Facial macrosomatognosia and pain in a case of Wallenberg's syndrome: selective effects of vestibular and transcutaneous stimulations.

Macro- and micro-somatognosia refer to rare disorders of the cerebral representation of the body whereby patients perceive body parts as disproportionately large or small. Here we report the experimental study of a patient who, following a left lateral medullary stroke (Wallenberg's syndrome, including vestibular deficits) complained of a persistent somatosensory illusory sensation of swelling, confined to the left side of his face (i.e., left macrosomatognosia). This hemifacial somatosensory distortion was associated with a left facial anesthesia, and a neuropathic pain affecting the three branches of the left trigeminal nerve. In this study, we first document quantitatively the patient's somatosensory illusion by using a somatosensory-to-visual matching task in which the patient modified the picture of his own face to fit his left-sided somatosensory misperception. The patient's performance revealed that macrosomatognosia was confined to the second branch of the left trigeminal nerve. Perception of the size of visual objects was comparatively preserved. Second, we investigated the effects of two peripheral stimulations, which may affect the spatial component of somatosensory deficits (caloric vestibular stimulation, CVS; transcutaneous electrical nervous stimulation, TENS) and pain (TENS). Left CVS abolished the facial somatosensory illusion, for about 30min, but had no effect on the left facial pain. Conversely, left TENS substantially reduced the neuropathic pain during stimulation, but had no effect on macrosomatognosia, indicating a double dissociation between the two disorders. These results reveal that facial macrosomatognosia may be regarded as a high-order deficit of somatosensory perception of the shape and volume of the face, which fits the definition of 'hyperschematia' (i.e., when the body takes up too much room) originally proposed by Bonnier (1905). Our data also indicate that CVS may favor the restoration of the conscious representation of the shape and size of the face. Overall, these findings lend support to the view that afferent inputs from the vestibular system can affect in a specific fashion the activity of cerebral structures involved in the building up and updating of the topological description of body parts.