Effects of extraocular muscle tenotomy on congenital nystagmus in macaque monkeys.

INTRODUCTION Extraocular muscle tenotomy has been reported to damp congenital nystagmus in an achiasmatic sheepdog. We performed extraocular muscle tenotomy to evaluate its effects on congenital nystagmus in primates. METHODS Magnetic search coil eye movement recordings were used to document the presence of horizontal congenital nystagmus in 2 adult macaque monkeys that also had naturally occurring infantile strabismus. Extraocular muscle tenotomy was performed by operating on all 4 horizontal recti, surgically detaching the muscles from the globe and suturing them back to their original insertions without resection or recession. Eye movement recordings were repeated 4 months after the procedure, comparing the waveform, amplitude, retinal slip velocity, and intensity (frequency x amplitude) of the nystagmus before and after tenotomy. Visual acuity was also measured before and after surgery in 1 animal. RESULTS Preoperatively, a disconjugate, pendular nystagmus was evident in 1 monkey, and the other had a conjugate pendular-jerk nystagmus damped by convergence. After tenotomy, nystagmus mean amplitude decreased 18% to 52% in 1 monkey but increased 14% in the other (t test, P <.002). Retinal slip velocity and nystagmus intensity increased in both monkeys. After tenotomy, mean velocity increased 22% to 218%, while mean intensity increased 40% to 208% (t test, P <.002). Visual acuity measured after tenotomy decreased an average of 20% ( approximately 2.0 cycles per degree) in each eye. Tenotomy had no noteworthy effects on eye alignment or other aspects of visual behavior other than the congenital nystagmus. CONCLUSION Nystagmus velocity and intensity increased after extraocular muscle tenotomy in 2 monkeys. Further studies are required to establish the clinical value of this procedure as a treatment for various subtypes of congenital nystagmus in humans.