Results

Of the 15 pre-AMD or early AMD eyes, nine of them showed a mild decrease in visual acuity. The mean is 0.84 ranging from 0.7 to 1.0 (Table 1). In contrast, the contralateral asymptomatic eyes showed normal visual acuity with a mean of 1.06, ranging form 0.8 to 1.5 (Table 2). Fundus examination showed a majority of the AMD group had hypopigmentation (eight out of 15), five eyes had one to three hard macular drusen, and two eyes had soft drusen (Table 1) while the fellow eyes were normal in appearance (Table 2). Twelve of 15 in the patient group received an FA examination of both eyes. Window defect or hyperfluorescence was seen in 11 pre-AMD or early AMD eyes (Table 1) and three fellow asymptomatic eyes (Table 2). With the Amsler grid test, central distortion, wavy lines, or blurred areas were noted in four pre-AMD or early AMD eyes (Table 1), with no abnormality noted in all asymptomatic fellow eyes (Table 2). Using configurations A and B, no significant differences were detected in the P1 amplitudes or the N1 latencies between the different regions (nasal versus temporal and superior versus inferior) of the retinas in the normal control eyes, the early AMD eyes, and the fellow eyes. There were also no significant differences in the P1 amplitudes and the N1 latencies with each region between normal, AMD, and the fellow eyes (Table 3). Contrary to configurations A and B, with configuration C, P1 amplitudes decreased and N1 latencies increased as the rings were further from the fovea in all three groups of eyes (Table 4). In the normal eyes, the foveal (R1) P1 amplitude was the highest among the six ring groups (p<0.001). However, there was no significant difference among the latencies of different rings. In the pre-AMD or early AMD eyes, the foveal (R1) P1 amplitude was also the highest among all six ring groups (p<0.001), and it was significantly reduced when compared with the normal (p=0.005). Foveal (R1) N1 latency was also the longest among the six rings (no significant difference) and was significantly longer than the normal (p<0.001). The fellow asymptomatic eyes showed values approximating those of the pre-AMD or early AMD eyes. The foveal (R1) P1 amplitude was also reduced compared to the normal (p=0.017), and the foveal N1 latency was also significantly longer than the normal (p<0.001). Figure 2 represents typical tracings of MERG responses in the six concentric ring configuration in the normal eyes, asymptomatic fellow eyes, and pre-AMD or early AMD eyes from two age and sex matched individuals showing the above mentioned features. Results of MERG at 3 months of follow up remained essentially the same (not shown) when compared with the initial examinations.

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Figure 2

Illustration of the MERG responses and their calculated P1 amplitudes and N1 latencies in (a) normal, (b) fellow, (c) and pre-AMD or early AMD eyes.

Discussion

In this study using the first kernel calculation of concentric ring configuration in MERG recordings, we noted a significant reduction in the P1 amplitude as well as a significant delay in the N1 latency of foveal responses from pre-AMD or early AMD eyes and their asymptomatic fellow eyes when compared with normal control eyes. These findings suggest that both foveal P1 amplitude and foveal N1 latency in MERG measurements may be sensitive means to detect early foveal AMD changes.

Configurations A and B were initially used because it was reported that in normal individuals the nasal retina showed a lower response than the temporal retina while there was no difference in the superior or inferior retina.16 However, we did not find any significant difference between these different regions of the retina or among the three groups of eyes.

Our observation that pre-AMD or early AMD eyes showed suppressed foveal P1 amplitude and prolonged foveal N1 latency is consistent with earlier reports noting foveal electrophysiological abnormality in AMD eyes.17 Contrary to other studies, we used eyes showing early or pre-AMD symptoms such as drusen and window defects in FA. Whether these eyes would ultimately suffer AMD remains to be determined but our study showed that foveal parameters of MERG are sensitive means to detect foveal changes.

Unlike the regular ERG, the exact origin of the first order kernel measurement is controversial although it is generally accepted that it arises from the cones.1819 The P1 amplitude of the central fovea is believed to be linked to the number and function of photoreceptor cells.1819 Our observation that the pre-AMD or early AMD eyes have a lowered P1 amplitude than their fellow eyes and the normal eyes is consistent with the hypothesis that the AMD eyes may have a reduced number and/or functional deficiency of photoreceptors. According to Kondo et al, the negative and positive deflections in the first order kernel of MERG probably do not correspond to conventional a and b-waves of ERG.20 Various investigators have suggested that MERG may include responses from the inner retina.2122 It is possible that the noted delay in the N1 latency reflects damages of the inner retinal layers as well as the outer retina. Although both the P1 amplitude and the N1 latency were affected, because of the smaller variations in latency measurements, the latter may be a more useful indicator of the changes.