In a 5‑month‑old infant with generalized hypotonia, reduced muscle strength, preserved feeding, swallowing, and eye movements, and electromyography showing a myopathic pattern, what is the most likely diagnosis?

Diagnosis: Congenital Myopathy

In a 5-month-old infant presenting with hypotonia, decreased muscle power, preserved feeding/swallowing/eye movements, and myopathic EMG pattern, the most likely diagnosis is congenital myopathy (Option C). 1

Key Diagnostic Features Supporting Congenital Myopathy

Clinical Presentation Pattern

• The preservation of feeding, swallowing, and eye movements is characteristic of congenital myopathies, which typically spare bulbar and extraocular muscles in early infancy 1, 2
• Congenital myopathies classically present with hypotonia, decreased muscle power, and a static or slowly progressive clinical course from birth or early infancy 3, 2, 4
• The myopathic EMG pattern (polyphasic motor unit action potentials of short duration and low amplitude with increased insertional activity) confirms a primary muscle disorder rather than a neurogenic process 5, 1

Why Not the Other Options

Cerebral Palsy (Option A) is excluded because:

• Cerebral palsy typically presents with spasticity and increased tone rather than hypotonia and weakness 5
• EMG in cerebral palsy would not show a myopathic pattern, as it is an upper motor neuron disorder 5
• The preserved reflexes and lack of pyramidal signs argue against cerebral palsy 5

Congenital Muscular Dystrophy (Option B) is less likely because:

• Congenital muscular dystrophies typically present with significantly elevated creatine kinase (CK) levels, often >1000 U/L, whereas congenital myopathies characteristically have normal or only mildly elevated CK 5, 1, 6
• Muscular dystrophies show a dystrophic pattern on muscle biopsy with necrotic and degenerative changes, while congenital myopathies lack these features 6
• The question does not mention elevated CK, which would be a critical finding if present 5, 1

Diagnostic Confirmation Strategy

Initial Laboratory Testing

• Measure serum creatine kinase (CK) immediately—normal CK levels strongly favor congenital myopathy over muscular dystrophy 1, 7
• Normal CK occurs in approximately 95% of congenital myopathies but is rare in muscular dystrophies 1, 7

Definitive Diagnosis

• Muscle biopsy remains the gold standard for confirming congenital myopathy diagnosis and identifying the specific subtype (central core disease, nemaline myopathy, centronuclear myopathy, congenital fiber type disproportion, or myosin storage myopathy) 1, 4, 8
• Muscle biopsy will reveal characteristic structural abnormalities including nemaline rods, central cores or minicores, central nuclei, or type 1 fiber predominance 1, 3
• Genetic testing should follow muscle biopsy findings, with RYR1 mutations being the most common cause, though over 15 different genes are associated with congenital myopathies 1, 2, 4

Critical Management Considerations

Respiratory Monitoring

• Regular monitoring of respiratory muscle weakness is mandatory, even when limb weakness appears mild or static, due to risk of nocturnal hypoventilation 1
• Respiratory complications are a major cause of morbidity and mortality in congenital myopathies 1, 4

Multidisciplinary Follow-up

• Management requires a specialized neuromuscular team including pediatric neurology, pulmonology, cardiology, and physical therapy 4
• Cardiac screening should be performed, though cardiac involvement is less common in congenital myopathies than in muscular dystrophies 5, 1