Motor delay, hypotonia and the floppy infant — formative SAQs

SAQ 1 — Localisation and first-line safety (10 marks)

A 3-month-old is referred because she “feels floppy.” She smiles socially. Head control is poor. Limbs hang with limited antigravity movement. Deep tendon reflexes are difficult to elicit. The tongue shows fasciculations. Work of breathing is mildly increased with feeds. [1] [9]

Questions

1. Localise the likely pattern (central versus peripheral) and give three bedside discriminators that support your answer. (3 marks) [1] [2]
2. List the immediate safety and diagnostic actions you take today, in order of priority. (4 marks) [5] [9]
3. Explain why “watch and wait for tone to improve” is unsafe here. (3 marks) [9]

Model answer

1. Localisation (3)

• Peripheral / motor-unit pattern is most likely.
• Discriminators: true weakness with antigravity failure; hyporeflexia/areflexia; tongue fasciculations (anterior horn cell clue); relative early social alertness does not exclude SMA. [1] [2] [9]

2. Immediate actions (4)

• Protect airway/feeding: assess work of breathing, cough, secretions, aspiration risk; support feeds if swallow unsafe.
• Urgent SMN1 deletion testing with SMN2 copy-number context (or confirm newborn-screen result if already done).
• Rapid neuromuscular centre referral for disease-modifying pathway consideration.
• Baseline supportive plan: physiotherapy, respiratory review, family counselling; do not delay therapy waiting for every secondary test. [5] [9]

3. Against observation alone (3)

• Infantile-onset SMA is time-critical; motor neurons are lost while observation continues.
• Disease-modifying therapy improves motor outcomes when started early (ENDEAR-class evidence for nusinersen pathways).
• Progressive areflexic weakness is never a “benign congenital hypotonia” label. [9]