Medical Glossary

A neurotransmitter involved in the autonomic nervous system, particularly the parasympathetic branch. It acts on muscarinic and nicotinic receptors to regulate heart rate, digestion, and glandular secretion.

A rare esophageal motility disorder where the lower esophageal sphincter fails to relax, causing difficulty swallowing. Can co-occur with autonomic dysfunction.

A condition where the adrenal glands do not produce enough cortisol and sometimes aldosterone. Can cause fatigue, low blood pressure, and salt cravings similar to dysautonomia.

Also called epinephrine, a hormone and neurotransmitter released by the adrenal medulla in response to stress. Increases heart rate, blood pressure, and blood glucose.

Relating to nerve cells or fibers that release norepinephrine (noradrenaline) as a neurotransmitter, or to receptors activated by epinephrine and norepinephrine.

A nerve that carries sensory signals from the body to the central nervous system. Afferent autonomic fibers relay information about internal organ status to the brain.

A mineralocorticoid hormone produced by the adrenal cortex that regulates sodium and potassium balance, affecting blood pressure and blood volume. Low aldosterone can worsen orthostatic intolerance.

A receptor found on blood vessel walls that causes vasoconstriction when activated by norepinephrine or epinephrine. Dysfunction can contribute to orthostatic hypotension.

A protein that abnormally aggregates in Lewy bodies and is associated with Parkinson's disease and Multiple System Atrophy. A key biomarker in synucleinopathies.

A disease caused by abnormal protein (amyloid) deposits in organs and tissues. Cardiac and peripheral nerve amyloidosis can cause autonomic neuropathy.

A severe, life-threatening allergic reaction involving systemic mast cell activation. Patients with MCAS may experience anaphylaxis-like episodes.

A condition characterized by a deficiency of red blood cells or hemoglobin, reducing oxygen delivery to tissues. Can worsen fatigue and tachycardia in POTS patients.

The inability to sweat normally, which can lead to overheating. A sign of autonomic neuropathy affecting sudomotor function.

An autoimmune marker found in the blood that indicates the immune system may be attacking the body's own cells. Elevated ANA is common in connective tissue diseases like EDS.

A mental health condition characterized by excessive worry and fear. Autonomic dysfunction can mimic anxiety symptoms, and the two conditions frequently co-occur.

Narrowing of the aortic valve opening, restricting blood flow from the heart. Must be ruled out in patients with syncope and exercise intolerance.

An abnormal heart rhythm. Dysautonomia can cause various arrhythmias including tachycardia, bradycardia, and inappropriate sinus tachycardia.

A rapid feedback mechanism that detects changes in arterial blood pressure and adjusts heart rate and vascular tone to maintain stable blood pressure.

An irregular and often rapid heart rate caused by chaotic electrical signals in the atria. Can co-occur with autonomic dysfunction.

An autoimmune condition where antibodies attack the autonomic ganglia, causing widespread autonomic failure including orthostatic hypotension, anhidrosis, and GI dysmotility.

A severe form of autonomic dysfunction where the autonomic nervous system cannot maintain basic functions like blood pressure regulation, heart rate control, and sweating.

The division of the nervous system that controls involuntary body functions including heart rate, blood pressure, digestion, and glandular activity. Divided into sympathetic and parasympathetic branches.

Damage to the nerves that control automatic body functions. Can be caused by diabetes, autoimmune disease, hereditary conditions, or be idiopathic.

A sudden, extreme surge of autonomic activity causing dangerous spikes in heart rate, blood pressure, temperature, and sweating. Can occur in severe neurological injury.

Sensory receptors in the aortic arch and carotid sinus that detect changes in blood pressure and send signals to the brain to regulate cardiovascular function.

Loss of the normal blood pressure regulation mechanism, leading to extreme blood pressure fluctuations. A severe form of autonomic dysfunction.

A type of white blood cell that releases histamine and other inflammatory mediators. Closely related to mast cells and involved in allergic reactions.

A class of medications that block beta-adrenergic receptors, reducing heart rate and blood pressure. Low-dose beta blockers (e.g., propranolol) are commonly used for POTS.

The abnormal accumulation of blood in the lower extremities when standing, reducing venous return to the heart. A key mechanism in POTS and orthostatic intolerance.

The total amount of blood circulating in the body. Many POTS patients have reduced blood volume (hypovolemia), which worsens orthostatic symptoms.

A heart rate slower than 60 beats per minute. Can occur in some forms of dysautonomia, particularly vasovagal syncope and neurocardiogenic syncope.

Cognitive symptoms including difficulty concentrating, memory problems, mental fatigue, and slowed thinking. Extremely common in dysautonomia, ME/CFS, Long COVID, and MCAS.

The lower part of the brain connecting to the spinal cord. Contains autonomic control centers for heart rate, breathing, and blood pressure. Affected in MSA.

The volume of blood pumped by the heart per minute (heart rate × stroke volume). Reduced cardiac output contributes to exercise intolerance in POTS.

A class of neurotransmitters and hormones including epinephrine, norepinephrine, and dopamine. Catecholamine levels are measured in POTS workup to identify hyperadrenergic subtypes.

An autoimmune disorder triggered by gluten that damages the small intestine. Can cause autonomic neuropathy and is associated with increased MCAS prevalence.

A state of amplified neural signaling in the central nervous system causing hypersensitivity to pain and other stimuli. Common in fibromyalgia, ME/CFS, and CRPS.

The blood supply to the brain. Reduced cerebral blood flow when standing is a key cause of cognitive symptoms and presyncope in dysautonomia.

A structural defect where brain tissue extends into the spinal canal. Can cause autonomic dysfunction and is sometimes associated with POTS and EDS.

Relating to nerve cells that use acetylcholine as a neurotransmitter. Cholinergic dysfunction affects sweating, salivation, lacrimation, and GI motility.

See ME/CFS. A complex, multi-system illness characterized by profound fatigue, post-exertional malaise, cognitive dysfunction, and orthostatic intolerance.

A health condition that persists for a year or more and requires ongoing medical attention or limits activities of daily living.

The most abundant structural protein in the body, providing tensile strength to connective tissues. Defective collagen production is the hallmark of Ehlers-Danlos Syndrome.

Tight-fitting clothing (stockings, abdominal binders) worn to reduce blood pooling in the legs and abdomen. A first-line non-pharmacological treatment for POTS.

Tissue that supports, connects, or separates different types of tissues and organs. Includes tendons, ligaments, cartilage, and the extracellular matrix. Affected in EDS.

A group of diseases affecting the structural proteins of the body. Includes Ehlers-Danlos Syndrome, Marfan syndrome, and Loeys-Dietz syndrome.

Narrowing of the coronary arteries due to plaque buildup, reducing blood flow to the heart. Must be ruled out in patients with chest pain and autonomic symptoms.

A steroid hormone produced by the adrenal cortex in response to stress. Regulates metabolism, immune function, and blood pressure. Dysregulated in ME/CFS.

A chronic pain condition characterized by severe burning pain, swelling, skin changes, and autonomic dysfunction in a limb, typically following injury or surgery.

Small proteins released by immune cells that regulate inflammation and immune responses. Elevated cytokines contribute to symptoms in ME/CFS, Long COVID, and MCAS.

Physical deterioration from prolonged inactivity or bed rest. Can worsen POTS symptoms and must be addressed through carefully graded exercise rehabilitation.

Insufficient fluid in the body, reducing blood volume and worsening orthostatic symptoms. POTS patients are advised to maintain high fluid intake (2-3 liters/day).

See Gastroparesis. Food remains in the stomach longer than normal due to impaired stomach muscle contractions.

Damage to the myelin sheath surrounding nerve fibers, slowing nerve conduction. Occurs in multiple sclerosis and some forms of autonomic neuropathy.

A metabolic disease causing high blood sugar. Long-standing diabetes is the most common cause of autonomic neuropathy worldwide.

Nerve damage to the autonomic nervous system caused by chronic high blood sugar. Affects heart rate, blood pressure, digestion, and bladder function.

Excessive, abnormal sweating not caused by heat or exercise. Can be a symptom of autonomic dysfunction, particularly hyperadrenergic POTS.

Impaired relaxation of the heart's left ventricle during filling. Can contribute to exercise intolerance and is found in some POTS patients.

A neurotransmitter involved in reward, movement, and autonomic regulation. Dopamine dysregulation is implicated in MSA and some forms of dysautonomia.

An umbrella term for conditions involving dysfunction of the autonomic nervous system. Includes POTS, NCS, MSA, PAF, IST, AAG, and many other conditions.

Abnormal movement of the digestive tract muscles, causing symptoms like nausea, bloating, constipation, or diarrhea. Common in dysautonomia and gastroparesis.

Shortness of breath or difficulty breathing. A common symptom in POTS, particularly during orthostatic stress or exertion.

An ultrasound of the heart that evaluates structure and function. Used to rule out structural heart disease in patients with POTS and syncope.

Swelling caused by excess fluid in body tissues. Dependent edema (in legs and feet) can occur in dysautonomia due to blood pooling and venous insufficiency.

A group of inherited connective tissue disorders caused by defects in collagen synthesis. Characterized by joint hypermobility, skin extensibility, and tissue fragility. Strongly associated with POTS.

Minerals in the blood and body fluids (sodium, potassium, magnesium, chloride) that carry electrical charges and regulate fluid balance. Critical for blood volume management in POTS.

Also called adrenaline. A hormone and neurotransmitter that increases heart rate, blood pressure, and blood glucose in response to stress.

A blood test measuring inflammation. Elevated ESR can indicate autoimmune or inflammatory conditions that may cause autonomic dysfunction.

Inability to perform physical activity at normal levels due to abnormal physiological responses. A hallmark symptom of POTS and ME/CFS.

See Post-Exertional Malaise. Worsening of symptoms following physical or mental exertion.

See Syncope. A temporary loss of consciousness caused by reduced blood flow to the brain.

Persistent tiredness not relieved by rest. One of the most debilitating symptoms across all dysautonomia conditions, ME/CFS, Long COVID, and MCAS.

A chronic condition characterized by widespread musculoskeletal pain, fatigue, sleep problems, and cognitive difficulties. Frequently co-occurs with dysautonomia and ME/CFS.

The sympathetic nervous system's acute stress response, releasing adrenaline and noradrenaline to prepare the body for action. Dysregulated in hyperadrenergic POTS.

A temporary worsening of symptoms in a chronic condition. Dysautonomia flares can be triggered by illness, stress, hormonal changes, heat, or overexertion.

A synthetic mineralocorticoid that increases sodium and water retention, expanding blood volume. One of the most commonly prescribed medications for POTS and orthostatic hypotension.

Consuming large amounts of fluids (2-3 liters/day) to expand blood volume and reduce orthostatic symptoms. A cornerstone of POTS management.

A cluster of nerve cell bodies outside the central nervous system. Autonomic ganglia relay signals between the central nervous system and target organs.

An autoantibody targeting the autonomic ganglia, found in Autoimmune Autonomic Ganglionopathy (AAG). Its presence confirms autoimmune autonomic failure.

Delayed gastric emptying without mechanical obstruction, causing nausea, vomiting, bloating, and early satiety. Common in autonomic neuropathy and diabetes.

Analysis of DNA to identify mutations associated with hereditary conditions. Used to confirm EDS subtypes and other genetic causes of dysautonomia.

A class of corticosteroid hormones (including cortisol) that regulate metabolism and immune function. Used therapeutically in some autoimmune dysautonomias.

A medication that blocks H1 histamine receptors, reducing allergic symptoms. First-line treatment for MCAS alongside H2 antihistamines.

A medication that blocks H2 histamine receptors in the stomach, reducing acid secretion and some MCAS symptoms. Used alongside H1 antihistamines.

The variation in time between heartbeats. Low HRV indicates reduced autonomic flexibility and is a biomarker of autonomic dysfunction in dysautonomia and ME/CFS.

Difficulty tolerating warm environments, which causes blood vessels to dilate and worsens blood pooling and symptoms. Very common in POTS.

A chemical released by mast cells and basophils during allergic reactions. Causes vasodilation, increased vascular permeability, itching, and GI symptoms. Central to MCAS.

A portable ECG device worn for 24-48 hours to record continuous heart rhythm. Used to detect arrhythmias and heart rate patterns in dysautonomia.

A subtype of POTS characterized by elevated standing norepinephrine levels (>600 pg/mL), causing extreme tachycardia, hypertension, tremor, and anxiety when upright.

The most common EDS subtype, characterized by joint hypermobility, chronic pain, and connective tissue fragility. Strongly associated with POTS and autonomic dysfunction.

Joints that move beyond the normal range of motion. A hallmark of hypermobile EDS (hEDS) and associated with increased risk of POTS.

Persistently elevated blood pressure. In hyperadrenergic POTS, blood pressure can spike dangerously when standing.

Low carbon dioxide levels in the blood, often caused by hyperventilation. Can trigger cerebral vasoconstriction and worsen symptoms in POTS.

Abnormally low blood pressure. Orthostatic hypotension (drop of ≥20/10 mmHg on standing) is a key diagnostic criterion for several dysautonomia conditions.

Reduced blood volume. A common finding in POTS that contributes to orthostatic symptoms. Treated with increased fluid and salt intake.

Immunoglobulin E, an antibody involved in allergic reactions. Elevated IgE indicates classical allergy; MCAS can occur with normal IgE levels.

Stimulation of the immune system, leading to inflammation and cytokine release. Chronic immune activation is implicated in ME/CFS, Long COVID, and MCAS.

A condition where the sinus node fires faster than normal without an identifiable cause, resulting in a resting heart rate >100 bpm and symptoms of palpitations and fatigue.

The body's immune response to injury or infection. Chronic low-grade inflammation contributes to symptoms in ME/CFS, Long COVID, MCAS, and some dysautonomias.

A type of cytokine that mediates communication between immune cells. Elevated interleukins (IL-6, IL-8, IL-1β) are found in ME/CFS and Long COVID.

A medication that selectively reduces heart rate by blocking the If channel in the sinus node. Used off-label for POTS and IST to reduce tachycardia without lowering blood pressure.

A condition characterized by joints that move beyond normal range, causing pain and instability. Now considered part of the hypermobile EDS spectrum.

An anesthetic and analgesic used in low doses for treatment-resistant pain conditions including CRPS. Works by blocking NMDA receptors.

A metabolic byproduct produced during anaerobic exercise. Abnormal lactate metabolism is found in some ME/CFS patients, contributing to post-exertional malaise.

Abnormal protein aggregates (primarily alpha-synuclein) found in neurons in Parkinson's disease and Lewy body dementia. Also present in MSA.

Persistent symptoms lasting more than 12 weeks after acute COVID-19 infection. Commonly causes dysautonomia (especially POTS), fatigue, brain fog, and breathlessness.

A dietary approach that reduces intake of histamine-rich foods (aged cheeses, fermented foods, alcohol) to manage MCAS symptoms.

An immune cell found in connective tissues that releases histamine and other mediators in response to allergens and pathogens. Central to MCAS pathophysiology.

A condition where mast cells inappropriately release mediators causing recurrent multi-system symptoms including flushing, hives, GI distress, and anaphylaxis.

A complex, multi-system illness characterized by profound fatigue, post-exertional malaise, cognitive dysfunction, unrefreshing sleep, and orthostatic intolerance.

A region in the brainstem that integrates autonomic signals to regulate heart rate and blood pressure. Damaged in MSA.

Pooling of blood in the abdominal blood vessels after eating, which can worsen orthostatic symptoms. Contributes to postprandial hypotension.

An alpha-1 adrenergic agonist that causes vasoconstriction, raising blood pressure. Used to treat orthostatic hypotension and POTS.

A neurological condition causing severe headaches, often with nausea, light sensitivity, and aura. Frequently co-occurs with dysautonomia, POTS, and MCAS.

Impaired function of mitochondria, the cellular energy producers. Implicated in ME/CFS, Long COVID, and some forms of autonomic neuropathy.

Chronic Inflammatory Response Syndrome triggered by mold exposure. Can cause symptoms overlapping with MCAS and dysautonomia.

A rare, progressive neurodegenerative disease causing autonomic failure, parkinsonism, and cerebellar ataxia. Caused by alpha-synuclein accumulation in glial cells.

An unpleasant sensation of needing to vomit. Common in dysautonomia, particularly with gastroparesis, postprandial hypotension, and orthostatic stress.

A test measuring the speed and strength of electrical signals in peripheral nerves. Used to diagnose peripheral neuropathy including small fiber neuropathy.

Also called vasovagal syncope. A common cause of fainting triggered by prolonged standing, emotional stress, or pain, causing sudden drops in heart rate and blood pressure.

Inflammation of the nervous system. Implicated in ME/CFS, Long COVID, fibromyalgia, and some forms of dysautonomia.

Damage or dysfunction of peripheral nerves. Autonomic neuropathy affects the nerves controlling involuntary functions.

Small protein-like molecules used by neurons to communicate. Substance P and CGRP are neuropeptides involved in pain and MCAS.

A neurotransmitter and hormone released by sympathetic nerve endings and the adrenal medulla. Elevated standing norepinephrine is a marker of hyperadrenergic POTS.

A drop in blood pressure of ≥20 mmHg systolic or ≥10 mmHg diastolic within 3 minutes of standing. A key diagnostic criterion for autonomic failure.

Difficulty tolerating upright posture due to abnormal cardiovascular responses. An umbrella term encompassing POTS, orthostatic hypotension, and related conditions.

An abnormal increase in heart rate upon standing. The defining feature of POTS (increase ≥30 bpm in adults, ≥40 bpm in adolescents within 10 minutes of standing).

A self-management strategy for ME/CFS and dysautonomia involving staying within one's energy envelope to prevent post-exertional malaise and symptom flares.

Awareness of the heartbeat, often described as racing, pounding, or fluttering. A very common symptom of POTS, IST, and other dysautonomias.

The 'rest and digest' division of the autonomic nervous system. Slows heart rate, stimulates digestion, and promotes recovery. Dysfunction causes tachycardia and GI problems.

A neurodegenerative disease causing motor symptoms (tremor, rigidity, bradykinesia) and autonomic dysfunction. Related to MSA through alpha-synuclein pathology.

Damage to the peripheral nerves outside the brain and spinal cord. Can affect sensory, motor, or autonomic nerve fibers.

The level of norepinephrine in blood plasma. Elevated levels (>600 pg/mL standing) define hyperadrenergic POTS.

A potent phospholipid mediator released by mast cells and other immune cells. Involved in severe allergic reactions and MCAS.

A hallmark symptom of ME/CFS and Long COVID where physical or mental exertion causes a disproportionate worsening of symptoms lasting 12-48+ hours.

A drop in blood pressure after eating, caused by blood pooling in the digestive system. Common in autonomic failure and elderly patients.

A form of dysautonomia characterized by an abnormal increase in heart rate upon standing (≥30 bpm), causing symptoms of lightheadedness, fatigue, brain fog, and palpitations.

The sensation of nearly fainting, including lightheadedness, tunnel vision, nausea, and weakness, without actual loss of consciousness.

A non-selective beta blocker used in low doses (10-20 mg) for POTS to reduce heart rate and sympathetic activity. One of the most commonly prescribed POTS medications.

Lipid compounds released by mast cells and other cells that mediate inflammation, pain, and fever. Elevated prostaglandins contribute to MCAS symptoms.

A rare neurodegenerative disease causing isolated autonomic failure without motor or cerebellar involvement. Characterized by severe orthostatic hypotension.

A measure of the time for the heart's electrical system to recharge between beats, corrected for heart rate. Prolonged QTc increases risk of dangerous arrhythmias.

Episodes of reduced blood flow to the fingers and toes in response to cold or stress, causing color changes. Associated with connective tissue disorders and dysautonomia.

Exercise performed lying down or in a reclined position (e.g., rowing, recumbent cycling, swimming). Recommended for POTS patients who cannot tolerate upright exercise.

A hormonal system that regulates blood pressure and fluid balance. Dysregulation of RAAS contributes to hypovolemia in POTS.

A heart rate above 100 bpm at rest. Characteristic of IST and can occur in hyperadrenergic POTS.

Increasing dietary sodium intake (3,000-10,000 mg/day) to expand blood volume and reduce orthostatic symptoms. A key non-pharmacological treatment for POTS.

A neurotransmitter involved in mood, sleep, and GI function. Serotonin dysregulation is implicated in IBS, fibromyalgia, and some dysautonomias.

The heart's natural pacemaker located in the right atrium. Dysfunction of the sinus node causes IST and sick sinus syndrome.

An autoimmune disease causing dry eyes and mouth due to immune attack on exocrine glands. Can cause autonomic neuropathy and small fiber neuropathy.

Damage to the small unmyelinated nerve fibers that carry pain, temperature, and autonomic signals. Causes burning pain, numbness, and autonomic symptoms.

An electrolyte essential for fluid balance and nerve function. POTS patients are advised to increase sodium intake to expand blood volume.

Pooling of blood in the abdominal (splanchnic) blood vessels when upright. A major contributor to orthostatic symptoms in POTS.

An injection of local anesthetic into the stellate ganglion to block sympathetic nerve activity. Used experimentally for hyperadrenergic POTS and CRPS.

The volume of blood pumped by the heart with each beat. Reduced stroke volume is a key mechanism in POTS, compensated by increased heart rate.

The autonomic control of sweat glands. Sudomotor dysfunction (anhidrosis or hyperhidrosis) is a sign of autonomic neuropathy.

The 'fight or flight' division of the autonomic nervous system. Increases heart rate, blood pressure, and blood glucose. Overactivation contributes to hyperadrenergic POTS.

A temporary loss of consciousness caused by reduced blood flow to the brain. Common in neurocardiogenic syncope, orthostatic hypotension, and POTS.

A heart rate above 100 beats per minute. The defining feature of POTS and IST. Can cause palpitations, lightheadedness, and chest discomfort.

A diagnostic test where the patient is strapped to a table that tilts from horizontal to vertical to evaluate blood pressure and heart rate responses. The gold standard for diagnosing POTS and NCS.

Involuntary rhythmic muscle contractions causing shaking. Can occur in hyperadrenergic POTS due to excess norepinephrine, and in MSA due to parkinsonism.

An enzyme released by mast cells during activation. Elevated serum tryptase (>20 ng/mL baseline) is a diagnostic criterion for systemic mastocytosis and supports MCAS diagnosis.

Hives — raised, itchy welts on the skin caused by histamine release. A common manifestation of MCAS.

The level of activity of the vagus nerve. High vagal tone is associated with good cardiovascular health; low vagal tone is associated with autonomic dysfunction.

The longest cranial nerve, carrying parasympathetic signals to the heart, lungs, and digestive system. Vagal dysfunction contributes to dysautonomia and gastroparesis.

The degree of constriction or dilation of blood vessels. Impaired vascular tone regulation is a key mechanism in orthostatic hypotension and POTS.

Narrowing of blood vessels, increasing blood pressure. Impaired vasoconstriction when standing is a key cause of orthostatic hypotension.

Widening of blood vessels, decreasing blood pressure. Excessive vasodilation contributes to blood pooling and orthostatic symptoms in POTS.

See Neurocardiogenic Syncope. Fainting caused by an exaggerated vagal response to triggers like standing, pain, or emotional stress.

The flow of blood back to the heart from the peripheral circulation. Reduced venous return when standing is the primary mechanism of orthostatic intolerance.

Problems with the inner ear balance system. Can cause dizziness and vertigo that overlap with dysautonomia symptoms.

A viral infection that precipitates the onset of a chronic condition. Many POTS, ME/CFS, and Long COVID cases are triggered by viral infections.

Devices worn on the body (smartwatches, fitness trackers) that monitor physiological parameters like heart rate, HRV, SpO2, and activity. Useful for tracking dysautonomia symptoms.

Dry mouth caused by reduced salivary gland function. A symptom of cholinergic autonomic dysfunction and Sjögren's syndrome.