What is the likely diagnosis for a 6-week-old child with weak anti-gravity (against gravity) movement, normal eye fixation, normal Creatine Kinase (CK) levels, and decreased reflexes?

Diagnosis: Congenital Myopathy

The most likely diagnosis for this 6-week-old child with weak anti-gravity movement, normal eye fixation, normal CK, and decreased reflexes is congenital myopathy (Option A). 1, 2

Diagnostic Reasoning

Key Clinical Features Supporting Congenital Myopathy

• Normal CK essentially rules out Duchenne muscular dystrophy (DMD), which characteristically shows markedly elevated CK levels (typically >1000 U/L) even in early stages 3, 2, 4

• Decreased reflexes with hypotonia points to a primary muscle disorder rather than upper motor neuron pathology, which would show increased reflexes 1, 2, 4

• The clinical triad of hypotonia, weak anti-gravity muscle power, and preserved/decreased reflexes with normal CK is the classic presentation of congenital myopathy 1, 2

• Normal eye fixation indicates preserved extraocular muscle function, which is typical of most congenital myopathies at this early age 1

Why Other Options Are Excluded

Spinal Muscular Atrophy (SMA) - Option B:

• SMA characteristically presents with absent or markedly diminished deep tendon reflexes (not just decreased) 1, 2
• Tongue fasciculations are a hallmark feature of SMA that should be actively sought 1
• The absence of fasciculations argues strongly against anterior horn cell disease 2

Duchenne Muscular Dystrophy (DMD) - Option C:

• DMD is effectively excluded by normal CK levels 3, 2, 4
• DMD typically presents later (around 5 years of age on average), though delays in walking may be noted by 16-18 months 3
• At 6 weeks, DMD would already show significantly elevated CK even before clinical symptoms become apparent 3, 4

Muscular Atrophy - Option D:

• This is a vague term that could encompass SMA, but the clinical presentation doesn't fit neurogenic causes given the pattern of reflexes 2, 4

Diagnostic Workup and Management

Immediate Next Steps

• Muscle biopsy is essential for definitive diagnosis and to determine the specific congenital myopathy subtype (nemaline myopathy, central core disease, centronuclear myopathy, etc.) 1, 2

• Refer immediately to pediatric neurology or genetics for comprehensive evaluation and genetic testing 2

• Cardiac evaluation should be performed, as certain congenital myopathies (particularly centronuclear and some nemaline subtypes) can develop cardiomyopathy and arrhythmias 1, 2

Concurrent Management

• Initiate early intervention services immediately while diagnostic workup proceeds—do not wait for biopsy results 1, 2

• Monitor feeding and respiratory function closely, as bulbar and respiratory weakness can progress in some congenital myopathy subtypes 1

Critical Pitfalls to Avoid

• Do not assume normal CK rules out all muscle disease—congenital myopathies characteristically have normal or only mildly elevated CK, distinguishing them from muscular dystrophies 1, 2, 5

• Do not delay referral waiting for CK to become abnormal—in congenital myopathies, CK remains normal throughout the disease course 2, 5

• Hypotonia with preserved reflexes rules out cerebral palsy, which would show increased tone and hyperreflexia due to upper motor neuron dysfunction 2, 4