Thursday, 22 November 2007

Treament of genetic diseases in the real world 9: familial hypokalemic periodic paralysis

What is hypokalemic periodic paralysis?
Hypokalemic periodic paralysis is a condition that causes episodes of extreme muscle weakness typically beginning in childhood or adolescence. Most often, these episodes involve a temporary inability to move muscles in the arms and legs. Attacks cause severe weakness or paralysis that usually lasts from hours to days. Some people may have episodes almost every day, while others experience them weekly, monthly, or only rarely. Attacks can occur without warning or can be triggered by factors such as rest after exercise, a viral illness, or certain medications. Often, a large, carbohydrate-rich meal or vigorous exercise in the evening can trigger an attack upon waking the following morning. Although affected individuals usually regain their muscle strength between attacks, repeated episodes can lead to persistent muscle weakness later in life.
People with hypokalemic periodic paralysis have reduced levels of potassium in their blood (hypokalemia) during episodes of muscle weakness. Researchers are investigating how low potassium levels may be related to the muscle abnormalities in this condition.
How common is hypokalemic periodic paralysis?
Although its exact prevalence is unknown, hypokalemic periodic paralysis is estimated to affect 1 in 100,000 people. Men tend to experience symptoms of this condition more often than women.

Diagnosis/testing. The diagnosis of HOKPP is based on a history of episodes of flaccid paralysis; low serum concentration of potassium (<0.9 to 3.0 mmol/L) during attacks; the absence of myotonia clinically and on electromyography (EMG) (with the exception of one family with heat-induced myotonia and cold-induced HOKPP); and a family history consistent with autosomal dominant inheritance. Molecular genetic testing identifies disease-causing mutations in CACNA1S or SCN4A in 80% of individuals meeting clinical diagnostic criteria. Of all individuals with HOKPP, about 55-70% have mutations in CACNA1S and about 8-10% in SCN4A. Such testing is clinically available.

Management. Treatment of a paralytic crisis by administration of potassium by mouth or IV aims to normalize the serum concentration of potassium and to shorten the paralytic episode. ECG and blood potassium concentration must be monitored during treatment. Surveillance depends on the affected individual's symptoms and response to preventive treatment. Neurologic examination should focus on muscle strength in the legs to detect permanent weakness associated with myopathy. A number of factors can trigger paralytic attacks: unusually strenuous effort, excess of carbohydrate-rich meals, sweets, and alcohol should be avoided; oral or intravenous corticosteroids should be used with care; glucose infusion should be replaced by another type of infusion.
Potassium in doses of 0.2 to 0.4 mmol/kg is administered orally every 15 to 30 minutes over one to three hours.
If the individual is unable to swallow or does not tolerate potassium by mouth, potassium may be administered intravenously. In that case, it must be diluted in 5% mannitol rather than in glucose or sodium chloride, which trigger crises in individuals with HOKPP. The concentration of potassium administered intravenously must not exceed 40 mmol/L and the flow must not exceed 20 mmol/hour or 200-250 mmol/day; administration must be stopped when the serum potassium concentration is normalized, even if weakness persists.
Because the hypokalemia and subsequent changes in potassium

Genetic counseling. HOKPP is inherited in an autosomal dominant manner. Most individuals diagnosed with HOKPP have an affected parent. The proportion of cases caused by a de novo gene mutation is unknown. Offspring of a proband have a 50% risk of inheriting the mutation. Penetrance is about 90% in males and may be as low as 50% in females depending on the causative mutation. Prenatal testing is possible if the disease-causing mutation has been identified in the proband; however, requests for prenatal testing for conditions such as HOKPP that do not affect intellect and have some treatment available are not common.


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