Critical Illness Polyneuropathy and Myopathy
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Direct answer
Critical illness polyneuropathy and myopathy are acquired weakness syndromes, collectively called ICU-acquired weakness, that develop during severe sepsis or multi-organ failure. They cause symmetrical limb weakness with reduced reflexes and difficulty weaning from the ventilator. Electrophysiology with direct muscle stimulation separates the two, and prevention rests on early mobilisation, glycaemic control and limiting neuromuscular blocking drugs.
What you must remember
- Spectrum: critical illness polyneuropathy, critical illness myopathy and a mixed form, all grouped as ICU-acquired weakness.
- Risk factors: sepsis, multi-organ failure, prolonged mechanical ventilation and hyperglycaemia, with high-dose corticosteroids linked to myopathy, especially with neuromuscular blockers.
- Presentation: symmetrical limb weakness with reduced reflexes, usually noticed once sedation is lifted and the patient fails to wean from the ventilator.
- Polyneuropathy: a typically length-dependent axonal sensorimotor neuropathy, in which sensory testing is harder to interpret in an unconscious patient.
- Myopathy: proximal and facial weakness with preserved sensation, and selective loss of myosin filaments on muscle biopsy.
- Key test: direct muscle stimulation gives a normal response in polyneuropathy but a reduced response in myopathy, while nerve stimulation is reduced in both.
- EMG: reduced compound muscle action potentials with normal conduction velocity, fibrillation potentials, and short-duration polyphasic motor unit potentials favouring myopathy.
- Prevention: early mobilisation, control of hyperglycaemia, treatment of sepsis, and avoidance of prolonged neuromuscular blockade and excess steroid exposure.
A pathway from suspicion to recovery
The pathway moves from recognising the pattern to excluding mimics, testing and then correcting the drivers. Recovery often takes weeks to months, so the pathway continues through rehabilitation.
- Recognise the pattern: Suspect ICU-acquired weakness when limb weakness persists after sedation is lifted, and when weaning fails despite recovery of the lungs.
- Exclude mimics: Check for Guillain-Barré syndrome, hypophosphataemia, hypokalaemia, hypermagnesaemia and the residual effect of sedatives or neuromuscular blockers.
- Perform electrophysiology: Nerve conduction studies and EMG are done, with direct muscle stimulation to separate polyneuropathy from myopathy.
- Biopsy when unclear: A muscle biopsy showing myosin loss confirms myopathy when electrophysiology is inconclusive.
- Correct the drivers: Sepsis is treated, glucose is controlled, and exposure to corticosteroids and neuromuscular blockers is reduced.
- Rehabilitate: Early physiotherapy and a structured weaning protocol support recovery, while the family is counselled about a slow course.
Worked example: a 60-year-old man recovers from septic shock, but his limbs remain weak after sedation is stopped and he cannot be weaned. Direct muscle stimulation is reduced, and muscle biopsy shows myosin loss, so the diagnosis is critical illness myopathy.
Where the examiner tests the weakness
Questions usually describe an ICU patient who fails to wean and ask for the test or the distinguishing feature.
- Trap: Attributing persistent weakness to the sedation alone. Correct: Weakness that persists after sedation is withdrawn points to ICU-acquired weakness, and electrophysiology defines its form.
- Trap: Diagnosing polyneuropathy from reduced nerve stimulation alone. Correct: Nerve stimulation is reduced in both conditions, so direct muscle stimulation is needed to separate myopathy from polyneuropathy.
- Trap: Expecting rapid recovery with ventilator weaning alone. Correct: Recovery usually takes weeks to months, and rehabilitation is part of the management.
Frequently asked questions
What are the risk factors for critical illness polyneuropathy?
Risk factors for critical illness polyneuropathy include sepsis, multi-organ failure, prolonged mechanical ventilation and hyperglycaemia. High-dose corticosteroids, especially combined with neuromuscular blocking drugs, are linked with critical illness myopathy. Early mobilisation and glycaemic control help reduce the burden of both.
How do polyneuropathy and myopathy differ on electrophysiology?
Direct muscle stimulation separates critical illness polyneuropathy from myopathy on electrophysiology. Responses to direct muscle stimulation are normal in polyneuropathy but reduced in myopathy, while nerve stimulation responses are reduced in both. Short-duration polyphasic motor unit potentials favour myopathy.
What does muscle biopsy show in critical illness myopathy?
Muscle biopsy in critical illness myopathy shows selective loss of myosin filaments, which is demonstrated with myosin staining. It is used when electrophysiology is inconclusive. The biopsy helps confirm myopathy rather than polyneuropathy.
Why does critical illness weakness delay ventilator weaning?
Critical illness weakness delays ventilator weaning because the respiratory muscles, including the diaphragm, are weakened along with the limbs. Patients fail spontaneous breathing trials even after the primary illness has resolved. Early rehabilitation and structured weaning protocols help the patient regain strength.