A new surgery for congenital nystagmus: Effects of tenotomy on an achiasmatic canine and the role of extraocular proprioception

Purpose:Human eye-movement recordings have documented that surgical treatme nt of congenital nystagmus (CN) also produces a broadening of the null zone and changes in foveation that allow increased acuity. We used the achiasma tic Belgian sheepdog, a spontaneously occurring animal model of human CN an d see-saw nystagmus (SSN), to test the hypothesis that changes induced by s urgical interruption of the extraocular muscle afference without a change i n muscle-length tension could damp both oscillations. Methods:An achiasmati c dog with CN and SSN underwent videotaping and infrared oculography in a s ling apparatus and head restraints before and after all extraocular muscles (stage 1: 4 horizontal rectus muscles and stage 2 [4 months later]: 4 vert ical rectus muscles and 4 oblique muscles) were surgically tenotomized and immediately reattached at their original insertions. Results: The dog had i mmediate and persistent visible, behavioral, and oculographic changes after each stage of this new procedure. These included damped CN and SSN, increa sed ability to maintain fixation, and increased periods of maintaining the target image on the area centralis over a broad range of gaze angles. Concl usions: Severing and reattaching the tendons of the extraocular muscles aff ect some as-yet-unknown combination of central nervous system processes pro ducing the above results. This new procedure may prove effective in patient s with CN with either no null, a null at primary position, or a time-varyin g null (due to asymmetric, (a)periodic, alternating nystagmus). We infer fr om our results in an achiasmatic dog that tenotomy is the probable cause of the damping documented in human CN after Anderson-Kestenbaum procedures an d should also damp CN and SSN in achiasma in humans. It may also prove usef ul in acquired nystagmus to reduce oscillopsia. The success of tenotomy in damping nystagmus in this animal suggests that the proprioceptive feedback loop has a more important role in ocular-motor control than has been apprec iated. Finally, we propose a modified bimedial recession procedure, on the basis of the damping effects of tenotomy.