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Potassium-Aggravated Myotonia

Last updated Oct. 29, 2018

Approved by: Krish Tangella MD, MBA, FCAP

Potassium Aggravated Myotonia is a group of diseases that causes tensing and stiffness (myotonia) of skeletal muscles, which are the muscles used for movement.

What are the other Names for this Condition? (Also known as/Synonyms)

  • Atypical Myotonia Congenita
  • Potassium-Aggravated Myotonia Congenita, Acetazolamide-Responsive
  • Potassium-Aggravated Myotonia Permanens

What is Potassium-Aggravated Myotonia? (Definition/Background Information)

  • Potassium Aggravated Myotonia is a group of diseases that causes tensing and stiffness (myotonia) of skeletal muscles, which are the muscles used for movement.
  • The three types of Potassium-Aggravated Myotonia include:
    • Myotonia fluctuans
    • Myotonia permanens
    • Acetazolamide-sensitive myotonia
  • Potassium Aggravated Myotonia is different from other types of myotonia because symptoms get worse when an affected individual eats food that is rich in potassium
  • Symptoms usually develop during childhood and vary, ranging from infrequent mild episodes to long periods of severe disease
  • Potassium Aggravated Myotonia is an inherited condition that is caused by changes (mutations) in the SCN4A gene
  • Treatment begins with avoiding foods that contain large amounts of potassium; other treatments may include physical therapy (stretching or massages to help relax muscles) or certain medications (such as mexiletine, carbamazapine, or acetazolamide)

(Source: Potassium-Aggravated Myotonia; Genetic and Rare Diseases Information Center (GARD) of National Center for Advancing Translational Sciences (NCATS), USA.)

The characteristics of the 3 subtypes of Potassium-Aggravated Myotonia are as follow:

  • Myotonia fluctuans: Episodes of muscle stiffness often during the resting period following exercise
  • Myotonia permanens: Continuous and severe symptoms
  • Acetazolamide-sensitive myotonia: Symptoms can be severe, and may be triggered by fasting, exercise or cold exposure

Who gets Potassium-Aggravated Myotonia? (Age and Sex Distribution)

  • Potassium-Aggravated Myotonia is a rare congenital disorder; its exact prevalence is not known
  • The onset of disease symptoms typically occurs in childhood
  • Both males and females may be affected
  • The age of onset is in childhood, either in the neonatal period or infancy

What are the Risk Factors for Potassium-Aggravated Myotonia? (Predisposing Factors)

  • A positive family history may be an important risk factor, since Potassium-Aggravated Myotonia can be inherited
  • Currently, no other risk factors have been clearly identified for Potassium-Aggravated Myotonia

It is important to note that having a risk factor does not mean that one will get the condition. A risk factor increases one’s chances of getting a condition compared to an individual without the risk factors. Some risk factors are more important than others.

Also, not having a risk factor does not mean that an individual will not get the condition. It is always important to discuss the effect of risk factors with your healthcare provider.

What are the Causes of Potassium-Aggravated Myotonia? (Etiology)

  • Potassium-Aggravated Myotonia is caused by missense mutation(s) in SCN4A gene
  • The SCN4A gene codes for the alpha subunit of the skeletal muscle voltage-gated channel Nav1.4
  • The proper functioning of this gene is crucial for the proper activation and inactivation of ion channels
  • Potassium-Aggravated Myotonia is inherited in an autosomal dominant manner

Autosomal dominant inheritance: Autosomal dominant conditions are traits or disorders that are present when only one copy of the mutation is inherited on a non-sex chromosome. In these types of conditions, the individual has one normal copy and one mutant copy of the gene. The abnormal gene dominates, masking the effects of the correctly function gene. If an individual has an autosomal dominant condition, the chance of passing the abnormal gene on to their offspring is 50%. Children, who do not inherit the abnormal gene, will not develop the condition or pass it on to their offspring.

What are the Signs and Symptoms of Potassium-Aggravated Myotonia?

The signs and symptoms of Potassium-Aggravated Myotonia may vary in type and severity among affected individuals, and include:

  • Apneic episodes in infancy
  • Laryngospasm
  • Muscle stiffness
  • Stridor

Based on the frequency of symptoms observed, the following information may be noted:

Very frequently present symptoms in 80-99% of the cases:

  • Hypertonia
  • Myotonia

Frequently present symptoms in 30-79% of the cases:

  • Elevated serum creatine phosphokinase
  • EMG abnormality
  • Feeding difficulties in infancy
  • Flexion contracture
  • Gait disturbance
  • Myalgia

Occasionally present symptoms in 5-29% of the cases:

  • Asthma
  • Chest pain
  • Hyperkalemia
  • Joint stiffness
  • Muscle cramps
  • Muscle weakness
  • Ophthalmoparesis
  • Respiratory insufficiency
  • Skeletal muscle hypertrophy

(Source: Potassium-Aggravated Myotonia; Genetic and Rare Diseases Information Center (GARD) of National Center for Advancing Translational Sciences (NCATS), USA.)

How is Potassium-Aggravated Myotonia Diagnosed?

Potassium-Aggravated Myotonia is diagnosed on the basis of the following information:

  • Complete physical examination
  • Thorough medical history evaluation
  • Assessment of signs and symptoms
  • Laboratory tests
  • Imaging studies
  • Biopsy studies, if necessary
  • Electromyographic (EMG) tests under normal conditions and in response to exercise
  • Molecular genetic testing to check for or confirm causative SCN4A gene mutation(s)

Many clinical conditions may have similar signs and symptoms. Your healthcare provider may perform additional tests to rule out other clinical conditions to arrive at a definitive diagnosis.

What are the possible Complications of Potassium-Aggravated Myotonia?

The complications of Potassium-Aggravated Myotonia may include:

  • Severe cramping
  • Incapacitating pain
  • Severe shortness of breath
  • Decreased quality of life

Complications may occur with or without treatment, and in some cases, due to treatment also.

How is Potassium-Aggravated Myotonia Treated?

There is no cure for Potassium-Aggravated Myotonia, since it is a genetic condition. The treatment is usually given to manage the signs and symptoms and any complication that develops. Treatment methods may include:

  • Avoiding potassium-rich foods
  • Depending on the subtype of the disorder, acetazolamide (for acetazolamide-responsive subtype) or mexiletine or carbamazepine
  • Physical therapy to improve muscle tone and for pain relief

How can Potassium-Aggravated Myotonia be Prevented?

Currently, Potassium-Aggravated Myotonia may not be preventable, since it is a genetic disorder.

  • Genetic testing of the expecting parents (and related family members) and prenatal diagnosis (molecular testing of the fetus during pregnancy) may help in understanding the risks better during pregnancy
  • If there is a family history of the condition, then genetic counseling will help assess risks, before planning for a child
  • Active research is currently being performed to explore the possibilities for treatment and prevention of inherited and acquired genetic disorders

Regular medical screening at periodic intervals with tests and physical examinations are recommended.

What is the Prognosis of Potassium-Aggravated Myotonia? (Outcomes/Resolutions)

  • The prognosis of Potassium-Aggravated Myotonia is good, with many affected individuals gaining control over their signs and symptoms over time
  • However, complications such as respiratory distress may reduce one’s quality of life. This is specially applicable to the subtype myotonia permanens
  • The prognosis is generally made on a case-by-case basis

Additional and Relevant Useful Information for Potassium-Aggravated Myotonia:

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What are some Useful Resources for Additional Information?

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Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: Oct. 29, 2018
Last updated: Oct. 29, 2018