The pattern electroretinogram in Parkinson's disease reveals lack of retinal spatial tuning

Spatio-temporal visual abnormalities, involving processing of medium coarse stimuli, are known to occur in Parkinson's disease (PD). While these deficits have been related to retinal dopaminergic deficiency, previous ERG studies in PD patients have provided conflicting results, probably due to differences in stimulus conditions. The influence of pattern element size (spatial frequency, SF) on the pattern electroretinogram (PERG) in PD has not been systematically studied. We recorded steady-state PERG to sinusoidal gratings of 50% contrast, counterphase modulated at 7.5 Hz with a series of SFs ranging from 0.5 to 6.9 c/deg in 20 PD patients and 20 healthy volunteers, subdivided in 10 'young' and 10 'age-matched' (AM) subjects. The PERG was analyzed by means of Fast Fourier Transform and the amplitude and the phase of the second harmonic response (15 Hz) were taken into account. We evaluated the medium-to-low SF amplitude ratio and termed it 'PERG tuning ratio' (TR). The results indicate that aging affects all the studied SF, but the pattern of age-related loss differs from that observed in PD. Compared to AM subjects, PD patients show a specific deficit at medium SF, with a distorted PERG SF response function. Consequently, all PD patients show an attenuated PERG TR and 17 of them (85%) have an inverted TR. A significant TR decrease is correlated with the clinical stage of PD. There is a marked TR difference between patients receiving and not receiving L-DOPA. We conclude that stimulus SF is a crucial variable of the PERG in PD. PERG measurements and the derived PERG TR may provide a simple tool to evaluate retinal dopaminergic mechanisms and could contribute to the clinical assessment and monitoring of dopaminergic therapy in PD.