Diagnosis: Spinal Muscular Atrophy (SMA) Type 1
The most likely diagnosis is Spinal Muscular Atrophy (SMA) Type 1, given the combination of decreased reflexes with hypotonia at 6 weeks of age, which is pathognomonic for lower motor neuron disease. 1
Key Diagnostic Features Supporting SMA
The clinical presentation is classic for SMA Type 1:
- Decreased or absent reflexes with hypotonia in early infancy 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 typically preserve or show normal to slightly decreased reflexes, not the markedly decreased reflexes seen in this patient 1, 2
- Hypotonia with preserved reflexes is the classic presentation of congenital myopathy, whereas SMA has decreased reflexes 1, 2
- The absence of preserved reflexes argues strongly against a primary muscle disorder 3
Duchenne Muscular Dystrophy (Option C) is excluded:
- DMD does not present at 6 weeks of age; symptoms typically begin between 2-5 years 1
- Normal CK rules out DMD, which shows markedly elevated CK levels (>1000 U/L) 1, 3
The term "Muscular Atrophy" (Option D) is non-specific and does not represent a distinct diagnosis
Clinical Context for SMA Type 1
SMA Type 1 (Werdnig-Hoffmann disease) presents before 6 months of age with:
- Profound weakness and hypotonia 1, 4
- Absent or markedly diminished reflexes 1, 4
- Normal cognitive development with expressive gaze contrasting with paralytic posture 4
- Respiratory involvement predominantly affecting intercostal muscles 4
The onset at 6 weeks places this patient in the Type IB category (onset before 3 months), which implies no head control 4
Immediate Management Steps
Urgent genetic testing for SMN1 gene deletion is the definitive diagnostic test and should be ordered immediately 1
Early diagnosis is critical because disease-modifying therapies (nusinersen, onasemnogene abeparvovec, risdiplam) are most effective when initiated early in the disease course 1
Common Pitfall to Avoid
Do not be misled by normal CK levels into considering muscular dystrophy. While mild to moderately elevated CK can occur in SMA (particularly Type III), normal CK is common in SMA Type I and does not exclude the diagnosis 5. The key distinguishing feature is the combination of decreased reflexes with hypotonia, which points definitively to lower motor neuron disease rather than primary muscle disease 1.