Neonatal Hypotonia

 

Reviewed by Nicky Webster, Carl Kuschel, Salim Aftimos (Genetics), and Melinda Nolan (Neurology)
December
2004
Clinical Guidelines Back Newborn Services Home Page
Aetiology History Physical Examination Investigations References

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):

Central
(most common)
Spinal cord
  • Birth trauma (especially Breech delivery)
  • Syringomyelia
Anterior Horn Cell
  • Spinal Muscular Atrophy
  • Pompe’s disease (acid maltase deficiency)
Neuromuscular junction
  • Myasthenia gravis (transient/ congenital)
  • Infantile botulism
Muscle
  • Muscular dystrophies (inc. congenital myotonic dystrophy)
  • Congenital myopathies (e.g. central core disease)
Peripheral nerves
  • Hereditary motor and sensory neuropathies
Metabolic myopathies
  • Acid maltase deficiency
  • Carnitine deficiency
  • Cytochrome-c-oxidase deficiency

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

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.

Central

Anterior Horn Cell

Nerve

Neuromuscular Junction

Muscle

normal strength generalised weakness weakness, distal>proximal weakness, face/ eyes/ bulbar  weakness, proximal>distal
normal/ increased DTRs + decreased/ absent DTRs decreased/ absent DTRs normal DTRs decreased DTRs
+/-seizures fasciculations +/- fasciculations no fasciculations  
+/-dysmorphic features often described as alert      

+ 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:

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.

Central causes

  • Neuroimaging
    • Ultrasound scan in the first instance.
    • MRI may be indicated if a structural abnormality of brain development is suspected and to exclude other abnormalities (for example, evidence of HIE)
  • EEG: prognostic information as to brain function, useful clinically if seizures suspected
  • Genetics review if any dysmorphic features present
  • Karyotype (if dysmorphic features)
  • TORCH screen
  • DNA methylation studies or FISH for Prader-Willi syndrome (if clinically indicated after a genetics review)
  • Metabolic workup

Peripheral causes

  • Neurology services review
  • Cervical myelopathies are an infrequent cause of hypotonia. The diagnosis is made by history and examination. Diagnostic studies are of limited value.
  • Molecular genetics – CTG repeats, deletions in SMN gene
  • Creatine kinase (levels need to be interpreted with caution in the newborn, as levels tend to be high at birth and increase in the first 24 hours, they also increase with acidosis).  If elevated in an early sample, repeat after a few days.
  • Nerve conduction studies and muscle biopsy (Depending on clinical situation, may be delayed until around 6 months of age as neonatal results are difficult to interpret)

References

1 Fenichel GM. Neonatal Neurology 3rd edition. Churchill Livingston Inc. 1990
2 Paro-Panjan D, Neubauer D. Congenital hypotonia: is there an algorithm? Journal of Child Neurology; Jun2004, Vol.19 (6): 439-43
3 Prasad AN, Prasad C. The floppy infant: contribution of genetic and metabolic disorders. Brain and Development; Oct 2003, Vol.25(7): 457-76