Patterns of neurophysiological abnormality in prolonged critical illness

Abstract

Objective

To describe the various patterns of neurophysiological abnormalities which may complicate prolonged critical illness and identify possible aetiological factors.

Design

Prospective case series of neurophysiological studies, severity of illness scores, organ failures, drug therapy and hospital outcome. Some patients also had muscle biopsies.

Setting

General intensive care unit (ICU) in a University Hospital. Patients: Forty-four patients requiring intensive care unit stay of more than 7 days. The median age was 60 (range 27–84 years), APACHE II score 19 (range 8–33), organ failures 3 (range 1–6), and mortality was 23%.

Results

Seven patients had normal neurophysiology (group I), 4 had a predominantly sensory axonal neuropathy (group II), 11 had motor syndromes characterised by markedly reduced compound muscle action potentials and sensory action potentials in the normal range (group III) and 19 had combinations of motor and sensory abnormalities (group IV). Three patients had abnormal studies but could not be classified into the above groups (group V). All patients had normal nerve conduction velocities. Electromyography revealed evidence of denervation in five patients in group III and five in group IV. There was no obvious relationship between the pattern of neurophysiological abnormality and the APACHE II score, organ failure score, the presence of sepsis or the administration of muscle relaxants and steroids. A wide range of histological abnormalities was seen in the 24 patients who had a muscle biopsy; there was no clear relationship between these changes and the neurophysiological abnormalities, although histologically normal muscle was only found in patients with normal neurophysiology. Only three of the eight patients from group III in whom muscle biopsy was performed had histological changes compatible with myopathy.

Conclusions

Neurophysiological abnormalities complicating critical illness can be broadly divided into three types — sensory abnormalities alone, a pure motor syndrome and a mixed motor and sensory disturbance. The motor syndrome could be explained by an abnormality in the most distal portion of the motor axon, at the neuromuscular junction or the motor end plate and, in some cases, by inexcitable muscle membranes or extreme loss of muscle bulk. The mixed motor and sensory disturbance which is characteristic of ‘critical illness polyneuropathy’ could be explained by a combination of the pure motor syndrome and the mild sensory neuropathy. More precise identification of the various neurophysiological abnormalities and aetiological factors may lead to further insights into the causes of neuromuscular weakness in the critically ill and ultimately to measures for their prevention and treatment.