Becker Disease

Prevalence is approximately 1:50,000

Myotonia

Myotonic episode leading to a difficult to ventilate/intubate situation

Genetic disease that results in muscle membrane hyperexcitability and delayed relaxation

Recessively inherited form of MC

Initial symptoms start around 4–12 y of age, with generalized myotonia and moderate to pronounced muscular hypertrophy from chronically increased muscle activity

Signs include muscle stiffness after voluntary contraction that improves with repetitive movement (“warm-up” phenomenon) and worsens after prolonged rest

Many experience transient weakness (<1 min) upon initiating movement; history of clumsiness, dropping objects, impaired postural control, or uncontrolled falling upon standing

Rarely, can have atrophy in the forearms and painful muscle cramps

Most have normal life expectancy without significant handicap

Aggravating factors: dietary insufficiencies, sleep deprivation, prolonged physical activity, and emotional stress

Menstruation, pregnancy, and hypothyroidism may alleviate or worsen symptoms in some individuals

No involvement in smooth and cardiac muscles, no extramuscular manifestations

It is important to differentiate this from myotonia with dystrophy, which is a multisystem disorder

Diagnosis:

• Characteristic symptoms (described previously)

• “Percussion myotonia”: reflex hammer produces obvious dimpling or fasciculation in prominent muscles, such as thenar eminence or thighs, that lingers for several seconds

• Objective evidence: electromyography

• Molecular genetic testing is commercially available, although not sensitive for less common mutations

Impaired functioning of skeletal muscle ClC-1

Skeletal muscle chloride channels serve to stabilize membrane potential at the resting level; impaired ClC-1 leads to sarcolemmal excitability and delayed muscle relaxation

More than 120 mutations have been described; most mutations are unique to individual families or isolated cases