Thrombosis is the accumulation of blood in a vein. In the case of haemorrhoid thrombosis, it occurs in the haemorrhoid veins.
Even if its name sounds like a serious illness, it does not have the same importance as thrombosis in another parts of the body.
Thrombosis is normally caused by intense constipation, diarrhoeas or childbirth.
It consists on the rip of a vein and the formation of a clot, felt by the patient as a very painful lump in the anus.
With local anaesthesia, the clot is removed leaving a small wound that will cicatrize after a few days.