Aetiology

The causes of haemorrhoids are not well understood but the following factors are important:

constipation and prolonged straining at stool - this puts downward pressure on the anal vascular cushions and causes engorgement and dilation of haemorrhoidal tissue

• the typical Western diet is low in fibre and haemorrhoids are more common
• haemorrhoids are uncommon in developing countries
  
  

• increased anal sphincter tone:
  • several effective treatments aim to reduce anal tone, including anal stretch procedures and nitroglycerine ointment
    
    
• obstruction of venous flow may play a role:
  • during pregnancy
  • in patients with portal hypertension