Hi,
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The most probable cause of this particular bleed is accelerated hypertension. Epistaxis or nasal bleeds are classified into anterior or posterior (front or back part of the nose) based on the region from where they bleed. In your case, I would definitely consider posterior epistaxis as typically these bleeds are more torrential as they usually happen from the woodruffs plexus (venous leash of blood vessels draining the back part of your nose). Moreover in posterior bleeds, the blood immediately trickles down into the throat which can come out from the mouth or you might end up swallowing some blood causing gastric irritation and consequent emesis or vomiting. The reason why I have considered accelerated hypertension is because of your age and moreover in these bleeds, it is always typical that by the time you reach the ER your blood pressure would have come back to normal and bleeding stops making everyone wonder why it bled in the first place. You would have never been hypertensive earlier and this entity, there are episodes when the blood pressure transiently shoots up to high levels and the easiest area for the body to exhaust this sudden increase in blood pressure is the nose, the posterior part precisely speaking. Moreover in these bleeds, every other parameter that you would have got investigated would be normal. My advice to you would be to measure your blood pressure at home every four hours and document it, so that if required appropriate oral antihypertensives can be started. I am sure with this, the bleeding should stop and there is absolutely nothing that you need to worry about as this is a treatable entity. One more aspect which is worth mentioning is please do not panic when it bleeds as it can further shoot up the blood pressure causing the bleeds to last for a longer duration. These episodes might continue for a while until appropriate treatment for the cause is initiated. I hope this helps.