Isaac syndrome is a rare condition that causes uncontrollable muscle activity. The muscle movement occurs continuously, even during sleep or while a person is under anesthesia.

Other names for Isaac syndrome, or IS, include:

  • neuromyotonia
  • continuous muscle fiber activity syndrome
  • Isaacs-Mertens syndrome

IS is an autoimmune disease, the cause of which is the body’s immune system attacking its own tissues. The symptoms of IS are lifelong, but they are manageable with treatment.

This article discusses the symptoms, causes, and treatments for IS.

A man with Isaac syndrome sleeps.Share on Pinterest
For people with IS, which is more common in males, uncontrollable muscle activity continues during sleep.

IS is a rare autoimmune disease. It occurs when the immune system, which is essential for keeping the body safe from infection, mistakenly attacks healthy tissues and organs. There are currently 100–200 reported cases of IS globally.

IS causes peripheral nerves to become hyperactive and fire continuously. This produces uncontrollable nerve and muscle activity, which causes various symptoms. For example, it might lead to muscle spasms or stiffness.

IS can develop in anyone at any age, but it is more common in males than females.

The syndrome usually develops between 15–60 years of age. Most people will experience symptoms before the age of 40 years.

The symptoms and severity of IS vary. Most symptoms relate to the uncontrollable nerve or muscle activity. The symptoms tend to occur continuously, even during sleep.

Common symptoms of IS include:

  • continuous contracting or twitching of muscles
  • muscle stiffness and cramping that worsens over time
  • increased or excessive sweating
  • muscle pain
  • delayed reflexes
  • gradual muscle loss
  • difficulty coordinating movements and walking
  • weight loss
  • increased calf muscle size
  • elevated heart rate

Around 20% of people with IS also have Morvan syndrome. The symptoms of Morvan syndrome include:

  • irregular heartbeats
  • memory loss, confusion, and hallucinations
  • excessive salivation
  • constipation
  • personality changes
  • sleep difficulties

Doctors are not sure what causes the body to attack healthy tissue, as it does in people who have IS. However, it appears to relate to genetics, among other health factors.

It is possible to inherit IS or to acquire it later in life. Acquired IS tends to develop with other conditions, such as:

Less commonly, IS might develop with:

Most treatment plans for IS focus on reducing symptoms, such as muscle stiffness and spasms.

Common treatments for managing IS symptoms include:

  • anticonvulsant medications, such as valproic acid, lamotrigine, and carbamazepine
  • plasma exchange
  • oral corticosteroids, such as prednisolone
  • nonsteroidal drugs that suppress the immune system, such as methotrexate and azathioprine
  • acetazolamide

The exact treatment will depend on the type and severity of symptoms. There may also be additional treatments if IS develops with other conditions.

To diagnose IS, a doctor will check a person’s medical history and do a physical examination. Doctors will typically look for signs that include:

  • continuous muscle contractions or spasms
  • tics or twitches
  • muscle cramps

Muscle spasms may affect the hands, feet, or face particularly.

If a doctor suspects someone has IS, they will use an electromyography (EMG) test. The EMG test examines muscle and nerve activity.

The doctor may also require blood or urine tests to check for signs of the condition, such as specific antibodies. They might use a medical imaging scan, such as an MRI, to check for signs of muscle and nerve damage.

As IS is a rare disease, it can be challenging to detect, and some of its symptoms may resemble those of other conditions.

The doctor will want to rule out the presence of commoner conditions, such as:

There is no cure for IS currently, but it is possible to manage symptoms with ongoing treatment. Each case of IS involves different symptoms of varying severity. IS is disabling but rarely life threatening.

People who develop Morvan’s syndrome may develop severe symptoms that can be fatal. In many cases, the outlook will depend on other conditions that someone may develop with IS.

IS is a rare autoimmune disorder that causes continuous and uncontrollable muscle and nerve activity. The syndrome can develop with other conditions, such as cancer or other autoimmune disorders.

It is unclear what exactly causes IS. There is no cure for the condition, but several treatments exist for managing its long-term symptoms. For example, some drugs suppress the immune system and reduce symptoms.

Treating IS may also prevent further damage to muscles and nerves. The outlook for IS may also depend on the presence of other conditions.