Development of new treatments for congenital nystagmus

The use of ocular motor data as the basis for the development of both nonsurgical and surgical therapies for congenital nystagmus (CN) has been underway since the mid-1960s. This paper presents three nonsurgical therapies (composite prisms, soft contact lenses, and afferent stimulation) and a new surgical therapy (four-muscle tenotomy) hypothesized from analysis of ocular motor data. The expanded nystagmus acuity function test was developed to both predict and measure the effectiveness of CN therapies and for intersubject comparisons. Base-out prisms may be used to damp CN during distance fixation in patients whose CN damps during near fixation and who are binocular (i.e., they have no strabismus). Soft contact lenses may be used in those whose CN damps with afferent stimulation of the ophthalmic division of the trigeminal nerve. Cutaneous afferent stimulation (rubbing, vibration, or electricity) of the forehead or neck damps CN in some individuals. Finally, as first demonstrated in an achiasmic Belgian sheepdog and later in humans, tenotomy of the four horizontal rectus muscles and reattachment at their original sites may also damp CN. Taken together, these findings suggest the existence of one or more proprioceptive feedback loops acting to change the small-signal gain of the extraocular plant. Four-muscle tenotomy provides a needed therapeutic option for the many individuals with CN for whom other surgical therapies are contraindicated. Tenotomy may also prove useful in see-saw nystagmus (it abolished it in the aforementioned canine) or other types of nystagmus; further studies of the latter are required.