Electrodiagnostic Eye Tests
Clinical Applications of Electrodiagnostic Tests

Expertise provided by Dr Dianne Sharp

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.

Electroretinogram (ERG)

The electroretinogram (ERG) is the most direct and objective test available for evaluating the function of the retina. Electrodes are placed on the skin around the eye and a soft gold foil electrode is 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 4 years old. Babies 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.

By changing the stimulus to a flickering chequer-board pattern on a TV monitor screen a pattern ERG is obtained. The resulting waveform reflects central vision or macular function and enables diagnosis and a quantitative assessment of early macular disease as well as the differentiation between localised macular disease and more widespread retinal disease.

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. The ERG can help to distinguish peripheral retinal disease from diseases localised to the central retinal or optic nerve.

The ERG is being used increasingly to identify vision-threatening changes in diabetes and in central retinal vein occlusions. The potential of the retina 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.

Electro-oculogram (EOG)

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, allowing the early diagnosis of some inherited macular diseases such as Bests disease.

Cortical Visual Evoked Potential (VEP)

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. Many people require a combination of these tests to give complete information about their visual problem. Ophthalmic Electro-Diagnostic Tests.

Ophthalmic Electro-Diagnostic Tests

Test Name	Area Tested	Time Taken	Dilating Drops
Flash ERG	Global retina(Rods and cones)	60 minutes	Yes
Pattern ERG	Macular (central retinal) function	30 minutes	No
EOG	Retinal Pigment epithelium	30 minutes	No
Cortical VEP	Visual pathway (Optic nerve to visual centre in brain)	45 minutes	No

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