35. Syncope

Definition

Syncope = sudden, brief loss of consciousness due to temporary global cerebral hypoperfusion, with rapid spontaneous recovery.

Think: not enough blood/oxygen reaching the brain for a few seconds.

Classification

1. Reflex / neurally mediated syncope

Most common and usually benign.

Main types:

Vasovagal syncope

Triggered by emotional stress, pain, fear, blood, heat, prolonged standing.

Situational syncope

Occurs during specific actions, for example:

• coughing

• urination

• defecation

• swallowing

• after meals

Carotid sinus syncope

Pressure on carotid sinus causes excessive vagal response.

Triggers:

• tight collars

• shaving

• turning the head

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2. Orthostatic hypotension syncope

Occurs when standing up because BP drops too much.

Common causes:

• dehydration

• blood loss

• prolonged bed rest

• autonomic dysfunction

• antihypertensive drugs

• diabetic neuropathy

• Parkinson disease

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3. Cardiac syncope

Most dangerous type.

Arrhythmias

Heart rhythm is too fast or too slow → reduced cardiac output.

Examples:

• ventricular tachycardia

• AV block

• sick sinus syndrome

Structural heart disease

Heart cannot pump blood effectively.

Examples:

• aortic stenosis

• hypertrophic cardiomyopathy

• myocardial infarction

• pulmonary embolism

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4. Cerebrovascular syncope

Rare.

Due to impaired blood flow to brainstem/cerebral circulation.

Examples:

• vertebrobasilar insufficiency

• subclavian steal syndrome

Etiology

Reflex causes

• emotional stress

• pain

• fear

• prolonged standing

• heat

• coughing

• urination

• defecation

• carotid sinus stimulation

Orthostatic causes

• dehydration

• hemorrhage

• diuretics

• vasodilators

• alcohol

• autonomic failure

• diabetic neuropathy

Cardiac causes

• arrhythmias

• aortic stenosis

• hypertrophic cardiomyopathy

• ischemic heart disease

• pulmonary embolism

• heart failure

Other possible mimics

These are not true syncope, but can look similar:

• epilepsy

• hypoglycemia

• panic attacks

• stroke/TIA

• intoxication

Pathogenesis

Core mechanism

Syncope happens when there is:

↓ cerebral perfusion → ↓ oxygen/glucose delivery to brain → transient loss of consciousness

The brain is very sensitive to reduced blood flow, so even a short drop can cause fainting.

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Reflex syncope pathogenesis

Trigger causes abnormal autonomic reflex:

trigger → ↑ vagal activity + ↓ sympathetic tone → bradycardia + vasodilation → ↓ BP → ↓ cerebral blood flow → syncope

In simple words:

vessels dilate + heart slows → blood pressure drops → brain gets less blood → fainting

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Orthostatic syncope pathogenesis

When standing:

gravity pulls blood into legs → ↓ venous return → ↓ cardiac output → ↓ BP → ↓ brain perfusion

Normally, the body compensates by increasing sympathetic tone.

In orthostatic hypotension, compensation fails because of:

• dehydration

• drugs

• autonomic dysfunction

So BP drops and the patient faints.

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Cardiac syncope pathogenesis

Heart fails to maintain adequate output.

Arrhythmia:

abnormal rhythm → ↓ cardiac output → ↓ cerebral perfusion

Structural disease:

obstruction/pump failure → ↓ forward blood flow → brain hypoperfusion

This type can occur suddenly, often without warning.

Clinical manifestations

Before syncope — prodrome

Common especially in vasovagal/orthostatic syncope:

• dizziness

• lightheadedness

• nausea

• sweating

• weakness

• blurred vision

• tunnel vision

• ringing in ears

• feeling hot

• pallor

Patient may say:
“I felt like I was going to faint.”

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During syncope

• sudden loss of consciousness

• loss of postural tone

• falling/collapse

• pale, sweaty skin

• weak pulse

• low blood pressure

• brief duration, usually seconds to a few minutes

There may be brief twitching, but this does not always mean epilepsy.

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After syncope

Typical syncope has:

• rapid recovery

• no prolonged confusion

• patient feels tired or weak

• memory returns quickly

Important: prolonged confusion after the event suggests seizure rather than syncope