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Dmd explain
Duchenne Muscular Dystrophy (DMD) is an X-linked recessive genetic disorder caused by mutations in the dystrophin gene, leading to progressive muscle wasting. It primarily affects males and is one of the most common and severe muscular dystrophies.
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Here is a thorough explanation of Duchenne Muscular Dystrophy (DMD):
Duchenne Muscular Dystrophy (DMD)
What Is It?
DMD is the most common severe neuromuscular disease in humans - an X-linked recessive disorder caused by mutations in the dystrophin gene (located at Xp21.2). It has an incidence of approximately 1 in 5,000 male births. About one-third of cases arise from de novo mutations with no family history.
Genetics & Pathophysiology
- The dystrophin gene is one of the largest in the human genome (~2.4 megabases, 79 exons)
- Over 7,000 variants are associated with DMD; exon deletions (80%) and duplications are most common
- A frameshift mutation causes complete absence of the dystrophin protein
- Dystrophin normally forms a mechanical link between the sarcomere and sarcolemma (muscle cell membrane)
- Without it, a calcium leak occurs across the membrane, triggering muscle fiber necrosis
- Repeated cycles of degeneration and regeneration lead to replacement of muscle by fat and fibrous connective tissue
Clinical Features
Early childhood (ages 2-5):
- Delayed motor milestones
- Difficulty running, frequent falls
- Calf pseudohypertrophy (calves look enlarged due to fat infiltration)
- Gowers' sign - child uses hands to "walk up" legs to stand from the floor
Progression:
- Progressive proximal muscle weakness (lower limbs first)
- Most patients lose the ability to walk by ages 10-15
- Scoliosis and joint contractures develop as weakness progresses
- By age 20: kyphoscoliosis worsens respiratory compromise
Cardiac involvement:
- Dilated cardiomyopathy (DCM) due to cardiac fibrosis (most common)
- Rhythm and conduction abnormalities
- Can lead to heart failure and fatal arrhythmias
Respiratory:
- Chronic respiratory insufficiency develops in all patients
- Cardiorespiratory failure is the primary cause of death
Cognitive/neurological:
- Lower IQ and nonprogressive cognitive impairment in some patients
- Associated with autism, ADHD, and learning disorders (correlated with mutation location in the gene)
Diagnosis
| Test | Finding |
|---|---|
| Serum CK | 20-100x elevated (markedly raised due to leaking muscle) |
| Genetic testing | Positive in 90-95% of patients - confirms frameshift mutation |
| Muscle biopsy | Used if genetic testing is negative; shows fiber necrosis, inflammation, fibrosis, variation in fiber size |
| Immunohistochemistry | Absent dystrophin staining (except "revertant fibers") |
Treatment
Multidisciplinary approach is essential.
Pharmacological:
- Glucocorticoids (prednisone 0.75 mg/kg/day or deflazacort 0.9 mg/kg/day) - prolong ambulation, improve muscle strength; deflazacort causes less weight gain
- ACE inhibitors + beta-blockers - slow cardiac fibrosis progression and manage DCM
Exon-skipping therapies (FDA conditionally approved):
- Eteplirsen - skips exon 51 (30 mg/kg IV weekly)
- Golodirsen - skips exon 53 (30 mg/kg IV weekly)
- Viltolarsen - skips exon 53 (80 mg/kg IV weekly)
- Casimersen - skips exon 45 (30 mg/kg IV weekly)
These work by restoring a partial reading frame so a truncated but partially functional dystrophin is produced.
Supportive:
- Physical therapy to prevent contractures
- Noninvasive ventilation for respiratory failure
- Spinal fusion for scoliosis when indicated
- Wheelchairs and assistive devices
Carrier Females
- Most are asymptomatic (one normal X chromosome compensates)
- Up to 20% of carrier females can show some symptoms (muscle weakness, elevated CK, or DCM) depending on X-inactivation patterns
DMD vs. Becker MD (BMD)
| Feature | DMD | BMD |
|---|---|---|
| Mutation | Frameshift - no dystrophin | In-frame - reduced/abnormal dystrophin |
| Onset | 2-5 years | Teens/adulthood |
| Ambulation loss | ~12-15 years | After 16 years (often later) |
| Severity | Severe | Milder |
Sources: Goldman-Cecil Medicine International Edition; Tietz Textbook of Laboratory Medicine, 7th Edition
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