Tenotomy Does Not Affect Saccadic Velocities: Support for the "Small–Signal" Gain Hypothesis

Abstract

Abstract: : Purpose: To investigate the effects of tenotomy on saccadic peak velocity in infantile nystagmus syndrome (INS) and acquired pendular nystagmus (APN). Background: The success of the tenotomy procedure in treating IN (canine and human) and APN in MS, was hypothesized to be due to changes in proprioceptive receptors in the extraocular muscle tendons that effectively reduce the gain to the relatively small signals (compared to saccadic burst signals) that drive nystagmus. Methods: Eye movements of six patients with INS and one with APN were recorded using infrared reflection, magnetic search coil, or high–speed digital video. Saccadic peak–velocities were plotted vs. saccadic amplitudes. We used the eXpanded Nystagmus Acuity Function (NAFX) to quantify tenotomy–induced changes in the INS. Saccadic characteristics were measured using methods previously defined for subjects with coexisting nystagmus. Results: We studied two INS patients each with: no NAFX improvement; moderate improvement (21–40%); and large improvement (73–124%). The NAFX improvement in the APN patient was 34%. Five INS patients exhibited no change in their saccadic peak velocity vs. amplitude curves; one showed an increase. As a group, the INS patients showed no change, independent of the NAFX change. The APN patient also showed no change. Conclusions: Although four–muscle tenotomy successfully reduced INS and APN, enabling higher visual acuity in both types of patients and reduced oscillopsia in MS, it did not affect the saccadic velocities in either. This supports its hypothesized mode of action –– i.e., peripheral, small–signal gain reduction via proprioceptive tension control. The finding of higher saccadic peak velocities in one INS patient (whose NAFX increased by 40%), suggests that idiosyncratic, non–linear factors may also produce a paradoxical effect on the larger burst signals.

Keywords: nystagmus • eye movements: saccades and pursuits • ocular motor control