Parathyroid hormone suppression in spinal cord injury patients is associated with the degree of neurologic impairment and not the level of injury

Objective: To demonstrate that after spinal cord injury (SCI) suppression of the parathyroid-vitamin D axis is associated with the degree of neurologic impairment and not the level of injury. Design: A retrospective analysis of clinical and biochemical data obtained from hospital records of patients with SCI compared to a control group of patients with traumatic brain injury (TBI). Setting: The inpatient rehabilitation unit of a tertiary care hospital. Subjects: The medical records of 82 consecutive admissions to the rehabilitation unit with a diagnosis of SCI or TBI were reviewed. Patients with SCI were classified by the American Spinal Injury Association (ASIA) impairment scale and then grouped based on the completeness and level of injury. Main Outcome Measure: Comparisons of serum parathyroid hormone (PTH), 25-hydroxyvitamin D, and 1,25-dihydroxyvitamin D (1,25-D) were planned. Multiple comparisons were performed for total and ionized serum calcium levels, serum phosphorus levels, and 24 hour urinary calcium excretion rates to reflect changes in mineral homeostasis. Multiple comparisons were also performed for serum albumin, prolactin, thyroid function tests, and AM cortisol levels, as well as 24-hour urinary urea nitrogen and cortisol excretion rates to reflect metabolic responses to stress. Results: Patients with SCI had significant suppression in PTH (p < .000009) and 1,25-D (p < .02) levels with elevated phosphorus (p < 0.03) and prolactin (p < .03) levels compared to patients with TBI. Also, more patients with SCI were hypoalbuminemic (p < .003) than patients with TBI. Patients with complete SCI (ASIA A) had more suppressed PTH (p < .03) and higher urinary urea nitrogen (p < .05) levels than SCI patients with incomplete injuries (ASIA B-D). Patients with complete, but not incomplete, SCI had lower albumin levels than patients with TBI (p < .05). These differences were not found between patients with tetraplegic and paraplegic SCI. ASIA motor scores did not correlate with any of the measured parameters but when used as a covariate did abolish differences in PTH and 1,25-D among the study groups by ANOVA. Conclusion: In patients with SCI, the degree of neurologic impairment, and not the level of injury, is associated with PTH suppression and markers of metabolic stress.