What is the diagnosis for a 5-month-old child with delayed milestones, hypotonia (low muscle tone), mild but preserved reflexes, weak antigravity muscle power, normal creatine kinase (CK) levels, and electromyography (EMG) findings of myopathy?

Diagnosis: Congenital Myopathy

Based on the clinical presentation of a 5-month-old with hypotonia, preserved reflexes, weak antigravity muscle power, normal CK, and EMG showing myopathy, the diagnosis is congenital myopathy (Option B).

Clinical Reasoning Algorithm

Step 1: Tone Assessment Excludes Upper Motor Neuron Disease

• Hypotonia with preserved (not increased) reflexes rules out cerebral palsy, which presents with increased tone and hyperreflexia in upper motor neuron dysfunction 1
• The presence of hypotonia directs investigation toward lower motor neuron or muscle disorders 1

Step 2: Normal CK Distinguishes Myopathy from Muscular Dystrophy

• Normal CK effectively excludes congenital muscular dystrophy (Option D), which typically presents with significantly elevated CK levels 1
• Congenital muscular dystrophies show CK elevations often >1000 U/L, similar to Duchenne muscular dystrophy 1
• The normal CK in this case is a critical distinguishing feature pointing toward congenital myopathy rather than muscular dystrophy 1

Step 3: EMG Pattern and Reflexes Differentiate Myopathy from SMA

• Preserved reflexes exclude spinal muscular atrophy (Option C), which characteristically presents with diminished or absent deep tendon reflexes due to lower motor neuron involvement 1
• While EMG can show neurogenic patterns in SMA, the preserved reflexes are incompatible with this diagnosis 2
• Tongue fasciculations, a hallmark of SMA, are notably absent in this case 2

Step 4: EMG Confirms Myopathic Process

• EMG showing myopathy with normal CK is the classic presentation of congenital myopathy 1
• Congenital myopathies are characterized by "hypotonia and weakness, usually from birth, and a static or slowly progressive clinical course" 1
• The clinical triad of hypotonia, weak antigravity muscle power, and preserved reflexes with myopathic EMG strongly supports congenital myopathy 1

Key Clinical Features Supporting Congenital Myopathy

Characteristic Presentation

• Hypotonia with preserved reflexes is typical of congenital myopathy, distinguishing it from neurogenic causes 1
• Good feeding despite motor delays suggests preserved bulbar function, common in many congenital myopathies 1
• Delayed milestones with hypotonia from early infancy align with the typical presentation 1, 3

Diagnostic Pattern Recognition

• Normal CK is characteristic of most congenital myopathies (nemaline myopathy, central core disease, centronuclear myopathy) 1, 3
• EMG may show myopathic changes even when muscle biopsy reveals subtle findings 4
• The absence of fasciculations further supports a primary muscle disorder rather than anterior horn cell disease 1

Critical Pitfalls to Avoid

Common Diagnostic Errors

• Do not be misled by normal or inconclusive EMG results in infants with myopathy - EMG sensitivity is limited in this age group, with 16 of 24 confirmed myopathy cases showing normal EMG in one series 5
• Early-onset myopathies can occasionally show neurogenic EMG patterns, which should not invalidate the clinical diagnosis 4
• Congenital myasthenic syndromes can mimic congenital myopathy but typically show fatigability and respond to pyridostigmine 5

Next Steps in Management

• Muscle biopsy is essential for definitive diagnosis and genetic classification of the specific congenital myopathy subtype 1, 4
• Refer to pediatric neurology for comprehensive evaluation and genetic testing 1
• Cardiac evaluation should be performed, as some congenital myopathies (particularly those with TTN or MYH7 mutations) can have cardiac involvement 1
• Early intervention services should be initiated immediately while diagnostic workup proceeds 1