Diagnosis: Congenital Myopathy
The diagnosis is B) congenital myopathy, based on the constellation of early-onset hypotonia, preserved reflexes, normal CK, and EMG evidence of myopathy in this 5-month-old infant. 1
Clinical Reasoning Algorithm
Key Distinguishing Features Present
Hypotonia with preserved reflexes strongly points to a muscle disorder rather than upper or lower motor neuron disease. 1
- Normal CK essentially rules out congenital muscular dystrophy (option D), which typically presents with significantly elevated CK levels (often >1000 U/L in conditions like Duchenne muscular dystrophy) 1
- Preserved reflexes exclude cerebral palsy (option A), as upper motor neuron lesions characteristically show increased reflexes and abnormal plantar responses 1
- Absence of fasciculations and preserved reflexes argue strongly against spinal muscular atrophy (option C), which presents with diminished or absent deep tendon reflexes and tongue fasciculations as hallmark features 1, 2
Congenital Myopathy Diagnostic Criteria
The American Heart Association defines congenital myopathies as genetic muscle disorders characterized by hypotonia and weakness from birth with static or slowly progressive course. 1
The clinical presentation matches congenital myopathy features:
- Hypotonia with hyporeflexia (mild but preserved reflexes) 1
- Weak antigravity muscle power indicating proximal muscle weakness 1
- Good feeding suggests relatively preserved bulbar function, consistent with milder forms 1
- Normal CK is characteristic of congenital myopathies, distinguishing them from muscular dystrophies 1
- EMG showing myopathy confirms muscle fiber involvement 3
Critical Diagnostic Pitfalls
EMG in infants under 2 years with myopathy can be misleading. Research shows that among 24 children ultimately diagnosed with myopathies, only 5 displayed myogenic alterations before age 2, while 16 had normal EMG and 3 showed neurogenic patterns 3. However, this patient has documented myopathic EMG, strengthening the diagnosis.
Muscle biopsy remains the gold standard for definitive diagnosis and subtype classification (nemaline myopathy, central core disease, centronuclear myopathy, etc.), as congenital myopathies show specific morphological abnormalities 1, 4
Why Other Options Are Incorrect
- Cerebral palsy (A): Requires increased tone and hyperreflexia from upper motor neuron damage; this child has hypotonia with preserved reflexes 1
- Spinal muscular atrophy (C): Presents with absent/diminished reflexes, fasciculations (especially tongue), and typically more severe weakness; normal CK is consistent but the preserved reflexes and absence of fasciculations exclude this 1, 2
- Congenital muscular dystrophy (D): Characterized by markedly elevated CK (typically >1000 U/L); normal CK makes this diagnosis highly unlikely 1
Next Steps
Immediate referral to pediatric neurology or genetics for muscle biopsy is indicated to determine the specific congenital myopathy subtype and guide prognosis 1. Concurrent referral to early intervention services should proceed while diagnostic workup continues 1.
Cardiac evaluation should be performed, as certain congenital myopathies (particularly those with TTN or MYH7 mutations) can develop cardiomyopathy and arrhythmias 1