Autonomic Testing for Dysautonomia

Autonomic nerve disorders (dysautonomia) refer to disorders of the autonomic nervous system (ANS) function. Dysautonomia is a general term used to describe a breakdown or abnormal function of the ANS. The autonomic nervous system controls much of your involuntary functions. Symptoms are wide-ranging and can include problems with the regulation of heart rate, blood pressure, body temperature, perspiration, and bowel and bladder functions. Other symptoms include fatigue, lightheadedness, feeling faint or passing out (syncope), weakness, and cognitive impairment. (1. Mayo Clinic, Autonomic Nerve Disorders 2019)

Some forms of dysautonomia include; Postural Orthostatic Tachycardia Syndrome (POTS), Orthostatic Hypotension (OH),  Orthostatic Intolerance (OI), Neurocarciogenic Syncope (NCS), Pure Autonomic Failure (PAF), Multiple System Atrophy/Shy-Drager syndrome (MSA). 

There are many advanced tests used for further diagnosis once some form of dysautonomia is detected.  The most common tests used are as follows.

The tilt table test is often the standard method used for detecting types of dysautonomia. A patient is strapped to a mechanical table, which will tilt them upright at an angle of at least 60 degrees. Measurements of heart rate and blood pressure are taken while lying flat and then again at intervals while tilted upright. The optimal duration of tilt testing in patients suspected of Autonomic Dysfunction is 40 minutes or until they faint, at which point the test is concluded (Carew, Cooke, O'Connor, Donnelly, Costelloe, Sheehy & Lyons, 2009). Some patients are given a drug called isoproterenol through an intravenous line during this test. Isoproterenol is used to test the sensitivity of beta-receptors. People who have POTS with beta-receptor supersensitivity and a hyperadrenergic state will generally have an exacerbation of symptoms due to this drug. Other drugs that are sometimes used in place of isoproterenol include nitroglycerin, edrophonium, adenosine triphosphate, ephedrine and nitroprusside (Grubb & McMann, 2001).

The tilt table test may be combined with transcranial doppler ultrasonography. Transcranial doppler ultrasonography is useful in detecting a reduction in cerebral blood flow (Fredman, Biermann, Patel, Uppstrom & Auer, 1995). A number of  patients with orthostatic issues will have a decrease in blood flow to the brain upon standing (Low, 2000). This can occur even if the patient does not become hypotensive (Low, 2000).

Bowel motility studies are useful in ascertaining the degree of gastrointestinal involvement present (Grubb, Kanjwal & Kosinski, 2006).

A catecholamine test is sometimes given to patients suspected of having chemical abnormalities. Testing one's levels of catecholamines and their metabolites is also useful in detecting certain tumors, such as pheochromocytoma. Some  patients will have abnormal catecholamine levels, particularly norepinephrine levels. Norepinephrine is the main chemical messenger of the sympathetic nervous system, and is often elevated in certain types of dysautonomia patients. A physician can determine one's total catecholamine levels through analysis of their blood or urine. 

The blood test is quite simple. The patient will have to lie quietly for a period of time (usually around 30 minutes) before their blood is drawn. Some doctors believe a more accurate reading is gained if an intravenous line is placed in the patient's arm before the resting period. This is because the fear and pain associated with needles may induce a rise in catecholamines that could obscure results.

Other doctors will use the 24-hour urine catch method to determine catecholamine levels. The patient is given a container to collect their total urine volume over a 24-hour period. The container is then returned to the physician and catecholamine levels are assayed. 

The cold pressor test is used to activate the blood pressure control system. The patient places their hand in ice water during this test. The physician will monitor blood pressure changes. This test can be used to localize a sympathetic lesion in the afferent or efferent limb of the baroreceptor reflex arc (Engstrom &  Aminoff, 1997). 

Deep breathing/Paced respirations are tests that evaluate autonomic function by measuring heart rate responses to controlled breathing.

Electromyography (EMG) is sometimes used to test the electrical activity of nerves and muscles. A needle electrode(s) is inserted through the skin into the muscle. The measured muscle response to nervous stimulation can detect nerve or muscle disorders. A nerve conduction study may be given concurrently with electromyography. During this test, muscles and nerves are stimulated with small bursts of electricity to determine if they are functioning in a normal manner.

The Hand-grip test is a simple and non-invasive test of sympathetic function. Some physicians feel its sensitivity and specificity compare favorably with the tilt table test (Khurana & Setty, 1996). During this test, a patient squeezes a hand-grip until their arm is fatigued. Those with autonomic dysfunction may have an excessive diastolic blood pressure response to this test.

Magnetic resonance imaging (MRI)/(MRA) can be used to diagnose some conditions thought to be associated with autonomic dysfunction, such as tumors, Chiari, cervical stenosis or the nutcracker phenomenon. This test encompasses a powerful magnet, low-intensity radio waves, and computer technology. The patient lies still inside the magnet. The magnetic field and different radio frequencies allow a specialized computer to generate detailed images of the inside of the body. The Stand-Up MRI may lead to interesting findings in POTS patients.

Microneurography is sometimes used to record traffic from the peripheral nerves. A small needle is placed into a nerve in the leg. Physicians can then measure nerve signals traveling from the brain to the blood vessels. Some POTS patients have nerve damage in their legs (Low, 2000)..

Sweat tests such as the thermoregulatory, resting sweat output and the quantitative sudomotor axon reflex are sometimes given to patients to determine if they are sweating appropriately. Many people with forms of dysautonomia will have impaired sweating abilities. Some patients report inappropriate sweating, including night sweats. 

During the thermoregulatory sweat test, the patient wears a disposable bathing suit and an orange powder is applied to the skin. The patient is then placed in a hot environment to induce sweating. The orange powder will turn purple where the patient sweats. In this way, physicians can determine if the patient has abnormal sweating patterns.

A small battery operated current may be used to stimulate the sweat glands directly during the resting sweat output test. 

The quantitative sudomotor axon reflex test (QSART) is used to determine sweat volume and latency of response. It can also be used to test for postganglionic lesions (Fealey, 1996, p. 297). A reduced or absent sweat response indicates postganglionic sympathetic sudomotor failure. Used in conjunction with the thermoregulatory sweat test, QSART can differentiate a pre- from the postganglionic lesion (Low, 2003, 407-421).

Some patients may be given a 24-hour urinary sodium test. This test is useful in determining if one's plasma volume is normal (Low, 2000). One study performed by researchers at the Mayo Clinic showed that 28.9% of POTS patients excreted less than 100 mEq in 24 hours and 66.4% excreted less than 150 mEq in 24 hours. These researchers concluded that an important first step in the assessment and treatment of POTS is to determine the patient's volume status and institute salt and fluid replacement in those with hypovolemia (Thieben, Sandroni, Sletten, Benrud-Larson, Fealey, Vernino, Lennon, Shen & Low, 2007).

A study of neurocardiogenic patients noted that those with a urinary sodium excretion <170 mmol/24 hrs. were more likely to have reduced plasma volume (El-Sayed & Hainsworth, 1996). These patients responded well to salt loading. 

A patient may also be given a test to determine total red cell mass and plasma volume. Hypovolemia is sometimes found in conjunction with POTS. Plasma volume may be reduced or red cell mass may be reduced in the POTS patient (Low, 1993).

Another test that is sometimes used to determine autonomic dysfunction is the Valsalva maneuver. This is a test of baroreflex responsiveness. A physician will have the patient blow into a tube while observing heart rate and blood pressure changes. Impaired baroreflex appears to be characteristic of moderate to severe forms of dysautonomia (Stewart, 2002). Patients may have an excessive blood pressure increment after cessation of the maneuver that is sometimes associated with headache (Sandroni, Novak, Opfer-Gehrking, Huck & Low, 2000). Some researchers report normal heart rate responses during the maneuver and conclude that the assessment of heart rate responses during this test does not contribute to the diagnosis of POTS (Braune, Wrocklage, Schulte-Monting, Schnitzer & Lucking, 1999).

A Patient may be given various tests to determine how dysautonomia is affecting their heart. Sinus tachycardia commonly occurs in those with dysautonomia. A patient may be asked to wear a Holter monitor to determine if their heart is functioning correctly. This device allows physicians to record the heart's activity over an extended period of time (usually 24 hours). Electrodes are placed at various locations on the patient's chest. These electrodes are attached to a small portable recorder. A patient may be asked to keep a written journal of any symptoms they experience while wearing the Holter monitor. Irregular heart rhythms are recorded and then evaluated by a physician.

Endless-loop recorders or event recorders are sometimes used instead of Holter monitors. These recorders are smaller and can be worn for months at a time. Patients will press a button after experiencing symptoms such as tachycardia or fainting. An ECG of the event will be recorded and transmitted via telephone for a physician to access.  

A stress test can be used to determine how exercise might affect the dysautonomia patient. The patient walks on a treadmill until a target heart rate is reached. The test is ceased if the patient becomes exhausted or develops complications. The heart's electrical activity is recorded while the patient exercises. Sometimes an echocardiogram will be performed along with the stress test. An echocardiogram is an ultrasound of the heart. This test helps physicians identify abnormalities in the heart's structure and function. 

The above are only some of the tests that may be given to a  patient suspected of having autonomic dysregulation. It is important for patients to ask their physician if they should discontinue their medication before taking specific tests. Some medications that have been reported to significantly affect autonomic testing results include chlorpromazine, thioridazine, tricyclic antidepressants, bupropion, mirtazapine, venlafaxine, clonidine, alpha blockers, beta blockers, calcium channel blockers, opiates and topical capsaicin (Sandroni, 1998). A patient taking medication to lessen symptoms will present with decreased symptoms upon testing. A patient should always consult their physician before discontinuing any medication.

Resources

1. Mayo Clinic, Autonomic Nerve Disorders, Full Text Link