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Cap myopathy
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Condition

Cap myopathy

Cap myopathy is a rare genetic disorder characterized by muscle weakness and poor muscle tone, caused by abnormal cap-like structures within muscle cells.

Overview

Cap myopathy is a disorder that primarily affects skeletal muscles, which are muscles used for movement. Individuals with this condition experience muscle weakness (myopathy) and poor muscle tone (hypotonia) throughout the body, with the most severe effects typically seen in the face, neck, and limbs. The weakness, which begins at birth or during childhood, can worsen over time.

Symptoms

  • Feeding and swallowing difficulties in infancy.
  • Delayed development of motor skills, such as sitting, crawling, standing, and walking.
  • Frequent falls, fatigue, and difficulty running, climbing stairs, or jumping.
  • Life-threatening breathing difficulties if the muscles used for breathing are affected.
  • High arch in the roof of the mouth.
  • Severely drooping eyelids.
  • Abnormally curved lower back or a spine that curves to the side.

Causes

Cap myopathy is caused by mutations in the ACTA1, TPM2, or TPM3 genes. These genes provide instructions for producing proteins essential for skeletal muscle function:

  • ACTA1: Provides instructions for making skeletal alpha (α)-actin. Mutations result in an abnormal protein that interferes with the assembly of thin filaments, leading to the characteristic "cap" structures seen under a microscope.
  • TPM2 and TPM3: Provide instructions for making tropomyosin proteins, which regulate muscle contraction by controlling the binding of actin to myosin. Mutations may impair this binding process, resulting in muscle weakness.

Inheritance

Cap myopathy is an autosomal dominant condition, meaning one copy of the altered gene in each cell is sufficient to cause the disorder. Most cases are not inherited and result from new mutations in the gene in people with no family history of the disorder.

Frequently Asked Questions

significant risk in cap myopathy is the involvement of the respiratory muscles, which can lead to breathing difficulties that may become life-threatening. Patients are monitored through pulmonary function tests and overnight oximetry to detect sleep-disordered breathing or hypoventilation. In severe cases, non-invasive ventilation or other respiratory support may be necessary. Regular follow-up with a pulmonologist is essential to manage these risks and ensure adequate oxygenation, particularly during sleep or illness.
Infants with cap myopathy often experience significant feeding and swallowing difficulties due to weakness in the facial and throat muscles, which can lead to poor weight gain and aspiration risk. Early intervention by a speech-language pathologist and a feeding therapist is crucial to develop safe feeding strategies, such as using specialized bottles or modifying food textures. Additionally, speech development may be delayed, and early speech therapy can help improve communication skills and support the child's developmental milestones as they grow.
Diagnosis typically begins with clinical evaluation of muscle weakness and low tone, followed by genetic testing for mutations in the ACTA1, TPM2, or TPM3 genes. However, a muscle biopsy is often required for a definitive diagnosis because it reveals the characteristic "cap" structures, which are abnormal accumulations of actin and other proteins at the ends of muscle fibers. These caps are visible under a microscope and distinguish cap myopathy from other congenital myopathies that may present with similar symptoms but different genetic causes.
The progression of muscle weakness in cap myopathy varies; while it often worsens over time, the rate of decline differs among individuals. Currently, there is no cure or specific medication to reverse the genetic defect or stop the progression of the caps. Management is supportive, focusing on physical and occupational therapy to maintain mobility and prevent contractures, as well as orthopedic interventions for scoliosis or joint deformities. Research into gene therapy and pharmacological interventions is ongoing, but current care remains focused on maximizing quality of life and functional independence.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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