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]

SAQ 2 — Botulism branch and maternal myotonia (10 marks)

Scenario A: A previously well 5-month-old develops constipation, weak cry, poor feeding and descending weakness over days.
Scenario B: A floppy neonate needs respiratory support; the mother has grip myotonia and undiagnosed neuromuscular symptoms. [15] [16] [17]

Questions

1. For Scenario A, state the working diagnosis pattern and the key management principle regarding laboratory confirmation. (3 marks) [15] [16]
2. For Scenario B, name the likely diagnosis family and one critical examination step beyond the neonate. (3 marks) [17]
3. Outline shared resuscitation priorities that apply to both scenarios. (4 marks) [5] [16]

Model answer

1. Scenario A (3)

• Pattern: infant botulism (neuromuscular junction; constipation, weak cry, descending paralysis).
• Principle: treat on clinical suspicion with the jurisdictional antitoxin/support pathway; do not wait for stool toxin confirmation before acting. Honey history helps when present but is not required. [15] [16]

2. Scenario B (3)

• Likely congenital myotonic dystrophy (maternal myotonic dystrophy type 1 phenotype revealed by a floppy neonate).
• Critical step: examine the mother (grip myotonia, facial features, history of cataracts/neuromuscular symptoms) and involve genetics/neuromuscular services. [17]

3. Shared resuscitation (4)

• Airway positioning, secretion management, ventilatory support as needed.
• Protect feeding: stop unsafe oral trials; nasogastric/IV support.
• Escalate to NICU/PICU early when cough is weak or fatigue rises.
• Parallel work-up without delaying stabilisation; clear family communication that action follows the dangerous pattern, not only the final gene name. [5] [16]