Neuromuscular Respiratory Failure

Neuromusclar respiratory failure (NM RF) occurs with weakness of the respiratory muscles e.g. diaphragm. The most common causes of it are GBS and Myasthenic Crisis. Other causes include:

• Diaphragmatic paralysis, which can be due to diaphragmatic weakness or loss of innervation by the phrenic nerve e.g. in ALS or MS

• Cervical cord lesion (C3,4,5)

It's often triggered by a respiratory infection.

Pathophysiology

• Weakness of respiratory muscles results in a decline of the lungs’ residual capacity – This increases the work of breathing because, at lower volumes, the lungs are less distensible, therefore affecting the ventilation/perfusion relationships = Less efficient gas exchange

• The adequacy of central respiratory drive is particularly important in those with respiratory muscle weakness – In normal REMs sleep, there’s transient reduction of the respiratory drive, together with hypotonia of the accessory respiratory muscles, therefore patients with subacute/chronic respiratory muscle weakness will usually develop hypopnoea, hypoxaemia and hypercapnia during sleep first.

Presentation

The symptoms seen depend on the onset of the respiratory muscle weakness.

Acute NM RF e.g. GBS, MG Crisis – Mainly dyspnoea and orthopnoea, often accompanied by symptoms of bulbar weakness and inability to clear respiratory secretions.

Gradual NM RF - Inadequate respiration usually occurs first during REMs sleep. Symptoms of nocturnal hypoventilation include a broken sleep pattern, nightmares, nocturnal confusion, morning headache, daytime fatigue, mental clouding and drowsiness.

Signs seen include:

• Use of accessory muscles - Inwards movement of abdominal wall on inspiration suggests significant weakness of the diaphragm

• Inability to count from 1 to 20 in a single breath - Indicates significant reduction of vital capacity

• Signs specific to the neurological condition e.g. limb weakness, bulbar signs

Investigations

The main investigation to do is Vital Capacity - A fall by > 15-20% when lying flat suggests diaphragmatic weakness. When measuring this in someone with facial muscle weakness, it's important to ensure a good seal between the lips and mouthpiece (may be better to use face mask instead as VC may be deceivingly low).

An ABG should also be done - Derangement is a late feature in NM disease and normal results are quite compatible with significant weakness of the respiratory muscles. Patients with established respiratory failure from neuromuscular weakness will show hypoxaemia and a compensated respiratory acidosis (raised pCO2 and HCO3 with a normal/mildly reduced pH).

CXR may show an infection as well as a phrenic nerve palsy (raised hemidiaphragm).

Guillain-Barre Syndrome

~25% of these patients require NIV or Intubation due to bulbar and respiratory muscle weakness.

Features that make ventilatory support more likely:

• Onset of symptoms to admission < 7 days

• Bulbar dysfunction

• Bilateral facial weakness

• Vital Capacity < 20 ml/kg – VC has to be checked frequently as it’s a good indication of deterioration

Motor Neuron Disease

Most of these patients eventually develop respiratory muscle weakness, which is measeured by the VC. Symptoms include exertional dyspnoea, orthopnoea, fatigue, non-refreshing sleep, morning headache.