Paediatrics: The floppy infant

2021-03-09 12:00 AM

It is often helpful to divide causes into those that are ‘central’ involving the CNS (so-called ‘floppy strong’) and ‘peripheral’ involving lower motor neurons, neuromuscular junction (NMJ), or primary muscle disease (‘floppy weak’).

The floppy infant

Range of clinical features

  • Common to ‘central’ and ‘peripheral’ diseases: generalized hypotonia;‘frog-leg’ posture; respiratory failure; obstetric problems (e.g. polyhydramnios due to impaired swallowing, breech presentation); HIE.
  • Central conditions: encephalopathy; dysmorphism; reasonable musclestrength; ‘rise’or normal tendon reflexes.
  • Peripheral causes: normal conscious level; muscle signs (weakness,myotonia, fasciculations, or fatiguing); ‘fall’ or normal tendon reflexes; little facial expression; micrognathia; high arched palate; ptosis; undescended testes; limb contracture/deformities (severe is arthrogryposis multiplex congenital); hip dislocation.


  • Exclude severe systemic illness: e.g. sepsis that requires prompttreatment.
  • Treat any respiratory failure with O2or ventilatory support as required.
  • Examine for above clinical features to help distinguish cause. Examineboth parents for possible disease, e.g. maternal myasthenia gravis or myotonic dystrophy (possibly undiagnosed!).
  • Elicit family history (e.g. maternal myotonic dystrophy); antenatal history(e.g. polyhydramnios).
  • ‘Central’ cause: consider—blood glucose; U&E; Ca2+; Mg2+; septicscreen; ESR/CRP; TFT; karyotype; cranial ultrasound; CT/MRI; EEG; IEM screen; maternal drug screen; genetics opinion if dysmorphic.
  • ‘Peripheral’ cause: consider—serum creatinine phosphokinase; specificcytogenetics (e.g. myotonic dystrophy); electromyogram (EMG), nerve conduction studies; muscle or sural nerve biopsy; muscle ultrasound; edrophonium 20micrograms/kg test dose l followed 30s later (if
  • no adverse reaction) with 80micrograms/kg IV (causes dramatic improvement in some forms of myasthenia gravis); echocardiogram (storage diseases).
  • Spinal cord damage (rare): consider in the infant who has flaccid paralysis from birth. Associated with rotational forceps delivery. Immobilize neck. Seek specialist advice. MRI.
  • Refer to paediatric neurologist.


Causation-dependent and very variable. Some causes are fatal, e.g. type 1 SMA.