PE27.11 | Floppy Infant — Summary & Reflection

KEY TAKEAWAYS

The floppy infant presents with generalised hypotonia identified by bedside manoeuvres: head lag on traction, rag-doll drape on ventral suspension, and slip-through on vertical suspension. The fundamental classification is central (brain/UMN — lethargic infant, preserved/brisk reflexes, causes: HIE, Down syndrome, Prader-Willi, metabolic) versus peripheral (LMN/NMJ/muscle — alert infant, absent reflexes, ± fasciculations, ± elevated CK, causes: SMA, neuropathy, myasthenia, congenital myopathy). SMA Type 1 (SMN1 deletion, autosomal recessive) is the most important peripheral cause — presenting with absent reflexes, tongue fasciculations, normal CK, and respiratory involvement; confirmed by SMN1 MLPA; now treatable with nusinersen/risdiplam/onasemnogene. Investigation follows the clinical level: central causes require MRI brain, karyotype, metabolic screen, thyroid function; peripheral causes require CK, EMG/NCS, SMN1 gene testing, and sometimes muscle biopsy. Management is directed at supportive care (respiratory support, feeding), specific treatment of treatable causes (thyroxine, nusinersen, pyridostigmine for myasthenia), and MDT rehabilitation.

REFLECT

Reflect on the transformative impact of early genetic diagnosis in the floppy infant: before disease-modifying therapies for SMA, SMA Type 1 was universally fatal within 2 years; now, with nusinersen and gene therapy started in the pre-symptomatic or early symptomatic phase, children are walking. Consider what this means for clinical practice: the 'alert infant with absent reflexes' is not a diagnostic curiosity but a medical urgency where an hour's delay in investigation may mean months' worth of motor neuron loss. How would you prioritise the investigation pathway in a resource-constrained district hospital setting where EMG and SMN1 testing are not available on-site? What is the value of clinical examination in such a context, and how would you ensure appropriate and timely referral?