Diagnosing transient loss of consciousness: Approach to a common clinical conundrum

Over 90 percent of transient loss of consciousness (TLOC) presentations are due to syncope, seizure or psychogenic causes, but up to 30 percent of them are misdiagnosed initially due to significant overlap in manifestations, according to Dr Charlie Chan of the Prince of Wales Hospital, who shared tips on diagnosis at AIM 2025.

“If you had to guess the cause of a TLOC presentation, syncope would be more likely as it is statistically more common than seizures or epilepsy,” said Chan. The age-adjusted prevalence of syncope is <5–<20/1,000 person-years, and that for epilepsy is approximately 0.25–1.1/1,000 person-years. The lifetime risk of syncope is approximately one in three, while that for seizure is one in 10, and epilepsy, one in 26. [N Engl J Med 2002;347:878-885; Pract Neurol 2022;Oct:40-44; Syncope: An evidence-based approach, 2020, Chapter 6, doi:10.1007/978-3-030-44507-2_6]

“Not all TLOC events with twitching are seizure or epilepsy. Convulsive syncope is another diagnostic challenge in TLOC,” noted Chan. “It is not a subgroup of syncope, but one of the many clinical manifestations of syncope where myoclonic jerks are caused by cerebral hypoxia.”  

Chan further noted that the onset of jerks in convulsive syncope occurs after but never before the TLOC event, whereas in seizures, jerk onset occurs before or during loss of consciousness. Other differences include the number (few and <10 with syncope vs many and >20 in some cases of seizure), duration (a few seconds vs minutes) and rhythm (arrhythmic at irregular intervals vs rhythmic with gradual slowing) of the jerks, as well as tongue biting (rare and usually at tip of tongue if present vs common and usually at the side of the tongue).

“Psychogenic causes [such as pain, fear, arguing, attention seeking] of TLOC further complicate matters,” said Chan. “One of the differentiating features of psychogenic TLOC is distractibility and, unlike epileptic seizures, stereotypy is not observed.”

“To date, none of the tools available to aid differential diagnosis of TLOC are ready for routine clinical use,” noted Chan. “History taking from the patient, with or without witness accounts, is thus of paramount importance for clinicians to arrive at a provisional diagnosis and guide subsequent management.”

Traffic-light risk stratification guide

Chan provided a comprehensive traffic-light risk stratification guide to aid physicians in handling TLOC cases. Risk stratification for subgroups of syncope is as follows:

·  Red light: Cardiac syncope (most lethal; abrupt onset; pre-ictal presentations include palpitations, exertional chest pain, reduced exercise tolerance, chest pain radiating to the back, history of recent long-haul flight);

·  Yellow light: Orthostatic hypotension syncope (gradual onset; pre-ictal presentations include postural dizziness and darkening/whitening vision; most identifiable risk factors include culprit medications such as antihypertensives and diuretics, volume depletion due to vomiting/diarrhoea/dehydration/gastrointestinal bleeding, and autonomic dysfunction due to parkinsonism/diabetes/vitamin B12 deficiency/alcoholism);

·  Green light: Reflex syncope (most common and benign cause of syncope; situational onset, such as due to hot/stuffy environment, straining upon  micturition/defaecation, coughing, head turning).

Differential diagnoses of seizures can similarly be stratified into:

·   Red light: Acute symptomatic seizures (refer to A&E);

·   Yellow light: Epilepsy (refer to neurologist);

·   Green light: First unprovoked seizure (arrange MRI/EEG).