Evaluation of multifocal visual evoked potentials in patients with Graves’ orbitopathy and subclinical optic nerve involvement
OBJECTIVE To determine the relationship between spatially localized multifocal visual evoked potentials (mfVEPs) and Humphrey visual fields (HVFs) in patients with unilateral field defects. METHODS Humphrey visual fields and mfVEPs were obtained from 20 patients with unilateral field losses due to either ischemic optic neuropathy or glaucoma. Monocular mfVEPs were obtained for each eye. The amplitude of the mfVEP responses was calculated using root-mean-square and signal-noise ratio measures. Estimates of the HVF loss in the same regions of the field used for the mfVEP were obtained by interpolating the 24-2 HVF data. RESULTS Monocular mfVEP amplitude decreased with HVF loss, although small mfVEP signals were not uniquely associated with poor fields. On average, the monocular mfVEP was indistinguishable from noise for field losses between -5 and -10 dB, and good monocular mfVEP amplitudes were never associated with extensive visual field loss. The interocular ratio of the mfVEP amplitudes correlated well with the difference between the HVF values of the 2 eyes, and this correlation improved with increased signal-noise ratio. CONCLUSIONS The monocular and interocular results were consistent with a linear relationship between the amplitude of the signal portion of the mfVEP response and linear HVF loss. One way to produce this relationship would be if both the signal in the mfVEP and linear HVF loss were linearly related to the percentage of local ganglion cells lost. The clinical limitations of the mfVEP technique can be understood by taking the signal-noise ratio, and the linear model proposed herein, into consideration.