THE CANADIAN JOURNAL OF NEUROLOGICAL SCIENCES Abnormal Visual Adaptation to Flicker in Multiple Sclerosis

A visual psychophysical adaptation procedure was used on patients with Multiple Sclerosis (MS) in an attempt to induce a temporary and local exacerbation of subclinical visual impairment. Using a flicker detection task, sensitivity before and after adaptation to a flickering stimulus was measured in 9 MS patients and 9 control subjects. Although only 22% of patient eyes had abnormal flicker sensitivity prior to adaptation, visual deficit was observed in 83% of eyes studied after adaptation. Of the 7 MS eyes studied for which no other sign or symptom of visual involvement was present, 5 were found to have visual deficits after flicker adaptation. In addition, 10 of the 11 eyes affected by MS showed an abnormal response to flicker adaptation. Recovery from the effects of adaptation was complete in all patients within 2 minutes. The results suggest that partial demyelination of visual pathway neurons may exist in patients without signs or symptoms of visual involvement. The prolonged stimulation provided during adaptation may produce a temporary fatiguing or conduction blockade of such neurons which may lead to reductions in sensory sensitivity. RESUME: Adaptation visuelle anormale a la stimulation lumineuse intermittente dans la sclerose en plaques Nous avons utilise une technique d'adaptation visuelle psychophysique chez des patients atteints de sclerose en plaques (SEP) dans le but d'induire une exacerbation temporaire et locale de l'atteinte visuelle subclinique. A l'aide d'une epreuve de detection du papillotement, nous avons mesure la sensibilite avant et apres adaptation a une stimulation lumineuse intermittente chez 9 patients atteints de SEP et 9 sujets controles. Meme si seulement 22% des yeux des patients avaient une sensibilite anormale a la stimulation lumineuse inteimittente avant adaptation, un deficit visuel a ete observe chez 83% des yeux etudies apres adaptation. Parmi les 7 yeux SEP etudies qui ne manifestaient aucun signe ou symptome d'atteinte visuelle, 5 presentaient des deficits visuels apres adaptation a la stimulation lumineuse intermittente. De plus, 10 des 11 yeux atteints de SEP presentaient une reponse d'adaptation anormale. La recuperation des effets de l'adaptation etait complete chez tous les patients en dedans de 2 minutes. Ces resultats suggerent qu'une demyelinisation partielle des neurones des voies de transmission de l'influx visuel peut exister chez des patients ne manifestant aucun signe ou symptome d'atteinte visuelle. La stimulation prolongee fournie pendant l'adaptation peut produire une fatigue temporaire ou un blocage de la conduction de tels neurones, pouvant induire une diminution de la sensibilite sensorielle. Can. J. Neurol. Sci. 1988; 15:286-291 Extensive demyelination of neurons in the primary visual pathways is almost universally found in patients with advanced multiple sclerosis (MS) who are examined at autopsy. However, investigations of visual disorders in MS patients have revealed abnormalities in only 30 per cent of patients when conventional testing procedures (e.g., Snellen acuity, static perimetry, fundoscopic examination) are used. In view of these findings, it is likely that visual system pathology is present in a considerable proportion of MS patients during life but that it is not always manifested as clinically evident fundoscopic changes or as obvious disturbances in visual function. Recently, psychophysical techniques for measuring visual function have been used experimentally as a means of detecting subtle visual disturbances in MS patients. One class of such visual tests have employed temporal visual discriminations in an effort to demonstrate abnormally slowed or delayed neural transmission in the visual pathways of MS patients. For example, in double flash and multi-flash campimetry, subjects view a test spot at various locations in the visual field and are asked to judge whether they perceive a single illumination of a test spot or multiple successive flashes. The duration between successive flashes of light is lengthened until the subject reports the perception of flicker. Using these techniques, it was found that a large proportion of MS patients with a history of optic neuritis could tolerate substantially longer inter-stimulus intervals before flicker perception was reported than healthy control subjects. These results are consistent with neurophysiological data on experimentally demyelinated neurons which suggest that the temporal order of neural signals transmitted by demyelinated axons is under considerable temporal disarray. However, abnormal multi-flash campimetry results may also be found in patients with amblyopia, cataract, macular degeneration and optic neuritis not associated with demyelinating disease. From the Department of Psychology, University of Calgary. Received November 5, 1987. Accepted in final form April 23, 1988 Reprint requests to: Dr. J. Raymond, Department of Psychology, University of Calgary, 2500 University Drive, Calgary, Alberta T2N 1N4 Canada.