Diagnosis: Congenital Myopathy (Answer B)
The diagnosis is congenital myopathy based on the classic triad of hypotonia with preserved reflexes, normal CK, and myopathic EMG findings in a 5-month-old infant. 1, 2
Why Congenital Myopathy is Correct
Hypotonia with preserved reflexes is pathognomonic for a primary muscle disorder rather than neurogenic or central causes. 1, 2, 3 The American Academy of Pediatrics states that preserved or increased deep tendon reflexes distinguish peripheral muscle disorders from central nervous system pathology. 3
Normal CK is characteristic of congenital myopathies (such as nemaline myopathy, central core disease, and centronuclear myopathy), distinguishing them from muscular dystrophies. 1, 2 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
EMG showing myopathy confirms muscle fiber involvement and, combined with normal CK, represents the classic presentation of congenital myopathy. 2 The clinical features of weak antigravity muscle power, delayed milestones, and good feeding (suggesting preserved bulbar function) all align with congenital myopathy. 1
Why the Other Options are Incorrect
Cerebral Palsy (Option A) - Excluded
Preserved reflexes definitively rule out cerebral palsy. 2, 3 Upper motor neuron lesions characteristically show increased reflexes, increased tone, and abnormal plantar responses—none of which are present in this patient. 2, 3 The American Academy of Pediatrics emphasizes that hypotonic cerebral palsy should not be diagnosed without excluding other causes, especially with normal reflexes. 4, 3
Spinal Muscular Atrophy (Option C) - Excluded
Preserved reflexes and absence of fasciculations exclude spinal muscular atrophy. 1, 2, 3 Spinal muscular atrophy characteristically presents with diminished or absent deep tendon reflexes due to lower motor neuron involvement, and tongue fasciculations are a hallmark feature. 1, 2, 3 The American Academy of Pediatrics states that fasciculations strongly suggest spinal muscular atrophy and require urgent referral. 3
Congenital Muscular Dystrophy (Option D) - Excluded
Normal CK essentially rules out congenital muscular dystrophy. 1, 2, 3 Congenital muscular dystrophies typically present with significantly elevated CK levels, often >1000 U/L, similar to Duchenne muscular dystrophy. 2 The American Academy of Pediatrics identifies CK >3× normal as a "red flag" indicating muscle destruction seen in muscular dystrophies. 4, 3
Critical Next Steps
Immediate referral to pediatric neurology or genetics for muscle biopsy is mandatory to determine the specific congenital myopathy subtype and guide prognosis. 1, 2 Muscle biopsy remains the gold standard for definitive diagnosis, showing specific morphological abnormalities such as cores, rods, or central nuclei. 1, 5
Concurrent referral to early intervention services should proceed immediately while diagnostic workup continues—therapy must not be delayed awaiting definitive diagnosis. 1, 2, 3
Cardiac evaluation is essential, as certain congenital myopathies (particularly centronuclear myopathy and some forms of nemaline myopathy) can develop cardiomyopathy and arrhythmias. 1, 2, 3
Common Pitfall
Do not diagnose "hypotonic cerebral palsy" in an infant with preserved reflexes and normal tone on examination—this represents a primary muscle disorder until proven otherwise. 4, 3