Autonomic function tests quantify sympathetic and parasympathetic nervous system function. The standard battery includes heart rate variability (parasympathetic), Valsalva maneuver (sympathetic vasomotor + parasympathetic), tilt table (orthostatic blood pressure response), and quantitative sudomotor axon reflex test (QSART, sympathetic cholinergic small fibers). These tests detect and characterize autonomic neuropathies — diabetic autonomic neuropathy, multiple system atrophy, Sjögren syndrome, hereditary autonomic neuropathies, and pure autonomic failure. This page covers each test’s technique, interpretation, and clinical utility.

Heart Rate Variability (HRV)

Deep Breathing Test

  • Patient breathes deeply at 6 breaths/min for 1 minute.
  • Records ECG and measures heart rate variation with respiration.
  • Tests cardiovagal (parasympathetic) function.
  • Normal: increase in HR during inspiration, decrease during expiration; expiratory:inspiratory ratio (E:I).
  • Decreased E:I ratio: cardiovagal failure.
  • Age-adjusted norms.

30:15 Ratio (Standing)

  • Patient stands up.
  • Heart rate measured at 30 seconds and 15 seconds (ratio of R-R interval at 30:15).
  • Tests parasympathetic response to active standing.
  • Normal: ratio >1.04 (tachycardia at 15, bradycardia at 30).
  • Lower values indicate parasympathetic dysfunction.

Valsalva Maneuver

Protocol

  • Patient blows against resistance maintaining intrathoracic pressure of 40 mmHg for 15 seconds.
  • BP and heart rate monitored continuously.
  • Four phases:
    • Phase I: brief BP rise at strain onset.
    • Phase II early: BP fall with strain.
    • Phase II late: BP recovery as compensation.
    • Phase III: brief BP fall at strain release.
    • Phase IV: BP overshoot above baseline.

Measurements

  • Valsalva ratio (VR): ratio of fastest heart rate during strain to slowest after release.
  • Normal: >1.5.
  • Decreased VR: cardiovagal dysfunction.
  • Absent Phase IV overshoot: sympathetic vasomotor failure.
  • Excessive Phase II BP drop: sympathetic dysfunction.

Clinical Interpretation

  • Tests both parasympathetic (HR changes) and sympathetic vasomotor (BP changes).
  • Comprehensive evaluation of autonomic function in single test.

Tilt Table Test

Protocol

  • Patient supine on tilt table; HR and BP monitored.
  • After baseline, tilted head-up to 60-80°.
  • Position maintained for 5–30 minutes.
  • Continuous BP and HR monitoring.

Normal Response

  • Mild BP drop in first minute (recovers).
  • Sustained HR increase of 10–20 bpm.
  • No syncope, presyncope, or sustained BP drop.

Orthostatic Hypotension (Diagnosis)

  • Sustained drop in SBP >20 mmHg or DBP >10 mmHg within 3 minutes of standing.
  • Categories:
    • Classic orthostatic hypotension.
    • Initial orthostatic hypotension: transient (15 sec) drop >40 mmHg.
    • Delayed orthostatic hypotension: occurs after 3+ minutes.

Postural Orthostatic Tachycardia Syndrome (POTS)

  • HR increase >30 bpm (or HR >120) within 10 minutes of standing.
  • WITHOUT sustained BP drop.
  • Often associated with symptoms of fatigue, palpitations, presyncope.
  • Most common in younger women.

Vasovagal Syncope

  • Symptomatic BP and HR drop after prolonged standing.
  • Cardioinhibitory (HR drop), vasodepressor (BP drop), or mixed type.
  • Common diagnosis on tilt testing.

QSART (Quantitative Sudomotor Axon Reflex Test)

Protocol

  • Acetylcholine iontophoresed into the skin via low-voltage current.
  • Stimulates postganglionic sympathetic cholinergic axons.
  • Axon reflex triggers sweating in adjacent skin (not directly under iontophoresis).
  • Sweat volume measured at 4 standard sites: forearm, proximal leg, distal leg, foot.

Interpretation

  • Normal: characteristic temporal sweat response.
  • Absent or reduced response: postganglionic sympathetic cholinergic dysfunction.
  • Distal predominance: small-fiber neuropathy with sympathetic involvement.
  • Generalized loss: severe autonomic failure.

Clinical Utility

  • Sensitive for small-fiber neuropathy with autonomic involvement.
  • Useful when routine NCS is normal but autonomic symptoms present.
  • Helps differentiate preganglionic from postganglionic dysfunction.

Other Autonomic Tests

Thermoregulatory Sweat Test (TST)

  • Patient placed in heated chamber.
  • Indicator dye applied to skin shows sweating pattern.
  • Maps anatomic distribution of sweat loss.
  • Useful for diagnosis of small-fiber and central autonomic disorders.

Sympathetic Skin Response

  • Stimulus (often electrical to wrist) triggers brief sympathetic-mediated skin potential.
  • Recorded from palms and soles.
  • Absence suggests sympathetic dysfunction.
  • Less reliable than QSART.

Catecholamine Levels (Supine and Standing)

  • Plasma norepinephrine measured supine and 5 min after standing.
  • Normal: 2–4 fold increase with standing.
  • Reduced or absent increase: sympathetic failure (pure autonomic failure, MSA).
  • Increased baseline: pheochromocytoma.

Pupillary Response

  • Pharmacologic pupillary tests (pilocarpine, hydroxyamphetamine).
  • Distinguishes parasympathetic vs sympathetic causes.
  • Adie’s pupil: parasympathetic denervation pattern.

Comprehensive Autonomic Function Test Battery

Standard “AFT” battery includes:

  • Deep breathing test (HRV).
  • Valsalva maneuver.
  • Tilt table.
  • QSART.

Composite Autonomic Severity Score (CASS) quantifies overall autonomic dysfunction.

Interpretation Patterns

Cardiovagal Failure

  • Decreased E:I ratio.
  • Decreased Valsalva ratio.
  • Decreased 30:15 ratio.
  • Examples: diabetic autonomic neuropathy (often earliest sign), amyloidosis, vagotomy.

Sympathetic Vasomotor Failure

  • Orthostatic hypotension.
  • Absent Phase IV Valsalva overshoot.
  • Reduced plasma NE response.
  • Examples: MSA, pure autonomic failure, diabetic, hereditary autonomic neuropathy.

Sympathetic Sudomotor Failure

  • Abnormal QSART.
  • Abnormal thermoregulatory sweat test.
  • Examples: small-fiber neuropathy, Sjögren syndrome, hereditary sensory autonomic neuropathies.

Multiple System Atrophy (MSA)

  • Combined parkinsonism + autonomic failure + cerebellar.
  • Tests: orthostatic hypotension, abnormal QSART, normal sweating in proximal vs distal abnormality.
  • Distinguished from Parkinson disease with autonomic features by severity and distribution.

Pure Autonomic Failure

  • Profound sympathetic failure without other neurologic features.
  • Severe orthostatic hypotension.
  • QSART abnormal.
  • Better prognosis than MSA.

Specific Disease Patterns

Diabetic Autonomic Neuropathy

  • Cardiovagal failure first (decreased E:I, Valsalva).
  • Then sympathetic involvement.
  • QSART abnormal distally.
  • Tracks neuropathy severity.

Sjögren Syndrome

  • Sensory ganglionopathy + small-fiber neuropathy.
  • QSART often abnormal.
  • Sicca symptoms.
  • Anti-Ro/SSA positive.

Amyloidosis

  • Severe autonomic dysfunction.
  • Both cardiovagal and sympathetic.
  • Specific patterns: ATTR (transthyretin) or AL (light chain).

Hereditary Autonomic Neuropathies

  • HSAN (hereditary sensory autonomic neuropathy) subtypes.
  • Variable patterns.
  • Familial dysautonomia (Riley-Day): severe autonomic failure from infancy.

Botulism

  • Acute autonomic failure (especially anticholinergic effects: dry mouth, urinary retention).

Acute Pandysautonomia

  • Subacute severe autonomic failure.
  • Often post-infectious.
  • Variable recovery.

Provocative Testing

Cold Pressor Test

  • Patient immerses hand in cold water (4°C) for 1–2 minutes.
  • Triggers sympathetic-mediated BP increase.
  • Reduced response: sympathetic dysfunction.

Isometric Exercise

  • Sustained handgrip increases BP.
  • Reduced response: sympathetic dysfunction.

Mental Stress Test

  • Mental arithmetic or stressful imagery.
  • Activates sympathetic system.
  • Reduced response: sympathetic dysfunction.

Treatment Implications

Orthostatic Hypotension

  • Non-pharmacologic: salt and fluid, compression stockings, head-of-bed elevation.
  • Fludrocortisone: volume expansion.
  • Midodrine: α1 agonist.
  • Droxidopa: NE precursor.
  • Pyridostigmine: AChE inhibitor.

POTS

  • Salt, fluids, exercise reconditioning.
  • Beta-blockers, ivabradine, fludrocortisone.

Vasovagal Syncope

  • Trigger avoidance.
  • Counter-pressure maneuvers.
  • Beta-blockers (variable benefit).
  • Salt, fluids.

🔍 Did You Know?

The recognition of postural orthostatic tachycardia syndrome (POTS) as a distinct clinical entity has revolutionized the evaluation of patients with chronic fatigue, lightheadedness, and palpitations on standing. POTS — defined by sustained HR increase >30 bpm (or HR >120) within 10 minutes of standing without orthostatic hypotension — affects predominantly young women (15:1 female-to-male ratio) and was historically often misdiagnosed as anxiety disorder, deconditioning, or “functional” symptoms. The pathophysiology is complex and heterogeneous, involving impaired venous return, hyperadrenergic state, hypovolemia, neuropathic POTS (autonomic small-fiber dysfunction), or combinations. The clinical implication is profound: POTS patients have a real, measurable autonomic dysfunction that can be quantified with tilt testing, and treatment (exercise reconditioning, salt and fluid, beta-blockers, ivabradine, sometimes fludrocortisone) can substantially improve symptoms. For practicing neurologists, the take-home is that chronic fatigue and orthostatic symptoms in young women warrant tilt testing before being attributed to psychiatric causes. The same principle generalizes: autonomic dysfunction is increasingly recognized as a contributor to many “unexplained” syndromes, including post-COVID syndrome, chronic fatigue syndrome, fibromyalgia, and others. Quantitative autonomic testing provides objective evidence that can validate symptoms, guide treatment, and document response to therapy.

Pitfalls and Pearls

  • Standard battery: deep breathing, Valsalva, tilt, QSART.
  • Deep breathing E:I ratio: tests cardiovagal (parasympathetic).
  • Valsalva ratio >1.5: normal cardiovagal.
  • Valsalva Phase IV overshoot: sympathetic vasomotor function.
  • Tilt table: diagnose orthostatic hypotension (SBP drop >20 within 3 min).
  • POTS: HR increase >30 within 10 min standing, no BP drop.
  • QSART: postganglionic sympathetic cholinergic small fibers; sensitive for small-fiber neuropathy.
  • Thermoregulatory sweat test: anatomic distribution of sweat loss.
  • Catecholamines: supine and standing; reduced rise in autonomic failure.
  • Composite Autonomic Severity Score (CASS): quantifies severity.
  • Diabetic autonomic neuropathy: cardiovagal failure first; QSART abnormal later.
  • MSA: parkinsonism + autonomic failure + cerebellar.
  • Pure autonomic failure: severe sympathetic failure without other neuro features.
  • Sjögren: ganglionopathy + small-fiber; anti-Ro/SSA.
  • Amyloidosis: severe autonomic dysfunction; ATTR or AL.
  • POTS treatment: salt, fluid, exercise reconditioning, beta-blockers, ivabradine.
  • Orthostatic hypotension treatment: fludrocortisone, midodrine, droxidopa.

References

  1. Low PA. Clinical Autonomic Disorders. 3rd ed. Lippincott Williams & Wilkins; 2008.
  2. Shibao C, Lipsitz LA, Biaggioni I. ASH position paper: evaluation and treatment of orthostatic hypotension. J Clin Hypertens (Greenwich). 2013;15(3):147-153.
  3. Sletten DM, Suarez GA, Low PA, Mandrekar J, Singer W. COMPASS 31: a refined and abbreviated Composite Autonomic Symptom Score. Mayo Clin Proc. 2012;87(12):1196-1201.
  4. Sheldon RS, Grubb BP 2nd, Olshansky B, et al. 2015 Heart Rhythm Society Expert Consensus Statement on the diagnosis and treatment of postural tachycardia syndrome, inappropriate sinus tachycardia, and vasovagal syncope. Heart Rhythm. 2015;12(6):e41-63.
  5. Freeman R, Wieling W, Axelrod FB, et al. Consensus statement on the definition of orthostatic hypotension, neurally mediated syncope and the postural tachycardia syndrome. Auton Neurosci. 2011;161(1-2):46-48.