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Alien hand syndrome (AHS) is an extremely rare disorder. It is so rare, there appears to be no available prevalence studies. In the AHS literature, case studies are therefore nearly universal. For all intents and purposes, it is reasonable to say that AHS appears to be a disease so rare that the empirical prevalence rates are in every listed case statistically indistinguishable from zero.

One recent paper by Sarva, Deik and Severt (2014) reviewed 109 studies on AHS patients. Their first and foremost result was the finding that there appears to be three overlapping types:

Frontal: Associated with involuntary groping, grasping and tool use.

Associated with involuntary groping, grasping and tool use. Callosal: Associated with intentional opposing movements.

Associated with intentional opposing movements. Posterior: Associated with avoidance responses and uncoordinated movements.

Interestingly, they also briefly report an even rarer alien limb phenomena involving the leg!

Based on their review, Sarva, Deik and Severt (2014) suggested that different types of AHS may be caused by different kinds of disruptions to an executive system which is partly responsible for resolving between-task conflicts and prioritizing actions, causing disconnects and impaired coordination between hemispheres. They conclude:

Our understanding of underlying mechanisms remains incomplete, but evidence from pathological and functional neuroimaging studies suggests a role for network disruption causing bihemispheric disinhibition and/or interhemispheric disconnection. Depending on the variant and the underlying pathology, apraxia, neglect, cortical sensory deficits, and other neurologic abnormalities may also be present in varying degrees, likely explaining the considerable phenotypic variability among patients. Functional imaging studies likely will continue to expand our knowledge of the pathophysiology of AHS, but the limited number of patients in these studies and the diverse underlying pathologies should raise caution when attempting to generalize their results.

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