Examining the autonomic nervous system and the autonomic syndromes

What it is

The autonomic nervous system works without conscious control: the sympathetic division prepares the body for effort and the parasympathetic division for rest and digestion. When it fails, patients feel faint on standing, sweat too much or too little, and have bladder, bowel or sexual problems. A few simple bedside tests, above all orthostatic blood pressure and heart rate, show whether it is working.

Landmark anatomy

• Sympathetic outflow leaves the thoracic and upper lumbar cord, relays in the sympathetic chain beside the spine and reaches the heart, vessels, sweat glands and pupil dilator.
• Parasympathetic outflow travels in cranial nerves (the oculomotor to the pupil, the vagus to the heart and gut) and in sacral roots to the bladder and bowel.
• The hypothalamus and brainstem integrate these reflexes, such as the baroreflex that keeps blood pressure steady on standing.
• The sympathetic pathway to the eye runs through the neck, so neck and chest lesions can cause Horner syndrome.

Technique, step by step

1. Ask about faintness on standing or after meals, heat intolerance, change in sweating, constipation or diarrhea, bladder problems and erectile dysfunction.
2. Measure blood pressure and heart rate after the patient has been lying down for about five minutes.
3. Repeat at one and three minutes after standing: a fall of 20 systolic or 10 diastolic (mm Hg) is orthostatic hypotension.
4. Check whether the heart rate rises on standing: little or no rise despite low pressure suggests neurogenic failure.
5. Heart rate with deep breathing: it should speed up on breathing in and slow on breathing out; loss of this variation means vagal damage.
6. Inspect the pupils for size and reaction, and look for Horner syndrome (small pupil, mild ptosis) or a large, poorly reactive pupil.
7. Feel the skin for dryness or excess sweating, and note its color and temperature in the feet.
8. Ask about urinary retention or incontinence, and check for a distended bladder.

Abnormal findings

• Neurogenic orthostatic hypotension: pressure falls on standing without a compensatory rise in heart rate; seen in diabetes, Parkinson disease, multiple system atrophy and pure autonomic failure.
• Postural orthostatic tachycardia syndrome: heart rate rises by more than 30 per minute on standing (more than 40 in adolescents) without hypotension.
• Horner syndrome: small pupil, mild ptosis and sometimes loss of facial sweating on the same side, from a lesion of the sympathetic pathway.
• Autonomic dysreflexia: in spinal cord injury at or above about T6, a stimulus below the lesion (a full bladder) triggers severe hypertension, headache and flushing above the lesion, often with bradycardia.
• Diabetic autonomic neuropathy: resting tachycardia, orthostatic hypotension, delayed gastric emptying, bladder dysfunction and hypoglycemia without warning symptoms.

Common errors

• Not resting long enough before the lying measurement, or measuring only once on standing.
• Blaming autonomic failure without first excluding dehydration, bleeding and medicines that lower blood pressure.
• Formal tests (tilt table, sweat tests, heart rate response to the Valsalva maneuver) are done in specialized laboratories.

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Educational content on the clinical exam; it is learned and practised under supervision. It does not replace assessment by a health professional and is not for self-diagnosis.

Sources: Overview of the Autonomic Nervous System (MSD Manual Professional Edition, 2024); Autonomic neuropathy (MedlinePlus, U.S. National Library of Medicine, 2024).

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Representative model, for educational purposes only.