Becker Myotonia

Characteristics

Becker myotonia is a hereditary muscle disorder characterized by temporary stiffness (myotonia) in the skeletal muscles. This stiffness often occurs suddenly after a period of inactivity. For instance, it may manifest when someone stands in line for a short time or reacts to a sudden shock, potentially causing a fall. The stiffness typically resolves within seconds to minutes as the muscles “warm up” through light activity.

Another symptom is temporary muscle weakness that occurs at the start of activity after rest. For example, upon standing, a person may briefly lose strength and buckle at the knees, but repeated muscle contractions restore normal strength.

These symptoms usually begin in the leg muscles but may later affect other areas, causing difficulty gripping objects or chewing. In some cases, patients may struggle to open their eyes. Becker myotonia also often leads to prominent muscle development in the legs and hips and a hollow back, while neck, shoulder, and arm muscles may remain underdeveloped, particularly with age. Importantly, the disease does not affect heart or intestinal muscles.

Prevalence and Disease Course

Becker myotonia affects approximately 2 in 100,000 people in the Netherlands. Symptoms typically appear between ages 10 and 20, though they may begin earlier or, in rare cases, as late as age 30. Symptoms may worsen during the initial years but generally stabilize thereafter. Some individuals report mild, persistent muscle weakness later in life, though this progression has not been extensively studied.

Cause

Becker myotonia is an autosomal recessive condition, meaning both parents must carry the defective gene for a child to inherit the disorder. Affected children have a 25% chance of inheriting the disease.

The condition results from a defect on chromosome 7, causing reduced permeability of chloride channels in the muscle fiber membrane. This impairs the relaxation of contracted muscle fibers, leading to the sensation of stiffness.

Treatment

There is no cure for Becker myotonia. Severe symptoms may be managed with medications such as mexiletine, quinine, procainamide, or phenytoin. Mexiletine is the most commonly used medication, though it requires special authorization in the Netherlands.

Mexiletine works by blocking sodium channels in the muscle fiber membrane, promoting a better balance that reduces the tendency of muscles to contract excessively. Response to mexiletine varies among patients, but when properly dosed, it has minimal side effects.

Key Considerations:

• Diagnosis and Monitoring: Early recognition of symptoms can guide appropriate management.
• Treatment Response: Effectiveness of mexiletine and other medications should be monitored and adjusted for individual needs.
• Lifestyle Adjustments: Warm-up exercises and avoiding prolonged inactivity may help alleviate symptoms.