Clinical Notes : Neurology

162. Syncope 

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Syncope is defined as Transient Loss Of Consciousness (TLOC) due to transient cerebral hypoperfusion, characterized by a rapid onset, short duration, and spontaneous complete recovery.

Syncope shares many clinical features with other disorders; it therefore presents in many differential diagnoses. This group of disorders is labelled TLOC.


TLOC is defined as a state of real or apparent LOC with loss of awareness, characterized by amnesia for the period of unconsciousness, abnormal motor control, loss of responsiveness, and a short duration.

The two main groups of TLOC are

  • ‘TLOC due to head trauma’

  • ‘non-traumatic TLOC’ 


This module deals with non-traumatic TLOC, and does not include Traumatic TLOC


Classification of TLOC

Syncope 1.png

TIA = transient ischaemic attack

TLOC = transient loss of consciousness



The pathophysiological classification centres on a fall in systemic blood pressure (BP) with a decrease in global cerebral blood flow as the defining characteristic of syncope


Pathophysiology of syncope

Syncope 2.png

ANS = autonomic nervous system

auton. = autonomic

BP = blood pressure

OH = orthostatic hypotension

periph. = peripheral

resist. = resistance


A sudden cessation of cerebral blood flow for as short as 6–8 s can cause complete LOC.

A systolic BP of 50–60 mmHg at heart level, i.e. 30–45 mmHg at brain level in the upright position, will cause LOC.

Systemic BP is the product of cardiac output and total peripheral resistance. A fall in either can cause syncope.

However, in syncope, both mechanisms often act together to a varying degree.

All forms of syncope (but mostly reflex syncope and OH) are more likely to occur (or are more severe) when multiple factors are present, e.g.

  • medication causing low BP (due to vasodilatation or hypovolaemia)

  • alcohol use

  • volume depletion (haemorrhage, low fluid intake, diarrhoea, vomiting)

  • pulmonary diseases causing reduction in brain oxygen supply

  • environmental factors (thermal stress)


Etiology of syncope

Syncope 3.png

BP = blood pressure

OH = orthostatic hypotension

POTS = postural orthostatic tachycardia syndrome

VVS = vasovagal syncope.


The initial evaluation should consist of :

  1. Careful history taking concerning present and previous attacks, as well as eyewitness accounts, in person or through a telephone interview.

  2. Physical examination, including supine and standing BP measurements.

  3. Electrocardiogram (ECG)


Evaluation of syncope

Syncope 5.png

BP = blood pressure

ECG = electrocardiogram
H&P exam = history and physical examination

TLOC = transient loss of consciousness.

It is critical to identify patients with a high risk of death.

The presence of structural heart disease or abnormal ECG places them at high risk of death. [9] Causes of syncope that need to be excluded urgently include :

  • Myocardial infarction and ischaemia

    • An acute myocardial infarction or a remote myocardial infarction, especially with left ventricular dysfunction, can result in syncope due to ventricular arrhythmia, which can be life-threatening

  • Cardiac arrhythmias

    • Bradyarrhythmia and tachyarrhythmia are potentially life-threatening conditions that present with syncope.

    • A history of CAD, medicines that promote AV block or torsades de pointes, and increased age increase the likelihood of arrhythmias.

    • Less common causes of cardiac arrhythmias associated with sudden death, such as Wolff-Parkinson-White syndrome and inherited cardiac ion channel abnormalities such as long QT syndrome and Brugada syndrome, should be excluded.


  • Occult haemorrhage

    • Significant haemorrhage from gastrointestinal bleeding, tissue trauma, ruptured aortic aneurysm, ruptured ovarian cyst, ruptured ectopic pregnancy, retroperitoneal haemorrhage, or ruptured spleen may present with syncope.


  • Aortic dissection

    • Needs to be considered in patients with chest and back pain. .


  • Cardiac tamponade

    • May be caused by acute myocardial infarction, aortic dissection, trauma, hypothyroidism, or pericarditis.

    • Increased pericardial pressure causes compression of the heart chambers and subsequently decreases cardiac output. Immediate pericardiocentesis is required.


  • Severe hypoglycaemia

    • Severe hypoglycaemia due to excessive administration of insulin, hepatic disease, or islet of Langerhans tumour may cause syncope.

    • All patients presenting acutely with syncope should have blood glucose level measured as part of the initial assessment.


  • Addison's disease

    • Acute, life-threatening addisonian crisis can present with syncope, nausea and vomiting, fever, hypotension, hyperpigmentation, and electrolyte abnormalities.

    • Presumptive treatment with hydrocortisone is required to correct hypotension.


  • Massive pulmonary embolism

    • Saddle pulmonary embolus produces cardiac outflow obstruction, resulting in decreased cerebral perfusion and syncope.

    • Older patients with pulmonary embolism are more likely to present with syncope (24% of patients above 65 years).

    • The history needs to include possible thromboembolic (TE) risk factors, such as previous TE disease, prolonged immobilisation (e.g., flight >4 hours), smoking, oral contraceptive pill or hormone replacement therapy use, known malignancy, or family history of TE.


The diagnostic process should answer 4 key questions:

1. Was the event TLOC ? 

  • TLOC has four specific characteristics:

    • short duration

    • abnormal motor control

    • loss of responsiveness

    • amnesia for the period of LOC ​

2. In case of TLOC, is it of syncopal or non-syncopal origin ?

  • TLOC is probably syncope when:

    • there are signs and symptoms specific for reflex syncope, syncope due to OH, or cardiac syncope,

    • signs and symptoms specific for other forms of TLOC (head trauma, epileptic seizures, psychogenic TLOC, and/or rare causes)

3. In case of suspected syncope, is there a clear aetiological diagnosis ?

4. Is there evidence to suggest a high risk of cardiovascular events or death ?


Diagnosis of syncope

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Conditions that may be

incorrectly diagnosed as syncope

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LOC = loss of consciousness

PPS = psychogenic pseudosyncope

TIA = transient ischaemic attack

TLOC = transient loss of consciousness


BP = blood pressure

OH = orthostatic hypotension

POTS = postural orthostatic tachycardia syndrome

VVS = vasovagal syncope.

Risk Stratification


Risk stratification is important, for two reasons:

  1. To recognize patients with a likely low-risk condition able to be discharged with adequate patient education.

  2. To recognize patients with a likely high-risk cardiovascular condition requiring urgent investigation


Risk Assessment in syncope

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AF = atrial fibrillation

ARVC = arrhythmogenic right ventricular ardiomyopathy

AV = atrioventricular

BP = blood pressure

b.p.m. = beats per minute

ECG = electrocardiogram
ED = emergency department

ICD = implantable cardioverter defibrillator

LQTS = long QT syndrome

LVEF = left ventricular ejection fraction

SCD = sudden cardiac death

SVT = supraventricular tachycardia

VT = ventricular tachycardia

Management and referral


Low risk syncope may be discharged home from OOH with appropriate education and lifestyle modification advice 

  • reassurance about the benign nature of the disease

  • education regarding awareness and the possible avoidance of triggers and situations (e.g. dehydration and/or hot crowded environments)

  • the early recognition of prodromal symptoms in order to sit or lie down and activate counter-pressure manoeuvres without delay.

  • If possible, triggers should be addressed directly, such as

    • cough suppression in cough syncope

    • micturition in the sitting position, etc.

  • Increased intake of oral fluids may be advised where appropriate advised

  • short term salt supplementation may be recommended where appropriate

Low risk syncope discharged home from OOH should all be :

  • carefully safety-netted and instructed to report back immediately if recurring.

  • referred back to family GP for

    • physical review

    • chronic medication review (especially blood pressure meds)

    • possible further special investigations.

All intermediate and high risk syncope should be referred to A+E for investigation,and/or observation and/or admission


Management and referral of syncope

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BMJ Best Practice.png

Guidelines for the diagnosis and management of syncope

Michele Brignole et al.

European Society of Cardiology (ESC).

European Heart Journal (2018) 00, 1–69 


Assessment of Syncope

BMJ Best Practice

Last reviewed: April 2019

Last updated: November  2018


Transient loss of consciousness ('blackouts') in over 16s

National Institute for Health and Care Excellence

NICE Guideline CG 109

Last updated: September 2014


2017 ACC/AHA/HRS guideline for the evaluation and management of patients with syncope American College of Cardiology; American Heart Association Task Force on Clinical Practice Guidelines; Heart Rhythm Society

Published date: August 2017



Incidence, etiology and predictors of adverse outcomes in 43,315 patients presenting to the emergency department with syncope: an international meta-analysis.

D'Ascenzo F, Biondi-Zoccai G, Reed MJ, et al.

Int J Cardiol. 2013 Jul 15;167(1):57-62.


‘Ten Commandments’ of ESC Syncope Guidelines 2018: The new European Society of Cardiology (ESC) Clinical Practice Guidelines for the diagnosis and management of syncope were launched 19 March 2018 at EHRA 2018 in Barcelona

Michele Brignole

European Heart Journal, Volume 39, Issue 21, 01 June 2018, Pages 1870–1871,


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