Amblyopia

Amblyopia (from Greek αμβλυωπία, "blunt vision"), also called lazy eye, is a disorder of sight.^[1] It involves decreased vision in an eye that otherwise appears normal, or out of proportion to associated structural problems of the eye. There is much more damage to, or impact on, vision in that eye than is predicted. This disorder has been estimated to affect 1–5% of the population.^[2]

In amblyopia visual stimulation either fails to be or is poorly transmitted through the optic nerve to the brain for a continuous period of time. It also can occur when the brain "turns off" the visual processing of one eye to prevent double-vision, for example in strabismus (crossed eyes). It often occurs during early childhood and results in poor or blurry vision.

Detecting the condition in early childhood increases the chance of successful treatment, especially if detected before the age of five. The earlier it is detected, and the underlying cause corrected with glasses or surgery, the better the long term outcomes.^[3]

Strabismus amblyopia

Strabismus, sometimes also incorrectly called lazy eye, is a condition in which the eyes are misaligned.^[9] Strabismus usually results in normal vision in the preferred sighting (or "fellow") eye (the eye that the person prefers to use), but may cause abnormal vision in the deviating or strabismic eye due to the difference between the images projecting to the brain from the two eyes.^[10] Adult-onset strabismus usually causes double vision (diplopia), since the two eyes are not fixed on the same object. Children's brains, however, are more neuroplastic, and therefore can more easily adapt by suppressing images from one of the eyes, eliminating the double vision. This plastic response of the brain, however, interrupts the brain's normal development, resulting in the amblyopia. Recent evidence points to a cause of infantile strabismus lying with the input to the visual cortex.^[11]

Those with strabismic amblyopia tend to show ocular motion deficits when reading, even when they use the nonamblyopic eye. In particular, they tend to make more saccades per line than persons with normal stereo vision, and to have a smaller reading speed, especially when reading a text with small font size.^[12][13]

Strabismic amblyopia is treated by clarifying the visual image with glasses, or encouraging use of the amblyopic eye with an eyepatch over the dominant eye or pharmacologic penalization of the better eye. Penalization usually consists of applying atropine drops to temporarily dilate the pupil, which leads to blurring of vision in the good eye. This helps to prevent the bullying and teasing associated with wearing a patch, although sometimes application of the eye drops is more challenging. The ocular alignment itself may be treated with surgical or non-surgical methods, depending on the type and severity of the strabismus.^[14]

Pathophysiology

Amblyopia is a developmental problem in the brain, not any intrinsic, organic neurological problem in the eyeball (although organic problems can lead to amblyopia which can continue to exist after the organic problem has resolved by medical intervention).^[20] The part of the brain receiving images from the affected eye is not stimulated properly and does not develop to its full visual potential. This has been confirmed by direct brain examination. David H. Hubel and Torsten Wiesel won the Nobel Prize in Physiology or Medicine in 1981 for their work in showing the extent of the damage to ocular dominance columns produced in kittens by sufficient visual deprivation during the so-called "critical period." The maximum "critical period" in humans is from birth to two years old.^[21]

Older age

Although the best outcome is achieved if treatment is started before age 8, research has shown that children older than age 12 and some adults can show improvement in the affected eye. Children from 9 to 11 who wore an eye patch and performed near point activities (vision therapy) were four times as likely to show a two line improvement on a standard 11 line eye chart than children with amblyopia who did not receive treatment. Adolescents aged 13 to 17 showed improvement as well, albeit in smaller amounts than younger children. It is uncertain whether such improvements are only temporary, however, particularly if treatment is discontinued.^[14][29]

There is tentative evidence that perceptual training may be beneficial in adults.^[30][31][32]

Virtual reality computer games where each eye receives different signals of the virtual world that the player's brain must combine in order to successfully play the game have shown some promise in improving both monocularity in the affected eye as well as binocularity.^[33][34]