The spatial tuning of steady state pattern electroretinogram in multiple sclerosis

In normal subjects, the steady-state electroretinogram in response to contr ast reversing gratings (PERG), is spatially band-pass tuned in amplitude, w ith a maximum at intermediate spatial frequencies and an attenuation at low er and higher ones. The amplitude attenuation at low spatial frequencies is believed to reflect centre-surrouud antagonistic interactions in the recep tive fields of inner retinal neurons. The aim of this study was to evaluate the PERG spatial tuning in multiple sclerosis (MS) patients without a prev ious optic neuritis history. Steady-state PERGs in response to counterphase -modulated (8 Hz) sinusoidal gratings of variable spatial frequency (0.6, 1 .0, 1.4, 2.2 and 4.8 c/deg), were recorded from 18 patients with definite o r probable MS and no history of optic neuritis (ON-). Nine of them had no s igns of subclinical optic nerve demyelination (asymptomatic) in either eye, while nine had symptoms or signs of optic pathways involvement (symptomati c) in one or both eyes. Results were compared with those obtained from 10 M S patients with a previous history of optic neuritis (ON+) in one or both e yes, as well as from 21 age-matched controls. The amplitudes and phases of the responses' 2nd harmonics were measured. Compared with the controls, asy mptomatic ON- patients showed selective losses in mean PERG amplitudes at m edium and high (1.0-4.8 c/deg) spatial frequencies. Symptomatic ON- patient s and ON+ patients had reductions in mean PERG amplitudes, with respect to controls, involving the whole spatial frequency range, but with greater los ses at medium-high (1.0-4.8 c/deg) than at lower spatial frequencies. In al l patients' groups, the average PERG spatial tuning function differed signi ficantly from that of the controls, assuming a low-pass instead of the norm al band-pass shape. The PERG phase was delayed in ON+ but not in ON- patien ts, as compared to controls. However, the phase delay was independent of sp atial frequency In both ON- and ON+ patients, losses in PERG amplitude and spatial tuning tended to be associated with corresponding abnormalities in perimetric sensitivity, visual acuity, colour vision and transient visual e voked potential (VEP) latency The results indicate that abnormalities of th e spatial tuning of steady-state PERG can be found in MS patients without e ither optic neuritis or signs of subclinical optic nerve demyelination. The se changes may reflect a retinal dysfunction, developing early in the cours e of MS, due to a loss of specific subpopulations of inner neurons, changes in lateral interactions of their receptive fields, or both. Eur J Neurol 6 :151-162 (C) 1999 Lippincott Williams & Wilkins.