Objective perimetry using the multifocal visual evoked potential in central visual pathway lesions

Abstract

Aims: To examine the ability of the multifocal pattern visual evoked potential (mVEP) to detect field loss in neurological lesions affecting the visual pathway from the chiasm to the cortex.

Method: The mVEPs recorded in the clinic were retrospectively reviewed for any cases involving central neurological lesions. Recordings had been performed with the AccuMap V1.3 objective perimeter, which used an array of four bipolar occipital electrodes to provide four differently oriented channels for simultaneous recording. 19 patients with hemianopias were identified. Of these there were 10 homonymous hemianopias with hemifield type loss, two bitemporal hemianopias, and seven homonymous hemianopias with quadrantanopic distribution. A comparison with subjective field results and CT/MRI findings was done to determine the relation between the two methods of visual field mapping and any relation with the anatomical location of the lesion and the mVEP results.

Results: In all hemianopic type cases (12) the defect was demonstrated on the mVEP and showed good correspondence in location of the scotoma (nine homonymous and two bitemporal). The topographic distribution was similar but not identical to subjective testing. Of the seven quadrantanopic type hemianopias, only four were found to have corresponding mVEP losses in the same areas. In the three cases where the mVEP was normal, the type of quadrantanopia had features consistent with an extra-striate lesion being very congruous, complete, and respecting the horizontal meridian.

Conclusions: The mVEP can detect field loss from cortical lesions, but not in some cases of homonymous quadrantanopia, where the lesion may have been in the extra-striate cortex. This supports the concept that the mVEP is generated in V1 striate cortex and that it may be able to distinguish striate from extra-striate lesions. It implies caution should be used when interpreting "functional" loss using the mVEP if the visual field pattern is quadrantic.

Figure 1

Stimulus pattern for mVEP. Note central check sizes and stimulus areas are much smaller than peripheral (cortical scaling of stimulus). External ring expands up to 26° of eccentricity with an additional nasal step up to 32°.

Figure 2

Average quadrantic amplitude of mVEP from normal quadrants on Humphrey visual field (HVF) Sita 24-2 compared with mVEP amplitude derived from quadrants which were affected on HVF (at least 75% of test points p<0.5%). Data from the 10 cases where mVEP was abnormal (p<0.0001).

Figure 3

Subject with hemianopia from head trauma (patient 6). Shows HVF grey scale and pattern deviation plots (Sita 24-2) for right and left eyes (A) and corresponding mVEP trace arrays and probability plots (B) from AccuMap. There is good correspondence between HVF and mVEP defects.

Figure 4

Subject with right superior quadrantic field defect (subject 13). The defect just crosses the horizontal meridian in the left eye making it not completely congruous.

Figure 5

Subject with an incomplete incongruous left quadrantanopia, with an additional small defect in the superonasal field of the left eye which was caused by an old retinal lesion (subject 15).

Figure 6

Subject with complete, congruous left superior quadrantanopia from a presumed vascular event with normal CT scan (subject 18). Repeat visual fields on more than five occasions confirmed distribution. A normal mVEP was recorded twice, with only a few paracentral points reaching borderline significance. Finding is consistent with an extra-striate lesion.

Figure 7

Artificial quadrant defect in a normal subject produced by occluding the upper left quadrant of the stimulus screen. Confirms loss of mVEP signal and excludes cross contamination that could mask field loss. Grey shading indicates areas where signal is indistinguishable from noise or <60 nV. There is slight signal spill over across the vertical (in one segment) possibly due to fixation shifts. It is important to realise that occluding a segment of the screen does not simulate a real scotoma, it only confirms that the software is not generating artificial signals by cross contamination or processing errors.