Neurogenic shock falls into the classification designated Distributive Shock--shock resulting from severe peripheral vasodilation. Neurogenic shock is a condition in which disruption of the autonomic pathways within the spinal cord or brain cause low blood pressure, decreased vascular resistance, and altered vagal tone that slows the heart. Neurogenic shock is usually due to trauma to the central nervous system.
The 10th cranial nerve, the vagus nerve, is very complex. One of its functions is the control of the parasympathetic nervous system function which is crucial in the control of heart rate and the constriction of blood vessels.
In acute spinal cord injury, neurogenic shock (hypotension + bradycardia) results from the interruption of sympathetic autonomic pathways that run through the spine. This damage causes the parasympathetic signals from the vagus nerve to become dominant which can decrease vascular resistance/heart rate and result in profound hypotension.
Signs and Symptoms
Loss of sympathetic autonomic vascular tone causes vasodilation: while pulse pressure and systolic BP are low, the skin stays warm and well perfused. Hypotension, bradycardia, altered mental status, and frank paralysis or one or more limbs are common in this condition.
Note that the hypotension, besides from autonomic interruption, may also be due to hypovolemia due to blood loss additionally seen in acute spinal injuries.
Treatment includes spinal immobilization and aggressive IV fluid repletion, with a goal mean arterial blood pressure (also known as MAP) of 90mmHg. MAP is found by using the following formula: MAP = Diastolic Pressure + 1/3(Systolic Pressure – Diastolic Pressure). In many cases, fluids alone will not sufficiently raise the blood pressure and pressor agents such as norepinephrine and medications to raise the heart rate (atropine) may be required.
Rapid transport to a trauma center is universally required for these patients, as they often require significant surgery to correct the spinal damage that leads to neurogenic shock.