What is the diagnosis for a 5-month-old patient with delayed milestones, hypotonia, mild but preserved reflexes, weak antigravity muscle power, normal Creatine Kinase (CK) levels, and Electromyography (EMG) findings showing myopathy?

Diagnosis: Congenital Myopathy

The diagnosis is B) congenital myopathy, based on the combination of infantile-onset hypotonia, normal CK levels, preserved reflexes, and myopathic EMG pattern. 1

Key Diagnostic Features Supporting Congenital Myopathy

Normal CK Levels Are Decisive

• Normal CK strongly favors congenital myopathy over muscular dystrophy, as congenital muscle dystrophies typically present with significantly elevated CK levels (often >10 times normal). 1, 2
• The American Academy of Pediatrics recommends considering congenital myopathy specifically in infants with hypotonia, decreased muscle power, and normal CK levels. 1
• In contrast, approximately 95% of patients with congenital muscular dystrophies show markedly elevated CK. 2

Preserved Reflexes Exclude Spinal Muscular Atrophy

• Preserved reflexes argue strongly against spinal muscular atrophy (SMA), which typically shows absent or markedly diminished reflexes due to anterior horn cell loss. 1
• The absence of fasciculations further supports a myopathic rather than neurogenic etiology, though fasciculations can be subtle in infants with SMA. 1

Myopathic EMG Pattern Confirms Primary Muscle Disorder

• The myopathic EMG pattern confirms a primary muscle disorder rather than a neurogenic process, characterized by polyphasic motor unit action potentials of short duration and low amplitude with increased insertional activity. 1
• This pattern is consistent with congenital myopathy, where normal or mildly elevated CK with myogenic EMG changes is typical. 3, 4

Why Other Options Are Excluded

Cerebral Palsy (Option A) - Incorrect

• Cerebral palsy typically presents with increased tone (spasticity) and hyperreflexia, not hypotonia with preserved reflexes. 1
• EMG would be normal in cerebral palsy as it is a central nervous system disorder, not a primary muscle disease.

Spinal Muscular Atrophy (Option C) - Incorrect

• SMA would show absent or markedly reduced reflexes, not preserved reflexes. 1
• Fasciculations would typically be present (though can be subtle in infants).
• EMG would show neurogenic rather than myopathic changes.

Congenital Muscle Dystrophy (Option D) - Incorrect

• Congenital muscle dystrophies present with significantly elevated CK levels, not normal CK. 1, 2
• CK elevations in muscular dystrophies are typically >10 times the upper limit of normal. 2

Next Steps for Confirmation

Muscle Biopsy Is Gold Standard

• The American College of Medical Genetics and Genomics recommends muscle biopsy as the gold standard for confirming congenital myopathy diagnosis and identifying the specific subtype. 1
• Characteristic structural abnormalities include nemaline rods, central cores or minicores, central nuclei, and type 1 fiber predominance or disproportion. 1, 3, 4

Genetic Testing Should Follow

• Genetic testing should follow muscle biopsy findings, with RYR1 mutations being the most common cause of congenital myopathy. 1
• Over 15 different genes are associated with congenital myopathies, demonstrating significant genetic heterogeneity. 1, 4

Critical Management Consideration

• Regular monitoring of respiratory muscle weakness is necessary, even when limb weakness appears mild or static, due to the risk of nocturnal hypoventilation, particularly with SEPN1 mutations and nemaline myopathy. 1
• The preserved feeding and swallowing in this patient is reassuring but does not eliminate the need for respiratory surveillance. 1