Motor delay, hypotonia and the floppy infant — branching viva

Stem

A 4-month-old is brought because she “goes through your hands” when held under the arms and is not rolling. The examiner starts with definitions, then branches into red-flag phenotypes. [1]

Branch 1 — Definitions

Examiner: What is the difference between tone and strength? [1]

Strong answer: Tone is the resistance I feel when I move a relaxed limb. Strength is the active force the infant can generate against gravity or resistance. An infant can be low-tone without being weak, or weak with reduced tone — I never use the words interchangeably. [1] [2]

Examiner: How do you separate central from peripheral hypotonia at the bedside? [1]

Strong answer: Central patterns often have better power than expected for the low tone, preserved or later brisk reflexes, and may show encephalopathy, seizures or dysmorphology. Peripheral patterns show true antigravity weakness, hyporeflexia or areflexia, and sometimes tongue fasciculations. I also check for ligamentous laxity mimics. [1] [2]

Branch 2 — SMA pathway

Examiner: The infant is socially bright, areflexic, with tongue fasciculations and progressive weakness. What now? [9]

Strong answer: This is an SMA pattern until genetics say otherwise. I protect airway and feeding, arrange urgent SMN1 testing with SMN2 copy-number context, and refer same-day to a neuromuscular centre because disease-modifying therapy is time-critical. A social smile does not clear neuromuscular disease. [9] [10]

Examiner: What evidence supports urgency? [9]

Strong answer: Infantile-onset trials such as ENDEAR showed motor-milestone benefit of nusinersen versus sham; gene-replacement experience also supports early treatment. My job is recognition and rapid pathway activation, not inventing a private dose card. [9] [10]

Branch 3 — Botulism and maternal myotonia

Examiner: A previously well infant develops constipation, weak cry and descending paralysis. [16]

Strong answer: Infant botulism until proven otherwise. I treat on clinical suspicion with the public-health antitoxin pathway and supportive care; I do not wait for stool toxin results. Absence of honey does not exclude the diagnosis. [16]

Examiner: A floppy neonate’s mother has grip myotonia. [17]

Strong answer: I consider congenital myotonic dystrophy and examine the mother carefully. The neonate needs respiratory and feeding support while genetics/neuromuscular services confirm the diagnosis. [17]

Branch 4 — Disposition and counselling

Examiner: How do you counsel the family today? [5]

Strong answer: I separate “we do not yet have every molecular answer” from “we are acting on the dangerous pattern now.” I name airway/feeding protection, which tests change management this week, who owns the referral, and explicit return precautions for breathing change, feed refusal or loss of skills. I never discharge progressive weakness with reassurance alone. [5] [9]

Examiner notes

• Fail if candidate equates tone with strength, or calls progressive areflexic weakness “benign hypotonia.” [1] [2]
• Fail if botulism management waits for laboratory confirmation. [16]
• Pass marks for ordered safety → localisation → targeted urgent tests → disease-specific pathway. [5] [9]