Autonomic dysreflexia (AD) is a clinical emergency in individuals with spinal cord injury (SCI). It commonly occurs in individuals with injury at level T6 and above (Mathias & Frankel 1988; Karlsson 1999; Teasell et al. 2000; Mathias & Bannister 2002). An episode of AD is usually characterized by acute elevation of arterial blood pressure (BP) and bradycardia (slow heart rate), which, on occasion, may be replaced by tachycardia (rapid heart rate). Objectively, an increase in systolic BP greater than 20–30mmHg is considered a dysreflexic episode (Teasell et al. 2000). Individuals with cervical and high thoracic SCI have resting arterial BPs that are approximately 15 to 20 mmHg lower than able-bodied individuals (Mathias & Bannister 2002; Claydon & Krassioukov 2006). As such, acute elevation of BP to normal or slightly elevated ranges could indicate AD in this population. Intensity of AD can vary from asymptomatic (Linsenmeyer et al. 1996), mild discomfort and headache to a life threatening emergency when systolic blood pressure can reach 300mmHg (Mathias & Bannister 2002) and symptoms can be severe. Untreated episodes of autonomic dysreflexia may have serious consequences, including intracranial hemorrhage, cardiac complications, retinal detachments, seizures and death (Yarkony et al. 1986; Pine et al. 1991; Eltorai et al. 1992; Vallès et al. 2005). During an episode of AD, a significant increase in visceral sympathetic activity with coronary artery constriction can result in myocardial ischemia, even in the absence of coronary artery disease (Ho & Krassioukov 2010).
It has been observed that the higher the level of the SCI, the greater the degree of clinical manifestations of cardiovascular dysfunctions (Mathias & Frankel 1992; Curt et al. 1997; Krassioukov et al. 2003). Another crucial factor affecting the severity of AD is the degree of completeness of spinal injury as only 27% of incomplete tetraplegics presented with signs of AD compared to 91% of tetraplegics with complete lesions (Curt et al. 1997). AD is three times more prevalent in tetraplegics with a complete injury, in comparison to those with an incomplete injury (Curt et al. 1997). It is important to note, however, that although autonomic dysreflexia occurs more often in the chronic stage of spinal cord injury at or above the 6th thoracic segment, there is clinical evidence of early episodes of autonomic dysreflexia within the first days and weeks after the injury (Silver 2000; Krassioukov et al. 2003).