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Neuromuscular disorders and the floppy infant
In a hypotonic infant first decide whether the problem is central (brain: floppiness with relative strength, brisk reflexes, seizures and cognitive delay) or peripheral (muscle, junction or anterior horn cell: true weakness, absent reflexes and fasciculations); SMA and Duchenne are the two peripheral diseases the exam loves.
Why it matters in the exam
SMA type 1: a 3-month-old who is alert and follows with his eyes, but lies "frog-legged", with absent reflexes, tongue fasciculations and paradoxical abdominal breathing — deletion of the SMN1 gene. Disease-modifying treatments now exist: intrathecal nusinersen, oral risdiplam and gene therapy (onasemnogene abeparvovec) at a young age — so early diagnosis is critical. Another question is infant botulism: constipation, weak feeding, ptosis and descending paralysis after honey or soil exposure.
Duchenne: a boy aged 3–5 with late walking, falls, a waddling gait, the Gowers sign (climbing up himself to stand) and calf pseudohypertrophy; CK very high (tens of times normal). Diagnosis is by genetic testing of the dystrophin gene (MLPA for deletions), and a muscle biopsy is only sometimes needed. Steroids (prednisone or deflazacort) slow the decline; cardiac (cardiomyopathy) and respiratory follow-up is essential. Becker is a milder form with partial dystrophin.
Also remember: congenital myotonic dystrophy in an infant whose mother has myotonia (CTG repeats and anticipation across generations), transient neonatal myasthenia in an infant of a mother with myasthenia, and Guillain-Barré syndrome in a child with ascending weakness and areflexia after an infection. Any child who loses motor milestones needs a neurological work-up — this is not "simple developmental delay".
Key points
- Floppy infant: central (brisk reflexes) versus peripheral (weakness, absent reflexes).
- SMA: alert infant, tongue fasciculations, no reflexes; disease-modifying treatments.
- Duchenne: Gowers, enlarged calves, very high CK, genetic test.
- Steroids and cardiac and respiratory follow-up in Duchenne.
- Infant botulism: constipation, ptosis, descending paralysis.
Summary table
| Disease | Clinical clue | Test | Treatment |
|---|---|---|---|
| Central hypotonia | Floppiness with relative strength, brisk reflexes, seizures, cognitive delay | Brain MRI, genetic and metabolic tests | According to the cause |
| SMA | Alert infant, weakness and floppiness, absent reflexes, tongue fasciculations | Genetic test for SMN1 | Nusinersen, risdiplam, gene therapy |
| Duchenne | Boy aged 3–5, Gowers, enlarged calves, waddling gait | Very high CK, MLPA of the dystrophin gene | Steroids, cardiac and respiratory follow-up |
| Infant botulism | Constipation, weak feeding, ptosis, descending paralysis, dilated pupils | Toxin detection in stool | Specific immunoglobulin, respiratory support |
| Congenital myotonic dystrophy | Floppy infant of a mother with myotonia | CTG repeat testing | Supportive |
| Guillain-Barré | Ascending weakness and lost reflexes after infection | Lumbar puncture: high protein without cells | IVIG, respiratory monitoring |
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Sources
- DMD Care Considerations Working Group: Diagnosis and management of Duchenne muscular dystrophy, 2018 — Diagnosis, corticosteroids and cardiac care
- Consensus statement for standard of care in spinal muscular atrophy, 2018 (updated with disease-modifying therapies) — Diagnosis and treatment
- Nelson Textbook of Pediatrics, 22nd edition (2024) — Neuromuscular Disorders; Hypotonia
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.