Autonomic function following cervical spinal cord injury

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Abstract

Spinal cord injury (SCI) is commonly associated with devastating paralysis. However, this condition also results in a variety of autonomic dysfunctions, primarily: cardiovascular, broncho-pulmonary, urinary, gastrointestinal, sexual, and thermoregulatory. SCI and the resultant unstable autonomic control are responsible for increased mortality from cardiovascular and respiratory disease among individuals with SCI.

Injury level and severity directly correlate to the severity of autonomic dysfunctions following SCI. Following high cervical SCI, parasympathetic (vagal) control will remain intact, while the spinal sympathetic circuits will lose their tonic supraspinal autonomic control. On the other hand, in individuals with injury below the 5th thoracic segment, both the sympathetic and parasympathetic control of the heart and broncho-pulmonary tree are intact. As a result of injury level, individuals with quadriplegia versus those with paraplegia will have very different cardiovascular and respiratory responses. Furthermore, similar relationships can exist between the level of SCI and function of other organs that are under autonomic control (bladder, bowel, sweat glands, etc.). It is also important to appreciate that high cervical injuries result in significant respiratory dysfunctions due to the involvement of the diaphragm and a larger portion of the accessory respiratory muscles. Early recognition and timely management of autonomic dysfunctions in individuals with SCI are crucial for the long term health outcomes in this population.

Introduction

In addition to devastating paralysis, spinal cord injury (SCI) also results in significant dysfunctions of the autonomic nervous system (ANS) (Krassioukov and Claydon, 2006). While the injury itself generally affects only a small area of the spinal cord tissue, the effect of this local disruption can commonly be seen in all spinal autonomic functions—both sympathetic and parasympathetic. There is also a very strong correlation between the level of SCI and the severity of autonomic dysfunctions. For example, individuals with cervical SCI face the daily challenge of managing an unstable blood pressure, which frequently results in persistent resting hypotension and/or episodes of uncontrolled hypertension, known as autonomic dysreflexia (Krassioukov and Claydon, 2006). These cardiovascular abnormalities have been well documented in human studies, as well as in animal models (Krassioukov et al., 1999, Mathias and Frankel, 2002, Claydon et al., 2006a, Claydon et al., 2006b, Inskip et al., 2009). Furthermore, cervical injury disrupts, not only respiratory motor output to the diaphragm, but coordinated autonomic control of the broncho-pulmonary functions as well, resulting in abnormal bronchial secretion, airway hypersensitivity, and other respiratory issues (Fein et al., 1998, Singas et al., 1999, Grimm et al., 2000). Both of these conditions are absent in individuals with low thoracic or conal injuries. The recognition and management of these cardiovascular and respiratory dysfunctions following SCI represent challenging clinical issues. Moreover, cardiovascular and respiratory disorders in the acute and chronic stages of SCI are among the most common causes of death in individuals with SCI (DeVivo et al., 1999, Garshick et al., 2005). Until recently, the majority of basic science and clinical investigations were focused on finding a cure for paralysis and reestablishing motor function. Unfortunately, we are only starting to understand the complexity of autonomic dysfunctions associated with SCI and the impact such dysfunctions have on those individuals (Krassioukov et al., 2007). This chapter will predominantly focus on the clinical issues associated with abnormal autonomic control of cardiovascular and broncho-pulmonary functions following SCI.

Section snippets

General organization of the autonomic nervous system

There are two components within the autonomic nervous system (ANS): sympathetic and parasympathetic (Krassioukov and Weaver, 1996a, Krassioukov and Weaver, 1996b). Most of the visceral organs are innervated by both components of the ANS, including the heart and broncho-pulmonary tree (Krassioukov and Weaver, 1996a, Krassioukov and Weaver, 1996b, Lefkowitz et al., 2007). However, with the exception of the cavernous tissue of the penis and clitoris (which have parasympathetic innervations) the

Acute period following SCI

Initially, following the injury, there is a marked reduction or abolition of sensory, motor, and reflex function of the spinal cord below the level of injury, known as spinal shock (Ditunno et al., 2004). This condition is also commonly accompanied by severe cardiovascular dysfunctions, especially with injury at the cervical level. Individuals with such injuries typically present with severe hypotension and persistent bradycardia—common components of the phenomenon known as neurogenic shock (

Effect of spinal cord injury on respiratory functions

Similar to their impact on cardiovascular functions, the level and completeness of injury can significantly alter the possible outcomes in respiratory functions following SCI. The coordination between somatic control of the respiratory muscles and autonomic control of the broncho-pulmonary tree is crucial for normal respiration and blood oxygenation. Loss of diaphragmatic voluntary inspiration is the most prominent feature of high cervical SCI. Specifically, the preservation of the integrity of

Conclusion

In addition to motor and sensory deficits, individuals with SCI face life-long abnormalities in blood pressure control and frequently present with various degrees of respiratory dysfunctions (Spungen et al., 1997, Mathias and Frankel, 2002, Cardenas et al., 2004, Claydon et al., 2006a, Claydon et al., 2006b, Chiodo et al., 2008). The severity of these dysfunctions depends on the level and completeness of injury of the spinal cord. Clinical evidence suggests that individuals with cervical SCI

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    This paper is part of a special issue entitled ‘Spinal cord injury—Neuroplasticity and recovery of respiratory function’, guest-edited by Gary C. Sieck and Carlos B. Mantilla.

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