Paediatrics chapters
The floppy infant
How to approach a baby with low tone: separating central (non-paralytic) from neuromuscular (paralytic) hypotonia, the common causes including SMA, and the tests that sort them out.
- A floppy infant has reduced tone; the key questions are whether the baby is weak and whether tendon reflexes are present.
- Non-paralytic (central) hypotonia: tone low but strength fair, reflexes present; commonest cause overall is perinatal asphyxia; also Down and Prader-Willi syndromes, hypothyroidism.
- Paralytic (neuromuscular) hypotonia: weak, reflexes reduced or absent; commonest is spinal muscular atrophy (tongue fasciculations, alert face); evaluate with CK, EMG/NCV, genetic tests and muscle biopsy.
On this page
Overview
A floppy infant is a baby with hypotonia: reduced resistance of the limbs to passive movement and poor posture against gravity. Hypotonia is a sign, not a diagnosis. It can come from the brain (central), the spinal cord, or any part of the lower motor unit: anterior horn cell, nerve, neuromuscular junction or muscle. In infants the usual upper and lower motor neuron signs are blurred because myelination is incomplete, so the bedside approach rests on two questions: is the baby weak, and are the reflexes present?
Recognising the floppy infant
- Lies in a frog-like posture: hips abducted and externally rotated, limbs flat on the bed.
- Pull to sit: marked head lag.
- Vertical suspension (held under the arms): the baby slips through the examiner's hands at the shoulders and does not take weight on the legs.
- Ventral suspension (held prone over a hand): head and all four limbs hang down, forming an inverted U; the baby cannot lift the head.
- Weak babies may have chest deformity and a see-saw (paradoxical) breathing pattern.
- Older infants show delayed motor milestones.
"Inverted U and frog legs" = floppy baby.
Paralytic vs non-paralytic hypotonia
| Feature | Non-paralytic (central) | Paralytic (neuromuscular) |
|---|---|---|
| Strength | Fair; moves limbs against gravity | Weak; little movement against gravity or to a painful stimulus |
| Tendon reflexes | Present or brisk | Reduced or absent |
| Level | Brain, chromosomes, connective tissue, metabolic | Anterior horn cell, nerve, junction, muscle |
| Other clues | Seizures, low consciousness, dysmorphic features, delayed social milestones | Alert face, fasciculations, ptosis, wasting |
| Example | Perinatal asphyxia, Down syndrome | Spinal muscular atrophy |
The commonest cause of a floppy baby overall is perinatal asphyxia (hypoxic-ischaemic encephalopathy). The commonest paralytic cause is spinal muscular atrophy. Paralytic causes are mostly genetic.
A baby with cerebral palsy may be floppy in early infancy and only later become spastic. Hypotonia with brisk reflexes is still a central problem.
Causes
Non-paralytic (central) hypotonia
- Brain injury: birth asphyxia, intracranial haemorrhage, hypotonic cerebral palsy, infection, structural brain malformations.
- Chromosomal and genetic: Down syndrome, Prader-Willi syndrome (these two are the commonest genetic causes).
- Metabolic and endocrine: hypothyroidism, hypercalcaemia, hypoglycaemia, kernicterus, inborn errors of metabolism.
- Drugs given to the mother before delivery: benzodiazepines (diazepam), magnesium sulphate, opioids, barbiturates, general anaesthesia.
- Connective tissue disorders: Ehlers-Danlos, Marfan, familial ligament laxity (strength and milestones otherwise normal).
- Benign congenital hypotonia: a diagnosis of exclusion.
Paralytic (neuromuscular) hypotonia
| Level | Causes |
|---|---|
| Spinal cord | Birth injury (cervical cord), tumour, malformation |
| Anterior horn cell | Spinal muscular atrophy, poliomyelitis |
| Peripheral nerve | Congenital neuropathies (rare) |
| Neuromuscular junction | Transient neonatal myasthenia, congenital myasthenia, infant botulism |
| Muscle | Congenital myopathies, congenital myotonic dystrophy, congenital muscular dystrophies, metabolic myopathies (Pompe disease, lipid storage) |
Key conditions
Spinal muscular atrophy (SMA)
- Progressive degeneration of anterior horn cells; upper motor neurons, heart and intelligence are normal.
- Autosomal recessive; SMN gene on chromosome 5; one of the commonest recessive diseases (Nelson gives a carrier frequency of about 1 in 50).
- Starts in fetal life or later; the earlier the onset, the worse the course.
| Type | Name | Onset and course |
|---|---|---|
| Type 1 | Werdnig-Hoffmann disease (severe infantile) | Birth to first few weeks or months; most die of respiratory failure within the first 2 years (Akbar: within about 18 months) |
| Type 2 | Intermediate | Presents between about 3 and 12 months; slower course; marked wasting, contractures, scoliosis |
| Type 3 | Kugelberg-Welander (juvenile) | Normal until about 2 years or later; girdle weakness, waddling gait, slow loss of walking; survives into adult life |
Werdnig-Hoffmann features:
- Severe hypotonia and generalised weakness, thin muscles, absent tendon reflexes.
- Tongue fasciculations (best seen when the baby is asleep).
- Alert, bright expression, normal social and language development; extraocular muscles and sphincters spared.
- Often reduced fetal movements.
- Breathing becomes rapid, shallow and abdominal (intercostal weakness); death from atelectasis and pneumonia.
Tests: SMN genetic test confirms; CK normal or slightly raised; EMG shows fibrillations and fasciculations (denervation); nerve conduction normal; biopsy shows grouped atrophy. Care is supportive: chest care, nutrition, preventing contractures and scoliosis.
Floppy + areflexic + tongue fasciculations + alert face = SMA type 1.
Prader-Willi syndrome
- Severe neonatal hypotonia and feeding difficulty with poor weight gain in infancy.
- Small hands and feet; in boys small genitalia and undescended testes.
- Later hyperphagia and obesity.
- Most have a deletion on the paternal chromosome 15 (15q11-q13).
Benign congenital hypotonia
- Non-progressive, cause unknown; diagnosis of exclusion.
- Floppy from early life with delayed sitting and crawling, but normal strength, brisk kicking, normal reflexes, good speech and social skills.
- Improves with age; most catch up.
Infant botulism
- Spores (dust, soil, honey) germinate in the infant's gut and make a toxin that blocks acetylcholine release.
- Constipation and poor feeding, then hypotonia, weak cry, ptosis, poor gag, sluggish pupils; may need ventilation.
- Diagnosis: organism or toxin in stool; treat with botulism immunoglobulin early.
Congenital myotonic dystrophy
- Baby of a mother with myotonic dystrophy; immobile, hypotonic, ptosis, no suck, breathing difficulty, often clubfoot or contractures and reduced fetal movements.
- Examine the mother: she cannot release her grip quickly after a handshake.
Diagnostic evaluation
Start with history (pregnancy and fetal movements, delivery and asphyxia, maternal drugs, family history, honey) and examination (strength, reflexes, fasciculations, dysmorphism, the mother).
| Test | Use |
|---|---|
| Serum CK | High in muscular dystrophies; normal or mildly raised in SMA and central causes |
| Thyroid function, calcium, glucose | Hypothyroidism, hypercalcaemia, hypoglycaemia |
| Karyotype / genetic tests | Down syndrome, Prader-Willi syndrome, SMN gene, myotonic dystrophy |
| Urine and metabolic screen | Inborn errors of metabolism |
| CT or MRI brain | Hypoxic damage (atrophy, cysts), malformations |
| Nerve conduction | Slowed in neuropathies; normal in SMA |
| EMG | Separates denervation (SMA) from myopathy |
| Muscle biopsy | Most important specific test in many neuromuscular disorders; shows neurogenic or myopathic change and enzyme defects |
| Edrophonium or neostigmine test | Suspected myasthenia |
| Stool for C. botulinum toxin | Suspected infant botulism |
In SMA the CK is near normal, while in muscular dystrophy it is very high. Both cause weakness, so the CK helps sort them.
Management principles
- Treat the cause where possible (thyroxine for hypothyroidism, botulism immunoglobulin, cholinesterase inhibitors for myasthenia).
- Feeding support (nasogastric tube), airway and chest care, physiotherapy, prevention of contractures and scoliosis.
- Genetic counselling for recessive and dominant conditions.
Expect: definition of hypotonia and bedside signs (frog posture, head lag, inverted U on ventral suspension); paralytic vs non-paralytic hypotonia judged by strength and reflexes; perinatal asphyxia as the commonest cause and SMA as the commonest paralytic cause; Down and Prader-Willi as common genetic causes; Werdnig-Hoffmann features (tongue fasciculations, areflexia, alert face, normal intelligence, AR on chromosome 5); benign congenital hypotonia as a diagnosis of exclusion; infant botulism with constipation and honey; and the work-up of CK, thyroid tests, EMG, nerve conduction, genetic tests and muscle biopsy.
Quick revision
- Floppy = low tone; ask: is the baby weak? are reflexes present?
- Central: strength fair, reflexes present; asphyxia, Down, Prader-Willi, hypothyroidism, maternal drugs.
- Neuromuscular: weak, reflexes absent; SMA, myasthenia, botulism, myopathies.
- Commonest overall: perinatal asphyxia; commonest paralytic: SMA.
- SMA 1: tongue fasciculations, areflexia, alert, normal IQ, chromosome 5, AR.
- Benign congenital hypotonia improves; exclusion diagnosis.
- CK high in dystrophies, near normal in SMA.
- EMG splits nerve from muscle; biopsy and genetics confirm.
Written by The House Job from standard references (Basis of Paediatrics by Pervez Akbar; Nelson Essentials of Paediatrics).
FAQ
What is the single most useful bedside sign to separate central from neuromuscular hypotonia?
The tendon reflexes, judged together with strength. A weak baby with absent reflexes has a neuromuscular cause; a fairly strong baby with brisk reflexes has a central one.
Why does a baby with Werdnig-Hoffmann disease look alert?
The disease destroys anterior horn cells only; the brain and intelligence are normal, and eye movements are spared, so the face looks bright despite profound weakness.
Is every floppy newborn neurologically abnormal?
No. Benign congenital hypotonia and connective tissue laxity cause floppiness with normal strength, and these babies usually catch up.
Which simple history point suggests infant botulism?
Constipation and poor feeding before the weakness, especially if the baby was given honey.