Neuromuscular: Anterior Horn & Peripheral Nerve
A 30-slide clinical learning module · The floppy infant, SMA + disease-modifying therapy, and childhood neuropathies
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Module Outline
🧭 Part 1
🧬 Part 2
💉 Part 3
⚡ Part 4
📋 Part 5
Slide 1/30
🧭 Part 1: Localizing the Weak Child
The Hypotonic Infant: Where Is the Problem?
Central vs peripheral — the bedside split that drives the entire workup
- 1The first question in every floppy infant is anatomic: is the weakness coming from ABOVE the spinal cord (central) or from the motor unit (peripheral)? This split changes every subsequent test you order.
- 2Central hypotonia clues: altered mental status or encephalopathy, dysmorphic features, brain MRI abnormality, hyperreflexia or clonus, seizures, brisk response to stimulation despite reduced tone. Examples: Prader-Willi, chromosomal aneuploidy, HIE, structural brain malformation.
- 3Peripheral hypotonia clues: ALERT infant who cannot move, areflexia, tongue fasciculations (SMA), paradoxical breathing (diaphragm sparing in SMA type 1), ptosis or ophthalmoplegia (MTM1, CMS), normal MRI. The alert/weak dissociation is the single most important clue.
- 4Within peripheral hypotonia, localize to: anterior horn (SMA — tongue fasciculations, absent reflexes, paradoxical breathing), nerve (CMT, hereditary neuropathy), NMJ (congenital myasthenic syndrome — fatigability), or muscle (congenital myopathy, CMD, dystrophinopathy — normal or elevated CK).
- 5Action: in any floppy newborn without a clear central cause, order SMN1 deletion testing on day 1. SMA type 0/1 is the most common genetic cause of infant death from weakness and is immediately treatable — do not wait for biopsy.
💡
Clinical Pearl
The alert/weak dissociation is pathognomonic for peripheral hypotonia. A baby who tracks your face with bright eyes but cannot lift a limb or cry effectively has motor unit disease until proven otherwise. Order SMN1 testing the same day.
DMT in SMA lecture deck; Wang et al., J Child Neurol 2007 (floppy infant evaluation consensus)