Amblyopia ex anopsia

Amblyopia, from the Greek word for “dullness of vision,” is a form of cortical visual impairment caused by degradation of the retinal image during a sensitive period of visual development (the first few years of life). It manifests as reduced acuity in the absence of a structural ocular or visual pathway abnormality (1,2).

• is usually secondary to another pathological process e.g. - refractive errors between the two eyes (anisometropia), ocular misalignment (strabismus or squint)
• most are unilateral but bilateral amblyopia may also occur, particularly if patient has high refractive error in both eyes

Amblyopia causes a range of abnormalities of visual functions which includes

• visual acuity
• vernier acuity - ability to detect the alignment or lack of alignment of the two parts of a broken line
• crowding - better acuity when a stimulus is presented in isolation than when a stimulus is presented surrounded by other stimuli
• contrast sensitivity- ability to distinguish between a stimulus and its background
• motion perception - ability to resolve or discriminate motion
• stereopsis- ability to use binocular cues to aid depth perception
• visual fields, colour vision, and pupillary responses are usually unaffected

The visual acuity of the affected eye is better when test letters are viewed singly rather than in series. Unlike reduced visual acuity due to organic disease, that of the amblyopic eye is unchanged by decreased illumination. However, pursuit movement is impaired when the amblyopic eye is used for fixation. The condition is often asymptomatic and goes undetected until the eyes are tested separately.

It affects 1-5% of the population and is the is the most frequently treated disorder in paediatric ophthalmic and orthoptic practice (1)

• the UK prevalence of amblyopia amongst children aged 4-5 years is likely to be within the range of 1% to 4% (2)

It may be corrected in some individuals.

Reference:

• (1) The Royal College of Ophthalmologists Guidelines for the management of amblyopia
• (2) Public Health England. Legacy screening portal. The UK NSC recommendation on Vision defects screening in children
• (3) Jefferis JM, Connor AJ, Clarke MP. Amblyopia. BMJ. 2015;351:h5811