The autonomic nervous system regulates certain body processes, such as blood pressure and the rate of breathing. This system works automatically (autonomously), without a person's conscious effort.

Disorders of the autonomic nervous system can affect any body part or process. Autonomic disorders may result from other disorders that damage autonomic nerves (such as diabetes), or they may occur on their own. Autonomic disorders may be reversible or progressive.

Anatomy: The autonomic nervous system is the part of the nervous system that supplies the internal organs, including the blood vessels, stomach, intestine, liver, kidneys, bladder, genitals, lungs, pupils and muscles of the eye, heart, and sweat, salivary, and digestive glands (see also Biology of the Nervous System: Autonomic Nervous System).

The autonomic nervous system has two main divisions: the sympathetic and the parasympathetic. After the autonomic nervous system receives information about the body and external environment, it responds by stimulating body processes, usually through the sympathetic division, or inhibiting them, usually through the parasympathetic division.

An autonomic nerve pathway involves two nerve cells. One cell is located in the brain stem or spinal cord. It is connected by nerve fibers to the other cell, which is located in a cluster of nerve cells (called an autonomic ganglion). Nerve fibers from these ganglia connect with internal organs. Most of the ganglia for the sympathetic division are located just outside the spinal cord on both sides of it. The ganglia for the parasympathetic division are located near or in the internal organs.

Function: The autonomic nervous system controls blood pressure, heart and breathing rates, body temperature, digestion, metabolism (thus affecting body weight), the balance of water and electrolytes (such as sodium and calcium), the production of body fluids (saliva, sweat, and tears), urination, defecation, sexual response, and other processes.

Many organs are controlled primarily by either the sympathetic or the parasympathetic division. Sometimes the two divisions have opposite effects on the same organ. For example, the sympathetic division increases blood pressure, and the parasympathetic division decreases it. Overall, the two divisions work together to ensure that the body responds appropriately to different situations.

Generally, the sympathetic division prepares the body for stressful or emergency situations—fight or flight. Thus, it increases heart rate and the force of heart contractions and widens (dilates) the airways to make breathing easier. It causes the body to release stored energy. Muscular strength is increased. This division also causes palms to sweat, pupils to dilate, and hair to stand on end. It slows body processes that are less important in emergencies, such as digestion and urination.

The parasympathetic division controls body process during ordinary situations. Generally, it conserves and restores. It slows the heart rate and decreases blood pressure. It stimulates the gastrointestinal tract to process food and eliminate waste. Energy from the processed food is used to restore and build tissues.

Both the sympathetic and parasympathetic divisions are involved in sexual activity, as are the parts of the nervous system that control voluntary actions and transmit sensation from the skin (somatic nervous system).

Two chemical messengers (neurotransmitters), acetylcholine and norepinephrine, are used to communicate within the autonomic nervous system. Nerve fibers that secrete acetylcholine are called cholinergic fibers. Fibers that secrete norepinephrine are called adrenergic fibers. Generally, acetylcholine has parasympathetic (inhibiting) effects and norepinephrine has sympathetic (stimulating) effects. However, acetylcholine has some sympathetic effects. For example, it sometimes stimulates sweating or makes the hair stand on end.

Symptoms

In men, difficulty initiating and maintaining an erection (erectile dysfunction) can be an early symptom of an autonomic disorder. Autonomic disorders commonly cause dizziness or light-headedness due to an excessive decrease in blood pressure when a person stands (orthostatic hypotension).

People may sweat less or not at all and thus become intolerant of heat. The eyes and mouth may be dry.

After eating, a person with an autonomic disorder may feel prematurely full or even vomit because the stomach empties very slowly (gastroparesis). Some people pass urine involuntarily (urinary incontinence), often because the bladder is overactive. Other people have difficulty emptying the bladder (urine retention) because the bladder is underactive. Constipation may occur, or control of bowel movements may be lost.

The pupils may not dilate and narrow (constrict) as light changes.

Diagnosis

Doctors can check for signs of autonomic disorders during the physical examination. They measure blood pressure and heart rate while a person is lying down or sitting and after the person stands. They examine the pupils for abnormal responses or lack of response to changes in light.

Other tests can provide additional information. Tilt table testing may be done to check blood pressure and heart rate responses to changes in position (see Diagnosis of Heart and Blood Vessel Disorders: Tilt Table Testing). Blood pressure is measured after the person, who is lying flat on a pivoting table, is tilted into an upright position. Blood pressure is also measured continuously while the person performs a Valsalva maneuver (forcefully trying to exhale without letting air escape, as during a bowel movement). Electrocardiography is done to determine whether the heart rate changes as it normally does during deep breathing and the Valsalva maneuver.

Sweat testing is also done. For this test, the sweat glands are stimulated by electrodes that are filled with acetylcholine and placed on the legs and wrist. Then, the volume of sweat is measured to determine whether sweat production is normal. A slight burning sensation may be felt during the test. In another test (thermoregulatory sweat test), a dye is applied to the skin, and a person is placed in a closed, heated compartment to stimulate sweating. Sweat causes the dye to change color. As a result, doctors can identify which areas of the body sweat too much or too little.

Other tests may be done to check for disorders that can cause the autonomic disorder.

Treatment

Disorders that may be contributing to the autonomic disorder are treated. If no other disorders are present or if such disorders cannot be treated, the focus is on relieving symptoms.

Simple measures can help relieve some symptoms:

• Orthostatic hypotension: People are advised to elevate the head of the bed by about 4 inches (10 centimeters) and to stand up slowly. Wearing a compression or support garment, such as an abdominal binder or compression stockings, may help. Consuming more salt and water helps maintain blood volume and thus blood pressure. Sometimes drugs (midodrine, pyridostigmine, or fludrocortisone taken by mouth) are used.
• Decreased or absent sweating: If sweating is reduced or absent, avoiding warm environments is useful.
• Urinary retention: If urinary retention is caused by inability of the bladder to contract normally, people can be taught to insert a catheter into the bladder themselves. They insert it several times a day and remove it after the bladder is empty. Bethanechol can be used to increase bladder tone and thus ease bladder emptying.
• Constipation: A high-fiber diet and stool softeners are recommended. If constipation persists, enemas may be necessary.
• Erectile dysfunction: Usually, treatment consists of drugs such as sildenafil, tadalafil, or vardenafil taken by mouth.

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Some Drugs Used to Treat Symptoms of Autonomic Disorders Symptom Drug Drug's Effect Constipation Fiber supplements (such as bran or psyllium) Stool softeners (such as docusate, lactulose, or polyethylene glycol) Fiber supplements add bulk to the stool and thus stimulate the natural contractions of the intestine. Fiber supplements and stool softeners help move food through the intestine more quickly. Fullness in the stomach Metoclopramide This drug stimulates contractions in the gastrointestinal tract and thus helps move food through it more quickly. Erectile dysfunction Sildenafil Tadalafil Vardenafil These drugs increase the frequency, rigidity, and duration of erections. Orthostatic hypotension Fludrocortisone This drug helps the body retain salt and thus helps maintain blood volume and blood pressure. Midodrine This drug causes small arteries (arterioles) to constrict and thus helps maintain blood pressure. Pyridostigmine This drug causes arterioles to constrict only when people stand and thus helps maintain blood pressure when people stand but does not increase blood pressure when they are lying or sitting down. Urinary incontinence Oxybutynin Tolterodine These drugs relax the muscles of an overactive bladder. Urine retention Bethanechol This drug stimulates contractions of the bladder and thus helps the bladder empty.

Last full review/revision November 2006 by Phillip Low, MD