Clinical rationale for manifest congenital nystagmus surgery.

Although electronystagmography is most useful, especially in furthering our knowledge of nystagmus, simple clinical study is the first step to any understanding of nystagmus. Ophthalmologists should be aware that they may understand a good deal about nystagmus merely by studying visual acuity in their surgery. Testing visual acuity at distance with both eyes open will lead to the qualitative and quantitative diagnosis of compensatory abnormal head postures. Testing at distance with 1 eye covered will help to distinguish between the concordant head turn, one eye fixing in abduction and the other fixing in adduction (ABD + ADD) as manifest congenital nystagmus, and the discordant head turn in which each eye fixes in adduction (ADD + ADD), as is seen in manifest latent nystagmus, for which surgery is totally different. Pseudolatent nystagmus found in congenital nystagmus with exophoria can be differentiated from latent nystagmus. This illustrates de visu (with the naked eye) the "beauty" of the movement of blocking fusional convergence and the nature of the involved mechanisms with artificial divergence surgery. Testing at near will bring out the existence of blocking convergence. Depending on the results, the clinical rationale for surgery is thus made clear: horizontal, vertical, rotatory Kestenbaum-like surgery should be used in cases of abnormal head posture, whereas artificial divergence surgery is our most satisfactory armamentarium in cases of blocking convergence.