Alien hand sign (AHS) is an interesting sign where patients have no control over the movements of their limbs. The limb may act as if it has a mind of its own, causing distress to the patients. We describe one such patient who complained of involuntary masturbation as part of frontal variant of AHS secondary to stroke. We also discuss the etiopathogenesis of this rare condition.
A 45-year-old man was brought by his wife with complaints of slow speech for the previous 5 days. He was a diabetic on irregular therapy. The patient informed that his right hand was grabbing objects without his control and he was using his left hand to control the right hand. He used to keep his right hand below his pillow to control it. He also stated that his right hand used to grab his genitals involuntarily, even in public places, often to his embarrassment and that of his wife.
His vital signs were normal. On the right side of body, power was 4+/5 with brisk reflexes, extensor plantar response, palmomental reflex, and exaggerated grasp reflex without any sensory loss. He could not release the right hand without the help of the left hand. Common, usual objects were kept in front of him and he was instructed not to touch them. However, his right hand (affected limb) reached for these objects but the left hand (normal limb) immediately removed these objects from the right hand [Video 1]. He used the left hand to control his right hand even when at rest.
Plain computed tomography showed a sub-acute infarct in the left anterior cerebral artery involving the left medial frontal lobe and anterior corpus callosum, which was confirmed by magnetic resonance imaging [Figure 1] and [Figure 2]. A final diagnosis of frontal variant of AHS was made. He was discharged and was advised medication with aspirin, metformin and atorvastatin. Two months later, although these abnormal movements diminished in intensity, they persisted.
Figure 1: Magnetic resonance imaging - left ACA infarct
Though this condition was previously described by Goldstein in 1908,  le signe de la main ιtrangθre (the sign of the foreign/strange hand) or the AHS was first introduced by Brion and Jedynak  to denote the phenomenon in which patients failed to recognize their limbs as their own. This scope of this definition has been broadened to include the condition where the limb performs commands that are not directed by the patient and often are contrary to what the patient intends to do, thereby acting as if it has a mind of its own.
AHS has been broadly classified as the anterior type (frontal and callosal subtypes) and the posterior type [Table 1].
In the frontal type, the lesion involves the medial frontal lobe and often involves the dominant hand. This consists of exaggerated grasp reflex [Video 2], involuntarily grasping objects in the visual field - variously termed as magnetic apraxia, visual groping, and tactile mitgehen (manual pursuit movement) and utilization behavior or the compulsive manipulation of objects. This may be a part of generalized frontal lobe dysfunction.
In the callosal type, the lesion is located in the corpus callosum, usually in the anterior region. This is characterized by inter-manual conflict - where the affected limb opposes the action of the normal limb, presence of mirror movements in the affected limb, enabling synkinesis- in which one arm can carry out an act only if both arms act together, and diagnostic dyspraxia - where the movement is contrary to the command given. This type often affects the non-dominant (left) hand.
In the sensory or posterior type,  the site of involvement is usually the non-dominant posterior circulation territory, characterized by hemineglect, optic ataxia, and sensory loss.
Other types include the mixed variant - presenting both frontal and callosal features as well as sensory and callosal features,  the transient type - occurring due to a cerebral ischemia, and the paroxysmal type-happening as a manifestation of seizures. 
It involves focal lesions such as surgical division of the callosum to treat seizures, stroke and aneurysm (ruptured, unruptured, and following clipping) on the anterior cerebral arteries, moyamoya disease, tumors of the parasagittal, callosal, and frontal lobes including lymphomas, herpes simplex infection, multiple sclerosis, neurocysticercosis, obstructive hydrocephalus, Marchiafava Bignami disease, trauma  as well as a part of degenerative disorders such as cortico basal degeneration (CBD) (progressively involving other limbs), Creuzfeldt Jacob disease, Alzheimer disease, and sudanophilicleukodystrophy.
Though the classical sites are the medial frontal lobe and the corpus callosum, lesions of the thalamus, basal ganglia, and pons have also been reported.
The dual premotor systems theory proposed by Goldberg and Bloom  states that there are two systems involved in limb movement: The medial system, consisting of the supplementary motor area (SMA) and the cingulated gyrus involved in anticipatory movements of the opposite limb, and the lateral system, consisting of the arcuate premotor area responsible for generating movements as a reaction to external stimuli in the environment.
These systems inhibit each other and supplementary motor area (SMA) on both sides interacts through the corpus callosum.
When the SMA is disconnected from the lateral system because of a callosal lesion, it causes callosal type of AHS, whereas damage to the SMA disinhibits the premotor area, thereby leading to frontal-type AHS.
The left SMA controls the right limb through the left motor cortex, whereas the right SMA controls the left limb and through the callosum, controls the right limb. When the lesion involves the left SMA and the callosum, the left limb is no longer under the control of the right SMA.
This explains why the left hand is more often involved in callosal-type AHS.
Damage to the parietal lobe causes sensory deafferentiation and the patient loses awareness of the limb due to loss of inputs from visual and proprioceptive systems.
Review of Indian literature regarding alien hand sign
We could find two reports with details, reported from India.
Panda  had reported transient left AHS of the frontal variant due to infarct of the right fronto parietal region, which lasted for 2 hours.
Pradhan et al.,  have described this phenomenon as a part of seizures secondary to neurocysticercosis.
Alien hand sign as a feature of involuntary masturbation
Involuntary grabbing and fondling of the genitals has been described previously in three reports. Della Sala et al.,  described a right AHS in a lady following recovery from clipping of an Anterior communicating artery aneurysm. Kischka et al.,  described a right AHS in a 66-year-old man following a left ACA infarct, and OngHai  described a left AHS in a 73-year-old man following left ACA infarct.
AHS as a part of degenerative disorders such as CBD progresses to involve other limbs; however, if the etiology is stroke, AHS usually improves to varying degrees by 1 year and the patient learns to control it by various tricks such as holding an object in the affected limb might allow normal activity of the uninvolved limb.
AHS is an interesting phenomenon that has not been fully explained; further studies are required to help understand the principles of free will.
Yuan JL, Wang SK, Guo XJ, Hu WL. Acute infarct of the corpus callosum presenting as alien hand syndrome: Evidence of diffusion weighted imaging and magnetic resonance angiography. BMC Neurol 2011;11:142. [PUBMED]