Learning Objectives
- Distinguish between Duchenne and Becker muscular dystrophies based on genetics and severity.
- Identify the role of Dystrophin in muscle fiber integrity.
- Recognize classic signs like Gowers sign and calf pseudohypertrophy.
- Understand the multi-system involvement of Myotonic Dystrophy.
1. Duchenne vs. Becker (X-Linked Recessive)
Both disorders involve mutations in the DMD gene, the largest protein-coding gene in humans, which increases the risk of spontaneous mutations.
| Feature | Duchenne (DMD) | Becker (BMD) |
|---|---|---|
| Genetics | Frameshift or nonsense mutation → Absent dystrophin. | Non-frameshift deletion → Partially functional dystrophin. |
| Severity | Severe onset < 5 years old. | Less severe; onset in adolescence/adulthood. |
| Clinical Signs | Waddling gait, Gowers sign, calf pseudohypertrophy. | Slower progression; “Becker is better.” |
Key Clinical Findings:
- Gowers Sign: Patient uses hands to “walk up” their own legs to stand.
- Pseudohypertrophy: Calves look large but are actually filled with fat and connective tissue (fibrofatty replacement).
- Death: Commonly due to dilated cardiomyopathy.
2. Myotonic Dystrophy (Autosomal Dominant)
Unlike DMD/BMD, this is an Autosomal Dominant trinucleotide repeat disease (CTG repeat in the DMPK gene).
- Myotonia: Inability to relax muscles (e.g., difficulty releasing a handshake).
- Systemic Features: Cataracts, Toupee (frontal balding), Gonadal atrophy (Mnemonic: CTG).
- Biopsy: Shows ring fibers and central nuclei.
Activity