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Difficulty: HardPediatric Neurologic Disorders

A four-month-old female infant is evaluated in the pediatric neurology clinic for progressive generalized weakness and an inability to hold her head up. She was born at term following an uncomplicated pregnancy and delivery. Over the past eight weeks, her parents have noticed a significant reduction in her spontaneous limb movements. Additionally, her feeding has become prolonged, and her cry sounds abnormally weak. On presentation, her respiratory rate is 52/min52/\text{min}, and oxygen saturation is 96%96\% on room air. Physical examination reveals an alert infant who maintains normal eye contact and social smiling. She is positioned supine with her hips abducted and externally rotated. Neurological assessment demonstrates profound global hypotonia and complete absence of deep tendon reflexes in all extremities. Gentle tactile stimulation elicits normal sensory responses. Careful observation of the oral cavity while the infant cries reveals small, rapid, undulating movements of the tongue mucosa. Which of the following is the most appropriate next step in the diagnostic workup?

  1. A
    Stool analysis for botulinum toxin spores
  2. B
    Measurement of serum thyroid-stimulating hormone
  3. Targeted molecular genetic testingAnswer
  4. D
    Serum creatine kinase measurement
  5. E
    Observation and reassessment of motor milestones in four weeks

Answer

Targeted molecular genetic testing
The correct answer is targeted molecular genetic testing. This infant's presentation of progressive profound hypotonia, areflexia, and tongue fasciculations in the setting of preserved cognitive function is highly characteristic of Spinal Muscular Atrophy (SMA) type 1 (Werdnig-Hoffmann disease). SMA is an autosomal recessive motor neuron disease caused by a mutation in the survival motor neuron 1 (SMN1) gene. The most appropriate and definitive diagnostic step is targeted molecular genetic testing for the SMN1 gene deletion, which provides a non-invasive confirmation of the disease.

Step-by-Step Solution

1
Analyze the neurological examination findings.
The infant exhibits profound global hypotonia, absent deep tendon reflexes, and tongue fasciculations, with preserved cognitive and social interactions.
Identifying the specific pattern of neurological deficits is essential for localizing the lesion in a 'floppy infant'.
2
Localize the neuroanatomical lesion.
The combination of severe hypotonia and areflexia without cognitive impairment points to a peripheral motor unit disorder. The presence of tongue fasciculations specifically localizes the defect to the anterior horn cells (lower motor neurons).
Different neuromuscular disorders affect different parts of the motor unit (e.g., anterior horn cells, peripheral nerves, neuromuscular junction, or muscle).
3
Formulate a primary diagnosis.
The clinical picture of progressive lower motor neuron degeneration in early infancy with sparing of extraocular muscles and cognition is pathognomonic for Spinal Muscular Atrophy (SMA) type 1 (Werdnig-Hoffmann disease).
Matching the localized lesion and clinical timeline to a specific disease entity.
4
Determine the most appropriate initial diagnostic test.
Targeted molecular genetic testing for the SMN1 gene deletion is the gold standard, non-invasive method to confirm SMA.
Genetic testing has replaced muscle biopsy and electromyography as the first-line diagnostic step for suspected SMA due to its high sensitivity and specificity.

Key Concept

Spinal Muscular Atrophy (SMA) type 1 presents in early infancy with severe, progressive lower motor neuron signs (hypotonia, areflexia, tongue fasciculations) and paradoxical breathing, while cognitive and social development remain intact. The definitive diagnostic test is targeted molecular genetic testing for the SMN1 gene deletion.
Estimated Time:1m 15s
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Pediatric Neurologic Disorders

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