Colonoscopies Demystified – The how, what and why


I have performed more than 25,000 colonoscopies to date in my career as a Gastroenterologist in Ireland, the UK and The Netherlands.

I know that going for a colonoscopy can be daunting but it can be central to helping diagnose the cause of digestive symptoms / gut problems.

Why is a colonoscopy important?

Colonoscopies are used to:

  • Investigate bowel symptoms: identify changes in the bowel that may be causing symptoms like a change in bowel habit (either towards constipation or diarrhoea), bleeding from the back passage, abdominal pain, problems with controlling bowel motions (incontinence) and a number of other symptoms. Conditions such as haemorrhoids, inflammatory bowel disease (IBD), diverticulosis (see our previous post) and bowel cancer are diagnosed in this way. If you have IBS, a colonoscopy will be entirely normal.
  • Screen for bowel (colon) polyps in people who have no symptoms: This is called colonoscopy screening. Polyps are benign growths, which are harmless if removed, but can slowly progress to a cancer (they are precancerous) if left to grow to a large size. Removing polyps at colonoscopy is usually straight-forward and prevents those polyps from causing potential problems down the line (because they’ve been removed). Not all polyps will develop into cancer even if they are not removed, but we generally don’t leave this to chance. Screening for polyps usually starts between the ages of 50-60 (depending on the country you live in), or younger in people who have a family history of polyps or colon cancer.
  • To investigate low iron or iron-deficiency anaemia, as sometimes there can be invisible bleeding (called occult bleeding) from the bowel, that you cannot see, but which can lead to blood and iron loss over time, resulting in anaemia. The most common cause of anaemia in young women however is heavy periods or low iron intake in the diet.

The removal of polyps reduces the risk of future bowel cancer. If bowel cancer is detected in its early stages it is easier to treat and a person’s recovery rate is significantly improved.

Bowel cancer is the third most common cancer in men and women. You can learn more about bowel cancer, the risk factors, red flag symptoms, and things you can do  to reduce your risk in our previous blog

How is a colonoscopy performed?

Colonoscopies are typically performed in a hospital’s Endoscopy Department. The procedure is done by a doctor or an endoscopy nurse specialist
  • Wearing a hospital gown you will lie on your left side and bring your knees up towards your chest. You will likely be asked to change position during the procedure (lie on your back or on your right side) as this can help the scope be advanced around the colon more comfortably
  • Conscious sedation is usually given for colonoscopy- this means that you are relaxed and a little drowsy, but not asleep, and often you are aware of what is going on and can look at the screen. People differ in their sensitivity to sedation, and we tend to start with a small dose and ‘top it up’ as necessary. Colonoscopy can be done with no sedation, but this can be a little uncomfortable, and sometimes an anaesthetic is given (this is more common in the US than in Europe), particularly if someone has a history of an uncomfortable procedure in the past.
  • A flexible, thin tube with a small internal camera is inserted into the anus
  • Some air (or CO2) is gently insufflated into your gut allowing the endoscopist to see the bowel wall more clearly (this can make you feel a bit bloated), During a full colonoscopy, the scope is inserted into the top of the colon (the caecum) and through the valve between the large and small bowel (called the ileo-caecal valve), into the end of the small bowel.
  • Once the caecum or small bowel has been reached, the endoscopist then slowly withdraws the scope whilst examining the colon very carefully (you may feel some abdominal cramping)
  • The procedure typically takes 15-30 minutes but may take longer if polyps need to be removed, or some other form of therapy is needed
  • You are then moved to the Recovery Room, where you will sleep for a short period. When you wake up, you are given something to eat and drink
  • You are usually discharged later that same day. If you have had sedation, you will need someone to collect you and you cannot drive for 24 hours. Most hospitals recommend that you have someone at home with you that evening, just until the sedation has fully worn off

What is colonoscopy prep and how does it work?

It’s really important that your bowel is completely empty for a colonoscopy so that the endoscopist gets has the best possible view of your bowel. This is where colonoscopy ‘prep’ comes in. For most people, the ‘prep’, which is taken the day before the procedure, is the worst part of a colonoscopy, as some people find the taste unpleasant

Specific prep instructions vary between health institutions but typically they involve:

  • Eating only plain foods like skinless chicken, potatoes, white, rice, eggs 2 days before your procedure. It’s particularly important to avoid foods containing seeds for a few days before the procedure, as these turn up in the colon several days later and block the scope!
  • Not eating any solid food the day before your procedure
  • Drink plenty of permitted fluids the day before your procedure and laxatives as per your prep instructions. You will need to have easy and close access to a toilet after consuming the laxatives 
  • Preps vary in volume but most involve between 1-3 L of the prep drink. The laxative effect usually starts within a couple of hours of starting. The onset of effect is typically quicker in people who suffer with diarrhoea, and slower if you suffer with constipation

If you don’t follow your bowel prep instructions properly it is possible your colonoscopy won’t be a success

The NHS have an excellent colonoscopy explainer video at the bottom of this page.

What can you eat after a colonoscopy?

The tea and toast that you are given in the endoscopy recovery room is usually the best thing you’ve tasted in years! As after over 1.5 days of fasting and 2L of bowel prep, it usually tastes like the food of the gods! Once you go home if you’re feeling well, you can eat normally. Avoid alcohol the night of the procedure as there may still be some sedation in your system and this can make you very sensitive to alcohol.

Complete colonoscopy

In the right hands, a full or complete colonoscopy (this means that it was possible to reach the top of the colon- the caecum) is usually possible (95-97% success rate) in most people. Nowadays colonoscopy is also usually a comfortable procedure. However, if you have had bowel surgery in the past or gynaecological surgery (a Caesarian section or hysterectomy for example), or have a history of endometriosis, then there can be scar tissue around the bowel and this can cause significant pain and discomfort during the procedure. This pain/ discomfort may be a reason why the procedure cannot be completed. Other reasons why it may not be possible to for a colonoscopy to be completed include a poor bowel prep (all the faeces have not been cleared and it is difficult for the endoscopist to see), or a very twisty colon (we call this a tortuous colon), or severe diverticulosis, or a narrowing in the bowel (this is called a stricture), which is narrower than the width of the scope.

Complications of colonoscopy

Colonoscopy is an invasive procedure and while generally very safe, is associated with some potential complications. There can be pain and discomfort during the procedure itself. A tear can occur in the bowel (the risk of this is typically less than 1/1000), particularly if a large polyp has been removed, or some other intervention has been carried out. Bleeding can occur, typically after removal of a polyp, but the risk of significant bleeding is low. In general, nowadays colonoscopy is a safe procedure. Before you undergo a colonoscopy, you will be given information about the procedure, explaining everything, including any risks, so that you can give ‘informed’ consent to undergo the procedure. This means that you have had time to understand and think about the procedure, before you agree to undergoing it and you will be asked to sign a ‘consent form’.

Not the only way to investigate bowel symptoms

Colonoscopy is an extremely useful procedure for helping to diagnose (or rule out) certain conditions. However, a colonoscopy is often not warranted in younger people who have bowel symptoms, as often enough information can be gleaned from stool and blood tests to make a diagnosis. So for example, if you are young (<40) and have symptoms that are strongly suggestive of IBS, then stool and blood tests may be checked to rule out conditions such as IBD (Crohn’s or colitis); if these are normal, a colonoscopy may deemed unnecessary.

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