| Aspect | Progressive Muscular Dystrophies | Myotonic Syndromes |
|---|---|---|
| Definition | A group of genetic disorders characterized by progressive muscle weakness and wasting. | Disorders characterized by muscle weakness along with myotonia (difficulty relaxing muscles after contraction). |
| Main Types | Duchenne, Becker, Limb-Girdle, Facioscapulohumeral, etc. | Myotonic Dystrophy Type 1 (DM1) and Type 2 (DM2) |
| Inheritance | Often X-linked (e.g., Duchenne/Becker) or autosomal (e.g., Limb-Girdle). | Autosomal dominant. Trinucleotide repeat expansion diseases |
| Onset | Usually in childhood or adolescence. | Typically adolescence or adulthood. |
| Primary Symptoms | Muscle weakness, wasting, contractures, scoliosis, cardiac complications. | Muscle weakness, myotonia, cataracts, cardiac arrhythmias, endocrine issues. |
| Progression | Gradual; severity and speed depend on the subtype. | Gradual; multisystem involvement often worsens with age. |
| Pathophysiology | Defects in proteins like dystrophin (Duchenne/Becker) or other sarcolemmal proteins. | Expanded CTG (DM1) or CCTG (DM2) repeats causing RNA toxicity and protein misregulation. |
| Diagnosis | Genetic testing, muscle biopsy, creatine kinase (CK) levels, EMG. | Genetic testing, EMG (shows myotonic discharges). |
| Treatment | Symptomatic: physical therapy, steroids (e.g., Duchenne), assistive devices. | Symptomatic: medications for myotonia (e.g., mexiletine), physical therapy. |
| Cardiac Involvement | Common, especially in Duchenne and Becker (cardiomyopathy). | Common, often conduction abnormalities or arrhythmias. |
| Respiratory Complications | Can develop in advanced stages due to diaphragm involvement. | Can occur due to muscle weakness or central control issues. |
| Cognitive Impact | Rare | Can include mild cognitive and behavioral changes. |
| Prognosis | Varies by type; some forms (e.g., Duchenne) are life-limiting. | Varies by severity; generally better than PMD but depends on systemic involvement. |
Etiology
- Autosomal dominant inheritence
- Type 1: CTG trinucleotide repeat expansion in the DMPK gene → changes in myotonin protein kinase expression
Clinical features
- Myotonia: delayed muscle relaxation following normal muscle contraction
- Classically manifests as difficulty releasing a handshake
- Skeletal muscle weakness (due to muscle atrophy)
- Myalgia
- Arrhythmia
- Cataracts
- Testicular atrophy or features of ovarian insufficiency (i.e., infertility)
- Frontal balding
- Cognitive and behavioral impairment