The relationship between infantile strabismus and latent nystagmus

The so-called infantile strabismus syndrome consists, among other signs, of (1) strabismus, (2) a defect of pursuit and optokinetic tracking with particular involvement of temporally directed responses on monocular viewing, (3) latent nystagmus and (4) adduction preference of the fixating eye. The following causal relationship between these three phenomena is suggested. (1) Binocularity in the visual cortex is impaired, either as a primary defect or as a consequence of misalignment of the eyes. (2) The reduced binocularity prevents maturation of signal transmission from the visual cortex to the brainstem such that slip control is evident in poor pursuit and optokinetic responses, particularly to monocular, temporally directed stimuli. (3) The asymmetry of the pursuit and optokinetic systems is also evident in latent nystagmus which reflects a tonic preponderance, directed nasally with reference to the fixating eye. The directional preponderance drives the slow phases of latent nystagmus if the visual input is unbalanced in favour of one eye. Because of the maldeveloped slip control latent nystagmus is not inhibited by visual contours. When both eyes are open the better-functioning nasally directed pursuit and optokinetic control systems of the two eyes complement each other and largely prevent drifting of the eyes. The defect responsible for the abnormal motor control cannot be located between the retina and the visual cortex because perception of motion is only slightly impaired and a nasal-temporal asymmetry of the motion VEP, typically encountered in infantile strabismus, does not correlate quantitatively with the asymmetry of the motor control. Rather, the defect is located between the cortex and the brainstem. (4) Adduction preference of the fixating eye with a compensatory headturn is due to a gaze-evoked component added to the latent component of the nystagmus. The gaze-evoked component is a purposeful reaction that allows dampening of the nystagmus in adduction at the expense of an increase in abduction.