A syndrome of paroxysmal, episodic sympathetic hyperactivity after acquired brain injury has been recognized for almost 60 years. This project sought to simplify the confused nomenclature for the condition (>31 eponyms) and simplify the nine overlapping sets of diagnostic criteria. A consensus-developed questionnaire based on a systematic review of the literature was circulated to a widely representative, international expert group utilizing a Delphi approach. Diagnostic criteria were dropped if group consensus failed to agree on their relative importance, with a goal of reaching a Cronbach α of 0.8 (suitable for research purposes). The resulting criteria were combined into an assessment measure for clinical and research settings. The consensus group recommend that the term "paroxysmal sympathetic hyperactivity" replace previous terms to describe the "syndrome, recognised in a subgroup of survivors of severe acquired brain injury, of simultaneous, paroxysmal transient increases in sympathetic [elevated heart rate, blood pressure, respiratory rate, temperature, sweating] and motor [posturing] activity." An 11 point probabilistic diagnostic scale was developed with reference to published criteria, yielding an acceptable Cronbach α of 0.8. These 11 items were proceduralized and combined with a symptom severity index to produce a diagnostic tool for use with adults (the paroxysmal sympathetic hyperactivity assessment measure [PSH-AM]). Development of a pediatric version of the scale and further research into the validity of the PSH-AM is recommended. The consensus position builds on previous literature to establish diagnostic definitions and criteria, an important move to standardize research and management of this condition.

A substantial minority of patients who survive an acquired brain injury develop a state of sympathetic hyperactivity that can persist for weeks or months, consisting of periodic episodes of increased heart rate and blood pressure, sweating, hyperthermia, and motor posturing, often in response to external stimuli. The unifying term for the syndrome-paroxysmal sympathetic hyperactivity (PSH)-and clear diagnostic criteria defined by expert consensus were only recently established. PSH has predominantly been described after traumatic brain injury (TBI), in which it is associated with worse outcomes. The pathophysiology of the condition is not completely understood, although most researchers consider it to be a disconnection syndrome with paroxysms driven by a loss of inhibitory control over excitatory autonomic centres. Although therapeutic strategies to alleviate sympathetic outbursts have been proposed, their effects on PSH are inconsistent between patients and their influence on outcome is unknown. Combinations of drugs are frequently used and are chosen on the basis of local custom, rather than on objective evidence. New rigorous tools for diagnosis could allow better characterisation of PSH to enable stratification of patients for future therapeutic trials.

Severe excessive autonomic overactivity occurs in a subgroup of people surviving acquired brain injury, the majority of whom show paroxysmal sympathetic and motor overactivity. Delayed recognition of paroxysmal sympathetic hyperactivity (PSH) after brain injury may increase morbidity and long-term disability. Despite its significant clinical impact, the scientific literature on this syndrome is confusing; there is no consensus on nomenclature, etiological information for diagnoses preceding the condition is poorly understood, and the evidence base underpinning our knowledge of the pathophysiology and management strategies is largely anecdotal. This systematic literature review identified 2 separate categories of paroxysmal autonomic overactivity, 1 characterized by relatively pure sympathetic overactivity and another group of disorders with mixed parasympathetic/sympathetic features. The PSH group comprised 349 reported cases, with 79.4% resulting from traumatic brain injury (TBI), 9.7% from hypoxia, and 5.4% from cerebrovascular accident. Although TBI is the dominant causative etiology, there was some suggestion that the true incidence of the condition is highest following cerebral hypoxia. In total, 31 different terms were identified for the condition. Although the most common term in the literature was dysautonomia, the consistency of sympathetic clinical features suggests that a more specific term should be used. The findings of this review suggest that PSH be adopted as a more clinically relevant and appropriate term. The review highlights major problems regarding conceptual definitions, diagnostic criteria, and nomenclature. Consensus on these issues is recommended as an essential basis for further research in the area.

There is a high rate of acute PCS in both mTBI and non-brain injured trauma patients. PCS was not found to be specific to mTBI. The use of the term PCS may be misleading as it incorrectly suggests that the basis of PCS is a brain injury.