Familial adenomatous polyposis (FAP): symptoms, testing, surgery and family screening

Bowel polyps are small growths on the lining of the large intestine (colon) or rectum. The NHS says they are very common and not usually serious, but sometimes they can lead to bowel cancer, and that polyps are usually harmless and do not become cancerous, though in a small number of cases a polyp can become cancerous over many years. FAP is different because of scale and certainty: people with FAP develop a large number of adenomas, and Bowel Cancer UK says that if the polyps are not removed there is a very high chance of developing bowel cancer.

FAP is one of a handful of genetic conditions linked to bowel cancer. Bowel Cancer UK lists Lynch syndrome, FAP, MUTYH-associated polyposis (MAP) and MBD4-associated neoplasia syndrome among the conditions it knows about, and says that people with these conditions have a much higher chance of getting bowel cancer and are more likely to be diagnosed at a younger age. It also says around 5 to 10% of all bowel cancer cases are thought to be caused by a change in a known gene.

The NHS says bowel polyps do not usually cause symptoms, so most people with polyps will not know they have them, and they are often found during a colonoscopy. Rarely, larger polyps can cause a small amount of slime or blood in the poo, bleeding from the bottom, a change in bowel habit or weight loss.

The University Hospital Southampton genetics leaflet describes other features. Polyps can also form in the stomach and small intestine. Some people have skin lumps and cysts, or bone cysts that do not usually cause problems. Some have harmless dots at the back of the eye called CHRPEs (more than five, or a large distinctive area, suggests FAP, but not everyone with FAP has them). Some develop desmoid tumours, non-cancerous growths in connective tissue that can be slow-growing or aggressive. When these extra features are present, the term Gardner syndrome is sometimes used.

The NHS says bowel polyps do not usually cause symptoms, so most people with polyps will not know they have them, and they are often found during a colonoscopy. Rarely, larger polyps can cause a small amount of slime or blood in the poo, bleeding from the bottom, a change in bowel habit or weight loss. For bowel cancer itself, the NHS lists changes in poo, blood in the poo, bleeding from the bottom, feeling the need to poo even if you have just been, tummy pain, a lump in the tummy, bloating, losing weight without trying, and feeling very tired or short of breath as signs of anaemia.

The NHS advises seeing a GP if you have any symptoms of bowel cancer, and getting an urgent GP appointment or NHS 111 help if your poo is black or dark red or you have bloody diarrhoea. It says to call 999 or go to A&E if you are bleeding non-stop from your bottom or there is a lot of blood. If you know FAP runs in your family, tell the GP, as Bowel Cancer UK lists a relative with a known genetic condition linked to bowel cancer, such as FAP, among reasons to tell your GP.

The Southampton leaflet says a clinical diagnosis of FAP is made when large numbers of adenomas are found in the colon and rectum, usually at colonoscopy, and that diagnosis can also be made with a genetic test. The test looks for a variant in the APC gene from a blood sample; it starts with an affected person, and if a variant is found, other family members can be tested. It says this test finds a variant in 90 to 95% of people who clinically have FAP.

The NHS describes a colonoscopy as a test using a long, thin flexible tube with a small camera passed into the bowel via the bottom. You are given a laxative so your bowels are empty, are usually awake and are offered medicine to make you more comfortable. It should not be painful but can feel uncomfortable.

Bowel Cancer UK says that if you have FAP your surgeon may advise removing all of the colon and sometimes the rectum as well, to prevent bowel cancer, and your doctor will talk to you about the best type of surgery and the best time for you to have it. The Southampton leaflet says that after a person with a gene variant has polyps found, the clinician will usually recommend surgery. It describes the main operations:

Bowel Cancer UK says if you have FAP your surgeon may advise removing all of the colon and sometimes the rectum as well, to prevent you getting bowel cancer, and that your doctor will talk to you about the best type of surgery and the best time for you to have it. The pages read for this guide do not give ages or specific operations, so those decisions are for the specialist team.

Support is available. Bowel Cancer UK names Polyposis patient, an online support group for people with polyposis syndromes, the St Mark's Polyposis Registry, which supports people with a polyposis condition and their relatives, and the Ileostomy and Internal Pouch Association, which supports people who have had their colon removed.

The Southampton leaflet says everyone at risk of FAP should be examined for bowel polyps once a year, usually from the age of about 10 years, and that a person carrying the gene variant should continue yearly screening until polyps are found. If someone chooses not to have genetic testing they should still have regular bowel screening: a colonoscopy every one to three years depending on how many polyps are found, usually starting between ages 10 and 14. It also recommends a gastroscopy by about age 30 to look at the upper gut, repeated every three years.

Bowel Cancer UK says relatives of someone with an inherited condition may be offered testing, and those who carry the gene change are offered regular bowel screening. Relatives who do not want a test can still have regular screening if they have a 1 in 2 chance of carrying the changed gene.

Every cell contains genes that act like instructions for the body. Bowel Cancer UK explains that if a gene is changed, which may also be called a variant, mutation, gene fault or gene error, the instructions may not be correct. FAP is caused by a change in the APC gene, which helps to stop polyps from developing, so when it is changed polyps are more likely to develop and this can lead to bowel cancer.

The inheritance pattern is autosomal dominant. Only one copy of the changed gene needs to be inherited for a person to have FAP. Most people with FAP inherit the changed gene from one of their parents, but it is not always passed on from a parent: about 1 in 4 cases (25%) are caused by a new change in the APC gene. If you have FAP there is a 1 in 2 chance of each child having it, and if you inherited it from a parent each of your siblings also has a 1 in 2 chance.

MAP is a similar but separate condition, caused by changes in a gene called MUTYH, with a different pattern of inheritance in which two copies of the changed gene are needed. Patients with MAP usually have fewer polyps. A genetics team can explain which applies to a family.

Bowel Cancer UK says if you are planning to start a family and have questions about the risk to your children, your genetics team can answer them and discuss other fertility options. The Human Fertilisation and Embryology Authority has information about the options available.

None of the UK sources used here describe a diet that prevents or treats FAP polyps. FAP is caused by a change in a gene, and the Southampton leaflet says people with the condition will eventually develop bowel cancer without screening and treatment. Follow the advice of your specialist team on nutrition, particularly after bowel surgery, when a dietitian may help.

In the UK, FAP is usually managed by clinical genetics services and specialist bowel surgeons, often with a polyposis registry, and testing and screening for relatives is offered through the NHS. Because it is rare and lifelong, continuity matters more than the setting: ask which team coordinates your surveillance, how relatives are invited, and who to contact if symptoms change. Private consultations may speed up the first appointment, but long-term follow-up in a specialist service is the usual pattern.