Congenital hypotonia is a relatively common diagnosis in the newborn period. It is defined as a subjective decrease of resistance to passive range of motion in a newborn and can be due to a defect at any level of the nervous system.

Aetiology

Causes include (but are not limited to):

The first goal in diagnosing the source of neonatal hypotonia is to ascertain if it is central (upper motor neuron) or peripheral (lower motor neuron).  Central causes are the most common.  This delineation will determine the investigations most likely to yield a diagnosis.

History

• Any significant family history – affected parents or siblings, consanguinity, stillbirths, childhood deaths
• Maternal disease – diabetes, epilepsy, myotonic dystrophy (may not be recognised)
• Pregnancy and delivery history – drug or teratogen exposure
• Decreased fetal movements
• Abnormal presentation
• Polyhydramnios/ oligohydramnios
• Apgar scores
• Resuscitation requirements
• Cord gases
• History since delivery
  • Respiratory effort
  • Ability to feed
  • Level of alertness
  • Level of spontaneous activity
  • Character of cry

Physical Examination

A detailed physical examination should be performed, assessing muscle tone, any asymmetry, the infant’s strength, deep tendon reflexes (DTR), and any dysmorphic or unusual features.

⁺ At times babies with profound central hypotonia may have absent DTR, therefore absent DTR at least in the first few days of life would not rule out a central cause for the hypotonia

* Note that the presence of profound weakness as well as hypotonia suggests a disorder of the lower motor neuron. A sign of this may be a weak cry. Weakness is uncommon in central hypotonia except in the acute stages.

Arthrogryposis (the fixation of joints at birth) may be associated with neonatal hypotonia, more commonly with lower motor neuron unit or multisystem abnormalities.

Additional clues which may direct to a specific diagnosis:

• Hepatosplenomegaly – storage disorders, congenital infections
• Renal cysts, high forehead, wide fontanelles – Zellweger’s syndrome
• Hepatomegaly, retinitis pigmentosa – neonatal adrenoleukodystrophy
• Congenital cataracts, glaucoma – oculocerebrorenal (Lowe) syndrome
• Abnormal odour – metabolic disorders
• Hypopigmentation, undesceded testes – Prader Willi

Examination of the mother is also important in suspected cases of congenital myotonic dystrophy or myasthenia gravis.

Most studies have found that central causes account for 60-80% of cases and that the diagnosis can usually be made by a careful history and examination. However, there may be a mixed picture. Infants with a peripheral cause for their hypotonia may be at increased risk for problems during labour, delivery and resuscitation and develop hypoxic ischaemic encephalopathy.

Investigations

Further investigation needs to be guided by history and examination.

• If the infant is hypotonic but has a degree of  strength, a central cause is most likely and investigations should be directed toward this.
• If the infant is hypotonic and weak a peripheral cause is possible and an early review by the neurology service is warranted.

References