Acute neuromuscular respiratory failure

Pathogenesis

Definition: inadequate alveolar ventilation resulting from a lesion along the neurologic axis, anywhere from the cortex to the muscle.

Causes (by anatomical location):

  • Cortex
    • Epilepsy
  • Brainstem
    • Congenital Ondine’s curse
    • Vascular
      • Pontine tegmentum stroke à apneustic breathing
      • Laterally medullary syndrome à acute resp. failure
      • Basis pontis >> Locked-in = loss of voluntary breathing (but maintained regular involuntary breathing)
    • Post-infectious encephalomyelitis
      • Post-varicella
      • Post-rubella
      • ADEM
    • Paraneoplastic, e.g. anti-Hu
  • Autonomic failure
    • MSA
  • Formane magnum lesion
    • Cerebellar ectopia
    • Syringobulbia
    • Rheumatoid atlantoaxial subluxation
  • Cervical cord lesions (C3-C5 = phrenic nerve origin)
    • Trauma
    • Demyelination
    • Vascular
  • Anterior horn cell
    • Poliomyelitis
    • MND
  • Peripheral nerve
    • GBS
    • Critical illness polyneuropathy
    • Brachial neuritis
    • Paraneoplastic
    • Porphyria
    • Vasculitis
    • Toxic
    • HSMN
  • NMJ
    • MG
  • Muscle
    • Idiopathic inflammatory myopathies
    • Muscular dystrophies
    • Mitochondrial myopathies

Presentation

Neuromuscular respiratory failure can present as a gradually-worsening problem or acutely as an emergency.

Chronic neuromuscular respiratory failure presents initially with features of nocturnal hypoventilation:

  • Insomnia
  • Excessive daytime sleepiness
  • Lethargy
  • Morning headache (due to CO2 accumulation)
  • Inattention
  • Irritability

Acute neuromuscular respiratory failure may occur on a background of chronic neuromuscular respiratory failure or may occur de novo. People in chronic neuromuscular respiratory compensate for chronic alveolar hypoventilation by raising their haematocrit and increasing renal acid excretion, producing the clinical picture of slowly-progressive symptoms.

In contrast, acute neuromuscular respiratory failure is a sudden, potentially fatal presentation which carries a high mortality, even when treated.

Investigation

  • Assessing severity
    • Forced Vital capacity (sitting and upright)
      • Mainly due to diaphragm weakness
    • CXR to look for
      • signs of aspiration pneumonia
      • diaphragm paresis
    • ABG
      • Abnormal picture is usually one of type II (hypoventilatory) respiratory failure
  • Determining the cause: often this is obvious from the history if there is a known background of neuromuscular disease. Helpful investigations may include:
    • EMG
    • Nerve conduction studies
    • Lumbar puncture

Management

Depends on the underlying cause. Bilevel non-invasive ventilation is helpful acutely.