Haemorrhoids form when blood vessels in the lower section of the rectum and/or the upper anal canal enlarge.
Although it is uncertain precisely what causes haemorrhoids to form, contributing factors include regularly straining when have bowel movements and/or sitting on the toilet for long periods. Women often develop haemorrhoids during pregnancy and after giving birth.
Haemorrhoids are very common indeed, and can be present without causing any pain, with the main symptoms being bleeding from the anus and where they have prolapsed, that is where the enlarged blood vessels protrude from the anus.
Another common symptom is the presence of skin tags or patches of stretched skin around the anus.
Occasionally haemorrhoids may be accompanied by the development of an external lump which causes severe pain - in most cases this is caused by a blood clot in an external blood vessel, which is called a 'thrombosed perianal haematoma'.