Clinical approach to the weak patient in the intensive care unit

Motor weakness in a patient in the intensive care unit (ICU) may be related to (1) pre-existing neuromuscular disorder that leads to ICU admission, (2) new-onset or previously undiagnosed neurological disorder, or (3) complications of non-neuromuscular critical illness. Neuromuscular syndromes related to ICU treatment consist of critical illness polyneuropathy, critical illness myopathy, and prolonged neuromuscular blockade, and are now recognized as a frequent cause of newly acquired weakness in ICU patients. Clinical features include quadriparesis, muscle wasting, and difficulty weaning from the ventilator. Evaluation of these patients is based on knowledge of clinical setting and predisposing factors, focused neurological examination, detailed electrophysiological investigation, serum creatine kinase level, other laboratory studies as needed, and histological examination of muscle biopsy. If a central nervous system (brain or spinal cord) lesion is suspected, neuroimaging studies are required. In addition to conventional nerve conduction and needle electromyography, phrenic nerve conduction, diaphragm electromyography, blink reflex, and (recently) the technique of direct muscle stimulation have been employed. Critical illness polyneuropathy is an axonal motor and sensory neuropathy that often follows sepsis and multiorgan failure. Risk factors for critical illness myopathy are corticosteroids and neuromuscular blocking drugs, acute respiratory illness, and organ transplant. Three subtypes (acute necrotizing myopathy, thick myosin filament loss myopathy, and type II fiber atrophy) are recognized. Major differential diagnoses of critical illness related paralysis are incidental Guillain-Barré syndrome and unmasked myasthenia gravis. Rarely, atypical presentation of amyotrophic lateral sclerosis, polymyositis or other myopathies, and precipitation of porphyria or rhabdomyolysis due to drugs used in the ICU have been described. Recently a poliomyelitis-like flaccid paralysis due to West Nile virus infection was reported. A subgroup of patients with myasthenia gravis with muscle-specific tyrosine kinase antibody is noted to present as respiratory crisis. Muscle biopsy in ICU paralysis syndromes may be helpful in arriving at a specific diagnosis or to classify the type of critical illness myopathy. Nerve biopsy is only rarely indicated.