Attempts to measure electrical changes throughout the body have been going on for the past 100 years or more, and developments in electronics have allowed the appearance of highly sophisticated measuring devices. These devices have permitted measurement of ever smaller changes, sometimes at a marked distance from the organ of origin. The visual pathway has been an important center for investigation in this respect, and minute electrical impulses can now be recorded from the eye and visual cortex in a manner that would have astonished our Victorian forebears.
The basis of all these electrical changes is the bioelectrical potential, which can be de-fined as the electrical pressure difference between the inside and the outside of a cell—that is, the potential difference across a cell wall. All cells show this resting poten-tial; a marked change in the potential may occur when a cell is stimulated, causing an electrical current to flow in the surrounding region. Bioelectrical potentials are often very small, in the region of a millivolt (mV) or very much less, they must be amplified before they can be detected by a suitable recording instrument, such as a penwriter or an oscilloscope.
One reason the recorded potential is so small Is that often it must be picked up from a site remote from its source. If, during the course of clinical investigation, we were able to Insert inicroelectrodes into individual cells in the body, we would be able to obtain some exact information about the function of that particular cell or group of cells. Such techniques have, so far, been limited to the laboratory, and in the clinic we must rely on placing electrodes at some point as near as possible to the organ under investigation.
The eye itself provides the clinician with a view of tissues that are normally covered by opaque skin. With the ophthalmoscope, one can examine blood vessels and nerves directly. It is also possible to place electrodes on and around the eye and record electrical changes that occur when diffuse light is flashed on the retina or when the retina is exposed to different forms of light stimulus. So far, at least, the electrical changes in the optic nerve have not been recorded directly, and changes in the optic tracts, lateral geniculate bodies, and optic radiations still remain beyond the reach of the clinician; however, electrical changes over the visual cortex in response to visual stimuli can now be measured.
Electrical changes over the visual cortex, especially in combination with the changes recordable from the eye, are becoming increasingly useful in clinical practice, and there would seem to be enormous scope for the development of this means of measuring body function in the future. It is important to understand that these measures are indicators of function rather than structure. They should never be regarded as an alternative to ultrasound, computed tomography, or other ways of detecting structural changes in the body. Instead, they complement the subjective tests of visual function that are carried out in the clinic. By itself, an electroretinogram (ERG) or visually evoked response (VER) is nearly useless, and the clinical reporting of these electrical changes heeds to be backed up by a full history and the results of all the other relevant tests in a particular case. Electrodiagnosis usually provides one small piece of extra evidence that sometimes may be conclusive in reaching a firm diagnosis.
In this chapter, two types of electrical changes related to seeing are discussed: those recorded around the eye and those recorded from the surface of the scalp over the occipital cortex. Following this discussion, the clinical application of electrodiagnostic tests is considered.