The autonomic nervous system
The autonomic nervous system (ANS) is the part of the nervous system that controls the body's involuntary functions — those that happen automatically, without conscious effort. It regulates heart rate, blood pressure, breathing, digestion, temperature, bladder and bowel function, sweating, and many other processes that keep us alive and functioning.[1]
"Autonomic" comes from the Greek for self-governing. The ANS operates largely below the level of conscious awareness, continuously adjusting bodily functions in response to internal and external demands.
The ANS has two main divisions that generally work in opposition to each other:
| Division | Also known as | General effect | Key functions |
|---|---|---|---|
| Sympathetic | "Fight or flight" | Activating — prepares the body for action | Increases heart rate and blood pressure; dilates pupils; redirects blood to muscles; inhibits digestion; stimulates sweating |
| Parasympathetic | "Rest and digest" | Calming — conserves and restores energy | Slows heart rate; lowers blood pressure; promotes digestion; constricts pupils; stimulates glandular secretion |
| Enteric | The "gut brain" | Independent gut regulation | Controls gastrointestinal motility, secretion, and blood flow independently of central input — often affected in dysautonomia |
What happens when you stand up?
Standing upright is one of the most demanding challenges the ANS faces. Approximately 500–700 mL of blood shifts into the lower limbs and abdomen on standing — equivalent to draining a significant portion of the circulatory volume away from the heart and brain. In a normally functioning person, the ANS compensates within seconds: it increases heart rate slightly, constricts blood vessels in the legs and gut, and raises blood pressure to maintain adequate perfusion.[2]
When this compensatory response fails — for any reason — the result is orthostatic intolerance: the collection of symptoms that arise from inadequate circulation on standing. PoTS is the most common form of orthostatic intolerance.[3]
What is dysautonomia?
Dysautonomia is an umbrella term for any disorder of the autonomic nervous system. It is not a single diagnosis but a family of conditions that share a common thread: the ANS is not working as it should, causing a wide range of symptoms that affect multiple organ systems simultaneously.[1]
The symptoms of dysautonomia are notoriously varied and non-specific — which is a major reason why diagnosis is so frequently delayed. A person might present to cardiology with palpitations, to neurology with dizziness, to gastroenterology with nausea, and to psychiatry with anxiety — each presentation reflecting a different facet of the same underlying problem.
How common is it?
Dysautonomia as a category is considerably more common than is widely recognised. PoTS alone is estimated to affect around 1 in 100 people[4] — making it more prevalent than many well-known conditions. Vasovagal syncope (the "common faint") affects up to 40% of the population at some point in their lives.[5] Even rarer forms such as multiple system atrophy, though individually uncommon, cause significant disability.
Despite this, dysautonomia receives little attention in medical education, and many healthcare professionals have limited familiarity with the spectrum of conditions it encompasses. This contributes directly to diagnostic delays that frequently exceed three to five years for PoTS and related disorders.[4]
Who gets dysautonomia?
The answer varies by condition. PoTS predominantly affects women of reproductive age (roughly 80–90% female)[4]; vasovagal syncope affects both sexes equally; multiple system atrophy is slightly more common in men and presents in middle age or later. Dysautonomia can be primary (arising without a known underlying cause), secondary to another condition (such as diabetes, autoimmune disease, or Parkinson's disease), or triggered by an event such as viral illness, pregnancy, or surgery.
The spectrum of dysautonomia
The following covers the major forms of dysautonomia. They range from very common and generally benign (vasovagal syncope) to rare and life-limiting (multiple system atrophy). PoTS sits in the middle of this spectrum — common, chronic, significantly disabling for many, but not life-threatening in itself.
Postural Orthostatic Tachycardia Syndrome (PoTS)
Common · ChronicThe most common chronic form of dysautonomia. Characterised by an abnormal heart rate rise on standing (≥30 bpm, ≥40 bpm in adolescents) without significant blood pressure fall, accompanied by symptoms of orthostatic intolerance for ≥3 months.[6]
Predominantly affects women aged 15–50. Causes significant disability but is not life-threatening. Multiple subtypes exist. Frequently underdiagnosed — average delay of 3–5 years to diagnosis.[4]
Vasovagal syncope (VVS)
Very common · Usually benignThe common faint. A reflex-mediated event in which a paradoxical response to a trigger (pain, fear, prolonged standing, heat, emotional stress) causes sudden drop in heart rate and blood pressure, leading to transient loss of consciousness.[5]
Affects up to 40% of people during their lifetime. Usually benign but can be disabling if frequent. Can coexist with PoTS. Managed with trigger avoidance, physical countermanoeuvres, and occasionally medication.
Orthostatic hypotension (OH)
Common · Often secondaryA sustained fall in blood pressure on standing — defined as a drop of ≥20 mmHg systolic or ≥10 mmHg diastolic within 3 minutes of standing. In contrast to PoTS, heart rate does not rise appropriately (or may not rise at all).[7]
Often secondary to dehydration, medications (antihypertensives, diuretics, alpha-blockers, antidepressants), Parkinson's disease, diabetes, or ageing. Primary OH can occur as part of neurogenic autonomic failure. Important to distinguish from PoTS.
Inappropriate sinus tachycardia (IST)
Uncommon · Not orthostaticA persistently elevated resting heart rate (>100 bpm) and/or exaggerated heart rate response to minimal exertion, in the absence of an identifiable cause and without the postural relationship that characterises PoTS.[6]
Can be difficult to distinguish from PoTS. Key difference: in IST the resting HR is elevated in all positions; in PoTS the HR is often normal supine but rises excessively on standing. Both can coexist. Managed similarly to PoTS in terms of rate control.
Pure autonomic failure (PAF)
Rare · ProgressiveA rare neurodegenerative condition characterised by widespread sympathetic and parasympathetic failure, causing severe orthostatic hypotension, often without compensatory tachycardia, plus urinary, bowel, and sexual dysfunction.[8]
Onset typically in middle to later life. Caused by alpha-synuclein deposition in peripheral autonomic neurons. About 35% of cases progress to multiple system atrophy or Parkinson's disease over time. Managed symptomatically.
Multiple system atrophy (MSA)
Rare · SevereA rare, rapidly progressive neurodegenerative disorder combining autonomic failure (severe OH, urinary dysfunction) with cerebellar ataxia (MSA-C) or Parkinsonism (MSA-P). Caused by alpha-synuclein accumulation in the central nervous system.[8]
Mean age of onset 55–60 years. Median survival 6–10 years from symptom onset. Managed symptomatically — no disease-modifying treatment currently exists. Autonomic features often precede motor symptoms. Supine hypertension is common.
Autoimmune autonomic ganglionopathy (AAG)
Very rare · AutoimmuneAn autoimmune disorder in which antibodies against ganglionic acetylcholine receptors (gAChR) disrupt transmission at autonomic ganglia, causing subacute-onset severe autonomic failure. May present with OH, gastrointestinal dysmotility, urinary retention, and anhidrosis.[9]
Can be paraneoplastic (associated with underlying cancer) or idiopathic. Important to identify as it is potentially treatable with immunotherapy (IVIG, plasma exchange, rituximab). Testing for anti-gAChR antibodies is available via specialist centres.
Small fibre neuropathy (SFN)
Underdiagnosed · VariableA neuropathy affecting small unmyelinated (C) and thinly myelinated (Aδ) nerve fibres, which carry pain, temperature sensation, and autonomic signals. Results in neuropathic pain, dysaesthesia, and autonomic dysfunction — often all three simultaneously.[10]
Strongly associated with PoTS — an estimated 40–60% of PoTS patients have evidence of SFN on skin biopsy. Causes include diabetes, autoimmune disease, genetic mutations, and idiopathic. Standard nerve conduction studies are normal; diagnosis requires skin punch biopsy or corneal confocal microscopy.
Where PoTS fits within dysautonomia
PoTS is by far the most common chronic dysautonomia encountered in primary and secondary care, particularly in women of reproductive age. It is important to understand how it relates to other conditions in the spectrum — both for accurate diagnosis and because the appropriate management differs substantially across conditions.
| Feature | PoTS | Orthostatic hypotension | Vasovagal syncope | Pure autonomic failure / MSA |
|---|---|---|---|---|
| HR on standing | ↑↑ ≥30 bpm rise, sustained | May rise mildly or not at all | Variable; drops in event | Absent compensatory rise (neurogenic) |
| BP on standing | Maintained (no significant drop) | ↓ ≥20/10 mmHg within 3 min | Drops during episode; normal between | Falls markedly, often severe |
| Consciousness | Preserved (presyncope common) | May lose consciousness if severe | Transient loss of consciousness | Can lose consciousness; falls common |
| Typical age | 15–50, female predominance | Any age; more common in elderly | Any age; often younger | Middle-older age |
| Course | Chronic; variable; often improves | Often secondary; treat cause | Episodic; often improves with age | Progressive; life-limiting |
| Diagnosis | Active stand test / tilt table | Active stand test / lying-standing BP | Clinical + head-up tilt | Specialist autonomic testing; MRI |
Conditions that commonly overlap with dysautonomia
Several non-autonomic conditions occur alongside dysautonomia — particularly PoTS — at rates far exceeding chance. Understanding these overlaps is important for complete diagnosis and for avoiding management errors (for example, prescribing graded exercise in someone with coexisting ME/CFS).[11]
Hypermobile EDS (hEDS)
Joint hypermobility contributes directly to blood pooling via lax vessel walls and reduced venous tone. hEDS is the most common heritable connective tissue disorder and is substantially over-represented in PoTS populations.[11]
ME/CFS
Significant symptomatic overlap exists, and PoTS may drive some ME/CFS symptoms via cerebral hypoperfusion. Post-exertional malaise is the critical distinguishing feature — its presence changes management fundamentally.[12]
Mast cell activation (MCAS)
Mast cell mediators (histamine, prostaglandins) can trigger autonomic symptoms and contribute to the vasodilation that worsens PoTS. The triad of PoTS, hEDS, and MCAS is increasingly recognised as a distinct cluster.[11]
Small fibre neuropathy
Damage to the small autonomic fibres innervating blood vessels directly impairs the vasoconstriction needed on standing. SFN is found in skin biopsies of a substantial proportion of PoTS patients.[10]
Long COVID
Autonomic dysfunction — predominantly PoTS — is present in approximately one-third of people with long COVID. Post-COVID PoTS may have distinct features including autoimmune mechanisms and coexisting ME/CFS.[13]
Autoimmune conditions
Sjögren's syndrome, lupus, and other autoimmune conditions can cause autonomic neuropathy. Autoantibodies against adrenergic and muscarinic receptors have been identified in a subgroup of PoTS patients without classical autoimmune disease.[9]
How dysautonomia is diagnosed
Diagnosis begins with clinical suspicion — often prompted by a combination of symptoms (dizziness, palpitations, presyncope, fatigue) that are worse upright and improve lying down. The exact investigations depend on the suspected condition.
Tests used in dysautonomia
| Test | What it assesses | Setting | Relevant for |
|---|---|---|---|
| Active stand test (NASA lean test) | HR and BP response to standing over 10 minutes | Primary care or outpatient clinic | PoTS, orthostatic hypotension |
| Head-up tilt table test (HUT) | Passive orthostatic challenge at 60–70°; more controlled than active stand | Specialist — cardiology / autonomic clinic | PoTS, vasovagal syncope, orthostatic hypotension |
| 24-hour ambulatory BP monitoring | Supine hypertension; circadian BP patterns | GP / outpatient | Hyperadrenergic PoTS, PAF, MSA |
| Quantitative sudomotor axon reflex (QSART) | Sweat gland innervation — reflects postganglionic sympathetic function | Specialist autonomic lab | Small fibre neuropathy, PAF, MSA |
| Skin punch biopsy (intraepidermal nerve fibre density) | Direct quantification of small fibre loss | Specialist (Neurology / Dermatology) | Small fibre neuropathy |
| Standing plasma noradrenaline | Elevated (>600 pg/mL) suggests hyperadrenergic PoTS | Specialist | Hyperadrenergic PoTS subtype |
| Anti-ganglionic AChR antibody | Autoimmune autonomic ganglionopathy | Specialist (Neurology / Immunology) | AAG — subacute-onset severe dysautonomia |
| MRI brain and spine | Cerebellar or brainstem atrophy, MSA pattern | Neurology | MSA, other neurodegenerative dysautonomia |
Red flags — when to act urgently
Most dysautonomias, including PoTS, are not life-threatening emergencies. However, certain features should prompt urgent investigation to exclude more serious pathology.
Features requiring urgent or expedited assessment
- Syncope associated with exertion — raises concern for structural cardiac or arrhythmic cause; urgent ECG and cardiology review
- New neurological symptoms alongside autonomic dysfunction — especially cerebellar signs, Parkinsonism, or rapid cognitive decline — consider MSA
- Subacute onset of severe autonomic failure (weeks rather than months) — consider AAG; urgent specialist review and anti-gAChR antibody testing
- Family history of sudden cardiac death with syncope — exclude channelopathy; urgent cardiology and ECG
- Syncope with no prodrome and injury — raises cardiac risk; needs ECG, Holter monitoring, and specialist review
- Abnormal ECG — QT prolongation, Brugada pattern, pre-excitation, AV block — needs cardiology review before diagnosis of vasovagal or PoTS is accepted
- Severe postural hypotension with no obvious drug cause in middle-older age — consider PAF or MSA; neurology referral
- Possible paraneoplastic context — weight loss, night sweats, subacute autonomic neuropathy — malignancy must be excluded
Features that are consistent with benign dysautonomia (PoTS / VVS)
- Young woman with gradual onset of positional symptoms, normal ECG and bloods
- Clear postural relationship — symptoms better lying down, worse standing
- Symptoms triggered by heat, prolonged standing, large meals, or dehydration
- Post-viral onset (especially post-COVID)
- History of joint hypermobility or known hEDS
- Positive active stand test with symptom reproduction