Neuromuscular Respiratory Failure (2*)

Question 1

When does this occur?

What are its most common causes?
→ What are some other causes?

What is it often triggered by?

What is its pathophysiology?

Answer 1

➊ With weakness of the respiratory muscles

➋ GBS and Myasthenic Crisis
→ • 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)

➌ Respiratory infection

➍ • 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

Question 2

The symptoms seen depend on the onset of the resp. muscle weakness:
What is expected in Acute NM RF?

What is expected in Gradual NM RF?

What are the signs seen?

Answer 2

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

➋ 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.

➌ • 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

Question 3

Investigations:
What is the main investigation to do?
→ What result suggests diaphragmatic weakness?
→ What is important to do for those with facial muscle weakness?

What else should be done?
→ What could be seen?

What may a CXR show?

Answer 3

➊ Vital Capacity
→ A fall by > 15-20% when lying flat
→ Ensure a good seal between the mouthpiece and the lips – May be better to use face mask instead as VC may be deceivingly low

➋ ABG
→ Derangement is a late feature in NMD 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).

➌ Any infection as well as a phrenic nerve palsy (raised hemidiaphragm)

Question 4

Guillain-Barre Syndrome:
Who do ~25% of pts require?
→ Why is this?

Which certain features make ventilatory support more likely?

Answer 4

➊ NIV or Intubation
→ Bulbar and respiratory muscle weakness

➋ • 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

Question 5

Motor Neuron Disease:
What do most develop?
→ How is this measured?

What symptoms would they have?

Answer 5

➊ Respiratory muscle weakness
→ VC

➋ Exertional dyspnoea, orthopnoea, fatigue, non-refreshing sleep, morning headache