The multifocal visual evoked potential in neuro-ophthalmic disease

摘要

The use of multifocal multichannel VEPs has added a new tool to the investigation of disorders of the visual pathway. It has been used to objectively map field defects in glaucoma, optic neuropathies and cortical lesions. It has particular relevance to the investigation of optic neuritis where relative changes in amplitude and latency can identify the underlying pathology as demyelinating. Monocular mVEPs are recorded using the Accumap V2 system (Objecti Vision, Sydney, Australia). An array of four bipolar occipital electrodes provides four differently oriented channels for simultaneous recording. Visual stimulus consists of 56 closely packed segments in a black and white dartboard configuration, with 16 checks per segment. The segments are cortically scaled to stimulate approximately equal areas of striate cortex, and extend to 26 degrees of eccentricity in the visual field. Both amplitude and latency, with inter-eye asymmetry for each point in each eye is calculated. The Accumap normal patient database is used for comparison of amplitudes and latencies throughout the visual field, and a probability plot used to identify possible scotomas and areas of signal delay. The mVEP can detect changes of acute optic neuritis with greater sensitivity than Humphrey visual field testing. By analyzing the mVEP latency data we can distinguish between patients with demyelination and those with optic atrophy. In our clinical study latency delays were longer in those patients with a diagnosis of clinically definite MS compared with those with a high risk, which were in turn longer than those with a low risk of MS. Twelve month follow-up of these patients indicated that there was a high rate of conversion to clinically definite MS in those with greater latency delays. The new latency algorithm of the mVEP has enabled us to localise the abnormality to a specific area of the visual field and thus demonstrate exacerbations of existing lesions and/or new areas of demyelination. Furthermore, the mVEP can be used to track recovery after an acute attack of optic neuritis, and to detect possible remyelination, and would be helpful in evaluating future therapies aimed at this goal. Case examples of mVEPs recorded over 12 months follow-up (30 subjects) show initial recovery of amplitude in most subjects, and later partial recovery of latency. The pattern of this recovery may have prognostic significance.