What are the causes of dizziness and loss of consciousness (LOC)?

Causes of Dizziness and Loss of Consciousness

Dizziness and loss of consciousness (LOC) stem from two fundamentally different mechanisms: syncope (true LOC from global cerebral hypoperfusion) versus conditions that mimic LOC, with the distinction being critical for appropriate management and risk stratification.

Primary Categories of True LOC (Syncope)

Reflex (Neurally-Mediated) Syncope

The most common cause of true LOC, characterized by transient global cerebral hypoperfusion triggered by autonomic reflexes 1:

• Vasovagal syncope: Triggered by emotional distress (fear, pain, blood phobia) or orthostatic stress 1
• Situational syncope: Cough, sneeze, gastrointestinal stimulation (swallowing, defecation), micturition, post-exercise, post-prandial 1
• Carotid sinus syndrome: Pressure on carotid sinus causing bradycardia/hypotension 1
• Key features: Prodromal symptoms (nausea, vomiting, cold sweating, lightheadedness, visual blurring), brief LOC (<20-30 seconds), rapid recovery with appropriate orientation 1, 2

Orthostatic Hypotension

Failure of autonomic compensation when upright, causing cerebral hypoperfusion 1:

• Primary autonomic failure: Pure autonomic failure, multiple system atrophy, Parkinson's disease with autonomic failure, Lewy body dementia 1
• Secondary autonomic failure: Diabetes, amyloidosis, uremia, spinal cord injuries 1
• Drug-induced: Antihypertensives, diuretics, tricyclic antidepressants, phenothiazines, alcohol 1
• Volume depletion: Hemorrhage, diarrhea, vomiting 1

Cardiac Syncope (Cardiovascular)

Associated with significantly higher morbidity and mortality than reflex syncope 3:

• Arrhythmias:
  • Bradycardia (sinus node dysfunction, AV block, device malfunction) 1
  • Tachycardia (supraventricular or ventricular, including channelopathies) 1
• Structural cardiac disease: Valvular disease, acute MI/ischemia, hypertrophic cardiomyopathy, cardiac masses, pericardial tamponade 1
• Other cardiovascular: Pulmonary embolus, acute aortic dissection, pulmonary hypertension 1

Conditions That Mimic LOC (Non-Syncopal)

Disorders With True LOC But Different Mechanism

Epileptic Seizures 1, 4:

• Distinguishing features:
  • Prolonged tonic-clonic movements (>1 minute) with onset coinciding with LOC 1, 5
  • Specific auras (rising epigastric sensation, unusual unpleasant smell) 4
  • Automatisms (chewing, lip smacking, frothing) 1, 4
  • Lateral tongue biting 1, 4
  • Prolonged post-ictal confusion (>several minutes) 1
  • Blue face during event 1
• Duration: Typically 74-90 seconds versus <30 seconds for syncope 5

Metabolic Disorders 1:

• Hypoglycemia 1
• Hypoxia 1
• Hyperventilation with hypocapnia 1

Intoxication 1

Vertebrobasilar TIA 1:

• Always accompanied by focal neurological signs (limb weakness, ataxia, oculomotor palsies, oropharyngeal dysfunction) 1
• Isolated LOC without focal signs essentially excludes TIA 1

Disorders Without True Impairment of Consciousness

Psychogenic Pseudosyncope 1:

• Eyes typically closed during apparent unconsciousness (versus open in true LOC) 5
• Prolonged duration (>5 minutes) 2
• Eye fluttering, pelvic thrusting 4

Other Mimics 1:

• Cataplexy 1
• Drop attacks 1
• Falls 1
• Carotid TIA (does not cause LOC) 1

Dizziness Without LOC

Vertigo (Vestibular System Disorders)

True spinning sensation from vestibular pathology 6:

• Benign paroxysmal positional vertigo (BPPV): Most common cause, triggered by position changes 6
• Meniere disease: Episodic vertigo with hearing loss and tinnitus 6
• Vestibular neuritis/labyrinthitis: Acute prolonged vertigo 6, 7
• Central causes: Stroke (especially posterior circulation), though bedside examination more sensitive than imaging 7

Presyncope

Sensation of impending faint without actual LOC 6:

• Often medication-related (review all medications) 6
• Orthostatic hypotension 6
• Cardiac arrhythmias (including sick sinus syndrome causing episodic vertigo and presyncope) 8

Disequilibrium

Imbalance without vertigo or presyncope 6:

• Parkinson disease 6
• Diabetic neuropathy 6
• Multiple sensory deficits 6

Lightheadedness (Non-Specific Dizziness)

Vague sensation not fitting other categories 6:

• Psychiatric disorders (depression, anxiety, hyperventilation syndrome) 6
• No diagnosis reached in approximately 20% of dizziness cases 6

Critical Diagnostic Approach

History is the most powerful diagnostic tool and often the only test needed besides physical examination and ECG 1. Key discriminating features:

• Duration of LOC: <30 seconds strongly suggests syncope; >1 minute suggests seizure; >5 minutes suggests psychogenic 2
• Triggers and position: Supine LOC suggests cardiac cause; upright with triggers suggests reflex syncope 1
• Prodrome: Nausea, sweating, visual changes favor syncope; aura favors seizure 1
• Movements: Brief (<15 seconds), starting after LOC onset favor syncope; prolonged, synchronous movements favor seizure 1, 5
• Recovery: Rapid with immediate orientation favors syncope; prolonged confusion favors seizure 1

Common Pitfalls

• Avoid routine brain imaging (CT/MRI) in uncomplicated syncope 1
• Brief myoclonic jerks during syncope do not indicate epilepsy and should not trigger unnecessary neurological workup 4, 2
• Carotid Doppler has no value in typical syncope 1
• EEG is not indicated unless non-syncopal T-LOC is suspected 1
• Incontinence, injury, and post-event sleepiness have low specificity for distinguishing syncope from seizure 1
• Cardiac causes of syncope carry much higher mortality risk than benign reflex syncope, making accurate diagnosis imperative 3