Why hypovolemia cause headache
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Hypovolemia means reduced circulating blood volume, commonly from dehydration, vomiting, diarrhea, bleeding, or diuretics. It can cause headache through several overlapping effects:
- Reduced cerebral perfusion: Lower blood volume can lower blood pressure and reduce blood flow to the brain, particularly when standing. This may produce a dull headache, light-headedness, or near-fainting.
- Changes in blood vessels and pain pathways: Fluid loss alters vascular tone and can activate headache-sensitive blood vessels and nerves around the brain.
- Electrolyte disturbances: Losses of sodium and other electrolytes can contribute to headache, fatigue, nausea, confusion, or weakness.
- Relative brain dehydration: With dehydration, shifts in water balance can affect tissues and pain-sensitive structures, contributing to headache.
Symptoms often improve with appropriate oral fluids and electrolytes if dehydration is mild. But seek urgent care for a severe or sudden headache, fainting, confusion, chest pain, black/bloody stools, ongoing vomiting, very little urine, or suspected significant bleeding.
Do not confuse ordinary body hypovolemia with CSF hypovolemia. Low cerebrospinal-fluid volume causes a characteristic positional headache that is worse upright and improves lying down. Goldman-Cecil Medicine, p. 1023-1025.
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