A provisional diagnosis often needs proof. Sometimes it needs more detail before treatment can be planned. This is where investigations help. This chapter covers five investigations used often in anorectal disease. These are manometry, endoanal ultrasound, MRI fistulogram, biopsy, and colonoscopy. Each has its own purpose. Each has its own place in the diagnostic path.
8.1 Why Investigations Are Needed
Not every patient needs a test. Many anorectal conditions are diagnosed by history and examination alone. A test is needed when the diagnosis is not clear. A test is also needed when the exact anatomy must be known before surgery. A test may confirm a suspected cancer, or rule one out.
A test should always answer a specific question. Before ordering any test, the doctor should ask: what will this test tell me, and how will it change my plan? A test ordered without a clear question rarely helps the patient.
Investigations also have a cost. Some cause discomfort. Some need special preparation, or sedation. Some are expensive, and not always easily available in every centre. These factors matter. A test should only be ordered when its benefit clearly outweighs its cost to the patient.
8.2 Choosing the Right Investigation
Each investigation has a specific role. Choosing the wrong test wastes time. It may also cause the patient needless discomfort or expense. The sections below explain when each test is useful, and what it can, and cannot, show.
As a quick guide: manometry answers questions about sphincter pressure and function. Endoanal ultrasound answers questions about sphincter structure and small tears. MRI fistulogram answers questions about complex fistula anatomy. Biopsy answers questions about the exact nature of a tissue, at cell level. Colonoscopy answers questions about disease anywhere in the large bowel, not just the anal canal.
8.3 Anorectal Manometry
Anorectal manometry measures the pressure inside the anal canal. A thin catheter is passed into the rectum. It records resting pressure, and squeeze pressure, at different points along the canal.
Used mainly to assess sphincter function.
Helpful in anal incontinence, to measure how weak the sphincter is.
Helpful before sphincter-cutting procedures, such as sphincterotomy, to check that resting tone is not already low.
Helpful in obstructed defecation, to check for a failure of the sphincter to relax on straining.
Manometry needs no anaesthesia. It causes only mild discomfort. It is best done in a dedicated unit, with trained staff. The result depends on correct catheter placement, and on patient cooperation.
Explain the test to the patient beforehand. Tell them it involves a thin tube, not a rigid instrument. Reassure them that most patients tolerate it well, with only brief discomfort during catheter placement.
8.4 Endoanal Ultrasound
Endoanal ultrasound uses a small probe, inserted into the anal canal, to build a picture of the sphincter muscles. It shows the internal and external sphincters as separate rings.
The main use is to detect a sphincter tear, especially after childbirth injury.
It shows the exact position and size of a tear, described by the clock face.
It helps assess a fistula tract, and its relationship to the sphincter muscles, though MRI gives a more complete picture for complex fistulas.
It is quick, and does not need general anaesthesia in most patients.
Endoanal ultrasound depends on the skill of the person performing it. A poor scan can miss a tear, or wrongly suggest one. It should be performed by someone with specific training in this technique.
Endoanal ultrasound is often the first-line test for a suspected sphincter injury, since it is quicker and cheaper than MRI. MRI is reserved for cases where the ultrasound findings are unclear, or where the fistula anatomy is complex.
8.5 MRI Fistulogram
MRI fistulogram is the most detailed way to study a fistula tract. It uses magnetic resonance imaging, without radiation, to show soft tissue in fine detail.
It shows the full course of the fistula tract, from the internal opening to the external opening.
It shows any side branches, or hidden pockets of infection, that may not be found on examination alone.
It shows the tract's relationship to the sphincter muscles, which guides safe surgical planning.
It is the investigation of choice before surgery for a complex, or recurrent, fistula-in-ano.
A simple, low, first-presentation fistula rarely needs MRI. The findings on examination are usually enough. MRI is reserved for complex, high, recurrent, or multiple tracts, where the risk to the sphincter during surgery is higher.
MRI takes 20 to 30 minutes. It needs no injection in most cases, though a contrast agent may be used if inflammation needs to be assessed. It needs no bowel preparation. The patient must simply lie still for the scan.
8.6 Biopsy
A biopsy takes a small piece of tissue for examination under the microscope. It gives a tissue diagnosis, which no other test can replace.
Always taken from any growth or ulcer that looks suspicious.
Always taken when a fissure does not heal with standard treatment, or looks atypical.
Taken to confirm or exclude cancer, before planning major surgery.
Taken to diagnose specific conditions, such as Crohn's disease, or a rare skin condition affecting the perianal area.
A biopsy should never be delayed out of fear that it may worry the patient. A missed cancer, from a biopsy not taken, causes far greater harm than the short discomfort of the procedure itself.
A biopsy can often be taken in the clinic, under local anaesthesia, using proctoscopy or sigmoidoscopy for access. Send the tissue promptly, in the correct fixative, with clear clinical details on the request form. A poorly labelled or poorly fixed specimen can make the pathologist's report less reliable.
8.7 Colonoscopy
Colonoscopy examines the full length of the large bowel, using a flexible instrument passed through the anus. It goes far beyond the reach of the rigid sigmoidoscope.
Needed in any patient with red flag symptoms, such as unexplained weight loss, or a change in bowel habit.
Needed when rectal bleeding cannot be fully explained by a local finding, such as piles or a fissure.
Needed in patients over 50 years old, with new bowel symptoms, even when a local cause is found.
Allows biopsy, and removal of polyps, at the same sitting.
Colonoscopy needs proper bowel preparation, and is usually done under sedation. It carries a small risk of bleeding or perforation. This risk is far outweighed by its value in detecting early cancer and polyps.
Explain the bowel preparation clearly to the patient in advance. A poorly prepared bowel gives a poor view, and may mean the test has to be repeated. Clear, simple, written instructions help patients prepare correctly.
8.8 Preparing the Patient for Any Investigation
Good preparation improves every investigation. It also reduces the patient's anxiety. Follow a few simple steps before any test.
Explain, in simple words, what the test involves, and why it is needed.
Explain what the patient will feel, so there are no surprises.
Give clear instructions for any preparation needed, such as fasting or bowel cleansing.
Answer questions honestly. If a result may take time, say so clearly.
Arrange a clear plan for follow-up, so the patient knows when, and from whom, they will get the result.
A well-prepared patient cooperates better during the test. This often gives a clearer, more reliable result.
8.9 Combining Investigations
Complex cases often need more than one investigation. A patient with a complex fistula, for example, may need both endoanal ultrasound and MRI. A patient with rectal bleeding, and a normal local examination, may need both colonoscopy and biopsy. Choose each test for a clear reason. Avoid ordering a test simply because it is available.
When more than one test is planned, think about the order. A less invasive test often comes first. Its result may then make a more invasive test unnecessary, or may better target where that test should focus.
8.10 Common Mistakes to Avoid
Ordering a test without a clear question in mind.
Skipping colonoscopy in an older patient, because a local cause of bleeding, such as piles, has already been found.
Delaying biopsy of a suspicious lesion, out of reluctance to worry the patient.
Requesting MRI for a simple, low fistula, where examination findings are already sufficient.
Interpreting endoanal ultrasound findings without training in the technique.
Sending a biopsy specimen with poor labelling, or without enough clinical detail for the pathologist.
8.11 Recording Investigation Results
Record each investigation clearly. State the test performed, the date, and the key findings. State how the finding changes the diagnosis or the plan. For example: "MRI fistulogram: trans-sphincteric fistula tract, 2 o'clock position, no secondary tracts. Plan: seton placement, given sphincter involvement." This record links the test directly to the decision it supports.
8.12 Why This Chapter Matters
Investigations are tools, not routine rituals. Each one answers a specific question. Each one carries some cost, discomfort, or risk to the patient. Choose the right test, for the right reason, at the right time. This serves the patient far better than ordering every test available. The next chapter brings the diagnosis and the investigations together. It shows how to reach a final diagnosis, and plan treatment.