Episodic visual loss in early adult life, often hemianopic, is often the result of migraine. The other important cause of transient (weeks) monocular visual loss in this age period is optic neuritis, often a harbinger of multiple sclerosis. Amaurosis in the child or young adult may also be caused by systemic lupus erythematosus and the related antiphospholipid syndrome (Digre et al), or by migraine, or there may be no discernible cause. Later in life, transient monocular blindness, or amaurosis fugax, lasting minutes to hours is more common; it is caused by vascular disease, particularly stenosis of the ipsilateral carotid artery. Table 12-1 lists the main causes of episodic monocular visual loss. Of course, at any age, diseases of the retina and of other components of the ocular apparatus are important causes of progressive visual loss.
In the investigation of a disturbance of vision, one inquires as to what the patient means when he claims that he cannot see properly, for the disturbance in question may vary from nearor farsightedness to diplopia, partial syncope, dizziness, or a hemianopia. Fortunately, the patient’s statement can be checked by the measurement of visual acuity, which is an essential part of the ocular examination. Inspection of the refractive media and the optic fundi, the testing of pupillary reflexes, color vision, and the plotting of visual fields complete this part of the examination. Examination of the eye movements is also essential, particularly if amblyopia predicated on an early life strabismus is suspected, as discussed in Chap. 13.
In the measurement of distance visual acuity the Snellen chart, which contains letters (or numbers or pictures) arranged in rows of decreasing size, is used (Fig. 12-1A). Each eye is tested separately at a distance of 20 ft. Spectacles should be worn if they are required for distance. The letter at the top of the chart subtends 5 min of an arc at a distance of 200 ft (or roughly 60 m). The patient follows rows of letters that can normally be read at lesser distances. Acuity is reported as a nonmathematical fraction that represents the distance at which the patient has read the chart and the distance at which a person with normal vision would be able to read a letter of the same size. Thus, at distance of 20 ft if the patient can read only the top letter on the chart, which would be normally be visible at 200 ft, the acuity is expressed as 20/200 (if the distance is measured in meters, it is 6/60). If the patient’s eyesight is normal, the visual acuity will equal 20/20, or 6/6, corresponding to the eighth line on most charts. Many persons, especially during youth, can read at 20 ft the line that can ‘normally’ be read at 15 ft from the chart, and thus have acuity of 20/15.
For bedside testing, a “near card” or newsprint held 14 in from the eyes can be used, and the results expressed in a distance equivalent as if a distance chart had been used (Fig. 12-1B). Here, the Jaeger system is used (J1 is “normal” vision, corresponding to the line 20/20 on a Snellen chart, J7 to 20/50, J13 to 20/100, and J16 to 20/200). In young children, acuity can be estimated by having them match symbols on an eye chart to choices on a card or mimic the examiner’s finger movements at varying distances. The Teller acuity cards estimate visual acuity by evaluating a child’s preference (and hence ability) to view cards with increasingly fine stripes.