Ординатура / Офтальмология / Английские материалы / Electrodiagnosis of Retinal Disease_Miyake_2005
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1.2 Focal Macular ERGs 23
1.2.1.2Recording System
A Burian–Allen bipolar contact lens electrode (Fig. 1.20C) is used to record the ERGs. This lens allows the examiner to observe the fundus through the fundus camera clearly,and it allows a sharp image of the stimulus to be formed on the retina. When the ERG and visual evoked response (VER) are recorded simultaneously, the two responses are fed to two amplifiers, and the output of the amplifiers is fed to a signal
processor for signal summation. Usually, 256 or 512 responses are summed with a stimulus frequency of 4.5Hz. Using an artifact rejection system, baseline fluctuations larger than 40 mV are rejected from the summation. The luminances of the stimulus light and background illumination are 29.46 and 2.84cd/m2, respectively.
241 Principles and Methods
1.2.2 Proof of Focal Responses
To prove that the responses recorded by our system are really focal, a 5° diameter stimulus spot was moved in 7.5° steps from the optic disk through the fovea to 15° temporal to the fovea. The ERGs and VERs recorded simultaneously at each position from a normal subject are shown in Fig. 1.22 [13]. The mean amplitudes of the responses from four normal subjects are shown at the bottom of Fig. 1.22.
The amplitudes of the ERG and VER were largest when the stimulus spot was on the fovea,
and they became smaller as the spot was moved away from the fovea. Most importantly, a response was not present when the stimulus spot was on the optic disk, indicating that the responses were not contaminated by stray light responses.
Fundus photographs from three patients with unilateral differently sized macular colobomas are shown in Fig. 1.23 [13]. The sizes of the macular coloboma were approximately 8°–10°, 11°–12°, and 17°–20° in diameter in
Fig. 1.22. Left: A 5° diameter stimulus spot was moved in 7.5° steps from the optic disk through the fovea to 15° temporal to the fovea. A small spot on the fovea is a fixating target, which is useful for keeping the examining eye stable during each recording. Right, top: ERGs and VERs recorded simultaneously at each position from a normal subject. Right, bottom: Relative amplitudes of ERGs and VERs from four normal subjects. The ERG and VER amplitudes are maximum in the fovea and are absent at the optic disk. Filled circles, ERG; open circles, VER. (From Miyake [13])
1.2 Focal Macular ERGs 25
cases 1, 2, and 3, respectively. The full-field photopic ERGs and full-field 30-Hz flicker ERGs of the affected and normal fellow eyes are shown in Fig. 1.24. The amplitudes of full-field photopic ERGs of the affected eye are within the normal range but are smaller than those of the normal fellow eye in cases 2 and 3. However, the 30-Hz flicker ERGs from the two eyes did not differ significantly in any of the patients.
These results indicate that the full-field conemediated ERGs are of limited value but may be better than the 30-Hz flicker ERGs for evaluating macular function.
Additional information can be obtained by studying focal macular ERGs. The simultaneously recorded focal macular ERGs and VERs elicited from the affected and normal fellow eyes in these three cases are shown in Fig. 1.25
Fig. 1.23. Fundus photographs showing different sizes of macular colobomas. The sizes of the colobomas are approximately 8°–10° (case 1), 11°–12° (case 2), and 17°–20° (case 3). (From Miyake et al. [13], with permission)
Fig. 1.24. Full-field photopic ERGs (left) and full-field 30-Hz flicker ERGs (right) from the affected eye and the normal fellow eye in the three patients with macular colobomas shown in Fig. 1.23.The full-field photopic ERGs are smaller in the eye with the coloboma in cases 2 and 3, although the 30-Hz flicker ERGs from the two eyes did not differ significantly in any of the patients. (From Miyake [13], with permission)
26 1 Principles and Methods
[13]. The size of the stimulus spot was adjusted so it was approximately the same as that of the colobomas, and the stimulus spot was placed exactly on the coloboma by monitoring the fundus.
The ERGs and VERs were unrecordable in all cases, indicating that the stimuli for these recordings were stimulating the retina only underneath the spot, and the “responses” were indeed focal.
Fig. 1.25. Focal macular ERGs and VERs recorded simultaneously from the eyes with macular colobomas and normal fellow eyes in the three patients shown in Figs. 1.23 and 1.24. The diameters of the stimulus spots were 6° (case 1), 10° (case 2), and 15° (case 3). The ERGs and VERs are unrecordable in all cases, indicating that the stimuli were stimulating the retina only underneath the spot, and the “responses” were indeed focal. (From Miyake [13], with permission)
1.2 Focal Macular ERGs 27
1.2.3Macular Oscillatory Potentials
As described in the previous section on fullfield ERGs, oscillatory potentials (OPs) are wavelets superimposed on the ascending slope of the b-wave of the conventional ERG and are generated independently of the a-waves and b- waves. The site of generation of the OPs is not yet fully known, but experimental evidence indicates that OPs reflect the activity of inhibitory feedback synaptic circuits in the retina. Although many studies have investigated their physiological properties and clinical value, the OPs in humans were evaluated as
components of the total ERGs elicited by ganzfeld stimuli until we succeeded in recording the OPs from the human macula using the focal macular ERG recording system in 1988 [13, 14].
The focal macular ERGs seen in Fig. 1.26 were elicited by five different-diameter stimulus spots in 2.5° steps projected on the macula in a normal subject. The a-waves and b-waves of the ERGs were recorded with a time constant (TC) of 0.03s and a 100-Hz high-cut filter, and the OPs were recorded with a TC of 0.003s and
Fig. 1.26. Focal macular ERGs recorded simultaneously with two time constants (right) from a normal subject. The stimuli were differently sized spots centered on the macula (left). A time constant (T.C.) of 0.03 s with a 100-Hz high-cut filter was used to record a-waves and b-waves; and a T.C. of 0.003 s with a 300-Hz high-cut filter was used to recorded oscillatory potentials (Ops). (From Miyake [13])
28 1 Principles and Methods
a 300-Hz high-cut filter. The OPs consisted of three or four wavelets (O1–O4) and were clearly observed in the responses to all stimulus spot sizes.
To investigate the distribution of OPs in the human macular area, we used ring or annular stimuli, as shown in Fig. 1.27. We adjusted the stimulus conditions so the amplitudes of the a- waves and b-waves were essentially the same for the circular stimuli and annular stimuli (Fig. 1.28). Under these conditions, the OPs were significantly larger with the annular stimuli than with the circular stimuli, suggesting that the distribution of OPs is different from those of the a-waves and b-waves in the human macular region [13, 14]. The changes in the amplitude of response to the spot sizes and annuli indicated that the distribution of the neural elements generating the OPs is relatively sparse in the fovea. However, they become relatively more dense than those generating the a- waves and b-waves in the parafovea and even more dense in the perifovea.
Another unique property of the macular OPs is the nasotemporal asymmetry [15]. Semicircular stimuli were used to compare the ERGs elicited by stimulating the temporal and nasal macula, as shown in Fig. 1.29. Focal ERGs elicited by stimulating the temporal and nasal retina with semicircular stimuli and circular stimuli (15° in diameter) are shown in Fig. 1.30. The amplitudes and implicit times of the a- waves and b-waves in the nasal retina are almost identical to those from the temporal retina, whereas the amplitudes of the OPs are much larger in the temporal retina than in the nasal retina. The amplitude of the focal ERGs recorded with the circular stimulus was approximately the same as the sum of the amplitudes of the temporal and nasal ERGs.
These new properties of the macular OPs, as distinct from the a-waves and b-waves, have been confirmed by others recently with multifocal ERGs [16, 17]. The asymmetrical amplitudes of the OPs in the nasal and temporal retina may have resulted from the various
Fig. 1.27. Circular (top) and annular (bottom) stimuli on the macula
1.2 Focal Macular ERGs 29
retinal elements contributing to the OPs. The OPs reflect neuronal activity in the inner nuclear layer of the retina and are probably mediated by the amacrine cells or the interplexiform cells. It is interesting that there is some correlation between the distribution of dopamine-containing amacrine cells in macaque retina and that of OPs in the human macular region [18].
The significant temporal and nasal asymmetry only in OPs was surprising (Figs. 1.29, 1.30). Some reports have suggested that this asymmetry may be related to a nasotemporal difference in the number of cones and ganglion cells or to asymmetry of the optical density of photopigments in the foveal cones. However, we have not found any reports that provide evidence for asymmetry of the OPs.
Fig. 1.28. Comparison of focal macular ERGs elicited by a circular and an annular stimulus on the macula in four normal subjects. It was adjusted so there was little difference in the amplitudes of the a-waves and b-waves between the two recording conditions, but the OPs are much larger with the annular stimuli. (From Miyake et al. [14])
30 1 Principles and Methods
Fig. 1.29. Semicircular stimuli with the edge of the semicircle passing through the vertical axis
Fig. 1.30. Comparison of focal ERGs using semicircular stimuli on the nasal and temporal macular areas and a circular stimulus (15°). The OPs in the temporal macula are significantly larger than those in the nasal macula, and only the OPs show this significant asymmetry. (From Miyake et al. [15], with permission)
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1.2.3.1Components of Focal Macular ERG in Humans
Focal macular ERGs recorded from a normal human subject demonstrating the various components are shown in Fig. 1.31. The a-waves and b-waves, OPs, on and off components, and flicker responses are shown. Because these
components originate from the neural activity of different retinal neurons in different retinal layers, a layer-by-layer analysis of macular function can be performed objectively by analyzing the components.
Fig. 1.31. Components of the focal macular ERG recorded from a normal subject. ON and OFF responses recorded with 1-Hz stimulus frequency (top); a-wave, b-wave, and OPs recorded with 5-Hz stimulus frequency (middle); and 30-Hz flicker responses (bottom) are shown
32 1 Principles and Methods
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