Summary for the Clinician

››The VEP is a potential recorded with one or more electrodes placed on the skin over the occipital region.

››The mfVEP technique yields multiple small VEP responses and provides topographical information about the health of local regions of the visual field.

21.2  How Do I Interpret the Results

of mfVEP Tests?

21.2.1  Identifying Glaucomatous

Damage

Figures 21.2 and 21.3 contain portions of reports produced by the most commonly used analysis programs to date, our analysis (Fig. 21.2) and the Graham and Klistorner analysis (Fig. 21.3). In both figures, panel A shows the

24-2 SAP field for one eye of a patient with glaucoma. The mfVEP report should have at least two parts.

First, there should be a display of the actual mfVEP responses as in panel B of Figs. 21.2 and 21.3. Each little wiggly line is a response, i.e., a plot of voltage vs. time. In Fig. 21.2b, the responses from both eyes are shown, coded as red (left eye) and blue (right eye). This patient had asymmetrical field loss between eyes and the 24-2 SAP field for the right eye (not shown) was normal. A close inspection of Fig. 21.3b reveals that the responses from the two eyes are essentially identical in some locations, while in other locations the responses from the left eye are clearly smaller than those from the right eye. With a little practice the clinician can learn to identify poor recordings (e.g., records with line noise or alpha) by examining the individual responses.

Second, there should be a topographical map indicating which of the individual responses are abnormal, i.e., outside normal confidence limits. In Fig. 21.2c, d this information is presented in a form similar to the total deviation plot (Fig. 21.2a) of the 24-2 SAP field. In particular, the red colored squares indicate sectors with responses that are significantly small at the 5% (pale color) or 1% (saturated color) significance level.

In the monocular probability plot of Fig. 21.2c, the red squares indicate where the responses from the patient’s left eye are significantly smaller than a normative group’s response, while in Fig. 21.2d, the interocular probability plot, they indicate where the responses from the left eye are significantly smaller than those of the right eye. Because the responses from both eyes are essentially identical in an individual with normal vision (see Fig. 21.1b), the interocular comparison is particularly good for detecting unilateral damage.

Figure 21.3 shows similar plots produced by the Graham and Klistorner approach. Instead of colored squares, the sectors of the display are shaded in panels C and D to indicate significance at the 5% (light gray), 2% (dark gray), or 1% (black) level.

appreciated best if the results are presented in a form comparable to the results obtained from standard automated perimetry (SAP). The results in Figs. 21.2 and 21.3 illustrate two approaches. In Fig. 21.2c, each square represents the center of one of the mfVEP displays plotted on linear coordinates as is done in SAP (e.g., Fig. 21.2a). If the monocular and interocular probability plots are plotted on a linear scale, they can be easily compared with any perimetric printout. For this patient, the 24-2 SAP test detected visual field damage (gray ellipse) that was missed on the mfVEP test, while the mfVEP test picked up an arcuate defect (purple ellipse) missed by the 24-2 SAP test. A different approach is taken in Fig. 21.3c. Here the sectors of the mfVEP display are coded to indicate abnormal responses.

The mfVEP has two main assets. First, it is objective, in the sense that the patient’s state of attention has little or no influence on the responses [14]. Second, it provides topographical information. This information can be

Latency of the individual responses can also be useful. In general, glaucoma produces relatively small changes in local mfVEP latency, with some patients showing small increases in latency compared with healthy controls