Causes of Loss of Consciousness: A Comprehensive Mnemonic
Loss of consciousness (LOC) is primarily caused by global cerebral hypoperfusion (syncope), neurological disorders, metabolic derangements, and other conditions that impair brain function. 1, 2
SYNCOPE: The Most Common Cause of Transient LOC
Syncope is defined as a transient loss of consciousness due to global cerebral hypoperfusion characterized by rapid onset, short duration, and spontaneous complete recovery. 1
Pathophysiological Classification of Syncope (CRASH)
C - Cardiac
- Arrhythmias (bradyarrhythmias, tachyarrhythmias)
- Structural heart disease (aortic stenosis, hypertrophic cardiomyopathy)
- Other cardiovascular conditions (pulmonary embolism, aortic dissection) 2
R - Reflex (neurally-mediated)
- Vasovagal syncope (emotional distress, pain, blood phobia)
- Situational syncope:
- Cough, sneeze
- Gastrointestinal stimulation (swallowing, defecation)
- Micturition (post-micturition)
- Post-exercise
- Post-prandial
- Others (laughing, playing brass instruments, weightlifting)
- Carotid sinus syncope
- Atypical forms (without apparent triggers) 1
A - Autonomic dysfunction/orthostatic hypotension
- Primary autonomic failure (pure autonomic failure, multiple system atrophy, Parkinson's disease)
- Secondary autonomic failure (diabetes, amyloidosis, uremia, spinal cord injuries)
- Drug-induced orthostatic hypotension (alcohol, vasodilators, diuretics, antidepressants)
- Volume depletion (hemorrhage, diarrhea, vomiting) 1
S - Situational (overlap with reflex syncope but specifically triggered by activities)
H - Hypotension (various causes leading to inadequate cerebral perfusion)
NON-SYNCOPAL Causes of LOC (MIST)
M - Metabolic/toxic
I - Ictal/seizure disorders
S - Structural brain disorders
T - Trauma/psychiatric
Clinical Differentiation of LOC Causes
Duration and Recovery Pattern
- Syncope: Brief (typically 10-30 seconds), with rapid and complete recovery
- Seizures: Often longer (minutes), with post-ictal confusion
- Metabolic causes: Variable duration, may have gradual recovery 2
Associated Features
- Cardiac syncope: May occur during exertion or in supine position, preceded by palpitations or chest pain
- Reflex syncope: Often has prodromal symptoms (lightheadedness, nausea, sweating)
- Seizures: May have oral automatisms, prolonged head turning, lateral tongue biting
- Psychogenic: May have atypical features like eye closure during episode, prolonged duration with atypical triggers 2, 5
Special Considerations in Specific Populations
Elderly Patients
- Multiple causes often coexist
- Higher prevalence of cardiac causes and orthostatic hypotension
- Medication effects more common
- Higher mortality risk with cardiac syncope 6
Post-Traumatic LOC
- May involve complex disorders of consciousness (coma, unresponsive wakefulness syndrome, minimally conscious state)
- Prognosis varies significantly based on level of consciousness at 3 weeks post-injury 7
Diagnostic Approach
- Determine if true LOC occurred (complete loss of consciousness with inability to maintain posture)
- Assess timing, triggers, and recovery pattern
- Look for specific risk factors (structural heart disease, family history of sudden death)
- Consider initial testing: ECG, orthostatic vitals, basic metabolic panel
- Further testing based on suspected cause: neuroimaging, EEG, cardiac monitoring 2
Common Pitfalls to Avoid
- Misdiagnosing seizures as syncope and vice versa
- Overlooking cardiac causes (which carry higher mortality)
- Failing to recognize multiple contributing factors, especially in elderly
- Underestimating the importance of medication effects
- Missing rare but serious causes like pulmonary embolism 2, 6
Remember that drowning, which can lead to LOC, may be precipitated by various conditions including syncope, seizures, or cardiac arrhythmias, highlighting the importance of identifying underlying causes in all cases of LOC. 1