Abnormal stress responses in patients with diseases affecting the sympathetic nervous system.
Diseases that cause malfunction of the sympathetic nervous system provide insight into how the sympathetic nerves normally modulate responses to stress. This paper discusses insight from a number of such diseases. Transection of the cervical spinal cord leads to autonomic dysreflexia. This syndrome causes episodic hypertension in quadriplegic patients from excess sympathetic activity reflexly activated by bowel or bladder distention. These patients lack cerebral control of spinal sympathetic reflexes. Radiotherapy to the neck can destroy the arterial baroreceptors that monitor blood pressure fluctuations. Patients who lack baroreceptors have exaggerated blood pressure responses to stress. They have episodes of hypertension and hypotension that cause headaches and dizziness. Diabetics and uremics often develop a peripheral sympathetic neuropathy. They have postural hypotension and diminished blood pressure responses to stress. They are often unable to tolerate heat, exercise, or fluid deprivation. Patients with heart failure deplete sympathetic neuronal norepinephrine stores. The continual stress of heart failure diminishes their ability to respond to further stresses such as standing upright or exercising. Patients with diseases of the sympathetic nervous system illustrate that everyday occurrences such as a change in posture or ambient temperature are stresses requiring a marked change in sympathetic nervous activity. Both physical and psychological stresses elicit large initial sympathetic neuronal responses that are subsequently damped by feedback inhibition from structures such as the baroreceptors. Damage to part of these feedback loops leads to exaggerated pressor responses to stress.