List the potential causes of profound weakness in the critically ill, and outline how you would determine which factors were contributory.
Profound weakness in the critically ill can be as a result of a number of different pathophysiologic mechanisms. Residual paralysis should be considered and excluded by nerve stimulator examination (minimal response to train of four; post-tetanic facilitation, possibly some improvement with reversal of blockade). Residual sedation should also be considered, but once aroused, patient should be able to demonstrate more appropriate strength (and/or response to antidotes: e.g. naloxone, flumazenil). Critical illness polyneuropathy presents after a week or so of critical illness, typically with limb weakness and atrophy, reduced deep tendon reflexes, loss of peripheral sensation to touch and pin prick, but relative preservation of cranial nerve function. Electrophysiological studies reveal motor and sensory axonal neuropathy, and biopsies reveal axonal degeneration and denervation atrophy of muscles. An acute myopathy (especially in association with neuromuscular blockade and corticosteroids) may present with similar motor findings but with no sensory abnormalities. Creatine kinase should be elevated. Electrophysiological testing is consistent with a myopathy, and muscle biopsy reveals loss of thick filaments. Spinal cord lesions should be associated with a sensory level and hyperreflexia. Cranial nerve abnormalities suggest brain-stem problems or Guillain-Barre syndrome.
The topic of weakness comes up frequently in these exams.
A generic approach to the patient with generalised weakness in the ICU is discussed elsewhere.
The following tests and imaging studies could be useful:
Chapter 51 (pp. 568) Acute cerebrovascular complications by Bernard Riley and Thearina de Beer
Chapter 57 (pp. 617) Neuromuscular diseases in intensive care by George Skowronski and Manoj K Saxena
Yuki, Nobuhiro, and Hans-Peter Hartung. "Guillain–Barré syndrome." New England Journal of Medicine 366.24 (2012): 2294-2304.
Jani, Charu. "Critical Illness Neuropathy." Medicine (2011): 237.
Young, G. Bryan, and Robert R. Hammond. "A stronger approach to weakness in the intensive care unit." Critical care 8.6 (2004): 416.
Mareska, Michael, and Laurie Gutmann. "Lambert-Eaton myasthenic syndrome." Seminars in neurology. Vol. 24. No. 2. [New York]: Thieme-Stratton Inc.,[c1981-, 2004.
Reeves and Swenson have an excellent online textbook chapter (12) entitled "Evaluation of the Patient with Weakness"