What is the diagnosis for a 5-month-old patient with delayed milestones, hypotonia, mild but preserved reflexes, weak antigravity muscle power, no fasciculations, normal creatine kinase (CK) levels, and electromyography (EMG) findings indicating myopathy?

Diagnosis: Congenital Myopathy

The diagnosis is B) congenital myopathy, based on the characteristic clinical triad of hypotonia with preserved reflexes, normal CK, and myopathic EMG findings in an infant with delayed milestones. 1, 2

Key Diagnostic Features Supporting Congenital Myopathy

Clinical Presentation

• Hypotonia with preserved reflexes is pathognomonic for congenital myopathy, distinguishing it from both upper motor neuron lesions (which show hyperreflexia) and lower motor neuron disorders (which show hyporeflexia or areflexia). 1, 2
• Weak antigravity muscle power with good feeding indicates relatively preserved bulbar function, consistent with the static or slowly progressive course typical of congenital myopathies. 1, 2
• The absence of fasciculations further confirms a primary muscle disorder rather than anterior horn cell disease. 2

Laboratory Findings

• Normal CK is characteristic of most congenital myopathies (nemaline myopathy, central core disease, centronuclear myopathy), whereas congenital muscular dystrophies typically show CK elevations >1000 U/L. 1, 2
• EMG showing myopathy with normal CK represents the classic presentation of congenital myopathy. 2

Systematic Exclusion of Other Diagnoses

A) Cerebral Palsy - EXCLUDED

• Cerebral palsy presents with increased tone and hyperreflexia due to upper motor neuron dysfunction, not the preserved reflexes seen in this patient. 1, 2
• The presence of hypotonia with preserved reflexes rules out cerebral palsy. 2

C) Spinal Muscular Atrophy - EXCLUDED

• SMA characteristically presents with diminished or absent deep tendon reflexes due to lower motor neuron involvement. 1, 2
• Tongue fasciculations are hallmark features of SMA, which are absent in this patient. 1
• The preserved reflexes and absence of fasciculations strongly argue against SMA. 1, 2

D) Congenital Muscular Dystrophy - EXCLUDED

• Normal CK essentially rules out congenital muscular dystrophy, which typically presents with significantly elevated CK levels (often >1000 U/L, similar to Duchenne muscular dystrophy). 1, 2
• Congenital muscular dystrophies show CK elevations as a defining feature. 2

Immediate Management Steps

Diagnostic Confirmation

• Muscle biopsy is essential for definitive diagnosis and genetic classification of the specific congenital myopathy subtype (nemaline myopathy, central core disease, centronuclear myopathy, fiber type disproportion). 1, 2, 3
• Immediate referral to pediatric neurology or genetics for muscle biopsy to determine the specific subtype and guide prognosis. 1

Concurrent Evaluations

• Cardiac evaluation should be performed, as certain congenital myopathies can develop cardiomyopathy and arrhythmias. 1, 2
• Early intervention services should be initiated immediately while diagnostic workup proceeds. 1, 2

Common Pitfalls to Avoid

• Do not delay muscle biopsy waiting for CK elevation—normal CK is expected in congenital myopathies. 1, 2
• Do not dismiss the diagnosis based on good feeding; bulbar function is often relatively preserved in congenital myopathies compared to other neuromuscular disorders. 1
• EMG in infants under 2 years with myopathy can be misleading, so muscle biopsy remains the gold standard. 1