THE INCIDENCE AND NATURE OF VISUAL PATHWAY INVOLVEMENT IN FRIEDREICH'S ATAXIA

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Pattern-reversal visual evoked potentials (VEP) were recorded from 22 patients (mean age 33.7 years) with Friedreich's ataxia, 15 of whom also had a detailed neuro-ophthalmological assessment prior to the VEP examination. None had noted symptomatic visual impairment. Eleven of the 15 (73 per cent) examined clinically had one or more neuro-ophthalmic abnormality and 14/22 (64 per cent) had an abnormal VEP study which was always binocular and comprised absent responses, or most commonly, increased P100 component latencies. The maximum P100 latency was 143 ms and the group mean was 118 ms. The P100 amplitude was also generally reduced particularly in those patients with latencies less than 115 ms (upper limit of normal), while in those with latencies above the normal range there was a significant inverse correlation between the P100 amplitude and latency. The waveform, temporal dispersion and interocular differences were normal in almost all patients with identifiable responses, including those with prolonged VEP latencies. Electroretinograms recorded from three selected patients were either normal or minimally abnormal and suggested secondary rather than primary retinal involvement. The only VEP parameter to correlate with either the duration of the generalized disease or the visual acuity was the P100 amplitude. A good correlation was found between the VEP and the clinical neuro-ophthalmic findings. Temporal pallor of the optic disc was most often associated with an abnormal VEP result and impaired visual acuity or colour vision were uncommon in the absence of VEP abnormalities. The VEP changes and those obtained from 24 age- and acuity-matched cases of demyelinating optic neuritis are contrasted and the probable pathophysiology is discussed. Two main conclusions emerge from this study. First, there is a high incidence of asymptomatic visual pathway involvement in Friedreich's ataxia which can be demonstrated by both clinical and VEP examination. Secondly, the VEP changes in Friedreich's ataxia differ from those found in typical demyelinating optic neuropathy and are consistent with progressive nerve fibre loss and associated slowing of conduction, indicating that the visual pathway is affected by the same widespread process of axonal degeneration found throughout the nervous system.