Clinical meaning
Neurogenic shock is a distributive form of shock that results from the sudden loss of sympathetic nervous system tone following injury to the spinal cord, typically at the cervical or upper thoracic level (above T6). Under normal physiological conditions, the sympathetic nervous system maintains vascular tone by releasing norepinephrine from postganglionic sympathetic nerve fibers, which acts on alpha-1 adrenergic receptors on vascular smooth muscle to cause vasoconstriction. This tonic vasoconstriction is essential for maintaining systemic vascular resistance (SVR) and blood pressure. The sympathetic nervous system also innervates the heart through cardiac accelerator fibers originating from T1 through T4, which increase heart rate and contractility through beta-1 adrenergic receptor stimulation. When a spinal cord injury disrupts these sympathetic pathways, unopposed parasympathetic (vagal) activity dominates. The parasympathetic nervous system, carried by the vagus nerve (cranial nerve X), slows the heart rate and has no direct effect on peripheral vascular tone. The result is a triad of findings that distinguishes neurogenic shock from other forms of shock: hypotension (from massive vasodilation and loss of SVR), bradycardia (from unopposed vagal tone on the heart, unlike hypovolemic or cardiogenic shock which cause tachycardia), and peripheral vasodilation with warm, dry, flushed skin below the level of injury. This presentation is sometimes called warm shock because the skin remains warm and well-perfused initially, in contrast to the cold, clammy skin seen in hypovolemic shock. The cardiovascular collapse in neurogenic shock occurs because the heart cannot compensate for the profound decrease in SVR. Cardiac output may initially be maintained, but as venous return decreases due to venous pooling in the dilated peripheral vasculature, preload drops and cardiac output falls. Poikilothermia (inability to regulate body temperature) develops because the loss of sympathetic control prevents thermoregulatory vasoconstriction and shivering below the level of injury. The patient becomes essentially dependent on ambient temperature, making hypothermia a significant risk. It is critical to distinguish neurogenic shock from spinal shock, which is a separate phenomenon. Spinal shock refers to the temporary loss of all spinal cord function (motor, sensory, and reflex) below the level of injury, including loss of deep tendon reflexes and flaccid paralysis. Spinal shock may last days to weeks and does not necessarily involve hemodynamic instability. Neurogenic shock is the cardiovascular emergency that requires immediate hemodynamic support. The practical nurse must monitor for and promptly report the classic signs of neurogenic shock, assist with fluid resuscitation and vasopressor administration, prevent complications of immobility, and maintain normothermia.