Tenotomy and congenital nystagmus: a failure to answer the wrong question

Two companion papers used wavelet and dynamical systems analyses in an attempt to show that the new four-muscle tenotomy surgical procedure had little or no effect on either the ‘‘waveform structure’’ or the ‘‘underlying mechanism’’ of congenital nystagmus (CN). (Miura, Hertle, FitzGibbon, & Optican, 2003a; Miura, Hertle, FitzGibbon, & Optican, 2003b) We believe these papers (Parts I and II) asked the wrong question and misapplied the two analysis techniques to long stretches of uncontrolled fixation data taken from the NEI Clinical Trial of the Tenotomy procedure. Phase one of that study (ten adults) demonstrated that four muscle tenotomy successfully improved the CN waveforms, (Hertle et al., 2003) increasing potential visual acuity, as measured by the NAFX, and measured visual function. (Dell Osso & Jacobs, 2002) The two papers also contain unsubstantiated speculations about CN data analysis and tenotomy-induced waveform changes resulting in improved potential acuity; they created the false impressions in the minds of readers that our original hypothesis for mode of action of tenotomy was disproved and that the procedure was not successful. Because Dr. R.W. Hertle was an author on the companion papers and on the Phase-1 Tenotomy Clinical Trial paper, he chose not to be a signatory to either this Letter or any rebuttal it may induce. However, he was given the opportunity to read and edit it and agrees with both the need for, and content of, this Letter. The working hypothesis for the mechanism by which tenotomy improves CN waveforms is that it alters a proprioceptive feedback loop involved in maintaining resting muscle tension and reduces the small-signal gain of the ocular motor plant; that results in a reduced response to the basically unchanged CN signal. (Dell Osso, Hertle, Williams, & Jacobs, 1999; Dell Osso, 2002a) Neither tenotomy nor any other extraocular muscle nystagmus surgery has been hypothesized to affect the causal, brainstem, feedback mechanisms that result in CN. It is unlikely that the simple muscle surgeries commonly used therapeutically in CN could alter the underlying brain stem mechanisms. Therefore, the negative