Myosin Storage Myopathy

Definition

Myosin Storage Myopathy (MSM) is a rare inherited muscle disorder characterized by the abnormal accumulation of myosin protein within muscle fibers. This leads to muscle weakness and structural muscle abnormalities.

Cause

• MSM is caused by mutations in the MYH7 gene, which encodes the beta-myosin heavy chain, an essential protein for muscle contraction.

• The mutation leads to defective myosin proteins that accumulate inside muscle cells instead of functioning properly.

Symptoms

• Muscle weakness, typically progressive and primarily affecting proximal muscles (those closer to the trunk such as shoulders and hips).

• Muscle cramps or stiffness.

• Difficulty with activities requiring muscle strength (e.g., walking, climbing stairs).

• Symptoms often begin in childhood or early adulthood but can vary widely.

• Some patients may show mild or slowly progressive symptoms.

Types / Classification

Myosin Storage Myopathy is part of a group called myosinopathies and often overlaps clinically with other myopathies caused by MYH7 mutations, such as Laing distal myopathy or hypertrophic cardiomyopathy, depending on mutation specifics.

Diagnosis

• Clinical examination and family history.

• Muscle biopsy showing abnormal storage of myosin protein within muscle fibers.

• Genetic testing identifying mutations in the MYH7 gene.

• MRI of muscles may show characteristic patterns of muscle involvement.

Treatment

• No definitive cure currently available.

• Management is supportive and symptomatic:

  • Physical therapy to maintain muscle strength and prevent contractures.

  • Occupational therapy for daily activity support.

  • Monitoring for respiratory or cardiac complications if present.

• Genetic counseling for affected families.

Prognosis

• Variable, depending on mutation type and severity.

• Many patients have a slowly progressive course with preserved lifespan.

• Some mutations may lead to more severe weakness or associated heart problems.