Testing Visual Acuity in Children Libby Kelly DBO Orthoptist Taranaki Eye Centre

Introduction  Orthoptists are eye health care professionals who are specifically training to investigate, diagnose and treat defects of binocular vision and abnormalities of eye movement in patients of all ages.

Visual Acuity  Definition: a measure of your central vision, the ability to distinguish details and shapes of objects.  Measured by the smallest identifiable letter/picture that can be seen at a specified distance.  The result does not indicate other important aspects of sight such as peripheral vision (visual fields), the ability to identify colours or depth perception (stereopsis).

Why Test Visual Acuity?  Diagnostic tool: amblyopia or lazy eye means partial or complete loss of vision in one or both eyes caused by conditions that affect the normal development of vision, e.g. squint or strabismus, refractive error, cataract  Consideration for further investigations: refer to Ophthalmologist for cycloplegic refraction and fundus/media check  Baseline data  Evaluate treatment progression (glasses, occlusion)  Measures progression of eye condition (cataract, myopia)  Timing of strabismus surgery: better result when each eye has equal vision  Frequency of follow-up visits  Funding requirement (Ongoing Resourcing Scheme - ORS)  Referral criteria (Blind & Low Vision Education Network NZ – BLENNZ)

Visual Acuity Age

Type of Test

Example of Test

0 months – 2 years

Forced choice preferential looking;

Keeler cards

* optokinetic nystagmus

OKN drum

* Visually directed reaching (from approx 9 months, pincer grip)

100s & 1000s (approx 6/24) Rowe, 1997

Vanishing optotypes (from approx 6 months)

Cardiff Cards

* Objection to occlusion * Gross approximation

The Optokinetic reflex is a combination of a saccade and smooth pursuit eye movement. It is seen when an individual follows a moving object with their eyes. Presence of nystagmus indicates an intact visual pathway.

Keeler Cards

Visual Acuity Age

Type of Test

Example of Test

0 months – 2 years

Forced choice preferential looking;

Keeler cards

* optokinetic nystagmus

OKN drum

* Visually directed reaching (from approx 9 months, pincer grip)

100s & 1000s (approx 6/24) Rowe, 1997

Vanishing optotypes (from approx 6 months)

Cardiff Cards

* Objection to occlusion * Gross approximation

The Optokinetic reflex is a combination of a saccade and smooth pursuit eye movement. It is seen when an individual follows a moving object with their eyes. Presence of nystagmus indicates an intact visual pathway.

OKN Drum

Visual Acuity Age

Type of Test

Example of Test

0 months – 2 years

Forced choice preferential looking;

Keeler cards

* optokinetic nystagmus

OKN drum

* Visually directed reaching (from approx 9 months, pincer grip)

100s & 1000s (approx 6/24) Rowe, 1997

Vanishing optotypes (from approx 6 months)

Cardiff Cards

* Objection to occlusion * Gross approximation

The Optokinetic reflex is a combination of a saccade and smooth pursuit eye movement. It is seen when an individual follows a moving object with their eyes. Presence of nystagmus indicates an intact visual pathway.

Cardiff Cards

Visual Acuity

Age

Type of Test

Example of Test

2 – 3 years

Picture matching or naming

Crowded Kay’s Pictures 3m

Crowding phenomenon is a characteristic of amblyopic vision in which vision is better for single optotype presentation than multiple simultaneous optotype presentation.

Visual Acuity

Age

Type of Test

Example of Test

3+ years

Picture or letter matching or naming (crowded optotypes)

Keeler logMAR Crowded Test

Recording Visual Acuity  Results are recorded as a fraction (Snellen equivalent)  Numerator is the distance in metres at which the test was presented  Denominator is the position on the chart of the smallest picture or letter correctly identified by the child  “Normal” visual acuity considered to be the ability to recognise an optotype when it subtends 5 minutes of arc, i.e. 6/6 on Snellen chart  logMAR scale converts geometric sequence of a traditional chart to a linear scale

logMAR vs Snellen

6/60

6/36

6/24 6/18 6/12 6/9 6/7.5 6/6

Paediatric Visual Acuity  If the child is unable to identify the largest letter or picture at the testing distance, walk slowly towards the child until approx ½m away.  From 4 years:  counting fingers  Hand movements  Light perception (the child can tell if a bright light is shone directly into the eye)

References  Wikipedia  www.ssc.education.ed.ac.uk