A 62-year-old hospitalized woman is evaluated for muscle weakness. The patient was admitted 2 weeks ago due to septic shock from acute pyelonephritis. She was treated in the intensive care unit with intravenous fluids, vasopressors, and broad-spectrum antibiotics. The patient was also mechanically ventilated due to respiratory failure. Her hemodynamic status and infection gradually improved, but she has had difficulty being taken off the ventilator due to respiratory muscle weakness. She is also noted to have significant extremity weakness. Physical examination shows diffuse mild atrophy of the extremity muscles. Bilateral upper and lower extremity deep tendon reflexes are decreased. Which of the following is the most likely cause of this patient's current condition?
This patient has weakness, muscle atrophy, and decreased deep tendon reflexes after critical illness. Critical illness, especially due to sepsis, can result in neuromuscular weakness caused by one or both of the following:
Critical illness myopathy (CIM), which results in a decrease of muscle membrane excitability and loss of myosin with resultant atrophy of myofibers
Critical illness polyneuropathy (CIP), which results in decreased nerve excitability (due to inactivation of sodium channels) and axonal degeneration (possibly due to injury to the microcirculation by inflammatory mediators)
Clinical features of CIM and CIP typically include symmetric weakness greater in proximal, rather than distal, muscles with decreased deep tendon reflexes. It is often diagnosed when patients are unable to be liberated from the ventilator due to chest wall weakness. Treatment is focused on controlling underlying medical conditions, preventing further complications, and rehabilitation. Patients may improve over weeks to months with rehabilitation, but many have persistent weakness.
(Choice A) Corticospinal tract demyelination can occur at multiple levels including the pons (eg, central pontine myelinolysis) or spinal cord (eg, vitamin B12 deficiency). However, this would result in upper motor neuron signs (eg, hyperreflexia).
(Choice B) Patients with sepsis have an increased risk of stroke, which can result in motor cortex ischemic injury. However, patients with stroke often have asymmetric neurologic findings, and because it affects upper motor neurons, hyperreflexia (rather than loss of deep tendon reflexes) would be expected.
(Choice C) Myasthenia gravis (MG) can lead to antibody-induced motor endplate receptor loss; myasthenic crisis can be triggered by infection and lead to respiratory failure. However, patients with MG rarely have muscle atrophy, and deep tendon reflexes are classically preserved (MG causes fatigable weakness).
(Choice D) Myofiber lymphocytic inflammation occurs with inflammatory myopathies (eg, polymyositis), which classically present insidiously (rather than over 2 weeks) with symmetrical proximal muscle weakness. In addition, polymyositis does not typically affect the respiratory muscles, and deep tendon reflexes are usually normal.
Educational objective:
Critical illness can lead to weakness due to both myopathy (eg, atrophy of muscles) and polyneuropathy (eg, axonal degeneration, decreased nerve excitability). This can lead to both extremity weakness and difficulty breathing due to chest wall weakness.