Bowel movement, fainting and sweating: what to know

This pattern is sometimes called defecation syncope. Pressure and straining can stimulate the vagus nerve. The resulting reflex may widen blood vessels and slow the heartbeat, reducing blood flow to the brain for a short time. Warning symptoms can include warmth, nausea, blurred or narrowing vision, light-headedness, clammy skin and sweating before a person loses consciousness.

A single episode may have a straightforward trigger, but fainting can also be made more likely by dehydration, low blood sugar, anaemia, medicines that lower blood pressure or a heart-rhythm problem. The timing, recovery and any injury help a clinician work out the cause.

Stop straining and sit down immediately. If possible, lie flat and raise your legs. Turn onto your side if you feel sick or have vomited, loosen tight clothing and ask someone to stay with you. Do not stand quickly after the symptoms settle. Once fully alert, sip water unless a clinician has told you to restrict fluids. Prevent constipation with fluids, fibre and regular movement when these are suitable for you; avoid forcing a bowel movement.

Arrange a medical assessment if this has happened for the first time, keeps recurring or caused a fall. Write down what you were doing, whether you were straining, how long recovery took and any medicines or medical conditions.

Call emergency services if the person does not wake promptly, is not breathing normally, has chest pain, severe shortness of breath, a new irregular heartbeat, one-sided weakness, trouble speaking, a severe headache or a serious injury. Seek urgent medical advice for fainting during exercise, fainting without warning, black or bloody stools, severe abdominal pain or repeated episodes. These features can signal a problem beyond a bowel-related reflex.

Ask whether your blood pressure, heart rhythm, blood count, glucose level and medicines should be reviewed. Tell the clinician about palpitations, heavy bleeding, prolonged diarrhoea, poor fluid intake and any family history of sudden cardiac death. They can advise whether testing for anaemia, dehydration, hypoglycaemia or a rhythm condition is appropriate.