9.1 Corneal Topography and Tomography
9.2 Confocal Microscopy
9.3 Optical Coherence Tomography - Macula
9.4 Optical Coherence Tomography Angiography (OCT-A)
9.5 Optical Coherence Tomography - Glaucoma
9.6 Optical Coherence Tomography – Anterior Segment
9.7 Fundus Autofluorescence Imaging
9.8 Fundus Angiography - Fluorescein
9.9 Fundus Angiography - Indocyanine Green
9.10 B-scan Ultrasonography & UBM
9.12 Automated Visual Fields
The rationale of electrophysiology testing is to provide objective evidence of function at the different levels of the visual system, thus enabling accurate localisation and characterisation of dysfunction in the vast majority of patients. The three main arms of electrophysiological testing are the:
The ERG records the action potential produced by the retina when it is stimulated by light of adequate intensity. In particular, it assesses the photoreceptor and inner nuclear layers of the retina.
There are four types of waveforms:
The EOG is a measure of the function of the retinal pigment epithelium (RPE) and the interaction between the RPE and the (rod) photoreceptors. It achieves this by calculating the standing potential between the electrically positive cornea and the electrically negative posterior eye. Electrodes are placed at the medial and lateral canthi and the patient is asked to look from side to side in dark then light adaptation. The EOG is quantified by calculating the size of the light peak in relation to the dark trough as a percentage, known as the Arden index. The Arden index is typically over 175% but can be abnormally low in conditions like Best Vitelliform dystrophy.
The visual evoked potential is a measurement of the electrical signal recorded at the scalp over the occipital cortex in response to a reversing checkboard light stimuli. The VEP largely arises in the visual cortex, and thus enables assessment not only of those areas of the brain responsible for receiving the visual information, but also the intracranial pathways which transfer the information from eye to brain, especially the optic nerves and the optic chiasm. The VEP is primarily a function of central visual function, because such a large region of the occipital cortex is devoted to macular projections.
The typical VEP waveform contains an initial negative peak (N1 or N75), followed by a positive peak (P1 or P100). Second negative (N2 or N135) and second positive (P2) peaks follow. The latency of onset of a peak after light stimulus and the amplitude of the peak are the most useful features analysed.
Clinical uses of the VEP include:
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