What is the most likely diagnosis for a 5-month-old patient with hypotonia, decreased muscle power, preserved feeding, swallowing, and eye movements, and an electromyogram (EMG) showing myopathy?

Congenital Myopathy

The most likely diagnosis is C. Congenital myopathy, based on the characteristic presentation of hypotonia with preserved feeding/swallowing/eye movements, myopathic EMG findings, and the clinical pattern that distinguishes it from both cerebral palsy and congenital muscular dystrophy. 1, 2

Why Congenital Myopathy is the Answer

The clinical triad of hypotonia, decreased muscle power, and preserved reflexes with myopathic EMG strongly supports congenital myopathy. 2 This presentation is pathognomonic for primary muscle disorders rather than central or neurogenic causes. 1, 2

Key Distinguishing Features Present in This Case:

• Preserved feeding and swallowing indicates relatively intact bulbar function, which is characteristic of congenital myopathies and helps differentiate from more severe neuromuscular disorders. 1, 3

• Normal eye movements further support congenital myopathy, as many severe forms of congenital muscular dystrophy present with ocular involvement. 1

• EMG showing myopathy confirms muscle fiber involvement and localizes the pathology to the muscle itself. 1, 2, 4

Why NOT Cerebral Palsy (Option A)

Hypotonia with preserved reflexes rules out cerebral palsy, which presents with increased tone and hyperreflexia due to upper motor neuron dysfunction. 2, 3 The American Academy of Pediatrics states that preserved or increased deep tendon reflexes with abnormal plantar responses distinguish central from peripheral causes. 3

• Cerebral palsy characteristically shows increased reflexes and abnormal plantar responses, not the preserved reflexes seen in this patient. 1, 2

• The absence of perinatal complications or brain imaging abnormalities would further argue against cerebral palsy. 3

Why NOT Congenital Muscular Dystrophy (Option B)

Normal or only mildly elevated 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, 3 The American Academy of Pediatrics notes that significantly elevated CK (>3× normal) indicates muscular dystrophy and mandates immediate neurology referral. 3

• Congenital muscular dystrophies show CK elevations often exceeding 1000 U/L. 2

• Normal CK is characteristic of most congenital myopathies such as nemaline myopathy, central core disease, and centronuclear myopathy. 2

• While the question doesn't explicitly state CK levels, the EMG finding of "myopathy" in the context of preserved bulbar function strongly suggests congenital myopathy over muscular dystrophy. 1, 2

Clinical Reasoning Algorithm

The American Heart Association defines congenital myopathies as genetic muscle disorders characterized by hypotonia and weakness from birth with static or slowly progressive course, which matches this clinical presentation perfectly. 1

Step 1: Localize the lesion - Hypotonia with preserved reflexes points to a muscle disorder rather than upper or lower motor neuron disease. 1, 3

Step 2: Distinguish muscle disorders - The absence of significantly elevated CK (implied by the clinical context) distinguishes congenital myopathy from congenital muscular dystrophy. 1, 2

Step 3: Confirm with EMG - Myopathic EMG pattern confirms primary muscle involvement, though EMG in infants under 2 years can be misleading and may show normal or even neurogenic patterns in some myopathies. 1, 4

Critical Next Steps

• Muscle biopsy is essential for definitive diagnosis and genetic classification of the specific congenital myopathy subtype, as they show specific morphological abnormalities. 1, 2, 5

• Immediate referral to pediatric neurology or genetics for muscle biopsy to determine the specific subtype and guide prognosis. 1, 2

• Cardiac evaluation should be performed, as certain congenital myopathies can develop cardiomyopathy and arrhythmias. 1, 2, 3

• Early intervention services should be initiated immediately while diagnostic workup proceeds—therapy must begin without delay. 1, 2, 3

Important Caveats

• EMG findings in infants under 2 years with myopathy can be misleading—normal EMG is relatively common for confirmed muscle disorders in infants, and some early onset myopathies may even present with neurogenic EMG patterns. 1, 4

• The most frequent muscular cause of neonatal hypotonia is specific congenital myopathies, followed by congenital muscular dystrophy. 5

• Do not delay early intervention services while awaiting definitive diagnosis—physical and occupational therapy must begin immediately. 3