A critical review of the pathophysiology of dysautonomia following traumatic brain injury.

The management of Dysautonomia following severe traumatic brain injury (TBI) remains problematic, primarily due to an inadequate understanding of the pathophysiology of the condition. While the original theories inferred an epileptogenic source, there is greater support for disconnection theories in the literature. Disconnection theories suggest that Dysautonomia follows the release of one or more excitatory centres from higher centre control. Conventional disconnection theories suggest excitatory centre/s located in the upper brainstem and diencephalon drive paroxysms. Another disconnection theory, the Excitatory:Inhibitory Ratio (EIR) Model, suggests the causative brainstem/diencephalic centres are inhibitory in nature, with damage releasing excitatory spinal cord processes. Review of the available data suggests that Dysautonomia follows structural and/or functional (for example raised intracerebral pressure or neurotransmitter blockade) abnormalities, with the tendency to develop Dysautonomic paroxysms being more closely associated with mesencephalic rather than diencephalic damage. Many reports suggest that paroxysmal episodes can be triggered by environmental events and minimised by various but predictable neurotransmitter effects. This article presents a critical review of the competing theories against the available observational, clinical and neurotransmitter evidence. Following this process, it is suggested that the EIR Model more readily explains pathophysiological and treatment data compared to conventional disconnection models. In particular, the EIR Model provides an explanatory model that encompasses other acute autonomic emergency syndromes, accommodates 'triggering' of paroxysms and provides a rationale for all known medication effects.