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A proportion of patients surviving severe traumatic brain injury (TBI) have symptoms suggestive of excessive sympathetic discharge, here termed paroxysmal sympathetic hyperactivity (PSH). The goals of this study were: (1) to describe the clinical associations and radiological findings of PSH, its incidence, and features in subjects with severe TBI in the intensive care unit (ICU); (2) to investigate the potential role of increased intracranial pressure in the pathogenesis of PSH; and (3) to determine the prognostic influence of PSH during the ICU stay, on discharge from the ICU, and at 12 months post-injury. A prospective cohort study was undertaken of all ICU admissions with severe TBI older than 14 years over an 18-month period. The PSH symptoms consisted of paroxysmal increases in blood pressure, respiratory rate, and heart rate; worsening level of consciousness; muscle rigidity; and hyperhidrosis. Subjects demonstrating PSH episodes were compared with a group of non-PSH consecutive subjects studied over the first 6 months of the study period. Data were recorded on the clinical variables associated with PSH episodes, early post-injury cerebral CT findings, and neurological status at 1 year. Of 179 severe TBI patients admitted over the study period, 18 (10.1%) experienced PSH. Injury severity-related variables (e.g., initial APACHE II score, admission coma level, and proportion with intracranial hypertension) were similar between the two groups. The PSH group had a longer ICU stay and a greater incidence of infectious complications. At 1 year post-injury, 20% of this group demonstrated ongoing PSH episodes. Over 18 months, 10.1% of admissions following severe TBI demonstrated PSH features in ICU. Subjects with PSH had a longer ICU stay and higher rate of complications, although this did not appear to compromise their long-term neurological recovery.