Strabismus - a necessary condition for latent and manifest latent nystagmus

Our previous study of 31 cases of manifest latent nystagmus (MLN) suggested a one-to-one relationship between the occurrence of MLN and the presence of strabismus. This study of three patients with both latent nystagmus (LN) and MLN has elucidated the relationship between strabismus and these two intimately related types of nystagmus. Both LN and MLN have jerk waveforms whose fast phases are in the direction of the fixating eye and whose slow phases are decreasing-velocity exponentials; the nystagmus is simultaneously present in the two eyes. LN and MLN should not be confused with congenital nystagmus which differs in waveform, variability with gaze angle, mechanism, clinical signs and therapy. During binocular viewing, if the two eyes are aligned with normal retinal correspondence, the patient with true LN will have no nystagmus. Only if one of his eyes assumes a tropic position, might MLN develop. Under cover, the development of a phoria is synchronous with the occurrence of LN. Thus, strabismus in the form of a tropia or a phoria emerges as a necessary, but not sufficient, condition for both LN and MLN. That is, during the time when a nystagmus is present, the non-fixating eye (the eye behind cover or which is being suppressed) will be phoric or tropic, respectively. In LN cases with abnormal retinal correspondence, although no nystagmus may be present with the eyes in their normal fixed tropic position, cover will induce nystagmus along with a phoria. The intriguing and highly desirable possibility of converting MLN to LN, in a very young child with a fixed tropia, by early strabismus surgery is discussed.