There is a rare neurological condition called alien limb syndrome in which a person has one or more limbs that will often move without conscious control. Little is known about the exact cause of this rare and interesting disorder. The person usually can have conscious control of the limb at times but not always. A classic manifestation of this syndrome is when the left arm, for example, will reach out and grab objects, fiddle with clothing or other body parts, or just move straight up or out and stay there. When asked by a neurologist to perform a specific action with the good arm, the alien arm may often hinder or impede the action of the good arm. The arm then often needs to be wrestled back to a resting position by the other arm.

To the person with an alien limb, the limb really is foreign — it acts as if it were not their limb. Many people with alien limb syndrome name the limb or refer to it as “It.” The patients are sometimes upset that they don’t have control over the limb but most learn to just live with the rogue limb. However, not all people are aware of the alien limb’s movements until they look at the limb or until someone tells them about it.

So what causes someone to have an alien limb? The disorder is termed a syndrome because there is not one specific cause or even one specific symptom. There may be myriad causes and a constellation of symptoms. Lesions to a wide range of sites in the brain can result in an alien limb syndrome.

The most common neurological correlate is basal forebrain degeneration (i.e., atrophy of the bottom part of the frontal lobes). Damage to the corpus callosum — a wide swath of white matter connective fibers that provides much of the communication between both cerebral hemispheres — can also result in alien limb syndrome. Strokes or tumors are the most common cause of the lesions that produce this disorder. Widespread damage to the parietal lobe may also sometimes result in an alien limb syndrome. In cases of lesions to this posterior area of the brain, self-injurious behaviors are more common (e.g., the alien limb tries to strangle the person). There is no cure so neurologists and researchers will likely be fascinated by this disorder for many years to come.

Reference

Keith A. Josephs, Martin N. Rossor (2004). The alien limb. Practical Neurology, 4 (1), 44-45 DOI: 10.1111/j.1474-7766.2004.06-189.x