Diagnosis: Congenital Myopathy
The most likely diagnosis is C. Congenital myopathy, based on the constellation of hypotonia, decreased muscle power, normal CK, myopathic EMG pattern, and preserved feeding/swallowing/eye movements in a 5-month-old infant. 1
Clinical Reasoning
Key Distinguishing Features Supporting Congenital Myopathy
Congenital myopathies characteristically present with hypotonia and weakness from birth or early infancy, prominent facial weakness with or without ptosis, generalized hypotonic posture with hyporeflexia, and a static or slowly progressive clinical course. 1 The preserved feeding, swallowing, and eye movements in this patient are particularly important distinguishing features. 1
Normal CK levels strongly favor congenital myopathy over muscular dystrophy. 1 Congenital muscle dystrophies typically present with significantly elevated CK levels, often exceeding 1000 U/L. 1
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. 1, 2
Preserved reflexes (though decreased) argue against spinal muscular atrophy, which typically shows absent or markedly diminished reflexes due to anterior horn cell loss. 1
Why Not the Other Options?
A. Cerebral Palsy is excluded because cerebral palsy presents with upper motor neuron signs including increased tone (spasticity), hyperreflexia, and abnormal plantar reflexes—not hypotonia with decreased reflexes. 1 The EMG would not show myopathic changes in cerebral palsy.
B. Congenital Muscular Dystrophy is unlikely because these disorders characteristically present with markedly elevated CK levels, often 10-20 times normal or higher. 1 The normal CK in this patient effectively rules out muscular dystrophy. 1
D. Spinal Muscular Atrophy (SMA) is excluded by several features: SMA typically presents with fasciculations (absent in this patient), absent or severely diminished reflexes (this patient has decreased but present reflexes), and feeding/swallowing difficulties are common in severe infantile SMA (this patient has normal feeding/swallowing). 1 Additionally, EMG in SMA shows neurogenic rather than myopathic changes.
Diagnostic Confirmation Approach
Muscle biopsy remains the gold standard for confirming congenital myopathy diagnosis and identifying the specific subtype. 1, 2 The biopsy should be examined for characteristic structural abnormalities including:
- Nemaline rods (nemaline myopathy) 1, 3
- Central cores or minicores (core myopathies) 1, 3
- Central nuclei (centronuclear/myotubular myopathy) 1, 3
- Type 1 fiber predominance or disproportion (congenital fiber type disproportion) 1, 4
Genetic testing should follow muscle biopsy findings, with RYR1 mutations being the most common cause of congenital myopathy (approximately 1 in 90,000). 1 Genetic testing can identify causative mutations in over 15 different genes associated with congenital myopathies. 1, 5
Critical Clinical Pitfalls
Respiratory muscle weakness requires regular monitoring even when limb weakness appears mild or static. 1, 3 Nocturnal hypoventilation can develop insidiously, particularly with SEPN1 mutations and nemaline myopathy. 5
The absence of fasciculations does not exclude all neuromuscular disorders, but their presence would strongly suggest a neurogenic process like SMA. 1 This patient's lack of fasciculations supports a myopathic rather than neurogenic etiology.
Normal CK levels can occur in approximately 5% of late-onset Pompe disease patients, but infantile Pompe disease typically presents with severe cardiomyopathy, which is absent in this case. 1, 2