Diagnosis: Spinal Muscular Atrophy (SMA) Type 1
The most likely diagnosis is Spinal Muscular Atrophy (SMA) Type 1 (Answer B), based on the pathognomonic combination of decreased reflexes with hypotonia in early infancy, which specifically indicates lower motor neuron disease. 1
Key Diagnostic Features Supporting SMA
The clinical presentation is classic for SMA Type 1:
- Decreased reflexes with hypotonia at 6 weeks of age 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, 2
Why Other Diagnoses Are Excluded
Congenital Myopathy (Option A) is ruled out:
- Congenital myopathies characteristically present with hypotonia with preserved or normal to slightly decreased reflexes, not the markedly decreased reflexes seen in this patient 1, 2
- The presence of decreased reflexes excludes congenital myopathy, which shows preserved reflexes as a distinguishing feature from neurogenic causes 2, 3
Duchenne Muscular Dystrophy (DMD, Option C) is excluded:
- DMD does not present at 6 weeks of age; symptoms typically appear between 2-5 years 1
- Normal CK rules out DMD, which shows markedly elevated CK levels 1, 2
"Muscular Atrophy" (Option D) is non-specific:
- This is essentially another term for SMA, making Option B the more precise answer
Clinical Context of SMA Type 1
Disease characteristics at this age:
- SMA Type 1 presents in the first 6 months of life with profound weakness, hypotonia, and absent or markedly diminished reflexes 1, 4
- The expressive gaze and normal cognitive development contrast with the paralytic attitude 4
- Diminished or absent deep tendon reflexes occur with lower motor neuron disorders, distinguishing them from upper motor neuron dysfunction which shows increased reflexes 1
Immediate Management Required
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 (onasemnogene abeparvovec, nusinersen, risdiplam) are most effective when initiated early in the disease course 1, 5
- Treatment initiated at ≤6 weeks of age can significantly ameliorate outcomes, with all early-treated patients in recent cohorts achieving independent sitting 5
Common Pitfall to Avoid
Absence of fasciculations on examination supports but does not exclude SMA, as tongue fasciculations may be subtle or missed in young infants 1. Do not wait for fasciculations to make the diagnosis—the combination of hypotonia and decreased reflexes at this age is sufficient to warrant urgent genetic testing.