Educational scope notice: This is a study note for medical students, not medical advice, diagnosis, or treatment guidance. Clinical management should follow local protocols and current guidelines.
Muscular dystrophies are inherited, progressive diseases in which a defective structural protein leaves muscle fibres fragile. Every contraction then causes small membrane tears, calcium entry, fibre death, and gradual replacement by fat and connective tissue. The name suggests faulty nourishment, but the fault is mechanical: the link between the contractile apparatus and the surface membrane no longer distributes force safely.
Dystrophin and why its absence matters
Dystrophin is a very large protein that anchors intracellular actin to the extracellular matrix across the surface membrane. It is expressed in skeletal and cardiac muscle, brain, and retina, so dystrophin disease reaches beyond limb weakness. Without it, ordinary activity destroys fibres faster than they regenerate, and the muscle is slowly exchanged for tissue that cannot contract.
Duchenne muscular dystrophy
Duchenne dystrophy is the commonest paediatric muscle disease and follows X-linked inheritance, so boys are affected while carrier females show little or no disease. Frame-shifting mutations abolish dystrophin entirely. Subtle motor delay appears around age 3, diagnosis often follows around age 5 when an incidental CK measured in the tens of thousands prompts referral, and the CK elevation is chronic rather than episodic. Children rise from the floor by climbing up their own thighs, develop an exaggerated lumbar curve from pelvic-girdle weakness, and historically lost ambulation around age 10 or 11.
Because dystrophin is also expressed in brain, learning and coordination difficulties can accompany the weakness.
Corticosteroids stabilise the membrane and extend walking by a few years at the cost of weight gain, hypertension, bone effects, and growth slowing. Dissociated steroids aim to keep the benefit with less skeletal harm. Exon-skipping oligonucleotides restore the reading frame for specific deletions, producing a shortened dystrophin closer to the Becker state, and microdystrophin gene therapy delivers a compact functional gene by viral vector, with long-term results still under study. Newborn screening by CK or genetics is being introduced in some regions to allow intervention before irreversible replacement.
The milder end of the dystrophin spectrum
Becker dystrophy produces a reduced but partly functional dystrophin, so onset comes later, around early adolescence, progression is slower, and CK runs lower than in Duchenne. The two form one spectrum: the amount of working dystrophin predicts severity, which is exactly the gradient that exon-skipping therapy tries to climb.
Limb-girdle patterns
Limb-girdle muscular dystrophies weaken the shoulder and hip girdles and comprise more than 30 genetic subtypes, each disrupting a different membrane-cytoskeleton protein such as sarcoglycans, dysferlin, or calpain. The pattern is recognisable to the generalist, but assigning the exact subtype needs specialist assessment and genetics rather than bedside guesswork.
Facioscapulohumeral dystrophy
This dystrophy follows its name across face, shoulder blades, and upper arms. It is dominantly inherited and highly variable even within families. Detailed genetics and course belong to specialist references; the bedside contribution is recognising the distribution.
Myotonic dystrophy
Myotonic dystrophy is the commonest adult dystrophy and differs fundamentally from the structural diseases above: its cause is a nucleotide repeat expansion with toxic RNA effects, not a missing scaffold protein. Type 1 comes from a CTG expansion in DMPK, type 2 from a CCTG expansion in CNBP, with type 2 generally milder. Expansions grow across generations, so offspring can present earlier and more severely.
Because faulty RNA processing affects many tissues, the disease is multisystem: myotonia with progressive weakness, cataracts, endocrine disturbance including diabetes, and cardiac conduction disease. Facial involvement with ptosis and a transverse smile, and low-set ears in congenital forms, complete a recognisable picture. Electromyography shows waxing-waning discharges in myotonic disorders generally.
The practical discriminator sits in the channelopathy companion: myotonia with cataracts, endocrine, or cardiac disease points to myotonic dystrophy, while myotonia without systemic features points to a nondystrophic channel disorder.
Congenital myopathies, defined by histological signatures such as central cores or nemaline rods, present in infancy with hypotonia and belong to paediatric rather than adult practice.
Evidence anchors
- Birnkrant DJ, Bushby K, Bann CM, et al. Diagnosis and management of Duchenne muscular dystrophy, part 1: diagnosis, and neuromuscular, rehabilitation, endocrine, and gastrointestinal and nutritional management. Lancet Neurol. 2018;17(3):251-267. doi:10.1016/S1474-4422(18)30024-3
- Bird TD. Myotonic dystrophy type 1. GeneReviews [Internet]. University of Washington, Seattle. Available from: https://www.ncbi.nlm.nih.gov/books/NBK1165/