60
The hypotonic infant

The ‘hypotonic infant’ describes marked hypotonia or floppiness, i.e. less resistance to passive movement than normal, and is usually accompanied by muscle weakness (Fig. 60.1a, b and c). The cause of the hypotonia is either:

• central – central nervous system, or
• peripheral – lower motor neuron, neuromuscular junction or muscle disorders.

Fig. 60.1 (a) When held upright, the hypotonic infant slides through one’s hands. (b) When held prone, the infant flops like a rag doll. (c) On traction of the arms, there is marked head lag.

Transient hypotonia may result from systemic infection, electrolyte disorders, hypermagnesemia, seizures or drugs administered to the infant or mother. Preterm infants have reduced tone and strength compared to term infants. These circumstances are not considered in this chapter.

Clues from the history

• Family history – consanguinity, unexplained deaths, multiple miscarriages.
• May be increasing severity with succeeding generations, e.g. muscular dystrophy.
• Clinical features in mother – ptosis in myasthenia gravis, absence of facial expression and weak grip in myotonic dystrophy and family history of cataracts.
• Pregnancy – polyhydramnios and reduced fetal movements.

Causes and clinical features (Table 60.1)

Table 60.1 Causes and clinical features of central and peripheral hypotonia.

	Central hypotonia	Peripheral hypotonia
Causes	Cerebral malformation Encephalopathy: Hypoxic–ischemic encephalopathy Meningitis/encephalitis Hypoglycemia. Chromosomal/syndromes: Trisomy 21 (Down syndrome) Prader–Willi syndrome Metabolic: Hypothyroidism Inborn errors of metabolism, e.g. hyperammonemia, amino acidopathy	Spinal cord injuryAnterior horn cell: Spinal muscular atrophy (Werdnig–Hoffmann syndrome) Neuromuscular junction: Neonatal myasthenia gravis Muscles: Congenital myopathies Myotonic dystrophy
Clinical features	Antigravity movements present Normal or brisk tendon reflexes Features of brain dysfunction may be present	Weak or absent antigravity movements from severe muscle weakness Reduced or normal tendon reflexes Other features – see Fig. 60.2

Fig. 60.2 Clinical features that may be present with a peripheral neuromuscular disorder.

Investigations

May include:

• karyotype/DNA analysis
• gene tests for specific disorders
• imaging of brain/spinal cord – MRI or ultrasound
• blood glucose, calcium, magnesium and lactate
• acid–base status, urine and plasma amino acids, urine organic acids, plasma ammonia, lactate, acylcarnitines, CSF lactate
• CPK (creatine phosphokinase) – raised in muscular dystrophy
• thyroid function tests
• congenital infection screening tests
• EMG (electromyogram)
• nerve conduction studies
• muscle biopsy.

Some specific conditions

Central

Hypoxic–ischemic encephalopathy

Hypotonia may be replaced by spasticity when older.

Prader–Willi syndrome

• 70% have partial chromosomal (15q) deletion (imprinting, where the deletion occurs on the active paternal chromosome 15 and the maternal copy is inactive). Also uniparental disomy (two maternal copies of the 15q region).
• Characteristic facies with narrow forehead, almond-shaped eyes and triangular mouth (Fig. 60.3).
• Hypotonia.
• Hypogonadism/cryptorchidism.
• Obesity after the neonatal period.
• Behavior problems, developmental delay.

Fig. 60.3 Prader–Willi syndrome. Characteristic facies and hypogonadism. The nasogastric tube is required because of poor feeding.

(Courtesy of Dr Mike Coren.)

Peripheral (rare)

Spinal muscular atrophy type 1 (Werdnig–Hoffmann syndrome)

• Autosomal recessive – anterior horn cell degeneration.
• Pregnancy – decreased or loss of fetal movements.
• At birth – arthrogryposis (contractures) may be present.
• Characteristic feature – fasciculation of tongue.
• Severe, progressive disorder, with death from respiratory failure during first year of life.
• DNA test available.

Neonatal myasthenia gravis

• Affects 10–20% of infants of mothers with myasthenia gravis.
• Transient condition, from maternal anti-acetylcholine IgG antibodies.
• Presentation – generalized weakness, facial diplegia, rarely ptosis, weak suck and cry, tendon reflexes normal.
• Use neostigmine, not tensilon, to confirm diagnosis.

Myotonic dystrophy

• Autosomal dominant – inherited from the mother (trinucleotide repeat expansion mutations). Earlier and more severe presentation in successive generations (anticipation).
• Pregnancy – polyhydramnios and decreased fetal movement.
• Neonate – weakness, edema and petechiae at birth with or without arthrogryposis.
• Facial diplegia, ptosis, tent-shaped mouth, talipes equinovarus.
• Brain abnormalities present in some forms of muscular dystrophies.
• Feeding difficulties due to poor gut motility.
• CPK may be elevated, EMG and biopsy are diagnostic.

Congenital myopathies

• Most are recessively inherited.
• Clinical features – weak, hypotonic, areflexic.
• Abnormal swallowing, normal extra-ocular movements.
• Muscle weakness usually slowly progressive.