Often a patient will present with a reduction of vision in one or both eyes and yet the eyes themselves look quite normal. In the case of a child, the parents may have noticed an apparent difficulty in reading or the vision may have been noticed to be poor at a routine school eye test. The next step is to decide whether the fundus is also normal, but before dilating the pupil to allow fundus examination, it is important to check the pupil reactions and to eliminate the possibility of refractive error. Once the glasses have been checked and the fundus examined, the presence of a normal fundus narrows the field down considerably. The likely diagnosis depends on the age of the patient. Infants with visual deterioration might require an examination under anaesthesia to exclude the possibility of a rare inherited retinal degeneration or other retinal disease. Other children, particularly those in the 9-12-year age group, must first be suspected of some emotional upset, perhaps due to domestic upheaval or stress at school. This can make them reluctant to read the test type. Sometimes such children discover that exercising their own power of accommodation produces blurring of vision and they might present with accommodation spasm. The commonest cause of unilateral visual loss in children is amblyopia of disuse. This important cause of visual loss with a normal fundus is considered in more detail in Chapter 14 on squint. When, for any reason, one retina fails to receive a clear and correctly orientated image for a period of months or years during the time of visual development, the sight of the eye remains impaired. The condition is treatable if caught before the visual reflexes are fully developed, that is, before the age of eight years. Young adults who present with unilateral visual loss and normal fundi could, of course, have amblyopia of disuse and the condition can be confirmed by looking for a squint or a refractive error more marked on the affected side. We must also remember that retrobulbar neuritis presents in young people as sudden loss of vision on one side with aching behind the eye and a reduced pupil reaction on the affected side. This contrasts with amblyopia of disuse, in which the pupil is normal. Migraine is another possibility to be considered in such patients.
Elderly patients who present with visual loss and normal fundi might give the history of a stroke and are found to have a homonymous haemianopic defect of the visual fields caused by an embolus or thrombosis in the area of distribution of the posterior cerebral artery. Hysteria and malingering are also causes of unexplained visual loss, but these are extremely rare and it is important that the patient is investigated carefully before such a diagnosis is made.
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