I found a research article about AVM in the bowel. Basically what it says is that no one knows for sure why AVM occur or why they bleed. Sometimes they will bleed profusely and other times they are fine. If they are unable to get the bleeding to stop they may need to do something invasive to fix the abnormal collection of blood vessels. If there is just minor bleeding now and then they can treat the symptoms of blood loss with iron supplementation and/or transfusion. The transfusion does not treat the AVM, it just helps get the blood back up where it needs to be.
The problem with an invasive procedure is that they all come with risks. Not all 80 year olds are the same. In a very active healthy 80 year old they may feel they can be more aggressive in treating the AVM. In a more frail 80 year old there may be concerns that the treatment will do more harm than good.
When my grandmother was in her early 90s they knew from stool samples that she was bleeding internally in small amounts. The decision was made to not even do the testing (colonoscopy) because it was felt that the test itself was too traumatic given her age and health especially considering she wasn't bleeding excessively.
For your Mom - it's always good to ask the pros and cons of any test or procedure they will do and seek second opinions if you feel they are needed.
Best of Luck - Stephanie
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