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for a normal subject and a dark trough is evident after around 10 min followed by a light peak after a further period of light adaptation. If we form the ratio of light peak/dark trough then we obtain a measure of the change in voltage over the period of the test and this ratio is known as the Arden ratio.
3.8.2 The Electroretinogram (ERG)
The electroretinogram was the first physiological signal to be recovered from the human body by two young Scotsmen in 1877. Working independently Dewar and McKendrick recovered an evoked potential to a brief flash of light. This work formed the basis of modern electroretinography which gained popularity in the 1950s with ISCEV formulating a standard for the procedure in 1984.
The stimulus for the full field ERG is the Ganzfeld bowl shown in Figure 3.24. This device gives an even background illumination across the retina with the ability to superimpose short duration full field flashes of light. ISCEV specify the stimulus and recording conditions for the ERG response. A reference skin electrode usually made of Ag/AgCl is placed at the outer canthus. A Ground or indifferent electrode is placed on the forehead or earlobe and the active electrode is placed in contact with the eye. Figure 3.27 shows a range of ERG electrodes that are currently in use. Foils or fibres are placed under the lower eyelid (Figure 3.28) or special contact lenses with electrodes built in can be placed on the cornea.
The ERG response is dependent on a number of variables including the type of electrode used and it is therefore important for each individual laboratory or clinic to establish their own normative data. By appropriate selection of background light levels, stimulus luminance, dark or light adaptation it is possible to obtain a set of responses which give objective and complementary information on the integrity of retinal processing. The examples shown in this section were recorded using the disposable DTL fibre electrode. In the description of the responses a standard flash is defined as a stimulus luminance level of 1.5 to 3.0 Cd m−2 s.
Figure 3.27. ERG electrodes
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Figure 3.28. Gold foil ERG electrode in place
Response 1: The rod response
This response is recorded using a dim flash of light in the dark adapted eye. The period of dark adaptation should be at least 20 min. The stimulus intensity should be 2.5 log units below the SF. It is acceptable to perform serial averaging of a number of responses but inter-stimulus duration should not be less than 2 s to avoid light adapting the retina. A typical response is shown in Figure 3.29a. The positive wave is known as the b-wave and is generated by the on-bipolar cells in the retina. As signals are passed to the on-bipolar cells from the rod system, a normal waveform indicates intact rod and on-bipolar cell function. The key measurement is the amplitude from baseline to the peak of the response. Time to peak can also be of interest.
Response 2: The maximal response
This response is also performed on a dark adapted eye. In this case a bright flash of stimulus strength SF is used to evoke a response that is generated by both rod and cone systems. As this is a more intense flash of light inter stimulus duration should not be less than 15 s. An example is shown in Figure 3.29b. The trough is known as the a-wave and is generated by off-bipolar cells with a small contribution directly from the photoreceptors. The positive component is the b-wave and this is mainly generated by off-bipolar cells. The a-wave amplitude is measured from the baseline to the trough and the b-wave amplitude from the a-wave trough to the response peak.
Response 3: Oscillatory potentials
Oscillatory potentials are small oscillations on the rising edge of the b-wave. Stimulation is the same as for the maximal response but in order to emphasize the high frequency oscillations a different amplifier filter bandwidth is used. Instead of 0.5–300 Hz, a restricted bandwidth of 75–300 Hz is recommended. This removes the slow frequency component giving the oscillatory potentials as illustrated in Figure 3.29c.
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Figure 3.29a–e. Standard ERG response
The oscillatory potentials are believed to originate in the inner retina with horizontal and amacrine cells the most likely generators.
Response 4: Cone response
A 10 min period of light adaptation to a background luminance of 17–34 Cd m−2 is required before this photopic measurement is performed. In this case the SF is used and an example response is shown in Figure 3.29d. This is a response dominated by the cone pathway with the a-wave generated by the off-bipolar cells and the b-wave the on-bipolar cells. Waveform measurements are the same as in previous examples.
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Response 5: Flicker response
A pure response from the cone pathway can be obtained if a fast flickering stimulus is used. In this case, the standard flash is used at a stimulation rate of 30 Hz. The rod system cannot respond at these frequencies therefore the flicker response is a pure cone pathway response. A normal flicker response is shown in Figure 3.29e.
3.8.3 The Pattern Electroretinogram
This response is generated using a reversing checkerboard stimulus. The stimulus is shown in Figure 3.30 and the reversal should take place at a frequency less than 3 Hz. During the reversal, black elements will change to white at the same time as white elements change to black.
Recording electrodes can be the same as in the flash ERG but contact lenses should be avoided as these can interfere with the optics of the eye, degrading the stimulus presentation on the retina. The response is much smaller in amplitude than the standard ERG and is shown in Figure 3.31. The small amplitude of the response necessitates signal averaging with around 200 sweeps required to obtain a reasonable signal-to-noise ratio. The main components are marked on the figure with the N35 and P50 components having similar origins to the flash ERG. A new negative component, the N95 component is believed to originate from the inner retina at the ganglion cell layer. The response is highly dominated by the central retina and there is little contribution from the peripheral retina. This makes the procedure appropriate for objective quantification of macular function.
Figure 3.30. Pattern ERG stimulus