Background. Medically unexplained visual loss occurs in 1 to 5% of patients attending ophthalmology clinics and for many it runs a chronic course. A psychogenic aetiology is presumed in such cases, but little is known about the underlying neural mechanisms. Recent studies have established the value of functional magnetic resonance imaging (fMRI) in understanding the mechanisms of unexplained motor and sensory symptoms. The purpose of this study was to use a similar strategy (fMRI) to evaluate the cerebral responses to visual stimulation in a group of patients with medically unexplained visual loss, in an attempt to determine the underlying neural mechanisms.
Method. Brain activation induced by periodic (monocular) 8 Hz visual stimulation was detected by fMRI in five patients with unexplained visual loss who fulfilled DSM-IV criteria for conversion disorder, and seven normal volunteers. Between-group differences in mean power of activation were estimated by fitting a one-way analysis of variance (ANOVA) model at each intracerebral voxel in standard space.
Results. Compared with controls, patients showed reduced activation in visual cortices, but increased activation in left inferior frontal cortex, left insula-claustrum, bilateral striatum and thalami, left limbic structures, and left posterior cingulate cortex.
Conclusions. This preliminary study has identified novel neural correlates in patients with unexplained visual loss. The abnormal pattern of activation may reflect inhibition of primary visual cortex or a shift towards non-conscious (implicit) processing.