Hypotension in acute spinal injury is due to :

Hypotension, bradycardia and hypothermia occur following acute spinal injury due to loss of sympathetic control and tone. These are classic signs of spinal cord shock that occur immediately after injury. Spinal shock is a temporary neurological condition that occur after injury to the cord above the T6 level. Patients develop loss of motor, sensory, reflex and autonomic response below the site of injury. On examination there will be flaccidity and a positive Babinski sign. Spinal shock does not resolve abruptly but rather in a series of phases extending over a few hours to several weeks depending on the segment level and and the extent of cord injury. In areas where no function has returned flaccid paralysis becomes spastic with increased tone. Ref: Accident & Emergency: Theory and Practice edited by Brian Dolan, page 119.