Some 5 to 10 percent of patients encounter one or more recurrences of the acute polyneuropathy. An illness that in the beginning appeared to be an acute inflammatory polyradiculoneuropathy may fail to stabilize and continue to progress steadily, or there may be an incomplete remission followed by a chronic, fluctuating, slowly progressive neuropathy. These more chronic forms of inflammatory neuropathy are described in a later section of this chapter.
An acute or subacute symmetrical polyneuropathy is a frequent development in critically ill and septic patients, particularly in those with failure of multiple organs (Zochodne et al). This neuropathy causes difficulty in weaning a patient from the ventilator, even as the underlying critical illness comes under control. The neuropathic process, predominantly of motor type, varies in severity from an electrophysiologic abnormality without overt clinical signs, to quadriparesis with respiratory failure. Sensory symptoms and signs are variable but tend to be mild. Usually the cranial nerves are spared and there are few or no dysautonomic manifestations. In general, this type of polyneuropathy appears after several days or more of bacterial sepsis or other overwhelming infection (now called systemic inflammatory response syndrome [SIRS] ) and multiple organ failure, and is preceded in most instances by a confusional state or a depressed state of consciousness (“septic encephalopathy”).
The EMG and NCS findings of a primary axonal process with early denervation and a normal CSF distinguish this entity from the typical demyelinative form of GBS. Autopsy material has usually disclosed little or no inflammatory changes in the peripheral nerves. Differentiating critical illness polyneuropathy from critical illness myopathy (see just below) and from the axonal form of GBS is difficult and depends on the context in which the illness occurs. All of these processes that occur in the intensive care unit, when extreme, can eliminate the motor nerve action potentials and when this configuration is found, the problem is most often attributed to the neuropathy, although this is not always correct. The toxic effects of drugs and antibiotics and nutritional deficiency must be considered in causation, but rarely can they be established. The many systemic mediators of sepsis are toxic to the peripheral nervous system; tumor necrosis factor has been proposed as one such endogenous toxin in causing neuropathy.
Critical illness polyneuropathy must also be distinguished from an incompletely understood acute quadriplegic myopathy (critical illness myopathy) that also complicates critical illness (see Chap. 45). High doses of corticosteroids, particularly in combination with neuromuscular blocking agents, have been implicated. The acute myopathy, which affects both distal and proximal muscles, is sometimes heralded by an elevation in the serum creatine kinase (CK) concentration (at times up to several thousand units). Myopathic potentials in the EMG and a unique degeneration of myofilaments in all the muscles are found. This illness is described in more detail in Chap. 45.