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Difficulty: MediumPediatric Neurologic Disorders

A 5-month-old male infant is brought to the pediatric neurology clinic by his parents due to concerns about his motor development. The parents report that he "feels like a rag doll" when lifted and has never rolled over or been able to support his head. He was born at 39 weeks gestation via uncomplicated vaginal delivery and has been exclusively breastfed. On physical examination, the infant appears alert and tracks the examiner's face with normal extraocular movements. He exhibits severe generalized hypotonia and assumes a "frog-leg" posture when placed in the supine position. Deep tendon reflexes are absent bilaterally in both the upper and lower extremities. Examination of the oral cavity reveals fine, rapid, continuous twitching movements of the resting tongue. Which of the following is the most appropriate next step in the evaluation of this patient?

  1. Targeted molecular genetic testingAnswer
  2. B
    Analysis of stool for botulinum toxin and spores
  3. C
    Magnetic resonance imaging of the brain
  4. D
    Measurement of serum thyroid-stimulating hormone
  5. E
    Reassurance and referral to physical therapy

Answer

Targeted molecular genetic testing
The correct answer is targeted molecular genetic testing. This infant's presentation of severe generalized hypotonia, absent deep tendon reflexes, and pathognomonic tongue fasciculations in the setting of normal alertness and extraocular movements is highly characteristic of Spinal Muscular Atrophy (SMA) type 1 (Werdnig-Hoffmann disease). SMA is a progressive lower motor neuron disease caused by mutations in the SMN1 gene. The diagnosis is confirmed directly by molecular genetic testing showing a homozygous deletion or mutation of this gene, which has largely replaced electromyography (EMG) and muscle biopsy as the initial diagnostic step.

Step-by-Step Solution

1
Analyze the patient's age, milestone delays, and physical examination findings.
A 5-month-old infant presenting with severe generalized hypotonia, absent deep tendon reflexes, and fine tongue movements (fasciculations), alongside normal alertness and normal extraocular movements.
Accurate identification of the pattern of weakness is necessary to localize the lesion within the nervous system.
2
Differentiate between central and peripheral causes of infantile hypotonia based on the physical exam.
The presence of absent reflexes and tongue fasciculations localizes the lesion to the lower motor neurons (peripheral nervous system).
Central hypotonia (e.g., hypoxic-ischemic encephalopathy) typically involves altered mental status and eventual hyperreflexia, whereas peripheral hypotonia features profound areflexia and fasciculations.
3
Determine the most likely diagnosis and the corresponding, least invasive diagnostic test.
The pathognomonic presentation strongly suggests Spinal Muscular Atrophy (SMA) type 1, which is definitively confirmed by targeted molecular testing for the SMN1 gene.
SMA is an autosomal recessive lower motor neuron disorder caused by a deletion or mutation in the survival motor neuron 1 (SMN1) gene. Genetic testing is now the standard initial diagnostic step.

Key Concept

Spinal Muscular Atrophy (SMA) Type 1 presents with profound lower motor neuron signs, including generalized hypotonia, areflexia, and pathognomonic tongue fasciculations, with spared extraocular movements and normal cognition. It is diagnosed via SMN1 genetic testing.
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Topics

Pediatric Neurologic Disorders

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