Thermoregulation in people with spinal cord injury

Abstract

Thermoafferent information is integrated at multiple levels within the central nervous system. However, due to the difficulty in differentiating thermoregulatory functions of the spinal cord from those of higher centres in humans, the role of the spinal cord in certain aspects of thermoregulation remains unclear. Subjects with spinal cord injury have unique neural changes providing an opportunity to evaluate the role of the spinal cord, independently of higher thermoregulatory centres. Subjects with (N=11) and without (N=11) spinal cord injury were studied in a series of experiments, in which a wide range of local and whole-body temperature changes and postural manipulations were imposed. During these trials, various physiological (skin temperature, core temperature, local sweat rate and sweat expulsion frequency - a measure of central sympathetic drive), and psychophysical variables (thermal sensation and discomfort) were investigated. Six key observations arose from these experiments: (i) Subjects with spinal cord injury had a lower thermoafferent capacity (secondary to neural damage) and a corresponding reduction in thermoefferent drive (sudomotor sensitivity of 4.2 versus 8.8 expulsions•min-1•,°C-1;in able-bodied; P=0.03). (ii) Equations used to approximate thermoafferent drive in able-bodied subjects, overestimated thermal feedback in subjects with spinal cord injury. However, this could be corrected by modifying the skin area weightings to include only the sensate areas. (iii) No subjects with physiologically-confirmed thermoefferent spinal cord injury displayed sweating from insensate skin sites, indicating that a spinal cord that has been isolated from higher centres cannot induce thermal sweating. (iv) Subjects with spinal cord injury had higher forehead sweat rates (0.77 versus 0.52 mg•cm-2•min-1; P=0.03), but an equivalent sweat sensitivity (1.24 versus 1.27 mg•cm-2•min-1•,°C-1; P=0.94), indicating the presence of a peripheral adaptation to sustain thermal homeostasis, and secondary to reduced thermal afferent and efferent flow. (v) Respiratory frequency increased more for a given increase in body temperature in subjects with spinal cord injury (2.4 versus 1.1 breaths•min-1•°C-1; P=0.042), but this did not provide a thermoregulatory benefit. (vi) Subjects with spinal cord injury demonstrated greater changes in behavioural thermoregulatory indicators (thermal sensation and discomfort) in response to standardised local and whole-body thermal loads. Collectively, these observations indicate the unique nature of thermoregulation in people with spinal cord injury and the adaptive ability of the human thermoregulatory system