M08.028.004 Muscular dystrophy (MD)
Muscular dystrophy (MD) is a group of inherited muscle diseases characterized by progressive muscle weakness and wasting. This section focuses on two prominent X-linked forms: Duchenne Muscular Dystrophy (DMD) and Becker Muscular Dystrophy (BMD). It is crucial for medical students to understand their genetic basis, clinical presentations, and diagnostic approaches.
Duchenne Muscular Dystrophy (DMD)
Description:
- Most common and severe form of MD.
- Mutations in the dystrophin gene on the X chromosome cause X-linked recessive genetic disorders.
- Dystrophin protein is critical for muscle structure and function. Mutations lead to its virtual absence.
Clinical Features:
- Onset typically between ages 3-5.
- Progressive muscle weakness starts in the legs and pelvis and ascends to the upper body.
- Calf pseudohypertrophy (enlarged but weak calves due to fatty tissue infiltration).
- Proximal muscle weakness (shoulders and hips) affects daily activities like climbing stairs.
- Potential complications:
- Heart failure and arrhythmias.
- Respiratory insufficiency and increased risk of infections due to weak respiratory muscles.
Laboratory Findings:
- Elevated serum creatine kinase levels (indicative of muscle damage).
Muscle Biopsy:
- Shows a characteristic pattern:
- Variation in muscle fiber size.
- Necrosis, degeneration, and regeneration of muscle fibers.
- Fibrosis (scarring).
- Fatty infiltration.
- Immunostaining reveals a lack of dystrophin protein.
Diagnosis:
- Clinical features, laboratory tests, muscle biopsy, and genetic testing are used for confirmation.
| Feature | Description |
|---|---|
| Genetics | X-linked recessive |
| Gene | Dystrophin |
| Protein | Dystrophin (absent) |
| Onset | Age 3-5 |
| Weakness | Progressive, starts in the legs and pelvis |
| Muscle Findings | Calf pseudohypertrophy, proximal weakness |
| Complications | Heart failure, respiratory problems |
| Lab Tests | Elevated serum creatine kinase |
| Muscle Biopsy | Abnormal fiber size, necrosis, fibrosis, fatty infiltration, absent dystrophin protein |
| Diagnosis | Clinical features, labs, biopsy, genetic testing |
Points to Remember:
- DMD is a progressive and debilitating condition.
- Early diagnosis and management are crucial to optimize function and lifespan.
- Current treatment focuses on managing symptoms and slowing progression.
Becker Muscular Dystrophy (BMD)
Description:
- X-linked recessive genetic disorder similar to DMD, but with milder symptoms.
- Mutations in the dystrophin gene produce an altered, partially functional dystrophin protein.
Clinical Features:
- Later onset (childhood or adolescence) compared to DMD.
- Slower progression of muscle weakness, primarily affecting legs and pelvis.
- Cardiac involvement is rare.
- Patients can have a relatively normal lifespan with proper care.
Table 2. Summary of Becker Muscular Dystrophy Characteristics
| Feature | Description |
|---|---|
| Genetics | X-linked recessive |
| Gene | Dystrophin |
| Protein | Dystrophin (altered) |
| Onset | Childhood or adolescence |
| Weakness | Slower progression, affects legs and pelvis |
| Cardiac | Rare involvement |
| Lifespan | Relatively normal with proper care |
Points to Remember:
- BMD is a milder form of MD compared to DMD.
- Management focuses on physical therapy, respiratory support (if needed), and optimizing quality of life.
Genetic Counseling:
- Both DMD and BMD have significant implications for family planning.
- Genetic counseling is recommended for families with a history of these conditions.
Bibliography:
- National Institute of Neurological Disorders and Stroke: https://www.nichd.nih.gov/health/topics/musculardys
- Muscular Dystrophy Association: https://www.mda.org/
- Walton, J. N. (1960). The muscular dystrophies. Edinburgh: Livingstone.
- Engel, A. G., & Franzini-Armstrong, C. (2004). Myology (4th ed.). New York: McGraw-Hill.