What should you know about syncope due to dehydration?

Fluid loss from sweating, vomiting, diarrhoea, fever, or poor intake can reduce the amount of blood returning to the heart. Standing then becomes harder for the body to compensate for, producing thirst, dry mouth, weakness, dizziness, dark urine, or fainting. Heat exhaustion may add heavy sweating and weakness.

Dehydration is not the only possible explanation for a collapse. A urinary tract infection, kidney stones, gastroenteritis, or another illness may be responsible for fluid loss or may need separate treatment. Cloudy urine and constipation can also provide useful context but do not identify the cause by themselves.

Lie down in a cool place and rise slowly only after the dizziness has settled. If you are awake and able to swallow, take frequent small drinks; an oral rehydration solution may be useful when significant fluid loss has occurred. Follow medical advice if you have a condition that limits fluids.

Keep track of ongoing vomiting or diarrhoea, urine changes, fever, heat exposure, and how much you can drink. Do not drive or work at heights while faint or unsteady, and arrange a clinical review after a first collapse.

Seek emergency help if you cannot keep fluids down, pass very little urine, become confused, remain faint, have chest pain, breathing trouble, severe abdominal or side pain, blood in vomit or stool, or a serious injury. Heat-related collapse with confusion or worsening weakness is also urgent.

Ask whether the episode fits dehydration, whether you need blood or urine tests, how to replace fluids safely, and which symptoms would mean that infection, a urinary problem, or a heart cause needs review.