The most likely diagnosis is ICU-acquired weakness. ICU-acquired weakness includes critical illness polyneuropathy (with axonal nerve degeneration) and critical illness myopathy (with muscle myosin loss), resulting in profound weakness. These two conditions are difficult to differentiate and may overlap. Some experts recommend that biopsies and more formal electrophysiologic studies be reserved for patients in whom the diagnosis is more ambiguous and where other diagnoses are more likely to exist. ICU-acquired weakness is associated with long-term functional disability, prolonged ventilation, and in-hospital mortality. Risk factors include female sex, hyperglycemia, sepsis, multiple organ dysfunction and systemic inflammatory response, immobility, and long duration of mechanical ventilation. Strategies to limit or prevent ICU-acquired weakness include sedation limitation, early mobilization, and moderate glucose control. The strategies that have the most impact are not yet known.

Diabetes predisposes to multifactorial nerve injury due to nerve compression, ischemia, inflammation, and metabolic changes. Distal sensorimotor peripheral neuropathy is the most common disorder and presents with numbness, tingling, and burning pain in a stocking-glove distribution. Weakness may occur late in the course of the disease. However, this pattern is not present in this patient and would be unlikely to account for this patient's symmetric muscle weakness.

Guillain-Barré syndrome is the most common cause of acute diffuse neuromuscular paralysis. Affected patients initially experience rapid onset of symmetric weakness of the upper and lower limbs over days to weeks, generally in the setting of a recent infection, trauma, or surgery. The disorder generally progresses over 2 weeks, with 90% patients at their worst by 4 weeks. Although many patients describe paresthesias or neuropathic pain in the hands and feet, objective sensory loss is usually mild or absent. Neurologic examination reveals weakness and decreased or absent deep tendon reflexes. The time course and absence of paresthesias in this patient make Guillain-Barré syndrome an unlikely diagnosis.

Vasculitic neuropathy is usually found in association with a systemic vasculitis that involves other organs (skin, lungs, kidneys), but it can be found in isolation. Patients most commonly present with both sensory and motor nerve dysfunction that is asymmetric and distal, typically involving the longest nerves of the body first. This patient's painless and symmetric loss of muscle function is not compatible with vasculitis neuropathy.