Noninvasive clinical electrophysiological and psycho-physical measurements allow an assessment of the health of every segment of the visual system from retinal pigment epithelium (RPE) to the visual cortex. An understanding of each test and their interrelationships assists the diagnosis of a number of diseases.¹⁹ This is facilitated by the layered nature of the visual system and the assignment of electrical potentials from specific tests to particular cell layers.

Because most evoked responses stimulate the entire retina, regional loss of function might not be noticeable in the test results. By adjusting stimulus conditions and techniques of recording, a representation of the sequence of events along the visual pathway can be recognized, and the cone and rod systems can be measured separately. If the stimulus has an abrupt onset (as is usual), the delay between stimulus and response indicates additional signs of dysfunction that are not elicitable by other methods (e.g., psychophysics).

An understanding of the neuronal response order (from photoreceptor to visual cortex) and the interrelationships of the responses is important, since an abnormality at a lower level will usually give an abnormal response farther along the sequence chain, and misleading interpretations can be made if abnormal test results are inappropriately taken out of context. For instance, an abnormal visual evoked cortical potential (VECP) might be found in a number of retinal dystrophies and could be misinterpreted if an electroretinogram (ERG) is not performed to confirm that the defect is retinal, not in the optic pathway. Therefore, after taking an initial history from the patient, the tests to be performed on the patient should be ordered in such a manner as to maximize the success in diagnosing the level of dysfunction. For instance, if the patient has an abnormal ERG, then a VECP is seldom necessary. In practice, this approach must also be modified to maximize patient flow; in some clinics, tests are scheduled on different days, and patients who require further tests are recalled. In other cases, a VECP will be done before the ERG because the latter requires pupillary dilation and the patient cannot be recalled.

While the retina is composed of a complex neuronal matrix with both vertical and horizontal components, it is possible to detect abnormalities of the rods and cones, the photoreceptor layer generally, the middle retina, and the ganglion cell layer (table 49.1). If the central retina is reasonably healthy, then the VECP can detect abnormalities in the conduction of retina-generated signals to the visual cortex.

A directed approach to visual system evaluation is especially important for disorders that either have specific sites that have been shown to be abnormal on histopathology of the suspected disease or are believed to be abnormal on the basis of clinical appearance or other tests. For example, the ERG is well suited to evaluate generalized photoreceptor disease such as retinitis pigmentosa (RP) because the response is a mass one that is initiated by photoreceptors. Widespread photoreceptor dysfunction and loss are early features of RP. Best's macular dystrophy, on the other hand, presumably has a diffusely abnormal RPE, with inclusions of lipofuscin on histopathology but normal photoreceptors (except in regions of RPE loss or scarring). Therefore, it is not surprising that the ERG is normal in Best's dystrophy but the electro-oculogram (EOG), which tests potentials generated by the RPE, is abnormal.

While table 49.1 is helpful in forming a perspective on how the various tests can be used to evaluate cellular layers or “minisystems” in the visual system, it is useful, despite the risk of redundancy, to tabulate each test with the information that can be expected from doing it. These are presented in table 49.2.