Alien hand syndrome (AHS) is a rare neurological condition in which a single limb, a hand but sometimes a leg, moves involuntarily. It can signify a serious underlying issue, such as stroke or neurodegenerative disease.

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A person’s symptoms will depend on the specific region of the brain that the disorder affects. Symptom onset usually happens over time but may be sudden in the case of a stroke.

This article discusses alien hand syndrome in more detail, as well as the causes, symptoms, diagnosis, treatment, and outlook for the condition.

Alien hand syndrome is very rare and often involves underlying neurological conditions.

People with a type of Parkinson’s disease called corticobasal syndrome (CBS) are at especially high risk of developing the condition, and 30% of people with CBS have alien hand syndrome.

Alien hand syndrome can affect three regions of the brain: the frontal, callosal, and posterior regions.

  • Frontal region: People with the frontal variant of AHS experience grasping or groping of their hands, which they have difficulty controlling. They are often aware that the limb belongs to them but find it difficult to control the movements, which seem to happen independently.
  • Callosal region: When AHS affects the corpus callosum, which joins the right and left sides of the brain, it can cause agnostic dyspraxia. This is when one hand does not respond to a command, such as picking up an item with one hand, but the opposite hand performs the task instead.
  • Posterior region: In people with AHS that affects the posterior region, the brain may not recognize the limb as its own, and they can experience impaired motor control.

A 2021 study discusses a case study of an even rarer form of AHS called mixed alien hand syndrome. In mixed AHS, the frontal and callosal variants combine and can cause involuntary grasping, uncoordinated hand movements, and difficulty performing tasks with one hand, causing the other to move instead.

While AHS typically affects a single hand, most often the left hand, it can also occur in the lower limbs and may affect a person’s leg.

Experts do not completely understand how the brain can experience confusion between a person’s will and actions.

The most common causes of AHS include:

Stroke

In rare cases, a person may develop the posterior variant of AHS after an ischemic or hemorrhagic stroke.

In an ischemic stroke, a blood clot plugs a blood vessel in the brain, and the brain cells cannot receive oxygen and nutrients from the blood. This causes the cells to begin dying within a few minutes. Ischemic strokes are the most common type and account for around 80% of all strokes.

In a hemorrhagic stroke, a blood vessel breaks and bleeds into the brain, preventing oxygen and nutrients from reaching the brain cells, and the bleeding injures the nearby parts of the brain.

Neurodegenerative diseases

Neurodegenerative diseases, such as Parkinson’s disease or Alzheimer’s disease, are the leading cause of AHS. These diseases affect the body’s activities, such as:

  • heart function
  • movement
  • breathing
  • balance

Midline tumor

A midline tumor begins in the brain or spinal cord. They may cause:

Aneurysm

An aneurysm can occur when an area of an artery wall weakens, which causes it to widen and balloon outward.

Doctors do not always know what causes an aneurysm. Some people:

  • are born with them
  • have a disease or injury in the blood vessels

Cerebral aneurysms occur in a brain artery.

Damage to the corpus callosum

A traumatic brain injury can cause damage to this region of the brain, leading to alien hand syndrome and other complications.

Experts characterize alien hand syndrome by:

  • movements of a limb, usually the left hand, that appear intentional but are involuntary
  • no control over the movement of the limb
  • a sense of loss of ownership over the limb

The onset of the disorder usually happens over time but may be sudden following a stroke. It may also affect the lower limbs and, in rare cases, can affect more than one limb.

The symptoms may differ depending on which brain region the disorder affects.

There are no official criteria for the diagnosis of AHS.

A doctor will diagnose the condition based on the presence of symptoms.

If a person experiences their limb feeling foreign to them and complex, involuntary movement that does not fit within the symptoms of other movement disorders, a doctor may diagnose AHS.

There is not much evidence to support the most effective treatments for AHS, and no approved or recommended treatments currently exist.

Experts base management of AHS on anecdotal reports, and researchers find it difficult to complete large-scale, randomized studies that could help develop effective therapies due to the rarity of the disorder.

Some studies have found that certain tasks and exercises can improve the symptoms of AHS.

2021 research noted that people experienced the most improvement from doing bimanual tasks, which involved both hands, as well as tasks that involved different colors. The colors may help to attract the person’s attention to one particular side.

Another exercise researchers found helpful was to let the alien hand catch a cube, after which the person was able to better perform more exercises with the other hand.

There is no cure for AHS. A person may be able to manage their symptoms to a degree with the help of their doctor.

Medical professionals may suggest certain exercises to repeat, which can help train the brain and reduce the severity of the condition.

As AHS is typically a sign of an underlying disorder and is debilitating, a person must consult a doctor if they experience any of the disorder’s symptoms.

AHS is a neurological condition in which a single limb, most often a hand, moves involuntarily, although it may appear the person is intentionally performing actions with the limb.

AHS is often a sign of an underlying condition, such as stroke, a neurodegenerative disease, or damage to the brain. Different types of AHS relate to the specific region of the brain that the disorder may affect. Whether AHS affects the frontal, callosal or posterior regions can influence which symptoms a person experiences.

There is no recommended testament for AHS, and more research is required to develop an effective treatment. Anecdotal evidence supports using certain exercises and brain training to improve symptoms.