Electrophysiology in glaucoma assessment

Electrophysiological testing provides a valuable adjunct for assessment of function in glaucoma, providing an objective measure of visual function. While the conventional ERG is predominantly outer retinal, and thus has not been helpful in assessing glaucomatous loss, the PhNR is a promising inner-retinal ERG component, which deserves further study. The PERG has long been known to reflect glaucomatous damage, and appears to be largely ganglion-cell-driven, although the conventional PERG does not provide any topographic information. The mfVEP, using a multichannel bipolar recording technique, does shows a strong relationship between the locally recorded cortical response and subjective visual field loss at corresponding locations. The mfVEP can objectively demonstrate glaucomatous field defects. The mfERG has application to animal models of glaucoma, but as yet has not been able to demonstrate topographical correlations with other damage measures such as visual field sensitivity in human patients. In the clinical setting, electrophysiological testing with PERG and/or mfVEP, either supports the diagnosis or helps to rule-out subjective field loss. In some early or high-risk glaucoma cases it may detect functional damage earlier than other measures, enabling commencement of treatment at an earlier stage. Electrophysiological testing reassures the clinician and patient when perimetry results are inconsistent, variable or when unexpectedly excessive changes are seen, and may thus help prevent over-treatment. The latter situations are frequently encountered in clinical practice due to the large number of patients who have trouble with perimetry. Cases with excessive field loss might otherwise be sent for diagnostic imaging with CT scan or MRI, since the clinician is uncomfortable accepting the disparity between structural and functional loss. Less commonly, when both subjective and objective tests are out of proportion to disc changes, further pathology should be suspected (e.g. intracranial tumor) and CT/MRI should be considered. The mfVEP provides a new alternative approach for visual field assessment. It is very easy for patients to perform, even when it is their first time, and has a high level of patient acceptance. It is likely that this form of testing will continue to develop in the future as an objective means of monitoring glaucoma patients. With improved signal detection, greater reproducibility and shortened test times, it will provide the clinician with a valuable adjunct for assessing function in glaucoma.