Introduction
Transient loss of consciousness (TLoC) affects up to half the population at some point in their lives.1 Vasovagal syncope is the most common cause, with a global prevalence of at least 16%, but TLoC may also be the first presentation of a serious underlying pathology.2 Identifying individuals who require further investigation and evaluation is a challenging but important role of primary care clinicians. The National Institute for Health and Care Excellence (NICE) updated its TLoC guidelines in November 2023.3 This article summarises key aspects in differentiating causes of TLoC, outlines the recommended approach to assessment and referral, and provides an overview of the relevant Driver and Vehicle Licensing Agency (DVLA) rules.
Initial assessment
First, the clinician should try to establish whether a patient truly lost consciousness. A collateral history from a witness or video evidence is often vitally important to understand features of the event and duration of syncope. It may be necessary to ask patients if the clinician can ring friends and family to obtain this information. If an event was unwitnessed, TLoC should be suspected where a patient has a memory gap around a fall. When in doubt, it should be assumed that the patient had TLoC until proven otherwise.
The next step is to categorise TLoC as being potentially due to trauma, syncope, or a non-syncope cause (Figure 1) based on a thorough history of the events leading up to, during, and after the loss of consciousness.4 Syncope is defined as an episode of ‘cerebral hypoperfusion, characterized by a rapid onset, short duration, and spontaneous complete recovery.’4 Other important information to help risk stratify the event includes previous episodes of TLoC, personal and family history of cardiac or neurological disease, and a medication review.
Figure 1. A flow diagram outlining the initial approach to the categorisation and assessment of transient loss of consciousness. Adapted from Brignole et al.4 ECG = electrocardiogram. TIA = transient ischaemic attack. TLoC = transient loss of consciousness.
Non-syncope TLoC
There are numerous causes of non-syncopal TLoC, including hypoglycaemia, intoxication, or seizures, or psychogenic causes. Non-syncopal causes may be suspected from the history, such as illicit drug and alcohol exposure or TLoC in a patient with insulin-dependent diabetes. Sleep attacks occur where people fall asleep suddenly and without warning, often in people with underlying sleep disorders or neurological disease. Excessive daytime sleepiness is a common cause of road traffic accidents so the clinician should consider screening people for sleep apnoea and sleep disorders in this context.
People with seizures may experience an aura prior to TLoC, such as an unpleasant smell or taste. A case series study of 671 patients with TLoC reported that symptoms with a likelihood ratio >2.0 for predicting seizures rather than syncope included tongue biting, incontinence, post-ictal confusion, headaches, and convulsive movements.5
Categorisation of syncope
Syncope should be suspected where trauma and non-syncopal causes seem unlikely. Syncope can be sub-categorised into neurally mediated (including vasovagal), orthostatic, or cardiac causes (Box 1). The ‘3 Ps’ of posture, provocation, and prodrome can help differentiate between these and identify people who are likely to have had an uncomplicated episode of vasovagal syncope.
| Category of syncope | Mechanism | Conditions/events | Typical signs and symptoms |
|---|
| Cardiogenic | Arrhythmias | Bradycardia — sinus bradycardia, atrioventricular block
Tachycardia — ventricular or supraventricular tachycardia, Brugada syndrome, long QT syndrome, for example | Syncope with little or no prodrome
Palpitations and dizziness
Palpitations prior to syncope |
| Structural | Aortic stenosis, myocardial infarction, hypertrophic cardiomyopathy | Syncope during exercise |
| Cardiopulmonary | Pulmonary embolism, pulmonary hypertension, aortic dissection | Syncope during exercise
New breathlessness |
| Neurologic | Vasovagal | Known trigger, for example, pain or prolonged standing | Features of uncomplicated syncope — 3 Ps |
| Situational | Syncope following specific stimulus, for example, coughing, micturition, defecation, or vomiting. |
| Carotid sinus | Syncope following carotid artery pressure, for example, after head turning |
| Orthostatic | Drug induced | Starting new medication, for example, diuretics and vasodilators | Syncope while or immediately after standing, often after sitting or supine for a long period |
| Volume depletion | Following dehydration, vomiting, diarrhoea |
| Autonomic | Parkinson’s disease, spinal injury, diabetic autonomic neuropathy |
Box 1. Categorisation of syncope based on typical signs and symptoms
Vasovagal syncope typically occurs when standing, though it can occur when sitting or lying if there is a strong provoking factor, such as fear, sleep deprivation, or heat. Patients may experience prodromal symptoms such as their vision closing in, light-headedness, or a rising sensation in the abdomen. Syncope that occurs with a clear prodrome and provoking factor, for example, fear, is likely to be vasovagal.
Orthostatic syncope is characterised by the relationship to changes in posture. Reflex syncope should be considered if there was a specific trigger, such as coughing or micturition. Carotid sinus hypersensitivity should be considered if the patient rotated their head immediately before the TLoC or were wearing tight clothing around the neck. The same case series of patients with TLoC reported symptoms that were predictive of cardiac syncope, including chest pain, palpitations, breathlessness, prior cardiac history, and prolonged sitting or standing.5 Lack of prodrome and significant injury also point towards possible cardiac syncope.
A suspected diagnosis from the history can help guide the examination, such as assessing for a heart murmur, irregular pulse, or signs of heart failure. Lying and standing blood pressure (BP) is indicated if the history suggests orthostatic hypotension or where the diagnosis is uncertain. NICE explicitly recommends lying and standing BP with repeated BP measures over 1 to 3 minutes of standing, but recognise sitting to standing BP may be done for practical reasons.3 A drop in systolic BP >20 mmHg after 1 minute of standing is considered significant.6
Further investigations
An electrocardiogram (ECG) should be performed in all patients. NICE recommends that the following ECG changes should be considered as red flags; any conduction abnormality, including any degree of heart block, right or left bundle branch block, any ST segment or T wave abnormalities, and significant changes to the QT interval.3 This is not an exhaustive list and it is also important to compare the recent ECG with previous ones if available, as some of these changes can be longstanding normal variants, such as right bundle branch block.
Blood tests are often not required but may be considered depending on the history, including blood glucose, haemoglobin, electrolytes, thyroid function, and NT-pro-BNP.
Some primary care clinicians may have direct access to extended cardiac monitors or echocardiogram, which may be helpful to further investigate suspected cardiac syncope. However, it is important to recognise the potential limitations of these tests, and NICE recommends that all patients with suspected cardiac or neurological syncope based on the history, examination, and ECG should be referred to secondary care.
Primary care management and referral
Vasovagal, reflex, or orthostatic syncope can usually be diagnosed after the initial assessment and primary care investigations without the need for onward referral. The clinician should provide safety-netting advice, safety advice on how to avoid repeat episodes, and a driving recommendation (see below). For patients with orthostatic hypotension the mechanisms behind their syncope should be explained, medications reviewed, and potential treatment options discussed, if appropriate.
Patients with features that suggest seizure should be urgently referred to a specialist first-seizure clinic. When a cardiogenic cause is suspected, certain red-flag symptoms indicate that an admission or urgent referral is required. These include: ECG abnormalities, heart failure, TLoC during exertion, family history of sudden cardiac death, a murmur, and/or new breathlessness. An urgent referral can also be considered for those aged ≥65 years with TLoC without prodromal symptoms. The clinician should consider discussing patients with the on-call medical team where there is uncertainty and high clinical concern.
Driver and Vehicle Licensing Agency advice
Another critical role for clinicians is to give patients correct advice regarding driving. Road traffic collisions resulting from blackouts are 2–3 times more common than those resulting from seizures.7
There are two particularly important considerations in regard to TLoC and road safety. The first is whether the patient had a reliable prodrome that would be of sufficient time to allow them to safely stop the vehicle during a repeat event. The second is posture, and particular attention should be given if TLoC occurred while sitting.
Group 2 bus and lorry drivers must stop driving and notify the DVLA following any episode of TLoC, even if it was typical for vasovagal syncope. Group 1 car and motorcycle drivers who have typical vasovagal syncope or recurrent syncope with an avoidable trigger can continue to drive without notifying the DVLA, providing this happens when standing. If there is no reliable prodrome, unexplained syncope, cough syncope, a cardiovascular cause is suspected, or there is clinical suspicion of seizure then the patient must not drive and should notify the DVLA. DVLA guidance is continuously under review and these guidelines are likely to be revised in 2025/2026. It is therefore sensible to recheck the recommendations before providing individual patient advice.
Conclusion
TLoC is common and it is important that primary care physicians can accurately distinguish between benign causes and those that require further, urgent investigations and referral. Two-thirds of cases can be diagnosed on the basis of a history, examination, and ECG alone, with vasovagal syncope the most common cause. A witness history is often vital for this assessment. However, patients with a family history of sudden cardiac death, ECG abnormalities, suspected underlying heart disease, or where syncope occurs during exercise should be considered for urgent referral for suspected cardiac syncope. Anyone with a first seizure should also be urgently referred. The key points are listed in Box 2.
Transient loss of consciousness is very common, affecting nearly half the population at some point. While it is often benign, it is important to categorise the TLoC to help guide further investigations and management. Red-flag symptoms include new breathlessness, TLoC during exertion, murmur, abnormal ECG findings, and a family history of sudden death, and should prompt referral to secondary care. A witness history or video of the event is often a vital part of the assessment. It is important to remember to check DVLA guidance and provide patients with the correct driving advice following an episode of TLoC.
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Notes
Funding
No direct funding was received for this article. Nicholas R Jones is funded by an
NIHR Clinical Lectureship in General Practice.
Provenance
Freely submitted; externally peer reviewed.
Competing interests
Thomas Round is Associate Editor for the BJGP. Nicholas R Jones is a writer for Royal College of General Practitioners Essential Knowledge Updates (EKU) e-learning and wrote the EKU 2024.2 module on transient loss of consciousness. Thomas Round is clinical lead for the EKU programme. Kim Rajappan is a Member of the Secretary of State for Transport’s Honorary Medical Advisory Panel on driving and disorders of the cardiovascular system (specialising in electrophysiology).
- © British Journal of General Practice 2025
