve, which varies in length (usually as long as 5 cm). It is isointense relative to muscle on T1-weighted images, iso- to hyperintense on T2-weighted im-
ages, and typically does not enhance. Intra- and extraneural ganglion cysts appear as a thin-walled cystic lesion. They may feature internal septations and multilobulations, with minimal peripheral and septal enhancement. MR neurography adequately depicts the lesion and involved nerve branches in patients with an abdominal or pelvic tumor, tumor variant, or other type of focal lumbrosacral plexus involvement (Figs 12, 13).
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Figure 14. Lumbrosacral plexus avulsion injury in a 62-year-old man with right leg palsy who was involved in a boating accident. Coronal 3D MIP STIR SPACE MR image shows avulsion of the right L2, L3, and L4 dorsal root ganglia (arrows) with substantial inferior displacement of the L3 and L4 ganglia against a background of hemorrhage and edema in the right paraspinal muscles. The left dorsal root ganglia (arrowheads) appears normal. Minor lumbar vertebral contusions were also seen, with no fracture or dislocation.
Trauma
Systemic Diseases
In patients with acute traumatic neuropathy, the presence of hemorrhage and edema at the injury site may make it difficult to assess nerve discontinuity. It is critical to determine whether damage is merely the result of a stretch injury with nerve continuity or if neuroma formation, nerve root avulsion, or nerve discontinuity is present, which may necessitate surgical intervention (Fig 14) (5,35). MR neurographic findings depend on the severity of the nerve injury. Neurapraxia, the least severe type of nerve injury, is characterized by conduction block, with abnormally increased signal intensity in the involved nerve or nerves on T2-weighted SPAIR images and no associated muscle denervation changes. In axonotmesis, a loss of axonal continuity and its covering (endoneurium) is seen, with wallerian degeneration distal to the site of insult. MR neurography depicts muscle denervation changes and, as the injury becomes more severe, nerve enlargement, as well as disruption or effacement of nerve fascicles. In neurotmesis, axonal injury and disruption of the surrounding perineurium and epineurial layers are seen; MR neurography depicts a neuroma in continuity or complete transection of the nerve with formation of an end-bulb neuroma (7). Patients with neurapraxia undergo a full recovery, and those with axonotmesis may undergo full, incomplete, or poor recovery. Patients with neurotmesis do not recover; in these patients, prognosis depends on prompt and appropriate surgical intervention and postoperative support. In this setting, MR neurography may be used for anatomic mapping and grading of the nerve injury to help optimize the clinical outcome.
Amyloidosis rarely involves peripheral nerves. When peripheral nerves are involved, MR neurography depicts focal (amyloidoma) or diffuse nerve enlargement with associated multifocal masses related to long-segment infiltration, foci of hypointensity on T2-weighted images, and areas of variable contrast enhancement; disruption of or a prominent configuration of fascicles may also be seen (Fig 15) (37). Ischemic lumbrosacral plexus neuropathy or vasculitis may demonstrate long-segment nerve abnormalities, which are seen as areas of hyperintensity on T2-weighted images and fascicular prominence, a finding that usually is bilateral and symmetric unless local ischemia, a result of compression or entrapment, is present. Diabetic plexopathy is seen as mildly asymmetric and bilateral areas of enlargement and hyperintensity on T2-weighted images of the lumbrosacral plexus, commonly with involvement of the L4–S1 nerve roots and sciatic or femoral neuropathy.
Infectious, Autoimmune, and Hereditary Disorders In acute lumbrosacral plexus neuritis and Guillain-Barré syndrome, areas of mild hyperintensity or mild enlargement of multiple plexus branches are seen on T2-weighted images. Typically, affected plexus branches enhance after administration of gadolinium contrast material. MR neurographic findings of chronic infectious lepromatous neuropathy include moderately abnormal areas of hyperintensity on T2-weighted images and minimal to moderate enlargement of the involved nerve branches, which typically demonstrate enhancement. In acute and subacute stages of radiation neuropathy, nerves distributed within the radiation field demonstrate mild enlargement,
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Figure 15. Amyloid plexopathy in a 45-year-old man with weakness and numbness in the right leg. Coronal (a) and MIP (b) T2-weighted 3D STIR SPACE MR images at the level of the pelvis and thighs show enlargement of the right L5 to S2 nerves (arrowheads), which are heterogeneous and predominantly hyperintense at T2-weighted imaging, and diffuse enlargement of the right sciatic nerve (arrows in b). Amyloidosis was confirmed at histologic analysis.
Figure 16. Idiopathic lumbrosacral plexopathy in an 80-year-old woman with bilateral leg pain and weakness. Coronal (a) and MIP (b) 3D STIR SPACE MR images show mild diffuse enlargement of the right L5 nerve (arrow), which demonstrates abnormal asymmetric high signal intensity. Both L4 nerve roots and the left L5 nerve root (arrowheads) appear normal.
diffuse enhancement, and hyperintensity on T2weighted images, whereas chronic disease manifests with distorted nerves and regional fibrosis, with or without fatty bone marrow changes. In idiopathic lumbrosacral plexopathy, the affected side of the lumbrosacral plexus, particularly the sacral part, demonstrates asymmetric hyperintensity on T2-weighted images, with or without contrast enhancement (31,35). Associated peripheral nerve branch abnormalities may also be seen (Fig 16). In Charcot-Marie-Tooth
disease involving the lumbrosacral plexus, moderate to marked symmetric enlargement of multiple lumbrosacral plexus branches, with or without superimposed entrapments, is seen. Chronic in flammatory demyelinating polyneuropathy demonstrates multifocal, nearly symmetric, and bilateral moderate nerve enlargement with prominent
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or, less commonly, effaced fascicles (Fig 17). Contrast enhancement varies depending on the stage of disease. Because chronic inflammatory demyelinating polyneuropathy is commonly diagnosed in the subacute or chronic stage, it demonstrates no or minimal contrast enhancement compared with tumors (eg, perineurioma) that demonstrate intense enhancement.
MR Neurographic Findings in Individual Nerve Branches Iliohypogastric and Ilioinguinal Nerves Surgical procedures, such as abdominal incisions, suture entrapment, ilium harvesting for grafting, inguinal lymph node dissection, femoral catheterization, and orchiectomy, are the most common cause of iliohypogastric and ilioinguinal nerve injury (38,39). Less common causes include sportsrelated external oblique muscle tears and abdominal expansion in the third trimester of pregnancy. Iliohypogastric nerve entrapment manifests with pain and dysesthesia at the surgical site that radiates to the hypogastric area (38,39). Ilioinguinal nerve entrapment manifests with pain and dysesthesia at the surgical site that radiates to the inguinal area, labia majora, or scrotum (40–42). Occasionally, there may be tenderness medial to the anterior superior iliac spine. Electrodiagnostic testing is not reliable for diagnosing iliohypogastric or ilioinguinal neuropathy; positive nerve blocking with local anesthesia may be necessary to establish a diagnosis (40). It is noteworthy that a correctly performed negative nerve block is more specific for ilioinguinal nerve entrapment than a positive nerve block. However, only a few studies have shown the therapeutic value of these blocks. The iliohypogastric and ilioinguinal nerves are readily identified at MR neurography when they are enlarged or entrapped, although its main role is to exclude neoplastic or other space-occupying lesions along their anatomic course (Fig 9). Patients with iliohypogastric or ilioinguinal nerve injury undergo rehabilitation, with neurolysis and surgical excision reserved for those who do not respond to treatment.
Genitofemoral Nerve Genitofemoral nerve injury is most commonly iatrogenic, resulting from procedures such as abdominal incisions, hernia repair, lymph node biopsy, suture entrapment, and inguinal lymph node dissection; it also results from pregnancy and retroperitoneal hematoma (32,35). Genitofemoral nerve entrapment manifests with pain
Figure 17. Chronic inflammatory demyelinating polyneuropathy in a 38-year-old man. Axial T2-weighted SPAIR (a) and coronal 3D MIP STIR volumetric isotropic turbo spin-echo acquisition (VISTA; Philips Medical Systems, Best, the Netherlands) (b) MR images show asymmetric enlargement of both femoral nerves (arrows) and the left obturator nerve (arrowhead) with prominent fascicles, a finding in keeping with the clinical and laboratory findings of chronic inflammatory demyelinating polyneuropathy.
and dysesthesia at the surgical site that radiates inferior to the inguinal area over the anterior thigh, labia majora, or scrotum. Occasionally, localized tenderness medial to the anterior iliac spine is present. Because electrodiagnostic testing is not reliable, genitofemoral neuropathy may be diagnosed on the basis of positive blocking with local anesthesia (41–43). At MR neurography, the nerve is easily identified when it is abnormally enlarged or entrapped (Fig 18). MR neurography is also used to exclude neoplastic and other space-occupying lesions along the course of the nerve. Treatment of genitofemoral nerve injury includes rehabilitation, with neurolysis and surgical excision reserved for patients who do not respond to treatment. Complications of surgery include a numb scrotum or labium majora and a loss of cremasteric reflex.
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Figure 18. Genitofemoral neuralgia in a 50-year-old man with pain in the right side of the scrotum and pubic area after undergoing hernia repair. Axial T1-weighted (a) and T2-weighted SPAIR (b) MR images at the level of the lower pelvis show entrapment and enlargement of the right genitofemoral nerve (arrow) in the inguinal area. Focal fibrosis is seen at the previous surgery site. Figure 19. Wallerian degeneration of the right femoral and obturator nerves in a 45year-old woman with long-standing weakness in the right leg. At MR imaging of the lumbar spine, extrusion of the right posterolateral disk was seen at the level of the L3–L4 nerve roots and was compressing the L3 root. Axial T2weighted SPAIR (a) and coronal 3D MIP STIR VISTA (b) MR images show hyperintensity and enlargement of the right femoral (white arrow) and obturator (white arrowhead) nerves. The left femoral (black arrow in a) and obturator (black arrowhead in a) nerves appear normal.
Obturator Nerve Usually, obturator nerve injuries are associated with femoral nerve injuries. They are less common in isolation. Common causes of obturator nerve injury include pelvic trauma; hip fracture or dislocation; hip arthroplasty; pelvic tumor; paralabral cyst; obturator hernia; baseball pitcher–hockey goalie syndrome (nerve entrapment by muscle
hernia); and delivery of a fetus, which compresses the nerve between the head of the fetus and the pelvis. Obturator neuropathy manifests with adductor muscle and internal rotation weakness and ambulation difficulties. If the anterior branch of the obturator nerve is involved, pain and dysesthesia that are exacerbated by exercise may be present in the medial groin and the medial aspect of the thigh (44–47). Electrodiagnostic testing and nerve blocking may confirm the diagnosis of obturator neuropathy. MR neurography may be used to depict the abnormal nerve, a mass lesion along its course, and secondary changes in muscle denervation in the adductor compartment and exclude entities with a similar clinical picture, such as athletic pubalgia, osteitis pubis, stress fracture, and inguinal hernia (Fig 19). Most patients with obturator neuropathy undergo rehabilitation, with decompression, neurolysis, and surgical excision performed in those who do not respond to treatment or with a mass lesion.
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Figure 20. Diabetic amyotrophy in a 71-yearold man with pain and weakness in the right leg, a long-standing history of diabetes, and multifocal motor and sensory neuropathy changes at EMG; on the basis of these findings, the differential diagnosis included radiculopathy and diabetic amyotrophy. (a) Axial T2-weighted SPAIR MR image shows abnormal high signal intensity in and mild enlargement of the left femoral (white arrow) and genitofemoral (black arrow) nerves. The right femoral nerve appears normal (arrowhead). (b) Coronal 3D MIP STIR SPACE MR image shows diffuse neuropathy of the left femoral (white arrows) and genitofemoral (black arrows) nerves, a finding consistent with diabetic amyotrophy. Arrowhead = right femoral nerve.
Femoral Nerve Injury to the femoral nerve may be caused by direct trauma, such as a penetrating wound to the hip and pelvis fracture; compression by a pelvic mass; a psoas muscle hematoma or an aortic or iliac aneurysm; a stretch injury from the lithotomy position or excessive hip abduction with external rotation; or iatrogenesis from pelvic, abdominal, or spinal surgery (48,49). Femoral nerve injury manifests with quadriceps muscle weakness, mild pain near the inguinal ligament, and, potentially, numbness in the medial leg and calf. Sensory symptoms in the saphenous nerve distribution are rare in patients with injury to the main trunk of the femoral nerve. Femoral nerve injuries are diagnosed on the basis of clinical history and examination; the diagnosis may be confirmed with electrodiagnostic testing. MR neurography may depict abnormal areas of asymmetric signal intensity in the femoral nerve, nerve continuity or discontinuity in cases of trauma, the location of causative space-occupying lesions, and secondary muscle denervation changes in the extensor compartment, and it may be used to exclude entities with a similar clinical picture (Figs 19, 20). Conservative treatment is recommended in most patients, whereas surgery is performed in those with an extrinsic lesion or nerve discontinuity resulting from trauma (50).
Lateral Femoral Cutaneous Nerve Common causes of lateral femoral cutaneous nerve injury include seat belt injury from motor vehicle accidents, compression by tight garments, anomalous pelvic positioning resulting from a leg
length discrepancy, abdominal (eg, ovarian and uterine) masses, and diabetes. Entrapment usually occurs in patients who are middle-aged and is bilateral in 10% of patients. Neuropathy manifests with symptoms of anterolateral thigh dysesthesia that increases with hip extension, prolonged standing, or lying prone and alleviates when in a sitting position (meralgia paresthetica). Occasionally, tenderness medial to the anterior superior iliac spine may be present (51). Results of electrodiagnostic testing and nerve blocking performed for diagnostic purposes vary. In patients with abnormal results, the lateral femoral cutaneous nerve may appear hyperintense, or a neuroma may be seen, typically adjacent to the anterior superior iliac spine (Fig 21). In patients in whom neurolysis was unsuccessful, MR neurography may also depict re-entrapment. Patients with lateral femoral cutaneous neuropathy undergo rehabilitation, with surgical excision and decompression, with or without neurolysis, performed in those who do not respond to treatment (52).
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Figure 21. Neuroma of the right lateral femoral cutaneous nerve in a 38-year-old woman with clinical findings suspicious for right-sided meralgia paresthetica. (a, b) Sequential axial T2-weighted SPAIR images show progressive abnormal high signal intensity in the right lateral femoral cutaneous nerve at the anterior superior iliac spine and a focal neuroma (arrow). The contralateral nerve (arrowhead) appears normal. Figure 22. Split sciatic nerves and piriformis muscles in a 55-year-old man with clinical findings suspicious for right piriformis syndrome. Coronal T2-weighted 3D SPACE MR image shows that both piriformis muscles (arrowheads) and sciatic nerves (arrows) are split.
Sciatic Plexus The sciatic plexus is susceptible to involvement by lesions in the pelvis, gluteal region, hip, and thigh, distal to the lumbrosacral plexus and proximal to its bifurcation into distal branches. Causes of sciatic neuropathy include iatrogenesis, trauma, radiation therapy, piriformis syndrome, neoplasm, endometriosis, and, rarely, vascular malformations or anatomic factors, such as a prominent lesser trochanter. Sciatic neuropathy manifests with pain and paresthesia along the posterior thigh, the lower lateral leg, and the entire foot and with motor deficits, including difficulty flexing the knee and a flail foot with loss of dorsiflexion, plantar flexion, inversion, and eversion. Whereas electrodiagnostic testing is conducted to confirm the clinical diagnosis,
MR neurography may be used to (a) assess nerve continuity in cases of trauma, which is crucial for surgery planning; (b) localize the injury or entrapment (eg, in the piriformis muscle, which demonstrates splitting of the piriformis or intermuscular nerve course or a split nerve indented by the piriformis muscle); (c) depict other causative space-occupying lesions and secondary muscle denervation changes; and (d) exclude other diseases with similar clinical features, such as a hamstring tendon tear or lumbar spine condition that mimics sciatic neuropathy (Figs 22, 23) (9,53,54). Injury and compression are uncommon in the posterior femoral cutaneous nerve, which is easily identified at MR neurography, with areas of hyperintensity on T2-weighted images and other abnormalities rarely encountered. The role of MR neurography is to exclude lesions along the course of the nerve and guide administration of perineural medications.
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Figure 23. Re-entrapment of the right sciatic nerve in a 40-year-old woman with persistent right sciatica after undergoing partial resection of the piriformis muscle and neurolysis of the sciatic nerve. (a) Axial T1-weighted MR image at the level of the pelvis shows focal scarring at the site of resection (arrow). (b) Axial T2-weighted SPAIR MR image shows abnormal high signal intensity in and enlargement of the right sciatic nerve (arrow). The left sciatic nerve (arrowhead) appears normal.
Figure 24. Peripheral nerve sheath tumor of the dorsal nerve of the penis in a 44-year-old man with neurofibromatosis type 1 who presented with penile numbness after undergoing partial resection of a neurofibroma. Sequential axial T2-weighted SPAIR MR images at the level of the lower pelvis and perineum show residual tumor (arrow in a) along the course of the dorsal nerve of the penis, a distal branch of the pudendal nerve. Additional lesions (arrows in b) are seen along the perineal branches of the right pudendal nerve.
Pudendal Nerve The pudendal nerve is predisposed to entrapment at the level of the ischial spine, between the sacrotuberous and sacrospinous ligaments, and in the pudendal canal by way of the falciform process of the sacrotuberous ligament or a thickened obturator fascia. Less commonly, the peripheral
perineal branches may also become entrapped or involved. Local mass lesions, such as pelvic tumors and hematomas, may entrap the nerve, although it is more common for it to become injured or stretched during parturition, bicycling, or a pelvic procedure. Entrapment is a recognized cause of chronic perineal pain (pudendal neuralgia) and typically manifests with pain in the genitalia, perineum, or anorectal region. The
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Figure 25. Gluteal neuropathy in a 39-year-old woman with pain and weakness in the left leg. Axial T2-weighted SPAIR (a) and coronal 3D MIP STIR SPACE (b) MR images at the level of the pelvic floor show normal signal intensity in the pudendal nerves, asymmetric atrophy of the right gluteal muscle (arrows), and signal intensity similar to that of edema, findings indicative of isolated superior and inferior gluteal neuropathy.
diagnosis is established on the basis of a characteristic history of perineal pain that is aggravated by sitting, relieved by standing, and absent when recumbent or sitting on a toilet seat (55–58). Because adjacent neurovascular bundles have similar signal intensity, it is often difficult to appreciate changes in signal intensity in the affected nerve; however, nerve enlargement proximal to the site of entrapment may be seen. The role of MR neurography is also to exclude lesions along the course of the nerve (most commonly fibrosis) and guide administration of perineural medications (Fig 24) (23,35).
Superior and Inferior Gluteal Nerves The most common cause of superior and inferior gluteal nerve impairment is iatrogenic injury during hip replacement (59,60). The role of MR neurography is to exclude lesions along the anatomic course of the nerves. In most cases, neuropathy is indirectly indicated by secondary muscle denervation changes at imaging (Fig 25) (9).
Conclusion
In the evaluation of lumbrosacral plexopathy, 3-T MR neurography is a valuable adjunct to clinical examination and electrodiagnostic testing because it provides anatomic information that is not obtainable with other modalities and is useful for assessing lesions. Knowledge of the imaging patterns of the lumbrosacral plexus and the condi-
tions that affect it may enable radiologists to provide detailed reports and contribute to optimized patient-tailored treatment. Disclosures of Conflicts of Interest.—A.C.: Related
financial activities: grants from Siemens, Integra Life Sciences, and GE-AUR. Other financial activities: consultant for Siemens MSK CAD systems. G.A.: Related financial activities: none. Other financial activities: Patent pending for USPTO and payments for lectures. J.A.C.: Related financial activities: grant from Siemens. Other financial activities: board member of Vital and consultant for GE Healthcare, Medtronic, and Johnson & Johnson.
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Teaching Points
July-August Issue 2013
High-Resolution 3-T MR Neurography of the Lumbosacral Plexus Theodoros Soldatos, MD, PhD • Gustav Andreisek, MD • Gaurav K. Thawait, MD • Roman Guggenberger, MD • Eric H.Williams, MD • John A. Carrino, MD, MPH • Avneesh Chhabra, MD RadioGraphics 2013; 33:967–987 • Published online 10.1148/rg.334115761 • Content Codes:
Page 970 Typically, normal peripheral nerves are symmetrical in terms of size, morphologic characteristics, and signal intensity, accounting for variations in fat saturation. The fascicular pattern of peripheral nerves is consistently visualized in large nerve branches (those with a diameter larger than 3 mm), such as the femoral and sciatic nerves, but may also be seen in smaller nerves, such as the ilioinguinal and genitofemoral nerves, when they are affected by neuropathy and enlarged. Page 974 The lumbrosacral plexus may be locally involved by extrinsic compression or infiltration or associated with systemic conditions, such as metabolic, autoimmune, ischemic, and inflammatory disorders and vasculitis. ` Page 975 Similar to other peripheral areas in the body, evaluation of the lumbrosacral plexus at MR neurography relies on direct imaging features, such as changes in nerve size, fascicular morphologic characteristics, signal intensity, and nerve course, and indirect features, such as effacement of perineural fat planes, a result of focal fibrosis or mass lesions, and changes in regional muscle denervation. Page 978 It is critical to determine whether damage is merely the result of a stretch injury with nerve continuity or if neuroma formation, nerve root avulsion, or nerve discontinuity is present, which may necessitate surgical intervention. Page 979 In idiopathic lumbrosacral plexopathy, the affected side of the lumbrosacral plexus, particularly the sacral part, demonstrates asymmetric hyperintensity on T2-weighted images, with or without contrast enhancement. Associated peripheral nerve branch abnormalities may also be seen.
