The results indicate that, in NE-AMD, the FERG is altered in parallel with the extent and severity of fundus lesions. However, a functional impairment of outer macular layers, which is detected by FERG losses, could precede morphological changes typical of more advanced disease.
Nasal and temporal hemifield (14 x 24 degrees) pattern electroretinograms (PERGs) were recorded in eight patients (age range: 21-72 years) suffering from different post-geniculate lesions (documented by CT scan and/or MRI of the brain) and homonymous hemianopia at visual field testing. In total eight age-matched normal subjects served as controls. PERGs were elicited by alternating, sinusoidal gratings (90% contrast), whose spatial and temporal characteristics, 6 Hz-5 c deg-1 and 15 Hz-0.58 c deg-1, were chosen to enhance the relative contributions of parvo- (P) and magno- (M) retinal ganglion cell (GC) subsystems, respectively. Amplitudes and phases of the Fourier analyzed PERG 2nd harmonics were measured. In normal subjects, PERG amplitudes to 15 Hz-0.58 c deg-1, but not those of the 6 Hz-5 c deg-1 stimuli were on average larger (P < 0.05) in nasal than in temporal hemiretinae. In hemianopic patients, PERG amplitudes of 6 Hz-5 c deg-1 stimuli were on average reduced (P < 0.05) in the hemiretinae corresponding to blind hemifields, in comparison to those in the hemiretinae corresponding to functional hemifields. No differences between hemiretinae were observed for responses of the 15 Hz-0.58 c deg-1 stimuli. In both normal subjects and patients, average PERG phases did not differ between hemiretinae, while changing significantly (P < 0.01) across stimulus conditions. The PERG naso-temporal asymmetries observed in normal subjects are consistent with the reported asymmetries in GC density observed histologically (Curcio & Allen. (1990). Journal of Comparative Neurology, 300, 5-25). The results in patients indicate that the PERGs to specific spatio-temporal stimuli (i.e. of relatively low-temporal and high-spatial frequency), presented in the hemianopic field, are reduced in amplitude. This suggests, in agreement with the experimental findings in monkeys (Cowey & Stoerig. (1991). Trends in Neuroscience, 14, 140-145), that retrograde trans-synaptic dysfunction of P-GCs, with relative sparing of the M-subsystem, may occur in humans following lesions of post-geniculate pathways.
A subclinical visual dysfunction can be detected by psycho-physical methods in early-stage papilledema associated with idiopathic intracranial hypertension (IIH). We recorded steady-state pattern electroretinograms (PERGs) and visual evoked potentials (VEPs) to sinusoidal gratings of variable spatial frequency [0.6,1.0,1.4, 2.2 and 4.8 cycles/degree (cpd)] in 18 patients with IIH and early papilledema and in 21 age-matched controls. Spatial frequency selective reductions in the mean PERG (at 1–4.8 cpd) and VEP (at 4.8 cpd) amplitudes were found in patients in comparison with controls. The response functions of amplitude versus spatial frequency of patients’ PERG and VEP displayed a low-pass shape, whereas in normal subjects PERG and VEP functions showed a bandpass and a high-pass shape, respectively. PERG and VEP abnormalities were found in 14 (77.7%) and 10 (55.5%) out of 18 patients, respectively. Most of these abnormalities involved only selected spatial frequencies (1.4–4.8 cpd). These results indicate spatial-frequency-dependent functional losses on both PERG and VEP in early papilledema, and suggest a potential value of these responses for detecting subtle visual abnormalities in IIH.
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