Guía general para la administración del examen de comprensión

Electroretinogram Technique Development

6.6,1 Introduction

The c-wave is a slow developing comeal positive ERG response, which is believed to reflect the integrity o f the retinal pigment epithelium (see section 4.2.8). The RPE and outer retina are affected at an early stage in AMD, and thus the c-wave may have diagnostic potential. There have been no previous reports o f the effect of early AMD on the c-wave, but abnormalities in the waveform have been recorded in dogs and humans with diffuse RPE abnormality, whilst the a- and b-waves remain within normal limits (Nilsson & Wrigstad,

1997).

The ERG c-wave may be recorded from the cornea of humans, but is so slow to peak that the signal may be contaminated by electrode drift. Special non-polarisable electrodes can be used to counteract this problem, and the signal recorded on a DC amplifier. However, performing ERG recordings with DC amplification is technically difficult (Nilsson & Anderson, 1988). Hamasaki et al (1997) compared c-waves recorded using DC and AC equipment, and found that both systems would record a measurable response, although the amplitude of the c-wave was larger with the DC system. They recommended that AC amplifiers should have a low pass filter of 0.1 Hz or below.

Moschos et al (1993) successfully recorded the c-wave, using AC amplification, from 48 healthy controls and 15 subjects with cone dysfunction syndrome. The stimulus consisted of a 300 asb (95.4 cd.m'2) intensity flash o f 1.25 secs duration, presented to the dark-adapted eye. The time window o f recording was 5 seconds, and a ten minute interstimulus interval maintained the level o f retinal adaptation. All controls showed a recordable c-wave of mean amplitude 530.18 pV at a mean implicit time o f 1928.64 ms. Fourteen of the 15 subjects with cone dysfunction syndrome also produced a recordable c-wave, but of reduced amplitude and prolonged implicit time.

The optimal recording conditions to elicit a c-wave of maximal amplitude involve long duration, high intensity stimuli presented to the dark adapted eye (Skoog & Nilsson, 1974;

Hamasaki et al, 1997). A long timebase is required to record the peak of the response, which

Chapter 6 Electroretinogram Technique Development

The aim o f this study was to determine whether a practicable technique for reliably eliciting the ERG c-wave from healthy control subjects could be devised, within the confines o f the AC amplification equipment available.

6.6.2 Recording the c-wave

A pilot experiment was carried out to determine whether it was possible to replicate the results o f Moschos et al (1993). Responses were recorded from subject JB (age 56), who was dilated with 1.0% tropicamide, and prepared for ERG recording in the manner described in section 6.1. White flash stimuli were generated by a combination of red, green, blue and amber LEDs driven by the LED stimulator. Flash luminance was adjusted to 95.4 cd.m'2, with a flash duration o f 1.25 secs. An interstimulus interval o f 10 minutes was used. Figure 6.21 shows three consecutive ERG traces recorded on a 3 second timebase and bandpass filtered from 0.1-100 Hz.

B

500 pV

300 ms

Figure 6.21. ERG responses recorded from subject JB, in response to a full-field white flash (95.4 cd.m*2, 1.25 sec duration). Responses were recorded on a 3 second timebase, and bandpass filtered from 0.1-100 Hz. Trace A is a single (not averaged) ERG obtained after 30 mins dark adaptation. Traces B and C were recorded 10 and 20 minutes later respectively. A large positive going waveform can be seen following the b-wave in all three traces in Figure 6.20. This does not peak within the 3 second time window, but rather appears to continue rising. The amplitude o f this positive response, measured at 3 secs after stimulus onset, becomes progressively smaller in consecutive traces. The first ERG recorded shows the greatest amplitude of response (1103.6 pV), with smaller signals seen in the second (686.5 pV) and third (293.3 pV) traces. The timing o f this positive response suggests that it is

Chapter 6 Electroretinogram Technique Development

the c-wave described by Moschos (1993), although no sample traces were included in the original article, and consequently a direct comparison was not possible.

6.6.3 Conclusion

The progressive decrease in the amplitude o f the response in successive traces indicated that the ten minute interstimulus interval used in this protocol was insufficient to allow the level of retinal adaptation to return to baseline between flashes.

A further problem encountered in the recording o f the c-wave response using this technique was the difficulty experienced by the subject in avoiding blinking and eye movements in response to a long duration flash presented to a dark adapted eye. This was accentuated by the long timebase, with the subject having to maintain stable fixation for 3 seconds to prevent contamination o f the trace by extraneous signals. A number of responses were rejected due to such eye movements. This made the process particularly time consuming as a further period of dark adaptation was required before the next flash could be presented.

Attempts to repeat the recording on a further two control subjects, DA and AB (both aged 26) were unsuccessful. In both cases this was due to rejection o f the ERG traces due to blinking. The instillation o f a topical anaesthetic did not remedy the problem.

A large physiologic variability in the size o f the c-wave has been reported. It has been suggested that 10-15% o f normal subjects fail to have a recordable response (Hock &

Marmor, 1983). The repeatability o f the c-wave is also found to be poor, with a same-day

intrasubject variability o f 23-40% found in one study (Hamasaki et al, 1997). In view of such reports, and the problems experienced in eliciting this response from healthy adults, it was decided not to use the technique in this study.