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CATEGORY	DETAILS
RISK FACTORS	Male sex (X-linked recessive inheritance); positive family history of muscular dystrophy; maternal carrier status; spontaneous mutations account for approximately one-third of cases
ETIOLOGY	Mutation (most commonly large deletions) in the dystrophin gene located on chromosome Xp21, resulting in complete absence of functional dystrophin protein
PATIENT PRESENTATION	A 2 to 5 year old boy with progressive difficulty running, climbing stairs, and keeping up with peers; frequent falls; delayed motor milestones
CLASSIC PHYSICAL EXAM	Gowers sign (child uses hands to "walk up" his own thighs to stand); calf pseudohypertrophy; proximal muscle weakness greater than distal; waddling (Trendelenburg) gait; lumbar lordosis
KEY DIAGNOSTIC RESULTS	Serum creatine kinase (CK) markedly elevated (10 to 100 times normal); genetic testing reveals dystrophin gene mutation; muscle biopsy shows absent dystrophin on immunostaining
MANAGEMENT	Corticosteroids (prednisone or deflazacort) to slow disease progression; multidisciplinary care including cardiac monitoring, pulmonary function testing, physical therapy, and scoliosis screening
KEYWORDS	"Proximal weakness in a young boy," "calf pseudohypertrophy," "Gowers sign," "wheelchair-bound by age 12," "X-linked recessive," "absent dystrophin"

1. Pathophysiology

Duchenne muscular dystrophy arises from a mutation in the dystrophin gene, the largest known human gene, located on the short arm of the X chromosome at position Xp21. The most common mutations are large intragenic deletions (accounting for about 60 to 70 percent of cases), followed by duplications and point mutations. These mutations disrupt the reading frame of the gene, resulting in a complete absence of dystrophin protein in skeletal and cardiac muscle cells.

Dystrophin is a cytoskeletal protein that functions as a mechanical link between the internal actin cytoskeleton of the muscle fiber and the extracellular matrix through the dystrophin-associated glycoprotein complex (DGC). Without dystrophin, the sarcolemma (muscle cell membrane) becomes fragile and susceptible to contraction-induced damage. Each cycle of muscle contraction causes microtrauma to the sarcolemma, allowing excessive calcium influx into the myofiber. This calcium overload activates proteases and triggers a cascade of necrosis, inflammation, and attempted regeneration.

Early in the disease, the muscle attempts to regenerate, but the regenerative capacity of satellite cells is eventually exhausted. Over time, necrotic muscle fibers are progressively replaced by fibrosis and adipose tissue. This fibrofatty replacement explains two hallmark findings: the calf pseudohypertrophy (enlarged calves that feel firm or rubbery because they are filled with fat and connective tissue, not functional muscle) and the relentless progression of weakness.

Because the disease follows an X-linked recessive pattern, it almost exclusively affects males. Females who carry one mutant copy are typically asymptomatic carriers, though some manifesting carriers can show mild weakness or isolated cardiomyopathy due to skewed X-inactivation. The reason proximal muscles are affected earlier than distal muscles is that proximal muscles bear greater mechanical load during activities like standing and climbing, making them more vulnerable to contraction-induced sarcolemmal injury.

The involvement of cardiac muscle leads to dilated cardiomyopathy, which becomes clinically significant in the second decade of life. Similarly, progressive weakness of the diaphragm and intercostal muscles leads to restrictive lung disease and respiratory failure, which remains the leading cause of death.

2. Classification and Clinical Manifestation

STAGE	AGE RANGE	KEY CLINICAL FEATURES
Presymptomatic	Birth to 2 years	Mildly delayed motor milestones (late walking, typically after 18 months); incidentally discovered elevated CK on routine labs; language delay may be present
Early Ambulatory	2 to 5 years	Difficulty running, jumping, and climbing stairs; frequent falls; Gowers sign becomes apparent; calf pseudohypertrophy develops; waddling gait and toe-walking emerge
Late Ambulatory	6 to 11 years	Increasing difficulty with ambulation; progressive lordosis and contractures (hip flexors, iliotibial band, Achilles tendon); labored gait requiring assistive devices
Early Non-Ambulatory	12 to 15 years	Loss of independent ambulation (wheelchair dependence); progressive scoliosis; upper extremity weakness becomes functionally limiting; early signs of cardiomyopathy
Late Non-Ambulatory	Beyond 15 years	Severe restrictive pulmonary disease requiring ventilatory support (initially nocturnal, then continuous); progressive dilated cardiomyopathy; risk of cardiac arrhythmias; upper limb and hand function progressively lost

A critical concept for exam purposes: the clinical course of Duchenne is predictably progressive. The timeline of wheelchair dependence by approximately age 12 is a commonly tested benchmark that distinguishes Duchenne from Becker muscular dystrophy.

3. Diagnostic Workup

TEST	PURPOSE	KEY FINDINGS
Serum Creatine Kinase (CK)	Best initial test; screening	Markedly elevated, typically 10,000 to 20,000 IU/L (can reach up to 50 to 100 times normal); elevated from birth, even before symptoms appear
Genetic Testing (multiplex ligation-dependent probe amplification / sequencing)	Most accurate (confirmatory) test	Identifies deletions, duplications, or point mutations in the dystrophin gene; confirms diagnosis without need for biopsy in most cases
Muscle Biopsy with Immunostaining	Reserved for cases where genetic testing is inconclusive	Shows absent dystrophin on immunohistochemistry; reveals myofiber necrosis, regeneration, and fibrofatty replacement
Electromyography (EMG)	Supportive but not routinely required	Shows a myopathic pattern (short duration, low amplitude, polyphasic motor unit potentials) rather than a neuropathic pattern
Echocardiography	Cardiac screening (baseline and ongoing)	Monitors for dilated cardiomyopathy; recommended starting at diagnosis or by age 6
Pulmonary Function Tests	Respiratory monitoring	Tracks forced vital capacity; decline signals need for ventilatory support

The diagnostic approach begins with clinical suspicion in a young boy presenting with proximal weakness, Gowers sign, and calf pseudohypertrophy. The best initial test is serum CK, which will be dramatically elevated. An important teaching point: CK in Duchenne is not mildly elevated. Values are typically in the range of tens of thousands. This degree of elevation should immediately raise concern for a muscular dystrophy rather than an inflammatory myopathy, where CK elevation is usually more modest.

Once CK confirms a myopathic process, the next step is genetic testing, not muscle biopsy. Modern practice has shifted toward genetic confirmation as the gold standard because it is noninvasive, identifies the exact mutation (which has implications for emerging gene-targeted therapies), and is diagnostic in approximately 95 percent of cases. Muscle biopsy is reserved for the small percentage of patients in whom genetic testing does not identify a mutation, or when the clinical picture is atypical and a broader differential needs to be evaluated.

On muscle biopsy, immunohistochemistry for dystrophin shows complete absence of the protein, distinguishing Duchenne from Becker muscular dystrophy, where dystrophin is present but reduced in quantity or abnormal in size.

EMG is not routinely needed for diagnosis but may appear as a distractor on exam questions. Recognize that EMG findings in Duchenne are myopathic (short duration, low amplitude motor unit potentials with early recruitment), which helps exclude neuropathic conditions like spinal muscular atrophy.

4. Management and Treatment

INTERVENTION	DETAILS	TIMING
Corticosteroids	Prednisone 0.75 mg/kg/day OR deflazacort 0.9 mg/kg/day	Initiated when motor skills plateau (typically age 4 to 6); continued long-term
ACE Inhibitors or ARBs	Enalapril, lisinopril, or losartan at standard pediatric dosing	Started by age 10 or at earliest sign of cardiac dysfunction on echocardiography
Beta-Blockers	Carvedilol or bisoprolol	Added for progressive cardiomyopathy or reduced ejection fraction
Nocturnal Non-Invasive Ventilation (BiPAP)	Bilevel positive airway pressure	Initiated when forced vital capacity falls below 30 percent predicted or when nocturnal hypoventilation is documented
Scoliosis Surgery	Posterior spinal fusion	Considered when Cobb angle exceeds 20 to 30 degrees and the patient is non-ambulatory
Physical Therapy and Bracing	Stretching programs, ankle-foot orthoses	Ongoing from diagnosis to prevent and manage contractures
Exon-Skipping Therapies	Eteplirsen (exon 51), golodirsen (exon 53), viltolarsen (exon 53), casimersen (exon 45)	Applicable only to patients with mutations amenable to the targeted exon skip; FDA-approved under accelerated approval
Vaccinations and Bone Health	Pneumococcal and influenza vaccines; calcium, vitamin D supplementation; bone density monitoring	Ongoing, particularly important given chronic steroid use

Corticosteroids remain the cornerstone of treatment and the most commonly tested intervention. Prednisone at 0.75 mg/kg/day is the standard regimen. Deflazacort at 0.9 mg/kg/day is an alternative that may have a slightly better side effect profile regarding weight gain. Steroids slow the decline in muscle strength and function, prolong ambulation by approximately 2 to 3 years, reduce the need for scoliosis surgery, and delay the onset of cardiomyopathy and respiratory failure. Treatment is typically initiated when motor skills reach a plateau, usually around age 4 to 6, before the onset of decline.

The side effects of long-term steroid therapy are heavily tested. Expect questions about weight gain, growth suppression, osteoporosis, cushingoid features, behavioral changes, glucose intolerance, and adrenal suppression. Bone health management with calcium and vitamin D supplementation is essential, and patients should undergo periodic bone density assessment.

Cardiac management is critical because cardiomyopathy is a leading cause of death. ACE inhibitors or ARBs are started prophylactically by age 10 or at the first sign of myocardial dysfunction. Annual echocardiography and cardiac MRI are used to monitor ventricular function. Beta-blockers are added when ejection fraction declines.

Respiratory management follows a predictable trajectory. As respiratory muscles weaken, forced vital capacity declines. Patients progress from nocturnal hypoventilation (managed with BiPAP) to daytime respiratory insufficiency requiring continuous ventilatory support. Assisted cough devices and airway clearance techniques are added as cough effectiveness diminishes.

Exon-skipping therapies represent a newer pharmacologic approach. These antisense oligonucleotides promote skipping of a targeted exon during mRNA splicing, restoring the reading frame and allowing production of a truncated but partially functional dystrophin protein (essentially converting a Duchenne phenotype toward a Becker phenotype). These agents are mutation-dependent, meaning they benefit only the subset of patients whose deletion is amenable to the targeted exon skip.

5. Differential Diagnosis and Distractors

DIFFERENTIAL	WHY IT IS SIMILAR	KEY DISCRIMINATOR
Becker Muscular Dystrophy	Also an X-linked dystrophinopathy with proximal weakness and elevated CK	Later onset (typically after age 5, often in adolescence); slower progression; ambulation preserved beyond age 16; muscle biopsy shows reduced or abnormal dystrophin rather than absent dystrophin
Spinal Muscular Atrophy (SMA)	Proximal weakness in a young child; difficulty standing and walking	Lower motor neuron signs (hypotonia, areflexia, fasciculations, tongue fibrillations); CK is normal or only mildly elevated; EMG shows a neuropathic pattern; caused by SMN1 gene mutation
Limb-Girdle Muscular Dystrophy	Proximal weakness with elevated CK	Autosomal inheritance (affects both sexes); variable age of onset; biopsy shows presence of dystrophin but may show deficiency of sarcoglycans or other proteins
Polymyositis / Dermatomyositis	Proximal muscle weakness with elevated CK	Acquired inflammatory myopathy; older age of onset; associated with skin findings in dermatomyositis (heliotrope rash, Gottron papules); responds to immunosuppressive therapy; biopsy shows inflammatory infiltrate
Hypothyroid Myopathy	Can present with proximal weakness and mildly elevated CK	Associated with other signs of hypothyroidism (fatigue, weight gain, delayed relaxation of reflexes); CK is only mildly elevated; thyroid function tests are abnormal
Myasthenia Gravis	Weakness in a young patient	Fatigable weakness that worsens with repetitive use and improves with rest; involves ocular and bulbar muscles preferentially; CK is normal; positive acetylcholine receptor antibodies; responds to acetylcholinesterase inhibitors

The most commonly tested differential on exams is the distinction between Duchenne and Becker muscular dystrophy. Both are caused by mutations in the same dystrophin gene, but the critical difference lies in the reading frame. In Duchenne, the mutation disrupts the reading frame (out-of-frame deletion), producing no functional protein. In Becker, the reading frame is preserved (in-frame deletion), allowing production of a truncated but partially functional dystrophin. This is known as the reading frame rule and explains the milder phenotype in Becker.

The second most important distinction is from spinal muscular atrophy. Both present with a weak, floppy child. The key discriminators are: SMA presents with lower motor neuron signs (areflexia, fasciculations), CK is normal or near-normal, and EMG shows denervation. Duchenne presents with preserved reflexes early on, very high CK, and a myopathic EMG pattern.

6. Traps and High-Yield Pearls

The most common trap in exam questions on Duchenne muscular dystrophy involves confusing the diagnostic sequence. Students frequently select muscle biopsy as the confirmatory test, which was historically accurate but is no longer the preferred approach. In current practice, once CK is found to be markedly elevated in a clinically suspicious patient, the confirmatory step is genetic testing, not biopsy. Biopsy is reserved for genetically inconclusive cases. Expect the exam to test whether you understand this updated algorithm.

A second common pitfall is failing to recognize the presymptomatic presentation. A vignette may describe a toddler with isolated language delay or an incidental finding of dramatically elevated CK (or elevated AST and ALT that are actually of muscle origin, not hepatic). Students who do not consider a muscular dystrophy in the differential for unexplained transaminase elevation will miss this diagnosis.

Another frequently tested concept is the inheritance pattern and genetic counseling implications. A question may present a mother asking about recurrence risk. Remember: if the mother is a confirmed carrier, each son has a 50 percent chance of being affected, and each daughter has a 50 percent chance of being a carrier. Approximately one-third of cases arise from de novo mutations, meaning the mother may not be a carrier.

Finally, know the complications and causes of death. Students sometimes focus exclusively on the skeletal muscle disease and forget that dilated cardiomyopathy and respiratory failure are the leading causes of mortality. Questions about long-term management will test your understanding of when to initiate cardiac medications (ACE inhibitors by age 10) and respiratory support (BiPAP when FVC falls below 30 percent predicted). The core competency being tested across all Duchenne questions is your ability to recognize the classic presentation, order the correct diagnostic sequence, and understand the natural history well enough to anticipate and manage complications proactively.

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