Ординатура / Офтальмология / Английские материалы / Electrophysiology of Vision_Lam_2005

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dysfunction, because unlike the multifocal ERG, pattern ERG gives no topographical information about localized macular dysfunction. Although the pattern ERG is a more direct and specific measure of retinal ganglion cell function than the VEP, pattern reversal VEP is performed more frequently to detect optic nerve dysfunction perhaps because VEP is a more familiar established test. Like other visual electrophysiologic tests, the pattern ERG should be used in combination with a thorough ocular examination as well as other clinical ancillary tests such as visual field, fluorescein angiography, optical coherence tomography, optic nerve head imaging, and neuroimaging. Aside from maculopathies and optic neuropathies, pattern ERG can also serve as a useful objective measure in non-organic visual loss. However, pattern ERG testing requires good fixation and may not be possible in uncooperative patients or in patients with nystagmus or poor visual acuity.

BASIC CONCEPTS AND PHYSIOLOGIC

ORIGINS OF PATTERN ERG

The concept of recording retinal electrical signals in response to a pattern stimulus is attributed to Riggs et al. (2). Whereas conventional full-field ERG measures retinal activity in response to a change in luminance, pattern ERG detects

Figure 3.1 (Facing page) Schematic diagram showing the basic principles of pattern ERG. When the pattern stimulus reversal rates are slow enough ( 3 Hz) to allow the retina to recover, a transient pattern ERG response is recorded, and when the stimulus frequency is too fast ( 5 Hz) to allow the retina to reach resting state between stimuli, a steady-state pattern ERG response is elicited. Transient pattern ERG allows the identification of specific pattern ERG waveform components and their amplitudes and latencies. Steady-state ERG responses are sinusoidal, and Fourier analysis is required to determine amplitude and the amount of time phase shift relative to the stimulus. The amplitude of steady-state ERG response is similar to that of the N95 component.

Table 3.1 Summary of International Standard and Recommendations for Pattern Electroretinography (PERG)

Table 3.1 (Continued )

retinal activity in response to a reversing black and white checkerboard stimulus that has the same mean luminance throughout recording. Photoreceptor cells are more responsive to change in luminance, but cells involved in visual signal processing such as retinal ganglion cells and other inner retinal cellular elements generate responses to changing light–dark edges of a pattern stimulus.

Two categories of pattern ERG responses are recog- nized—transient and steady-state (Fig. 3.1). Transient pattern ERG responses are produced when the pattern stimulus reversal rates are slow enough to allow the retina to recover to its resting state between stimuli. Steady-state pattern ERG responses occur when stimulus rates are too fast to allow the retina to reach resting state between stimuli. Transient pattern ERG responses are elicited with a stimulus reversal rate of six reversals per second or less, equivalent to a phase frequency of 3 Hz or less. Steady-state responses are generated by a reversal rate of 10 reversals per second or greater, equivalent to a frequency of 5 Hz or more. Transient pattern ERG allows the identification of specific pattern ERG waveform components and their amplitudes and latencies. Steady-state ERG responses are sinusoidal, and Fourier analysis program is required to determine amplitude and the amount of time phase shift relative to the stimulus.

The transient pattern ERG waveform consists of a series of negative (N) and positive (P) components designated by their approximate latencies in milliseconds from the onset of the stimulus. Three components are recognized—N35, P50, and N95, but N35 is not always visible (Fig. 3.2). The amplitude of P50 is measured from the negative N35 peak to the positive P50 peak, and in cases of an ill-defined N35, the N35 peak amplitude is substituted by the average of the stimulus onset amplitude and the P50 onset amplitude. The amplitude of N95 is measured from the P50 peak to the N95 peak. The latency of P95 may be difficult to determine because its negative trough may be broad.

In general, P50 is produced by retinal ganglion cells and other retinal cellular elements, and N95 is generated predominantly by retinal ganglion cells (3,4). The P50 component

Figure 3.2 Normal transient pattern ERG response. The method of amplitude measurements and physiologic origins of the components are shown. In cases of ill-defined N35, the N35 peak amplitude is substituted by the average of the stimulus onset amplitude and the P50 onset amplitude. The latency of N95 may be difficult to determine due to its broad trough.

is partly reduced and N95 is severely reduced in monkeys treated with tetrodotoxin (TTX) which abolishes retinal action potentials generated by ganglion cells (5). This finding is also consistent with pattern ERG findings in humans after optic nerve resection (6). Luminance and contrast response studies also suggest that P50 receive contributions from cells other than retinal ganglion cells and is responsive at least to luminance. In contrast, N95 is dominated almost exclusively by retinal ganglion cell activity and responds well to contrast.

The different physiologic origins of P50 and N95 allow transient pattern ERG to differentiate between macular and optic nerve conditions (Fig. 3.3). In macular disorders, notable reductions of P50 are encountered with a concomitant reduction of N95 such that the N95 to P50 amplitude ratio is not reduced or in some cases even increased. In contrast, optic nerve diseases are associated with a relatively preserved P50 and a selective reduction of N95.