CIP and CIM have similar symptoms and presentations and are often distinguished largely on the basis of specialized electrophysiologic testing or muscle and nerve biopsy.

[3] Corticosteroids and neuromuscular blocking agents, which are widely used in intensive care, may contribute to the development of CIP and CIM,[4] as may elevations in blood sugar, which frequently occur in critically ill patients.

CIP/CIM typically develops in the setting of a critical illness and immobilization, so patients with CIP/CIM are often receiving treatment in the intensive care unit (ICU).

[citation needed] Weakness (motor deficits) occurs in generalized fashion, rather than beginning in one region of the body and spreading.

[citation needed] The causes of CIP and CIM are unknown, though they are thought to be a possible neurological manifestation of systemic inflammatory response syndrome.

[citation needed] Unlike Guillain–Barre syndrome, another neurological disorder that causes weakness, patients with critical illness polyneuropathy do not have loss of the myelin sheath that normally surrounds neurons (demyelination).

Current guidelines recommend a clinical diagnosis of ICUAW, made by manually testing the muscle strength with the use of the Medical Research Council (MRC) sum score or handgrip dynamometry.

Early detection of the condition is difficult, because these patients are often sedated and intubated, and thus unable to cooperate with a thorough neuromuscular physical examination.

[11] The use of conventional nerve conduction studies is time-consuming and requires specialized personnel; however, simplified electrophysiologic tests can be used as screening tools in the critically ill to confirm or exclude CIP/CIM.

The three main risk factors for CIP and CIM are sepsis and systemic inflammatory response syndrome (SIRS), and multi-organ failure.

[9] Additional risk factors for developing CIP/CIM include: female gender, high blood sugar (hyperglycemia), low serum albumin, and immobility.

[7] A number of terms are used to describe critical illness polyneuropathy, partially because there is often neuropathy and myopathy in the same person, and nerve and muscle degeneration are difficult to distinguish from each other in this condition.