Syncope

Definition

• Syncope: transient loss of consciousness from cerebral hypoperfusion with rapid onset, short duration, and spontaneous full recovery.
• Three main groups:
• Reflex syncope (neurally mediated): vasovagal, situational, carotid sinus syndrome.
• Orthostatic syncope: drug-induced, autonomic failure (Parkinson, MSA, diabetes), hypovolemia.
• Cardiac syncope: arrhythmia (brady/tachy), structural (AS, HOCM, PE, tamponade), ischemia — most dangerous group.

Aetiopathogenesis

• Reduced cerebral blood flow is the most common mechanism, usually due to reduced cardiac output: outflow obstruction (e.g. aortic stenosis, hypertrophic cardiomyopathy), systolic or diastolic dysfunction, arrhythmias or reduced venous return.
• Arrhythmias cause syncope when the rate is too fast for adequate ventricular filling (e.g. above 150–180/min) or too slow for adequate cardiac output (e.g. below 30–35/min).
• Vasovagal (neurocardiogenic) syncope: increased intrathoracic pressure, increased vagal tone or loss of sympathetic tone; common and benign.
• Orthostatic hypotension: failure of the normal compensatory mechanisms (sinus tachycardia, vasoconstriction) on standing; the most common cause in older people.
• Rare causes: basilar artery ischemia, pulmonary embolism, hypoglycemia (loss of consciousness rarely as abrupt as in syncope).
• Overall, the most common causes are vasovagal syncope and unexplained (idiopathic) cases; a smaller number have a serious, usually cardiac, cause.

Clinical features

Vasovagal syncope

• Classic triad: prodromes (nausea, sweating, pallor) → syncope → rapid recovery supine.
• Triggers: prolonged standing, warm rooms, pain, sight of blood, micturition, defecation, coughing.

Orthostatic syncope

• Classic orthostatic hypotension: sustained fall in systolic blood pressure of at least 20 mmHg or in diastolic pressure of at least 10 mmHg within 3 minutes of standing.
• Delayed orthostatic hypotension: sustained fall of the same magnitude that occurs only after more than 3 minutes of standing.
• Initial orthostatic hypotension: transient fall (systolic by more than 40 mmHg, diastolic by more than 20 mmHg) within 15 seconds of standing.

Clues to the cause

• Vasovagal: triggered by unpleasant physical or emotional stimuli (pain, fright), usually when upright, with warning symptoms such as nausea, weakness, yawning, blurred vision and sweating; recovery prompt but not immediate (usually 5–15 minutes).
• Cardiac: sudden onset and spontaneous termination without warning, immediate recovery; syncope on exertion suggests outflow obstruction or an exercise-induced arrhythmia, often with chest pain or palpitations; syncope while lying down suggests an arrhythmia.
• Red flags: syncope during exertion, several events within a short time, heart murmur or other signs of structural heart disease, older age, significant injury during syncope, sudden unexpected death in the family.
• Pointers to a seizure rather than syncope: jerking lasting more than a few seconds, incontinence, drooling, tongue biting, postictal confusion.

Diagnosis

• History and witness account: situation, posture and duration of standing before the event, warning symptoms, length of recovery, associated symptoms; past medical history, drugs taken, early sudden cardiac death in the family.
• Physical examination: heart rate and blood pressure supine and after 3 minutes of standing; pulse irregularity, heart murmurs and their change with the Valsalva maneuver, standing or squatting; signs of injury, neurological examination.
• ECG in all patients: arrhythmia, conduction abnormality, hypertrophy, pre-excitation, QT prolongation, Brugada pattern, pacemaker malfunction, ischemia or infarction.
• Pulse oximetry during or immediately after the event (hypoxemia suggesting pulmonary embolism).
• Rhythm recording: ambulatory ECG over at least 24 hours, event recorder or implantable loop recorder depending on the frequency of events.
• Echocardiography for unexplained or exercise-induced syncope, heart murmurs or suspected intracardiac tumors.
• Tilt table testing when an orthostatic or vasovagal cause is suspected.
• Targeted laboratory tests: pregnancy test in women of childbearing age, hematocrit if anemia is suspected, electrolytes, troponin if infarction is suspected.

Implantable loop recorder

• Subcutaneously implanted event recorder for infrequent events, such as syncope at intervals of more than 30 days; the battery lasts several years.
• A memory loop stores the ECG for seconds to minutes before and after activation; recording can be triggered, for example, with a small magnet.

Keep learning in the app

Further reading (open access)

1. MSD Manual Professional: Syncope
2. StatPearls: Syncope
3. MSD Manual Professional: Electrocardiography

Cross-references