Neonatal hypotonia and neuromuscular weakness — branching viva

Station opening

Examiner: "Define hypotonia and tell me why the first decision in a floppy neonate is whether the baby is also weak." [10]

Strong candidate (must-hit)

• Defines hypotonia as reduced resistance to passive movement (a loss of postural tone), distinct from weakness (a loss of active force); explains that the central/peripheral split turns on whether strength and reflexes are preserved — a central lesion lowers tone but keeps the motor unit intact, while a peripheral (motor-unit) lesion lowers tone, strength and reflexes together while sparing alertness. [10]

Weak candidate

• "Hypotonia is low muscle tone, usually from a muscle problem." [10]

Branch A — The floppy-but-strong, dysmorphic infant

Examiner: "A dysmorphic term infant with Down-syndrome features is profoundly hypotonic but has normal strength and brisk reflexes and is mildly lethargic. Central or peripheral, and what is the confirming test?" [10]

Strong

• Central hypotonia: tone is down but strength and reflexes are preserved and the infant is encephalopathic and dysmorphic, so the motor unit is intact and the problem is the brain. The confirming test is a chromosomal microarray or karyotype for the chromosomal aneuploidy, with a normal creatine kinase and electromyogram expected. [10]

Weak

• "It could be a muscle dystrophy — send a CK." [10]

Branch B — The weak, areflexic, alert infant (SMA)

Examiner: "A 3-month-old former term infant has progressive symmetric proximal weakness, tongue fasciculations and absent reflexes, and is alert and socially engaged. The CK is normal. What is the diagnosis, the genetic basis, and the disease-modifying therapy?" [1]

Strong

• Diagnoses spinal muscular atrophy type 1: biallelic SMN1 loss with severity set by SMN2 copy number (two copies → type 1); confirms with SMN1 deletion testing and SMN2 copy number; explains that nusinersen (intrathecal antisense that splices SMN2 to make functional SMN protein) and onasemnene abeparvovec (one-time IV AAV9 gene therapy) are the disease-modifying therapies, citing the ENDEAR trial (Finkel 2017, nusinersen motor and survival benefit versus sham) and the SPR1NT trial (Strauss 2022, pre-symptomatic onasemnene-treated infants sitting and walking); emphasises that therapy must start early — ideally pre-symptomatic via newborn screening — because motor neurons are lost irreversibly. [1] [2] [3] [5]

Weak

• "It is a floppy baby — reassure the family and review in clinic." [1]

Branch C — The floppy neonate of an affected mother (congenital DM1)

Examiner: "A profoundly hypotonic neonate has respiratory distress, a weak suck and facial weakness, born to a mother with mild grip myotonia and cataracts. Polyhydramnios and reduced fetal movements were noted. What is the diagnosis, the inheritance, and the two perioperative dangers?" [9]

Strong

• Diagnoses congenital myotonic dystrophy: autosomal dominant, from a maternal DMPK CTG-repeat expansion, with anticipation explaining the severe infant and the mild mother; confirms with a DMPK CTG-repeat assay on both infant and mother; names the two perioperative dangers — high anaesthetic and sedative risk (abnormal response to neuromuscular blockers and volatiles, prolonged respiratory depression) and cardiac conduction disease needing surveillance — because the toxic-RNA spliceopathy affects muscle, brain, heart and respiratory drive together. [9]

Weak

• "It looks like a metabolic problem — send an ammonia." [9]

Branch D — The respiratory-failure trap

Examiner: "A weak, areflexic neonate is being monitored on the ward. The saturations are 97% on room air. The nurse is reassured. What is your concern, and what do you monitor?" [10]

Strong

• Recognises that the saturations are a late and misleading sign in the hypoventilating weak neonate — oxygen saturations are preserved while the carbon dioxide climbs, so respiratory failure arrives unannounced; monitors the work of breathing and the capillary or transcutaneous CO2, and escalates ventilatory support (non-invasive ventilation, then intubation) early; identifies that respiratory failure is the proximate cause of death in untreated motor-unit disease. [9] [10]

Weak

• "Saturations are fine — continue ward observations." [10]

Close

Examiner: "Summarise your approach to the hypotonic neonate in one sentence." [10]

Strong

• "I make the central/peripheral split at the bedside on power, reflexes and alertness; for the peripheral cohort I send the discriminating genetic test and, if it is SMA, start nusinersen or onasemnene abeparvovec before symptoms because motor neurons are lost irreversibly; I pre-empt respiratory failure by watching the CO2 not the saturations, feed safely, and refer to neurology and genetics for the specific diagnosis, prognosis and family counselling." [1] [5] [10]