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Frank A. Carusone, Richard W. Hertle, Dongsheng Yang, Stephanie Knox; Ocular Motor Analysis of Vergence Damping in Infantile Nystagmus Syndrome (INS). Invest. Ophthalmol. Vis. Sci. 2012;53(14):519.
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Serra et al. discovered a hysteresis effect of vergence on INS that suggests that during divergence the same peripheral mechanism may be operating that damps INS during convergence. In an attempt to characterize and understand vergence damping in INS we prospectively analyzed the clinical and ocular motor characteristics of 28 patients with INS in whom con- or divergence damping was present.
Under IRB approval, data from 34 patients with INS who had oculographic evidence of vergence damping were analyzed. Clinical evaluation of binocular function, sensory system deficits and distance/near binocular best corrected visual acuity were measured according to previously published protocols. Eye movement recordings were obtained and evaluated qualitatively and quantitatively in response to step vergence stimuli.
Of 230 patients with INS evaluated during 2010-11, 15% (34) displayed vergence changes in their nystagmus. 24 (70%) were male, age ranged from 7 to 70 years (ave 17yrs) best corrected binocular acuity ranged from logMAR 0.0-0.9 (ave 0.4). Of these, all 34 patients, showed convergence damping and 13 showed additional divergence damping from primary position. NAFX measures of vergence were statistically improved during vergence changes. Patients with significant NAFX changes had larger distance/near acuity difference than patients without (T=4.5, P<0.001). There were significant waveform changes, broadening of the null zone and a positive relationship to binocular function associated with positive vergence damping.
Although the exact mechanism responsible for vergence damping in unknown, there is speculation that it might result from co-contraction of antagonist muscles of each eye during convergence. However, Miller found no co-contraction. When the eyes are converged by equivalent amounts, the muscle tension decreases which may be accomplished by lowered γ-innervation to a proprioceptive feedback loop controlling steady-state muscle tension. The observations of convergence-induced damping of other types of nystagmus support this "peripheral" mechanism in preference to one relying on an inherent property of the nystagmus.
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