Lazy Eye (Amblyopia)

Definition

Amblyopia is a reduction in the vision of one or both eyes (with optimum spectacles) which cannot be attributed directly to any structural disease of the eye or brain. It is a defect of central or reading vision, with the peripheral or side vision remaining normal.

Incidence/Age

Amblyopia results from the failure to detect and correct abnormal vision in childhood. It accounts for more reduced vision in childhood than all other causes combined, with a prevalence of around 2% in the UK population.

Causes

Amblyopia can be subdivided into different types depending on the disorder causing it to develop:

Strabismic amblyopia

This is the commonest form and develops in children with a constantly deviating eye (misalignment of the eyes is termed a squint). The brain ignores the image on the back of the squinting eye in order to avoid seeing two pictures (double vision). Over a short period the squinting eye will become lazy or amblyopic.

Anisometropic amblyopia

This develops when the eyes have unequal refractive errors (degrees of long or short sightedness), which causes the picture on the back of the worst eye to be chronically blurred. The brain becomes unable to process a clear image later in life if the situation is not corrected at a young age.

Ametropic amblyopia

This is a reduction in the vision of both eyes which results from large, approximately equal, uncorrected refractive errors in a young child. It most commonly occurs from the picture on the back of both eyes being constantly blurred from undetected long sightedness or astigmatism.

Deprivation amblyopia

This is the rarest form but usually the most severe. It is caused by an eye being deprived from seeing, eg, a droopy upper lid (ptosis) covering the pupil, or having a blurred picture due to an opacity present at birth or developing soon afterwards, eg, cataract, corneal scar.

Symptoms/Signs

It is usually very difficult for parents or teachers to detect amblyopia in infants unless the vision is obviously poor in both eyes or there is a noticeable squint, with one eye constantly turning inwards or outwards. It is most often discovered as a difference in the vision of the two eyes at an early vision screening test.

Complications of Disorder

Early detection and treatment is required to enable an improvement in central vision to be obtained. Vision develops up until around 8 years of age and therefore amblyopia detected after this age will result in permanently reduced vision in one or both eyes.

Tests

A full eye examination is required in all children who fail a visual screening program. Distant vision is measured often with the aid of symbols or picture charts. An orthoptic assessment is performed in order to detect any squint or abnormality using both eyes simultaneously. Eye drops are instilled to dilate both pupils which allows a light to be shone into the eyes and detect whether any significant long or short sightedness is present (refraction). Finally an internal eye examination is carried out using a different light source to rule out any structural abnormalities.

Treatment

Medical

The treatment depends on the exact cause of the amblyopia. The general principle is to prescribe the best possible glasses, if required, in order that the picture on the back of the eye is sharply focussed. When only one eye is affected then occlusion treatment is commenced whereby the "good" eye is covered with a patch (stuck over the eye or onto glasses) for part of the day to encourage the vision to improve in the lazy eye. The length of time recommended for this therapy depends on the age of the child and the severity of the amblyopia.

Surgical

Occasionally surgery is re