Neuromuscular Disorders in Critical Illness
Guillain-Barre syndrome and myasthenic crisis
Guillain-Barre presents with progressive symmetric weakness and areflexia, peaking within four weeks. Monitor vital capacity, negative inspiratory force, and bulbar function; admit to ICU for a vital capacity below 20 mL/kg, MIP less negative than -30 cmH2O, or swallowing failure. Treat with IVIG 0.4 g/kg/day for 5 days or plasma exchange; steroids are ineffective. Watch for dysautonomia and avoid succinylcholine because of hyperkalemia risk.
Myasthenic crisis requires identifying the trigger (infection, new medication), plasma exchange or IVIG, and avoidance of aminoglycosides, fluoroquinolones, magnesium, and beta-blockers. Extubate when vital capacity exceeds 15-20 mL/kg, inspiratory force is more negative than -30 cmH2O, and cough is strong. Pyridostigmine increases secretions and is often held while ventilated.
ICU-acquired weakness
Critical illness polyneuropathy and myopathy develop with sepsis and multiorgan failure; exam shows weakness with relatively preserved sensation, and myopathy may raise creatine kinase. Nerve studies show reduced compound muscle action potentials and prolonged durations. Treatment is supportive: minimize steroids and paralytics, control glucose, and mobilize early.
Trap: extubating on the basis of a normal blood gas while the patient cannot generate an adequate cough, or attributing new weakness in a septic patient to deconditioning.
Rule of thumb: the 20/30/40 rule for impending respiratory failure: vital capacity under 20 mL/kg, MIP weaker than -30 cmH2O, or maximal expiratory pressure under 40 cmH2O.