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.