Visual Electrophysiology Clinic

The Visual Electrophysiology Clinic is the state provider for visual electrodiagnostic services in Western Australia. All electrodiagnostic tests carried out comply with the standards and guidelines of the International Society for Clinical Electrophysiology of Vision (ISCEV)

Patient Referrals

Referrals may only be made by ophthalmologists using the Visual Electrophysiology request form or contacting the department. Request forms may be sent or faxed to the department. Phone or email bookings may be made but must be followed up with a request form.

Clinic Sessions

The clinic sessions operate from 9:00am until 5:00pm Monday to Friday.

The following tests are provided:

Psychophysical Tests

​A range of psychophysical tests is offered to assess visual function.

Amsler Grid

This test is used to record disturbances of central vision and involves identifying irregularities in a uniform pattern presented at reading distance.  All standard charts are available.

Colour

This test is used to assess colour discrimination and involves either arranging a series of coloured pots in colour order or identifying hidden numbers on a series of charts. The tests available include the Farnsworth 100 Hue (fully automated), the Ishihara and the Farnsworth-Munsell D15.

Contrast Sensitivity

This test is used to estimate how well the eye perceives subtle changes of contrast or “shades of grey”.  It usually involves picking out the lowest contrast pattern visible on a chart. The Arden Contrast Sensitivity test is currently used in this clinic.

Dark Adaptation

This test is used to estimate the ability of the eye to recover its sensitivity in the dark after being exposed to bright lights.  The subject simply identifies the lowest level of light seen in the dark at one-minute intervals for approximately 25 minutes using a Dark Adaptometer.

Visual Acuity

This test is used to estimate how well the eye perceives fine detail and usually involves reading progressively smaller sized letters on a chart from a distance. Tests available include the Snellen Acuity, Landolt C, Illiterate E, Ramsey symbols and Curpax Near Acuity.

Electrodiagnostic Tests

Electrodiagnostic tests use electrodes to assess the electrical function of the visual system.

​Electroculogram (EOG)

The purpose of the EOG is to examine the functional state of the outermost retinal layers, namely the retinal pigment epithelial and photoreceptor layers. Electrodes are applied at the inner and outer canthii before the patient is positioned in front of a Ganzfeld dome. The subject then tracks leftwards and rightwards, using their eyes, between two horizontally spaced red target lights for approximately 15 seconds.

This eye movement exercise is repeated every minute for 6 minutes in the light, 16 minutes in the dark and a further 10 to 15 minutes in the light.

The size of the signals recorded decreases with dark adaptation and increases with adaptation to light. The ratio between the highest point in light conditions and the lowest point in dark conditions (the Arden ratio) is of critical interest in the interpretation of the EOG.

The EOG is used to assist in the diagnosis of retinal disorders such as retinitis pigmentosa, drug-induced retinopathy and some macular dystrophies.

Electroretinography (ERG)

The ERG is a recording of the summed electrical response of the retinal potentials evoked by light flashes and reflects the functional state of the photoreceptor to the inner plexiform layers.

The recording is made by applying specialised fine wire electrodes (HK-Loops) to the surface of the eye, beneath the lower eyelid, and skin electrodes to the outer canthus and forehead. The light flashes are provided by a Ganzfeld dome light source for uniform illumination of the retina. The test is performed under dark adapted and light adapted conditions with pupils maximally dilated.  Various intensities, colours and background lighting conditions are used to assess the function of distinct cell types within the retina.

Normal and abnormal waveforms are differentiated by the amplitude and time-course or latency of the composite wavelets.

Multifocal ERG (mfERG)

Multifocal technology can rapidly derive the responses of a large number of small retinal areas by stimulating them simultaneously but independently. Responses from up to several hundred retinal areas can be obtained in the time it takes to derive the same information from a single retinal area, the result being a high resolution, three-dimensional topographic representation of retinal function. The high sensitivity of this technique allows the early detection of lesions which are missed by more conventional tests.

This test is useful in assessment of macular function and progression of drug-induced retinopathies as well as monitoring the effects of therapy.

Multifocal VEP (mfVEP)

The mfVEP is recorded in much the same way as the VEP, but uses sophisticated multifocal technology to obtain responses from many cortical areas in the time it takes to derive the same information from a single area.  The result is a high-resolution topographical representation of localised optic nerve function. The high sensitivity of this technique allows the early detection of lesions, such as glaucomatous damage, which are missed by more conventional tests.

Pattern Electroretinography (PERG)

The PERG is used to evaluate the function of the macula or central retina. Electrodes are applied in a similar manner to the ERG but, in this case, a gold foil electrode is placed so as to touch the surface of the eye.

Retinal responses to a reversing checkerboard pattern are measured. Two reversal rates are used to provide information on the functional integrity of the macula and optic nerve.

Visual Evoked Potentials (VEP)

The VEP provides information about optic nerve function.

Four surface electrodes are applied to the scalp and one to the forehead. The subject is required to fixate on a small target at the centre of a reversing checkerboard pattern on a video monitor. Light flashes from the Ganzfeld dome may also be used for stimulation. The eyes are tested one at a time with the other eye occluded.

A normal VEP has a main positive component, the peak of which occurs at approximately 110 ms after stimulation. The latency, amplitude and morphology of this component are important in the diagnosis of optic nerve defects.

The VEP is used to assist in the diagnosis of conditions such as multiple sclerosis, optic nerve tumours, optic atrophy, drug toxicity and amblyopia.

Paediatric Protocol (PP)

Usually the VEP and ERG are performed, although there may be some small differences from the standard protocols.  It is important that the child is awake and alert for the tests and it is usually recommended that, except for exceptional circumstances, they be performed without sedation or general anaesthesia.

Last Updated: 03/06/2020