Abstract
Abstract: :
Purpose: In theory, false positives on the multifocal visual evoked potential (mfVEP) test might occur due to inherently small signals in otherwise normal individuals, high levels of noise, a large alpha wave contribution, and/or poor fixation.[1] Here we explored whether the mfVEP test can be improved by taking into account one or more of these factors. Methods: Monocular mfVEPs were obtained from each eye of 91 control subjects. The mfVEPs from 50 patients with open–angle glaucoma [2] were also analyzed. The display was a pattern–reversal dartboard array, 44.5 deg. in dia., which contained 60 sectors. The mfVEPs were obtained using the VERIS system (EDI) with three channels of recording, and were analyzed with custom software as previously described.[3,4] Monocular and interocular analyses were performed using a method based upon signal–to–noise ratios (SNR) and mfVEPs from an original set of 30 control subjects.[3,4] Abnormal mfVEP results were identified with cluster criteria.[e.g. 4–7] A hemifield was classified as abnormal if 3 or more contiguous points were significant at <5%, with at least one at <1%. Four indices were devised per individual: 1. Signal Index (SI) was derived by first obtaining the best SNR at each location considering both eyes. The SI is the 3rd largest of these SNRs ; 2. Noise Index (NI) was derived as the average amplitude (RMS) in the noise window; 3. Alpha Index (AI) was derived by measuring the relative amount of alpha in the records; and 4. Fixation Index (FI) was derived as a cross–correlation between the central 4 responses from the two eyes. Results: Of the 91 control subjects, 9 (SI), 0 (NI), 7 (AI) and 8 (FI) indices (individuals) exceeded a 2 SD cutoff. All of the 15 individuals identified with these indices had significant clusters (false positives) in at least one eye. Of the 50 patients, 1 (SI), 1 (NI), 2 (AI) and 11 (FI) indices (individuals) exceeded a 2 SD cutoff. Conclusions:As with static automated perimetry, specialized indices should help improve the specificity of the mfVEP test. Taking sensitivity into consideration, the indices identifying small signals in both eyes (SI) and high alpha levels (AI) look most promising. 1. Hood & Greenstein (2003) Prog Ret Eye Res. 2. Hood et al (2004) IOVS. 3. Hood & Zhang (2000); 4. Hood et al (2002) AO. 5. Chauhan et al, (1988) Doc Ophth 6. Goldberg et al (2002) AJO. 7. Hood et al (2003) J. Glau.
Keywords: electrophysiology: clinical • visual fields • perimetry