Ординатура / Офтальмология / Английские материалы / Electrophysiology of Vision_Lam_2005

function with peak sensitivity at a wavelength of 555 mm, which is closely approximated by the sum of sensitivities of the longand middle-wavelength-sensitive cones. In contrast, the standard spectral sensitivity function under scotopic condition (Vl0) is a function with peak sensitivity at a wavelength of 507 mm in part to account for the peak sensitivity of the rods. Photopic troland (td) and scotopic troland (td0) are calculated based on the spectral distribution of the stimulus based on Vl and Vl0.

ERG Electrodes

Several ERG recording electrodes are available (Fig. 1.2), and their advantages and disadvantages are summarized in Table 1.2. The Burian–Allen and the Dawson–Trick–Litzkow (DTL) electrodes are probably the most popular. Proper placement of the electrodes is critical to obtain accurate and consistent ERG responses, and proper cleaning protocols as recommended by the manufacturer must be followed.

To record an electrophysiologic signal, the signal from the active recording electrode is compared with the signal

Figure 1.2 ERG recording electrodes. The characteristics of the electrodes are summarized in Table 1.2. Burian–Allen and Daw- son–Trick–Litzkow (DTL) electrodes are probably the most commonly used electrodes for the full-field ERG. Proper placement of the electrodes, as described in the text and Table 1.2, is critical to obtain accurate, consistent recordings. (Refer to the color insert.)

from a reference electrode which may be placed on several possible locations including the forehead or lateral regions of the orbital rim. A ground electrode is typically placed on an ear lobe or the forehead.

The Burian–Allen electrode is a contact-lens type electrode with a lid speculum to reduce the effect of blink and eyelid closure (4). Topical anesthesia with eye drops is required, and patient tolerance to the Burian–Allen electrode is less than that to the DTL electrode. The Burian–Allen electrode is available in several sizes. The bipolar version has a conductive coating on the outer surface of the lid speculum that serves as the built-in reference electrode eliminating the need for a separate reference electrode (5). The ERG responses obtained with the bipolar version are slightly smaller than the monopolar version, but the recording is more stable. However, the bipolar version is more vulnerable to recording artifact from light-induced reflexive contraction of the orbicularis called the photomyoclonic reflex (6). The view through the Burian–Allen electrode is blurred but this will not affect the full-field ERG. Newer modified versions provide clearer optics and are suitable for multifocal ERG.

The DTL electrode is a conductive Mylar thread typically placed in the lower fornix where the thread contacts the inferior bulbar conjunctiva or the corneal limbal region (7). This preferred position stabilizes the position of the thread (8). Topical anesthesia is not warranted, and patient tolerance is superior compare to contact-lens type electrodes such as the Burian–Allen and Jet electodes (9). Reproducibility of ERG recordings is favorable (10). Eyelid closure and blink artifacts with the DTL electrode are more pronounced than the Burian–Allen electrode, and the recorded ERG amplitudes are lower (11). Compared to the Jet electrode, the DTL electrode recordings have greater variability and recorded amplitudes are lower (12).

Skin electrodes are not recommended for routine ERG recording but may be useful in infants and young children intolerant to other electrodes. The ERG responses from skin electrodes are considerably smaller and more variable than conjunctival or corneal contacting electrodes.

System and Patient Set-Up

The pupils should be fully dilated with pharmacologic eye drops, because small pupils reduce retinal illuminance and ERG responses. To avoid electrical interference, the recording environment should be physically and electrically isolated from electrically noisy devices such as appliances, electrical equipment, and fluorescent lighting. The recording room should be light sealed and suitable for dark adaptation.

After electrode placement, impedance and baseline signals are evaluated to assess electrical noise interference and artifacts from eye movement and blink. If problems occur, electrode positions and connections are checked, and the patient is encouraged further to cooperate. The ERG signal is small compared to the electrical noise from power current and from the heart, muscles, and brain. Differential amplifiers amplify the difference in input between the recording and reference electrodes and reject signals common to both inputs. The impedance of the recording and reference electrodes should be matched for noise rejection to work properly. Impedance is the ratio of the measured voltage between the electrode and the ground electrode with respect to the input current: impedance ¼ V=I where V ¼ voltage and I ¼ input current. Most systems have built-in impedance meters that input a current from the active electrode towards the ground electrode allowing rapid check of impedance. A low impedance reading is desirable.

By adjusting the frequency filtering characteristics of the amplifier, the ERG recording is modified to reduce electrical noises and blink and eye movement artifacts. The low and high frequency cut-off ’s of the amplifier are set to exclude recording of signals with frequencies that are below the low frequency cut-off or above the high frequency cut-off. For full-field ERG, the low and high frequency cut-off ’s are typically set at 0.3 and 300 Hz, respectively, with the recording frequency range or bandpass being from 0.3 to 300 Hz. The exception is the recording of oscillatory potentials which require the bandpass to be adjusted from 75 to 300 Hz to exclude low frequency ERG components.

Recording Sequence

Binocular full-field ERG responses are usually simultaneously recorded. Whether scotopic or photopic recordings are recorded first is not critical as long as adequate adaptation periods are performed. If photopic responses are recorded first, the number and intensity of the photopic flash stimuli should not be excessive. Otherwise the subsequent dark adaptation will need to be longer to allow for retinal recovery. When contact-lens type electrodes are used, the scotopic responses may be performed first to minimize patient discomfort. The electrodes are placed at the end of the dark-adapta- tion period under dim red illumination to avoid altering rod sensitivity but this light exposure must be minimized. The patient then continues to wear the electrodes for the rest of the test through the subsequent shorter photopic recordings.

Five Standard Full-Field ERG Responses

Three standard scotopic responses are recommended (Fig. 1.3). First, a rod response is elicited by a dim white flash of 2.5 log units (25 dB) weaker the standard flash. Next, a maximal combined rod and cone response is elicited by the white standard flash. Third, the oscillatory potentials are elicited by the standard flash. The time interval between repeat flashes must be long enough to maintain dark adaptation and to allow retinal recovery. An interval of at least 2 sec between flashes is recommended for the rod response, and longer intervals of at least 10 and 15 sec are necessary for the combined rod–cone response and the oscillatory potentials, respectively.

Two standard photopic cone responses are recommended (Fig. 1.4). The single-flash and 30-Hz flicker cone responses are elicited with the standard flash and are recorded with the lit white photopic background of the full-field dome. The time interval between flashes for the single-flash cone response is not critical and can be as short as 0.5 sec. The stimulus flicker rate for eliciting the 30-Hz flicker cone response is close to but not exactly at 30 Hz (e.g., 30.3 Hz). This is because a flicker rate of exactly 30 Hz will produce a recording that is prone to electrical noise from the power supply which

Figure 1.4 (above) Standard photopic full-field ERG responses of a normal subject. The definitions of amplitudes and implicit times of the a- and b-waves are shown. Available computer programs allow automated calculation of the amplitude and implicit time for the 30-Hz flicker response.

Figure 1.3 (Facing page) Standard scotopic full-field ERG responses of a normal subject. The definitions of amplitudes and implicit times of the a- and b-waves are shown. The waveforms are all plotted to the same scale to demonstrate the relative size of the responses. Available computer programs allow accurate placement of cursors and rapid calculations. The a-wave of the standard scotopic combined rod–cone response for normal subjects may have a single negative peak or double negative peaks.

often has an oscillating rate in multiples of 30 (e.g., 60 Hz in the United States). The recording of the flicker response is not initiated until a few seconds after the start of the flicker since the first response to the flickering stimulus is a singleflash response rather than a flicker response.

At least two repeatable responses should be obtained for each of the five recommended responses. Averaging of multiple responses to the same stimulus produces an averaged waveform that has a smoother contour because random electrical noises cancel each other. The random noise level is decreased by the square-root of the number of responses averaged. Averaging helps to identify very weak responses buried by background electrical noise, but responses should not be averaged blindly, and poor quality responses with artifacts from blinks and eye movements should be excluded. Averaging of flash responses is not required as long as repeated recordings are reproducible and without artifacts. Flicker responses such as the 30-Hz cone response are routinely averaged from multiple flickers or sweeps because the responses are too rapidly for manual rejection.

Evaluating ERG Responses and Measuring

Amplitudes and Implicit Times

Electrophysiologic responses are evaluated based on appearance or morphology as well as measurable parameters such as amplitudes and implicit times of waveform components. The measurement of the amplitude and implicit time of each component is demonstrated in Figs. 1.3 and 1.4. The first negative component is called the a-wave and the ensuing positive component is called b-wave. The amplitude of the a-wave is measured from the baseline to the a-wave negative peak, and the amplitude of the b-wave is measured from the a-wave negative peak to the b-wave positive peak. The implicit time of each wave is the time from stimulus onset to its peak. The recording duration is usually in the range of 250 msec.

The scotopic rod response has a prominent b-wave but no a-wave because the electrical activity of the rod photoreceptors to the dim stimulus is very small, but this signal is

amplified tremendously by cells of the inner retina. The scotopic combined rod–cone response has distinct a- and b-waves. The a-wave may demonstrate double negative peaks likely due to the first wavelet of the oscillatory potential. In such cases, the a-wave amplitude may be measured from the baseline to the lower negative peak. Flashes brighter than the recommended standard flash elicit a larger better-defined a-wave.

The oscillatory potentials usually consist of three larger wavelets followed by a smaller one during the ascending phase of the b-wave. The wavelets are designated in order of occurrence as OP1, OP2, OP3, and so on. Several methods of measuring the oscillatory potentials have been proposed. In the peak-to-trough method, the amplitude of each wavelet is measured from the preceding trough to its peak. In the caliper-square method, the line connecting the troughs before and after each peak of the wavelet is calculated, and the amplitude is measured perpendicularly from the peak to the line. Alternatively, an index of the oscillatory potentials is provided by the sum of the amplitudes of all four major wavelets. The summed amplitude is similar regardless of which of two measuring methods is employed (13).

The photopic single-flash cone response has distinct a- and b-waves. The 30-Hz flicker cone response consists of only b-waves and provides a consistent measure of the cone response.

Recognizing Recording Noise and Artifacts

The electrical noise from the power supply and appliances is much larger than ERG signals and may interfere with ERG recording. Electrical noise often oscillates at a frequency related to that of the power supply (e.g., 60 Hz in the United States) (Fig. 1.5). Electrical noise may be reduced by checking the connections of the electrodes as well as unplugging appliances that are plugged into the local circuit. Electrical activities of the heart, muscles, and brain may also be a source of electrical noise.