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

Diagnosis: Spinal Muscular Atrophy (SMA) Type I

The most likely diagnosis is B - Spinal Muscular Atrophy (SMA), specifically Type I, based on the pathognomonic combination of decreased reflexes with hypotonia in early infancy, which indicates lower motor neuron disease. 1

Key Diagnostic Features Supporting SMA

The clinical presentation is classic for SMA Type I:

• Decreased reflexes with hypotonia at 6 weeks is pathognomonic for lower motor neuron disease, specifically SMA 1
• Weak anti-gravity movements at 6 weeks with decreased reflexes represents the hallmark presentation of early-onset SMA 1
• Normal eye fixation is characteristic of SMA, as extraocular muscles are typically spared 1
• Normal CK essentially excludes muscular dystrophies, which characteristically show CK elevations often >1000 U/L 1

Why Other Diagnoses Are Excluded

Congenital Myopathy (Option A) is ruled out:

• Congenital myopathies characteristically present with hypotonia with preserved or normal reflexes, not decreased reflexes 1, 2
• The markedly decreased reflexes in this patient are incompatible with congenital myopathy 1
• Hypotonia with preserved reflexes is the classic distinguishing feature of congenital myopathy versus SMA 2

Duchenne Muscular Dystrophy (Option C) is excluded:

• DMD does not present at 6 weeks of age; onset is typically 2-5 years 1
• Normal CK rules out DMD, which shows markedly elevated CK levels 1

"Muscular Atrophy" (Option D) is non-specific:

• This is essentially the same as SMA, making option B the more precise answer

Clinical Context of SMA Type I

Age and severity correlation:

• SMA Type I presents in the first 6 months of life with profound weakness, hypotonia, and absent or markedly diminished reflexes 1, 3
• Type IB specifically has onset before 3 months, which implies no head control 3
• The 6-week presentation places this patient in the Type IB category 3

Additional supportive features:

• Absence of fasciculations on examination supports but does not exclude SMA, as tongue fasciculations may be subtle or missed in young infants 1
• Diminished or absent deep tendon reflexes distinguish lower motor neuron disorders from upper motor neuron dysfunction, which shows increased reflexes 1

Critical Next Steps

Immediate diagnostic confirmation:

• Urgent genetic testing for SMN1 gene deletion is the definitive diagnostic test and should be ordered immediately 1
• Early diagnosis is critical as disease-modifying therapies (nusinersen, onasemnogene abeparvovec, risdiplam) are most effective when initiated early in the disease course 1

Common pitfall to avoid:

• Do not delay genetic testing while waiting for EMG or other confirmatory studies when clinical presentation is this classic 1
• The combination of decreased reflexes with hypotonia in a 6-week-old infant should trigger immediate SMN1 testing 1