[Virtual dystonia due to posterior ventrolateral thalamic infarct: case report].

Behaviors, actions and movements may take place as purely mental events, as in the obsessions of obsessive-compulsive disorder, phantom limbs or sensory tics. In the present paper we report on the case of a 43-year-old diabetic hypertensive man who developed an incomplete form of the Dejerine-Roussy syndrome. Whenever he lay down or withdrew the leg from the ground, he experienced the illusion that the left intermediate toes painfully twisted and mounted each other. Conversely, as he stood up or firm pressure was artificially exerted against the sole, there was a dramatic relief from the "cramp" whose illusory character could he be certain of only by looking down at the foot. By passively moving his toes into the referred position we realized that the experienced deformity conformed to the pattern of a fixed dystonia not outwardly expressed through the motor system. There was severe proprioceptive loss in the same toes that harbored the cramp. MRI showed the appropriate lesion in the posteroventrolateral thalamus (VPL) and wallerian degeneration of thalamo-cortical projections. SPECT showed hypoperfusion of the overlying ipsilateral parietal cortex as well as of the basal nuclei bilaterally, besides the expected image of thalamic exclusion. We hypothesize that the infarct disconnected the somatic sensory cortex (S-1) from critical proprioceptive input with relative sparing of superficial sensibility. Lifting the foot deprived S-1 of tonic inputs conveyed by undamaged contact-pressure pathways, a functional effect promptly reversed by placing the foot back against the ground. The case illustrates how a capricious deafferentation of S-1 by a discrete VPL thalamic infarct might facilitate the emergence of autochthonous activity.