Abnormal pattern electroretinogram in Alzheimer\'s disease: Evidence for retinal ganglion cell degeneration?

Abnormal Pattern Electroretinogram in Alzheimer’s Disease: Evidence for Retinal Cell Degeneration? *&on Barrett Katz, MD,‘P Steve Rimrner, MD,’ Vicente Iragui, MD,? and Robert Katzman, MDI ~~

We recorded pattern-reversal electroretinograms, flash electroretinograms, pattern-reversal visual evoked potentials, and flash visual evoked potentials in 6 patients with clinically diagnosed Alzheimer’s disease and 6 age- and sexmatched control subjects. The mean amplitude of the pattern-reversal electroretinogram in the Alzheimer patients was significantly less than that of the control group ( p = 0.004). This anomaly of the pattern-reversal electroretinogram may be a reflection of documented axonal depletion within the optic nerve and the degeneration of retinal ganglion cells seen in Alzheimer’s disease. We found Alzheimer patients to have normal pattern-reversal visual evoked potentials and flash electroretinograms, but a delayed second positive component of the flash visual evoked potential. This combination of findings may be of diagnostic import in Alzheimer’s disease. Katz B, Rimmer S, lragui V, Katzman R. Abnormal pattern electroretinogram in Alzheimer’s disease:

evidence for retinal ganglion cell degeneration?Ann Neurol 1989;26:221-225

Visual deficits in patients with Alzheimer’s disease (AD) have been thought to result from higher cortical impairment. Recently, Hinton and colleagues [l] reported significant axon depletion in the optic nerve and degeneration of retinal ganglion cells in patients with AD. Their findings suggest that preferential involvement of the anterior visual pathways may occur in AD. The traditional neurophysiological parameter of retinal function is the electroretinogram (ERG). The ERG is commonly elicited with flash stimulation (FERG) and is thought to be generated in the photoreceptors, cells of Miiller, and retinal pigment epithelium 12, 31. When a pattern-reversal stimulus is substituted for a bright flash, a different ERG is generated, one thought to arise from ganglion cells. Evidence for this is derived from work in vertebrate models which suggests that the pattern-reversal electroretinogram (PERG) parallels the functional integrity of ganglion cell axons [4, 5). While amplitude measurements in visual evoked potentials (VEP) have not been shown to be of clinical significance, amplitudes are of clinical significance in both FERGs 161 and PERGs. In humans, reduction in the amplitude of the PERG has been demonstrated in optic neuritis 17- 121, glaucoma 17, 13-16], ocular hypertension [17], optic atrophy 18, 181, surgical section of the optic nerve 1191, congenital optic neuropathy [7, lo], traumatic optic neu-

From the Departments of *Ophthalmology and TNeurosciences, University of California at San Diego, La Jolla, CA. Received Aug 2, 1988, and in revised form Jan 16, 1989. Accepted for publication Jan 18, 1989.

ropathy 17, 10, 121, compressive optic neuropathy 18, 121, ischemic optic neuropathy [S, 10, 121, multiple sclerosis 17, 13, 18, 20, 211, maculopathies 18, 181, central retinal artery occlusion 191, branch retinal artery occlusion 112, 22), and branch retinal vein occlusion 122). We recorded both PERGs and FERGs in patients with AD and in age- and sex-matched control subjects to determine if the expected change in optic nerve anatomy was reflected in this electrophysiological test of visual function. We also recorded simultaneous pattern-reversal visual evoked potentials (PVEPs) and sequential flash visual evoked potentials (FVEPs) to examine the effects such anatomical changes may have on more central electrophysiological function.

Materials and Methods Subjects Six subjects were selected at random from participants in the Alzheimer’s Disease Research Center at the University of California, San Diego, School of Medicine (Table 1). Informed consent was obtained after the nature of the study had been fully explained. Their ages ranged from 64 to 74 years (mean = 69.7, SD = 4.0); 4 were men and 2 were women. Each was examined by two independent neurologists and given a diagnosis of probable AD or possible AD based on the criteria of McKhann and associates 1231. All patients underwent neuro-ophthalmological evaluation which included determination of acuity, pupillary function,

Address correspondence to Dr Katz, Pacific Presbyterian Medical Center and The Smith-Kettlewell Eye Research Institute, 2340 Clay ’*‘15 Street’

Copyright 0 1989 by the American Neurological Association 221

Table 1. Descrzptive Charactmitics of Patients with Alzheimer’s Disease and Control Subjects Group Patients with Alzheimer’s disease 1 2

3 4

5 6 Control subjects 1 2 3 4 5 6

Age (yr)

Sex

Best Corrected Visual Acuity

64 66 69 72 73 74

M M M F F M

20120 20120 20120 20130 20125 20130

59 64 68 68 70 74

M M M F F M

20120 20120 20120 20120 20120 20125

slit lamp biomicroscopy, and dilated funduscopy. Visual acuity in all patients was 20130 or better. Ophthalmoscopic findings were normal in all 6 ; no patient showed clinical signs of optic nerve dysfunction or anatomical signature of optic nerve insult or glaucoma. A control group of 6 normal volunteer subjects matched for age (within 5 years) and sex was also studied. Their ages ranged from 59 to 74 years (mean = 67.2, SD = 5.2). None had ophthalmological or neurological dysfunction; all had visual acuity of 20125 or better.

Pattern-Reversal Electroretinograrn and Pattern-Reversal Visual Evoked Potential PERGs and PVEPs were recorded simultaneously from the left eye; the right eye was patched (in 1 control subject who had conjunctivitis of the left eye, we recorded from the right eye and patched the left). For the PERG, a gold foil electrode was inserted into the lower fornix of the eye, with foil touching the lower limbus. A cup electrode was placed on the ipsilateral ear to serve as reference, and a cup electrode was placed on the vertex of the scalp to serve as ground. The PVEP was recorded with a cup electrode placed 3 cm above the inion in the midline; a cup electrode was placed 3 cm above the nasion to serve as reference and a cup electrode was placed at the vertex to serve as ground. Impedance of each of the cup electrodes was below 5,000 a. All signals were recorded, amplified, and averaged using a commercially available system (Nicolet Compact-Four, Nicolet, Madison, WI). For the PERG, the preamplifier was set for a sensitivity of 100 pV with band-pass filters at 1 and 30 Hz. The artifact rejection system that was employed rejected large potentials caused by eye blinks and extraocular movement. For the PVEP, the preamplifier was set for a sensitivity of 100 pV with band-pass filters at 1 and 100 Hz. One hundred to 400 sweeps were averaged and plotted on an X-Y plotter. We employed a black and white checkerboard generated by a Nicolet Biomedical Visual Stimulator (NIC l015), projected on a cathode ray tube, for stimulus presentation. Checks reversed at a frequency of 2.1 reversals per second.

222 Annals of Neurology Vol 26

No 2 August 1989

Subjects were seated 70 cm from the screen and instructed to fixate on a target at the center of the cathode ray tube. Responses were measured employing checks subtending 30 minutes of visual arc. The cathode ray tube measured 16 by 20 degrees of visual arc. Subjects were tested with distance refraction corrected for 70-cm fixation, as refractive error is known to affect the PERG amplitude [24}.Contrast between black and white checks was 98% and mean luminance, 112.5 cd/m2. Average background luminance was 7.1 cd/m2.

Flash Electroretinograrn FERGs were recorded with the same electrode montage as the PERG (except for 2 patients with AD who could not tolerate continued use of a gold foil electrode, whose FERGs were therefore recorded with a gold cup electrode applied to the skin overlying the left inferior orbital rim). Subjects were dark adapted for at least 12 minutes. Flash stimulus (2.59 Jl cm2; pulse duration, 100 microseconds) was generated by a ganzfeld bowl (Cadwell VPA-10, Kennewick WA). Five sweeps were averaged; stimuli occurred at a frequency of 0.9 Hz. The preamplifier was set at a sensitivity of 1,000 p,V with band-pass filters at 1 and 1,000 H t .

Flash Visual Evoked Potential FVEPs were recorded with the same electrode montage as the PVEP. Background luminance was 7.1 cd/m2. Ganzfeld flash stimulus parameters were the same as for the FERG. Stimuli were presented at a frequency of 1.8 Hz. The preamplifier was set at a sensitivity of 100 FV with band-pass filters at 1 and 100 Hz. Two sets of potentials were generated for each subject; 100 sweeps were averaged for each trial.

Wav~oormAnalysis Figure 1 shows both a PERG and PVEP recorded simultaneously under the conditions just described; appropriate waveform components are labeled. In the PERG, latency of the b wave (or P1 component) was measured from onset of stimuli to the peak of the major positive component. Latency of the a wave (or N 1 component) was measured from onset of stimuli to the peak of the negative component immediately preceding the b wave. B wave amplitude was measured from the trough of the a wave to the peak of the b wave. FERG waveform components were labeled in the same fashion as the PERG. In the PVEP, latencies were measured from onset of stimuli to the peak of each wave. The major positive component occurring between 100 and 130 milliseconds was labeled P2. The negative component occurring before P2 was labeled N1. The positive component occurring before N1 was labeled P1. The negative component occurring after P2 was labeled N 2 . Only those waveforms that could be clearly identified were labeled. Retinocortical time was calculated as the latency difference between the PERG b wave and the PVEP P2 component. FVEP latencies were measured from the onset of stimuli to the peak of each wave. The initial negative component occurring at between 3 5 and 5 5 milliseconds was labeled N 1 . The large positive component occurring after the N1 was labeled P1. The negative component occurring after the P1

1 22& v

was labeled N 2 . The large positive component occurring after N 2 was labeled P2.

T 25ms

Results Descriptive statistics and statistical comparison between the group with AD and the control group for PERG and FERG are shown in Table 2. The mean PERG b wave amplitude in the group with AD was half that seen in the control group ( p = 0.004, Student’s unpaired t test); there was no significant difference in FERG amplitude between the groups. There was no significant difference in a or b wave latencies for both FERG and PERG between the two groups. Descriptive statistics and statistical comparison between the group with AD and the control group for PVEP are shown in Table 3. Of the PVEP waveform components recognized, there was no significant difference between groups in P1, N1, P2, or N2 latencies; there was also no significant difference between the retinocortical time of the two groups. Comparison between FVEP data revealed gross

P2 25ms

PVEP

Fig. I . Simultaneozls pattern-revmal electroretinogram (PERG) and pattm-reversal visual evoked potentiah (PVEP) recorded in our laboratory, with wavef m components labeled ( q k a r d dejection denotes relative positivity). See text for definitions of P I , N1 I P2, and N2.A = a wave, B = b wave.

Table 2 . Comparison of Pattern-Reversal and F b h Electroretinogram Waveform Components in Patients with Alzheime7is Disease and Age- and Sex-Matched Control Subjects ~~

~

PERG a-Wave Latency (ms)

b-Wave Latency (ms)

b-Wave Amplitude (kV)

a-Wave Latency (ms)

b-Wave Latency (ms)

b-Wave Amplitude (pV)

38.08 4.73

60.83 4.55

1.34 0.59

23.6 3.77

45.67 3.72

195.80 78.52

SD

35.83 2.81

Value”

NS

62.92 2.60 NS

2.57 0.53 0.004

24.2 1.48 NS

46.7 4.08 NS

286.13 4 1.04 NS

Group Patients with Alzheimer’s disease Mean

SD Control subjects Mean

p

FERG

~

~~

~

“Comparison of group with Alzheimer‘s disease and control group using Student’s unpaired t test

PERG = pattern-reversal electroretinogram, FERG

=

flash electroretinograms,NS = not significant ( p > 0.05).

Table 3 . Comparison of Pattern-Revwsal Visual Evoked Potential Waveform Components in Patients with Alzheimers Disease and Age- and Sex-Matched Control Subjects PVEP (ms)” Group Patients with Alzheimer’s disease Mean

SD Control subjects Mean

SD

p

Valueb

N 1 Latency

P2 Latency

N 2 Latency

Retinocortical Time (ms) (PVEP P2 - PERG b)

87.1 5.91

115.75 8.29

160.58 10.27

50.7 8.51

78.13

112.42 4.49 NS

148.83 9.22 NS

49.5 4.86 NS

8.16

NS

~

“See tern for definitions for N1, P2, and N2. bComparison of group with Alzheimer’s disease and control group using Student’s unpaired t test PVEP = pattern-reversal visual evoked potential, PERG = pattern-reversal elecrroretinogram, NS

=

not significant ( p > 0.05).

Katz et al: Abnormal PERG in Alzheimer’s Disease

223

FVEP

Alzheimer’s Disease

> / p,

Fig 2. Flash visual evokedpotentials (FVEP) generated by aueraging the FVEP of each control subject (top) compared with the FVEP of Patient 3 (bottom).Note the broad, delayed, positive component in the FVEP of the patient with Alzheimer’s disease. See text for definitions of Nl,N2, PI, and P2.

changes in waveforms in the group with AD, with a disappearance of some of the early peaks. Of the FVEP waveform components recognized, there was no difference between groups in N1, P1, or N2 latencies. While the identification and measurement of a purported P2 component are problematic with such florid changes in response, the major positive deflection of the PVEP in patients with AD was prolonged. Figure 2 compares a FVEP generated by averaging the FVEPs of our control group with the FVEP of a representative patient with AD.

Discussion Our data suggest that the PERG is affected in patients with AD. Our data provide indirect electrophysiological support for the anatomical changes reported by others. Hinton and colleagues [1) reported widespread axonal degeneration in the optic nerves of patients with AD, with 15 to 80% depletion of axons, loss of axons with a large diameter, loss of retinal ganglion cells, degenerative changes in residual ganglion cells, and diminution of the nerve-fiber layer. None of the other pathological abnormalities seen in AD brains (helical filaments, neuritic plaques, neurofibrillary tangles, or arnyloid angiopathy) were found in the optic nerves or retina. Sadun and co-workers [251 subsequently reported that loss of axons in these patients was correlated with the clinical severity of dementia. Our findings of a normal retinocortical time and normal PVEPs in the group with AD suggest that the pathological processes occurring do not affect conduction velocity in the primary visual pathways. Our

224 Annals of Neurology Vol 26 No 2 August 1989

findings of a normal PVEP seen with a delayed P2 component of the FVEP are consistent with results in previous reports 126-341. Wright and colleagues [34] specifically showed that patients with AD have a marked delay of the P2 component of the FVEP, yet no PVEP abnormalities. These authors suggest that the PVEP and the P1 component of FVEP are generated in the geniculostriate pathway and primary visual cortex, but the P2 component of FVEP may be transmitted by nongeniculate pathways and generated in the visual association cortex. This postulate is consistent with the known anatomical predilection of AD, as it affects associative areas with relative sparing of the primary cortex [35-381. We conclude that the PERG may be abnormal in patients with AD, with smaller amplitude than is seen in normal subjects. As the PERG may be a mass phenomenon dependent on the total number of functioning ganglion cells, our data may be clinical confirmation of previously described histopathological changes of AD [l]. Whether disturbed ganglion cell function may also be secondary to neurotransmitter paucity is yet to be explored. Our study sample was small; the full extent of clinical relevance of the PERG is controversial and still to be determined. Yet, the altered PERG and P2 component of the FVEP in association with a normal FERG and PVEP are consistent with the described anatomical predilection of AD as it affects the visual system. We suspect this electrophysiological profile may serve as a useful clinical marker of AD. The authors gratefully acknowledge the Alzheimer’s Disease Research Center of the Universiry of California at San Diego for their support of this research, and Ellen Taylor and Ray Whitney for their technical assistance.

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