Orthostatic hypotension is a condition characterized by a reduction in systolic blood pressure of at least 20mmHg (or a reduction in diastolic blood pressure of at least 10mmHg) in response to a change in body position from supine (lying) to upright (seated or standing) (“Consensus statement on the definition of orthostatic hypotension, pure autonomic failure, and multiple system atrophy. The Consensus Committee of the American Autonomic Society and the American Academy of Neurology,” 1996). Orthostatic hypotension typically affects individuals with cervical or high-thoracic SCI who lack the ability to vasconstrict the vascular beds in the abdomen due to the loss of supraspinal sympathetic control (Teasell et al. 2000). The presence of orthostatic hypotension can have a significant burden of morbidity as it impairs activities of daily living and can be associated with feelings of tiredness as well as reduced cognition and cerebral perfusion (Sahota et al. 2012). In addition to altered neural control following SCI, a number of other risk factors can precipitate an increased risk for OH including low blood volume, low sodium levels, and deconditioning of the heart and blood vessels (Claydon & Krassioukov 2006). Within those with cervical SCI, it is reported that up to 82% suffer from orthostatic hypotension (Illman et al. 2000).
Orthostatic hypotension is traditionally managed pharmacologically using medications similar to that in the general population, despite no SCI-specific validation. Of the drugs most commonly used Midodrine has the most evidence supporting efficacy in the SCI population. Despite relatively widespread use, there are side effects including potential urinary bladder dysreflexia. As such, a number of non-pharmacological avenues are being explored to manage OH in people with SCI, including pressure stockings, abdominal bindings, and various forms of exercise, the latter of which is reviewed below.
The majority of studies that have investigated the efficacy of exercise to abrogate the severity of orthostatic hypotension have conducted an acute study in which an individual with SCI is either tilted into the upright position, moved from supine to standing, or encased within a lower-body negative pressure (LBNP) chamber. Both the tilt-test and the LBNP chamber simulate the blood pressure change associated with orthostasis. In response to these acute hemodynamic challenges, investigators have subsequently demonstrated that stimulation of the lower limbs with either functional electrical stimulation exercise or neuromuscular stimulation improves blood pressure control and/or central hemodynamics.
There is insufficient evidence in the literature to determine whether chronic exposure to exercise training improves orthostatic intolerance in those with cervical SCI. Indeed, only a single study has compared the severity of orthostatic tolerance between highly-trained and untrained individuals with cervical SCI. This study demonstrated that there is level 5 evidence that individuals with SCI who are highly trained (i.e., competitive athletes) have better cardiovascular stability (i.e., fewer changes in the spectral components of heart rate and blood pressure during an orthostatic challenge) than those who are untrained.
There is insufficient evidence in the literature to determine whether a BWSTT intervention improves orthostatic intolerance in those with cervical SCI. Indeed, only a single study has compared the severity of orthostatic tolerance pre- and post-BWSTT. This study demonstrated that there is no improvement in the severity of orthostatic intolerance in individuals with cervical SCI who underwent BWSTT.
There is level 4 evidence (Ditor et al. 2005) from 1 study that 6 months of BWSTT does not reduce (or worsen) the severity of orthostatic intolerance in individuals with incomplete cervical SCI.
There is level 5 evidence (Otsuka et al. 2008) that chronic exposure to athletic training (at least 2hr/wk) improves cardiovascular stability during an orthostatic challenge in individuals with cervical SCI.