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PE27.11 | Floppy Infant — SDL Guide (Part 2)
Management and Specific Therapies
Management of the floppy infant is organised around three sequentially applied but overlapping principles: immediate stabilisation and supportive care (especially respiratory and feeding support, which are frequently compromised in both central and peripheral hypotonia), specific treatment of the underlying cause where a disease-modifying therapy exists, and long-term multidisciplinary rehabilitation to maximise functional outcomes for the ongoing deficits that persist after the acute phase. The urgency and nature of management depends critically on the identified cause. Acute central causes — HIE, metabolic crises, neonatal sepsis — demand immediate resuscitation and cause-directed critical care. Chronic peripheral causes — SMA, congenital myopathies — require urgent respiratory assessment and, for SMA, prompt initiation of disease-modifying therapy because motor neurons continue to be lost with each passing day. In all cases, a multidisciplinary team must be assembled promptly once the aetiology is established, and family counselling including genetic guidance must be integrated into the management plan from the outset.
1. Immediate and general supportive care:
• Respiratory support: Many infants with peripheral hypotonia (particularly SMA Type 1 and congenital myopathies) have intercostal muscle weakness but relatively spared diaphragm — this produces a characteristic paradoxical breathing pattern (chest wall draws in on inspiration). Respiratory assessment (pulse oximetry, transcutaneous CO2, polysomnography in older infants) guides the need for non-invasive ventilation (NIV/CPAP). BiPAP is commonly required in SMA Type 1.
• Feeding support: Bulbar weakness causes poor suck, aspiration risk. Nasogastric (NG) tube feeding is started early; gastrostomy is considered for long-term nutritional support in severe cases.
• Positioning: Anti-gravity positioning devices prevent aspiration, contractures, and scoliosis.
2. Specific treatments:
SMA (Spinal Muscular Atrophy):
Three approved therapies work by increasing functional SMN protein:
• Nusinersen (Spinraza): Antisense oligonucleotide delivered intrathecally every 4 months (after loading doses); modifies SMN2 splicing to produce more full-length SMN protein. Approved for all SMA types; best outcomes when started pre-symptomatically or in early symptomatic phase.
• Risdiplam (Evrysdi): Oral SMN2 splicing modifier; once-daily liquid formulation; suitable for all ages.
• Onasemnogene abeparvovec (Zolgensma): Single intravenous infusion of AAV9 vector carrying SMN1 gene; approved for infants under 2 years or up to 13.5 kg. The most definitive (gene replacement) approach but currently extremely costly.
Congenital hypothyroidism: Oral thyroxine (10–15 mcg/kg/day) — started immediately on diagnosis; normal intellectual development achievable if started within 2 weeks of birth.
Neonatal myasthenia gravis (transient): Supportive feeding and respiratory management; pyridostigmine (acetylcholinesterase inhibitor) for symptomatic relief; resolves within 2–8 weeks as maternal antibodies are cleared.
Down syndrome: No disease-modifying therapy; management focuses on early developmental intervention, comorbidity screening (congenital heart disease, hypothyroidism, atlantoaxial instability, hearing, vision, sleep apnoea).
3. Long-term rehabilitation (MDT):
• Physiotherapy: Positioning, stretching, respiratory physiotherapy, assisted mobility.
• Occupational therapy: Adaptive equipment, communication devices.
• Speech and language therapy: Feeding management (texture modification, positioning), AAC.
• Neurologist/neuromuscular specialist: Monitoring disease progression, managing respiratory failure.
• Dietitian: Nutritional management — hypotonic infants often have high caloric needs and poor intake.
• Genetic counselling: SMA is autosomal recessive (25% recurrence risk); carrier testing of parents; prenatal diagnosis options.
CLINICAL PEARL
The single most important bedside clinical distinction in the floppy infant is the status of deep tendon reflexes combined with the infant's level of consciousness. An ALERT infant with ABSENT deep tendon reflexes and TONGUE FASCICULATIONS = PERIPHERAL hypotonia (anterior horn cell = SMA) — this is a medical urgency in the modern era because early nusinersen/risdiplam therapy significantly improves prognosis. An encephalopathic (lethargic, seizure, reduced consciousness) infant with PRESERVED or BRISK reflexes = CENTRAL hypotonia (brain lesion — HIE, chromosomal, metabolic). The rule is simple but life-changing: ABSENT REFLEXES + ALERT INFANT = peripheral cause — check CK and request SMN1 gene test immediately. A normal serum CK in the context of peripheral hypotonia with fasciculations strongly points to SMA (not a muscle disease).
SELF-CHECK
A 3-month-old infant with suspected SMA Type 1 has confirmed homozygous deletion of SMN1 exon 7 on MLPA testing. She has paradoxical breathing and SpO2 falling to 88% during sleep. Which management priority is MOST immediately important?
A. Start intrathecal nusinersen immediately — gene therapy is the only management needed
B. Establish respiratory support (BiPAP/NIV) urgently — respiratory failure is the immediate life threat, alongside initiating SMA-specific therapy
C. Perform muscle biopsy to confirm the diagnosis before starting any treatment
D. Observe on room air — SpO2 of 88% is acceptable in a sleeping infant with SMA
Reveal Answer
Answer: B. Establish respiratory support (BiPAP/NIV) urgently — respiratory failure is the immediate life threat, alongside initiating SMA-specific therapy
In SMA Type 1, respiratory muscle weakness (intercostal > diaphragm) causes progressive respiratory failure, which is the primary cause of mortality — typically before age 2 years without intervention. SpO2 of 88% during sleep indicates nocturnal hypoventilation requiring immediate BiPAP/NIV support. Simultaneously, SMA-specific therapy (nusinersen or risdiplam) should be initiated urgently — combined respiratory support + disease-modifying therapy gives the best outcomes. Muscle biopsy is NOT required when SMN1 deletion is confirmed by genetic testing. Observing a SpO2 of 88% in an infant with SMA is clinically unsafe.
Self-Assessment: Floppy Infant
Use these case vignettes to test your ability to apply the central-versus-peripheral hypotonia framework and to direct appropriate investigations and management in a stepwise clinical manner. The floppy infant is one of the most commonly examined paediatric neurology presentations in final-year examinations and postgraduate entrance assessments — practise your reasoning rigorously from clinical features to anatomical level to specific aetiology. Work through each scenario independently before consulting the key concept list. The goal is to ensure that you can reach a justified clinical conclusion from bedside examination alone, even before laboratory results are available, which is the essential skill in resource-limited or emergency settings.
Case vignette 1: A 2-week-old male infant is noted to have severe hypotonia since birth. He is alert and responsive to voices, but has a very weak cry and suck, frog-leg posture, and paradoxical breathing. His tongue shows fine fasciculations. Deep tendon reflexes are absent throughout. Serum CK is 85 U/L. His 3-year-old sister is healthy.
1. Is this central or peripheral hypotonia? Which level of the neuraxis is affected?
2. What is the most likely diagnosis and its genetic basis?
3. What single investigation confirms the diagnosis?
4. What immediate management is required?
Case vignette 2: A 2-day-old female infant is born at term after uneventful pregnancy. She is lethargic, has seizures on day 1, and is hypotonic with brisk deep tendon reflexes. Her older sibling has Down syndrome.
1. Is this central or peripheral hypotonia?
2. List two investigations you would request.
3. What treatable cause must be excluded urgently?
Key concepts checklist:
• Bedside manoeuvres: traction (head lag), ventral suspension (rag-doll drape), vertical suspension (slip-through), scarf sign.
• Central hypotonia: lethargic/encephalopathic + preserved/brisk reflexes + no fasciculations; causes: HIE, Down syndrome, Prader-Willi, metabolic.
• Peripheral hypotonia: alert + absent DTRs + ± fasciculations + ± elevated CK; causes: SMA (anterior horn, SMN1 deletion), neuropathy, NMJ (myasthenia), muscle (myopathy/dystrophy).
• SMA Type 1: most severe; onset <6 months; never sit; SMN1 deletion; normal CK; tongue fasciculations; AR.
• Investigations: CK (muscle), EMG/NCS (neuropathic vs myopathic), SMN1 MLPA (SMA), MRI brain (central), karyotype/microarray (chromosomal), TFTs (hypothyroidism).
• Disease-modifying SMA therapies: nusinersen (intrathecal), risdiplam (oral), onasemnogene (IV gene therapy) — early treatment is critical.
• Treatable causes must never be missed: hypothyroidism, neonatal myasthenia, SMA (now treatable), metabolic disorders.