Functional Examinations of the Visual Pathway System with Electrophysiological Methods

Abstract

A fundamental task of neuro-ophthalmology is to demonstrate the integrity or detect the impairment of the visual pathway. Determining the nature of the impairment and precisely localizing it is very important, since these may have therapeutic consequences, and a wrong diagnosis has an impact on the entire life of the patient and fundamentally influences their lifestyle. A broad range of electrophysiological methods are available for the assessment of the functional status of the visual system. The proper selection and application of these methods help localize the lesion from the retina to the visual cortex. Information about the functional integrity of the pigment epithelium is provided by electrooculography (EOG). Standardized electroretinography (ERG) is suitable for the precise measurement of the rod and cone function, as well as the function of the bipolar Müller cells, the amacrine cells and the cells in the inner retinal layers. The central, 30° area of the retina – especially the function of the cones – is assessed with multifocal ERG (mfERG). The function of the central ganglion cells sensitive to contrast changes is shown with pattern electroretinography (PERG). The axons of the retinal ganglion cells send the visual information through the lateral geniculate body to the visual cortex, of which the visual evoked potential (VEP) test provides information. Damage to the ganglion cells results in ascending optic atrophy; a lesion of the optic nerve, in turn, has an impact on the functioning of the ganglion cells through descending atrophy. It is, therefore, obvious that the PERG and VEP examinations must always be used in conjunction. The development of the multifocal technique (multifocal ERG and multifocal VEP) has enabled the functional assessment of localized areas of the visual field with regard to the retina and the visual cortex. The entire range of the electrophysiological methods is rarely used by the clinical physician. The many types of examinations would take a lot of time and would be burdensome for the patient. The most necessary examinations that may lead closer to the accurate diagnosis or to answering the questions raised must be chosen based on the clinical symptoms.