What should you know about unheralded syncope?

Fainting may follow dehydration, prolonged standing, heat, pain or a sudden drop in blood pressure. Low blood sugar can cause collapse, particularly in someone using diabetes medicine. Anaemia may reduce oxygen delivery and contribute to light-headedness. Atrial fibrillation and other heart rhythm problems can reduce effective circulation; fainting during exertion or while lying down is especially concerning. A witness may notice pallor, brief jerking or confusion afterward, but these details do not establish the cause. A seizure, head injury or another problem can resemble syncope.

If someone faints, lay them flat, raise the legs when safe, loosen tight clothing and check breathing. Do not give food or drink until they are fully alert. Turn them on their side if they are vomiting or unconscious but breathing, and call emergency services if they do not wake promptly. After recovery, avoid driving, heights and swimming alone until a clinician has assessed the event. Record the circumstances, warning symptoms, medicines, injuries and a witness account.

Call emergency services for fainting with chest pain, severe breathlessness, palpitations, ongoing confusion, a serious injury, pregnancy, or failure to regain consciousness. Urgent assessment is also needed when it occurs during exercise, without warning, or repeatedly. Arrange medical review after any unexplained blackout, even if recovery was quick, because an ECG and other checks may be needed.

Could this have come from dehydration, low blood sugar, anaemia or a heart rhythm? Do I need an ECG, blood tests or medication review? When is it safe to drive and return to exercise?