Orthostatic hypotension following SCI is commonly seen in patients with high SCI, especially early after injury (Sampson et al., 2000). Arterial hypotension is defined as a systolic BP below 90 mmHg and a diastolic BP of less than 60 mmHg (Popa et al., 2010; Wecht et al., 2013). OH occurs when there is a significant decrease in BP during postural changes, and may be symptomatic or asymptomatic (Illman et al., 2000). More specifically, OH is characterized by a decrease in systolic BP of 20 mmHg or more or a reduction in diastolic pressure of 10 mmHg or more on assuming an upright posture from a supine position (Claydon et al., 2006); it is often accompanied by an increase in HR as well. Postural hypotension for injuries above the thoracic SNS output is related to the absence of reflex vasoconstriction and pooling of blood in extremities due to reduced SNS efferent activity in response to sudden postural changes (Teasell et al., 1996).
The incidence of OH during acute SCI has been evaluated by two observational studies. Illman et al. (2000) observed patients who were undergoing initial physiotherapy treatments. BP was monitored during the baseline rest period, mobilisation treatment period, and upon resumption of the resting period. OH occurred during 73.6% of treatment sessions, and was present for all patients on at least one treatment occasion. The incidence of OH was higher among tetraplegic patients compared to paraplegic patients. Sidorov et al. (2008) performed a retrospective review of patients admitted to an acute SCI unit, beginning from the time of hospital admission and continuing for the duration of the first month following injury. The observed incidence of OH in this population was 60%, with a significantly higher occurrence among patients with cervical and upper thoracic SCI than among those with lower thoracic and lumbar SCI (p<0.01).