I’ve noticed that some of my patients who have only one eye that sees well seem to suffer ocular trauma or some other ocular disorder to the good eye and not the bad eye. Why is that?

The main reason why screening for amblyopia in early childhood is done is to ensure that affected children are detected and treated within the critical period. This is important so a level of vision in their amblyopic eye can be achieved that would allow the amblyopic eye to provide useful vision should the non-amblyopic eye be damaged or affected by disease later in life. In other words, the main aim of amblyopia screening and treatment is to allow a person to have a backup eye should anything happen to one of their eyes.

As a study in the UK about 20 years ago looked at  370 patients with amblyopia worse than 6/12 in the amblyopic eye and recently acquired loss of vision in the non-amblyopic of worse than 6/12 over a 2-year period. 27% did not meet the vision driving standard, nearly half were visually impaired (43%) and almost a third were severely visually impaired or blind (30%). The most common cause of visual loss was age-related macular degeneration (27%) vein occlusion (23%) and cataract (16%). Trauma was the most common cause in those under 65 (21%).

It is not clear why people with one good eye and one poor eye suffer trauma to the good eye when a projectile flies towards the face. Hitting metal with metal is a common source of high-velocity hot shards of metal. It could be that the person tilts their head in such a way that the good eye is closer to the object that is being hit and the amblyopic eye is occluded and protected by the nose. Anything flying up into the face is therefore likely to hit the good and not the amblyopic eye.

The key here is to advise all patients with an amblyopic eye to wear eye protection where there is a chance, no matter how slight, that something may hit them in the face. Activities which may cause ocular trauma are the sports squash, tennis and badminton and any activity that could produce a high-velocity projectile, such as hitting metal with metal and recreational fishing.