Just as an electrocardiogram (ECG) offers valuable information to the physician treating a patient with heart problems, ophthalmic electrodiagnostic tests provide information about the function of the visual system from the retina at the back of the eye, through the visual pathways to the visual centre in the brain.
Information from electrodiagnostic tests can assist the eye specialist with the diagnosis and recommendations for treatment for patients with retinal and visual pathway disorders. The tests also provide the specialist with information from which the prognosis and inheritance pattern of their disorder can be made.
The retina, lining the back of the eye, is composed of 3 layers of nerve cells: the outermost layer is the light-sensitive cells of the retina, called photoreceptors. There are two types of photoreceptors in the human eye: rods and cones. Cones provide central reading vision, and are responsible for colour vision. Rods provide night vision and detect motion. Neural signals from the rods and cones undergo processing by nerve cells in the middle layer of the retina and finally a signal is sent to the brain through the ganglion cells which form the innermost layer with the axons of the ganglion cells forming the optic nerve.
All tests are done according to the ISCEV (International Society of Clinical Electrophysiology of Vision) standards.
The electroretinogram (ERG) is the most direct and objective test available for evaluating the function of the retina. Information can be obtained about the function of the rods and cones as well as the inner retinal layers.
Minute electrical voltages are produced by the eye. Electrodes are placed on the skin around the eye with a soft gold foil electrode positioned over the lower lid to be in contact with the cornea through the tear film. This is readily tolerated by adults and children as young as 6 years old. Younger children can be tested with an electrode taped to the lower lid. Flashes of light are presented under dark and light-adapted conditions to separate the rod (night vision) and cone (colour vision) systems of the retina. Both the timing and the size of the ERG responses give valuable information about the extent and nature of retinal abnormalities and disorders affecting both the outer and inner retina.
The ERG has been the gold standard for evaluating night blindness, pigment changes in the retina, and colour vision disorders, enabling the diagnosis of inherited retinal diseases before changes are seen in the eyes. It is also recognised as an important tool for the diagnosis and management of a wide range of common retinal diseases including inflammatory diseases and retinal vascular disorders. The ERG can help to distinguish peripheral retinal disease from diseases localised to the macula or optic nerve.
Retinal function can be assessed through opacities such as advanced cataracts, vitreous haemorrhage or in eyes after severe trauma and early toxic damage from various causes involving the retina can be detected with the flash ERG or pattern ERG.
Pattern ERG (PERG)
By changing the stimulus to a flickering checker-board pattern on a TV monitor screen, a pattern ERG is obtained. The resulting waveform provides information about macular and retinal ganglion cell function. It enables diagnosis and a quantitative assessment of early macular disease as well as the differentiation between localised macular disease and more widespread retinal disease.
Multifocal Electroretinogram (mfERG)
This is a new technology that is capable of assessing focal electrophysiologic responses in the central retina including the macula. The recording involves specialized stimuli and mathematical modelling of the response, leading to a topographical map of tracings across the tested area of the retina. It is useful to assess localised abnormalities at or around the macula. It has been found clinically useful in identifying early hydroxychloroquine (plaquenil) retinopathy or toxicity and is used in a variety of retinal dystrophies, macular dystrophies and other macular disorders.
Multifocal ERG in patient with AMD compared with normal response.
Other areas of the visual pathway can be investigated by specific electrical tests. The electro-oculogram (EOG) tests abnormalities of the outermost layer of the retina, the retinal pigment epithelium. It is particularly useful in the early diagnosis of some inherited macular diseases such as Bests disease.
Cortical Visual Evoked Potential (VEP)
Visual evoked potentials (VEPs) can provide important diagnostic information regarding the functional integrity of the visual system. The cortical visual evoked potential (VEP) provides information about the health and function of the visual pathways from the optic nerve as it leaves the back of the eye, to the visual centre in the brain.
- Pattern reversal VEP – is the preferred stimulus most commonly used as the responses are less variable in waveform and timing than the VEPs elicited by other stimuli. The standard pattern stimulus is a high contrast black and white checkerboard with large 1° and small 0.25° checks.
- Pattern onset/offset VEP – is used for young children, people with nystagmus and to assess a variation in the pathway of the fibres in the brain which is present in albinism. It also uses a checkerboard stimulus.
- Flash VEP – elicited by a brief flash of light, is useful when poor optics, poor co-operation or poor vision makes the use of pattern stimulation inappropriate.
Many people require a combination of these tests to give complete information about their visual problem and the total testing time may take 1-2 hours. In all, the visit may take 2-3 hours as a consultation to explain the results will follow the testing process. For most tests it is necessary to dilate the person’s pupils.