Hypothetical explanation for the role of proprioception in the damping of infantile nystagmus by tenotomy surgery: The small-signal gain hypothesis

摘要

Objective Tenotomy damped and otherwise improved infantile nystagmus syndrome (INS) waveforms, first in a canine and then in a masked, clinical trial. The damping was hypothesized to result from changing the proprioceptively controlled tension in the extraocular muscles; tenotomy did not appear to affect post-operative saccades. We examined the pre- and post-operative saccadic characteristics in INS and acquired pendular nystagmus (APN) to test the hypothesis that tenotomy affected only the small-signal ocular motor plant gain. Methods: Ten subjects with INS and 1 with APN were studied. All the analysis except for the statistics was done in MATLAB using custom-written OMtools software (including the expanded Nystagmus Acuity Function, NAFX, www.omlab.org). A new statistical procedure, "ctest," tested the significance of differences in the pre- and post-surgical saccadic data. Voluntary and braking saccades were used to provide a statistical pool at each saccadie amplitude. Results Six of 10 INS subjects showed no changes in saccadic peak velocity (P value > 0.05), acceleration, or trajectory (dwell time) despite nystagmus-amplitude decreases (14.6% to 39.5%) and NAFX increases (22.2% to 162.4%). In 4, the small saccadic peak-velocity decreases (8.9% to 18.7%) were less than either their NAFX increases or nystagmus-amplitude decreases. Eight of the 10 INS subjects showed no changes in saccadic duration. The APN subject showed no saccadic peak-velocity changes despite a 50% nystagmus-amplitude decrease and a 34% NAFX increase. The "small-signal" changes far exceeded any "large-signal" changes. Conclusions Tenotomy reduced INS and APN, enabling higher visual acuity without adversely affecting saccadic characteristics, supporting the peripheral, small-signal gain reduction (via proprioceptive tension control) hypothesis. Current simple, linear plant models cannot explain these differential effects; more accurate models of the plant are needed. A tentative preliminary model is provided. It contains two pathways, one for high-frequency bursts (saccades) and the other for the lower frequency signals (slow phases).