Visual Electrophysiology –Outline of Tests

Introduction:

Correct diagnosis requires:

1. Patient & Family history
2. Refraction, Visual Acuity, Fixation  Pattern
3. Optic Media & Ophthalmoscopy
4. Visual field, Colour Vision, Autofluorescence, Angiography

Patterns of Referrals

1. Referred for Specific Tests
2. Referred for investigation

Electrophysiology

2. Electrooculogram:

A Measure of Retinal Pigment Epithelium (RPE) function and interaction between RPE and the Receptor Cells (Rods And Cones).
This test measures the change in the Corneo-Retinal Potential as the
The eye changes from dark adaptation to light adaptation.
Light Adapted peak/dark adapted trough expressed as a %. Normal >175%.

1. Electroretinogram

This measures the mass action potential of the retina when exposed to a bright flash of light. It is measured with corneal electrodes and a standard flash (ISCEV Standards).

1. Measurement in dark and light adaptation allows estimation of rod and cone and function.
2. Varying intensity of flash and rate of stimulation allows vertical dissection of the retina including amacrine cell function and bipolar layers
3. Ganglion Cells contribute little to the full field ERG.

1. Electroretinogram

a.   Objectively detect, describe & differentiate retinal functional abnormalities
b.   Topographical Analysis of affected retinal layer (cell system)
c.   Differentiation of affected receptor cell system        

1. Pattern Electroretinogram

This is the response of the central retina to an isoluminant reversing black and white checkerboard stimulus.

1. Measures central retinal function including ganglion cell function.
2. It differentiates between optic nerve and macular function
3. This test should be read in conjunction with ERG and VEP (see diagram 1)

1. Visually Evoked Potential

This measures the change to the EEG responses when a visual stimlus is presented. This is measured with electrodes over occipital cortex. The stimulus is usually a reversing checkerboard (a flash or offset/onset stimulus may be used)

1. Measures optic nerve and chiasm function,
2. Calculates visual acuity objectively
3. Assesses misrouting of chiasm fibres
4. Visual field estimation.
5. Visual Pathway Diagnosis

Interrelationship between tests (Diagram 1)

MultiFocal VEP

Indications:
a.   Identifying functional problems
b.   Optic Neuritis- optimum way to follow local demyelination
c.   Confirming unreliable fields
d.   Early detection Glaucoma- future

Multifocal ERG

Indications:

• Early detection of macular abnormalities
• Regional distribution of retinal Function
• No differential diagnosis of maculardystrophies
• Follow-up of retinal dystrophies