The pathogenesis of anosognosia for hemiplegia.

We compared patients with unawareness of hemiplegia lasting more than 1 month after right hemisphere stroke with other patients with right hemisphere stroke who became aware of hemiplegia within a few days after onset. Patients with persistent unawareness invariably had severe left hemisensory loss and usually had severe left spatial neglect. They were almost always apathetic; their thought lacked direction, clarity, and flexibility, and they had at least moderate impairment of intellect and memory. Their right hemisphere strokes were large and always affected the central gyri or their thalamic connections and capsular pathways. In addition, there was evidence of at least mild left hemisphere damage, most commonly caused by age-associated atrophy. The pathogenesis of anosognosia for hemiplegia may involve failure to discover paralysis because proprioceptive mechanisms that ordinarily inform an individual about the position and movement of limbs are damaged, and the patient, because of additional cognitive defects, lacks the capacity to make the necessary observations and inferences to diagnose the paralysis. We discuss the implications of this "discovery" theory and contrast it with other explanations of anosognosia.