The mfVEP has been used in clinical research on patients with various types of glaucoma to evaluate the glaucomatous functional loss objectively. Their results confirmed the ability and credibility of mfVEP test to detect glaucomatous visual field loss. Whether mfVEP was assessed alone to detect visual field defects or in comparison to HFA, it has been always able to detect glaucomatous defects. In this study the two tests were used and their results were compared to each other in the three groups. The hemifield sector analysis protocol applied to the mfVEP test showed the ability to detect glaucomatous visual field defects confirmed by the HFA tests. In addition, mfVEP was also able to confirm normal responses in normal subjects. Most of the differences between the two tests were clearly shown in the glaucoma suspect group and the suspicious HFA results, where mfVEP added important information about the assessment of these subjects’ visual field.
4.6.1 Confirming normal visual field results
The hemifield sector analysis was performed on all normal subjects included in the control group. When sectors and hemi-rings were compared across the field all normal subjects were confirmed to have normal visual field. No single mfVEP test was reported as abnormal or suspicious in normal subject group. Figures 34 & 35 are examples of normal subjects’ recordings. The SITA standard test was also normal in those subjects. In addition, the hemifield sector analysis was also normal and showed no significant difference between the two hemifields, confirming their normal status. This finding is crucially important, as confirming normal visual field is as important as confirming glaucomatous visual field defects. Most of the previous research work confirmed the ability of mfVEP tests to detect glaucomatous visual field defects but without the important comment on its ability to confirm a normal field. In the presence of good signals and analysis evaluated by SNR
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values instead of the amplitude or latency analysis, the outcome is better and increases the reliability of the results.
4.6.2 Confirming glaucomatous visual field defects
As expected and mentioned in the initial hypothesis, mfVEP was able to detect already established glaucomatous visual field defects in glaucoma patients. All patients with variable degrees and severity levels of glaucoma were assessed using the hemifield sector analysis protocol and their results confirmed the defects. Interestingly, even in patients with advanced glaucomatous defects, where both hemifields are damaged, probably equal, the hemifield analysis protocol confirmed the defect indirectly by the generalized low SNR values on both hemifields. The advantage of this protocol is that it can detect the defect if significant SNR difference between the two hemifields is recorded, and in addition, it can prompt the clinician to a state of generalized depression in SNR values if advanced glaucomatous visual field defects are present. Figure 35 shows that clearly. In the top recording, the significant difference between each pair of corresponding sectors pointed to the existence of focal glaucomatous visual field defect. While in the bottom recording the advanced glaucomatous defect in both hemifields did not give any significant difference between corresponding sectors and hemi-rings, but instead it showed generalized depression in all sectors and hemi-rings across the midline. Figures 38-42 show examples of glaucomatous visual field defects of variable severity and grades. The hemifield sector analysis was performed in all subjects and confirmed the focal defects, regardless of its size or location; it was detected by the direct comparison or by the symmetrical SNR depression in one sector or more of the visual field. These results are in agreement with many studies. For example, Goldberg (134) used the mfVEP test to detect glaucomatous visual field defects already documented in glaucoma patients. He found good correlation with the HFA field results in glaucoma patients, and reported that mfVEP can assess the visual field and identifies glaucomatous visual field defects, and that it may have the potential for identifying defects earlier than conventional perimetry. Graham et al (159) also reported similar results, and showed evidence that mfVEP is an effective method for detecting visual field loss in glaucoma. They suggested that the mfVEP test provides a valuable aid to the clinician in categorizing patients with unreliable, variable, unconfirmed, or excessive subjective field loss.
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4.6.3 Early detection of glaucomatous visual field defects
The role of mfVEP in the early detection of glaucomatous visual field defects, and in confirming an unreliable visual field test is considered the main area of recent research. Recent evidence suggests that the mfVEP can have clear role in monitoring and detecting progression of glaucoma based on good repeatability figures (19,159,170,171). Glaucoma suspect patients and subjects with suspicious visual field tests are usually the main source of confusion and uncertainty in glaucoma practice. Most of glaucoma specialists depend in their day-to-day practice on the SAP techniques to diagnose and monitor their glaucoma patients, and when SAP results are unreliable or inconclusive it reflects on the management plan and the follow up visits. In this study, the hemifield sector analysis protocol was able to provide interesting results in some patients with glaucoma suspect diagnosis or patients with unreliable SAP results. Figures 43-47 are examples of glaucoma suspect patient with inconclusive visual field results. The mfVEP test results showed in almost all of them confirmed significant focal depression in one or two sectors. The focal depression detection usually reflects glaucomatous visual field defect, as the SNR value represent the response of each segment, and theoretically if the response in consistently low or depressed this should be a good indicator that there is a focal defect in this location. Modest visual field losses lead to very small mfVEP responses. Thus, the presence of a good SNR indicates that the visual field should be relatively good. This observation has an important clinical implication. Monocular field analysis measures focal defects precisely, the interpretation of these defects and how reliable it is could be answered by comparing the results of the two tests. Most of the cases shown in these figures did not show significant SAP changes or defects, in fact the GHT was within normal limits. While there was a good reason for those patients to be flagged as suspects; either by high intraocular pressure or family history, the presence of additional information from the mfVEP test should alert the clinician to modify his management plan or give him an indication to closely monitor this patient as there might be an early ongoing process of glaucomatous damage. Up until this point, there has been no conclusive role of the mfVEP as a solid parameter confirming a visual field defect due to glaucoma. Most likely its role can be as an adjunct to existing data, and may be used to aid the doctor when making a decision whether early glaucomatous damage may be expected. One exception is shown in the case presented in figure 46, showing a glaucoma patient on treatment, where the size and depth of visual field defect shown in the SAP test result did not match the clinical picture of this patient, as he should have deeper and larger defect. The mfVEP sector analysis protocol showed a much more extensive defect involving more sectors and the SNR values markedly reduced which could give an indication of deeper glaucomatous visual field defect. However, these mfVEP tests need to be repeatable and the defects are accurately
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reproducible in order to finalize the diagnosis. Long term follow up of this patient would reveal if the mfVEP is predictive of future SAP visual field loss.