Anal fistula is a health condition involving the presence of fistula in the anal or rectal area that can be diagnosed based on the presenting symptoms and physical findings. There are also several diagnostic tests that can be used to confirm the location of fistula and guide treatment decisions.

Patient Consultation

The first step in the diagnostic process is a patient consultation to discuss the presenting symptoms and medical history of the patient that may affect the diagnosis. The symptoms that patients with anal fistula may report include:

• Pain and inflammation
• Perianal discharge
• Change in bowel movements
• Skin excoriation

Additionally, some health conditions are closely linked to anal fistulae such as:  

• anal abscesses
• Crohn’s disease
• Diverticulitis
• HIV or other sexually transmitted infections.

Physical Examination

The physical examination will involve an inspection of the anus and surrounding are for visible signs of a fistula. The opening of a fistula is usually a red and inflamed spot that may ooze pus. Sometimes gentle pressure on the skin around the tract will be sufficient to express a purulent or bloody discharge.

If a fistula is identified, the path of the fistula may be able to be uncovered, as the tunnel structure is often hardened underneath the skin. This can then help to find any secondary tracts that may branch off of the original fistula tract.

It is important to understand the complete path of anal fistulas as this will affect treatment decisions.

A rectal examination may also be required to assess the function of the sphincter muscles.

This involves the practitioner inserting a finger into the anus and rectum in order to palpate the fistula track and surrounding tissue, for evidence of infection or extension of the fistula.

The patient is also asked to squeeze the sphincter muscles on the examining finger to demonstrate their function, so that the need for further testing may be determined.

Key Findings

Key findings that may be indicative of anal fistula include:

• Visible opening of the fistula onto the skin
• Evident pain and inflammation in rectal area
• An area of thickened skin due to chronic infection, referred to as induration
• Discharge of blood or pus from the anus or the surrounding skin

Further Testing

Further tests may be required to investigate the symptoms, particularly if there are several fistula tracts involved. Referral to a specialist such as a colorectal surgeon may be warranted for the following tests:

• Anoscopy involves endoscopic visualization of the inside of the anus, including the internal opening of the fistula if present.
• Proctoscopy may be carried out under general anesthetic, using a proctoscope and a fistula probe to see inside the rectum and visualize any fistula present.
• Flexible sigmoidoscopy may be useful in patients who are suspected to have Crohn’s disease or ulcerative colitis
• Anal endosonography (ultrasound) may be used to visualize the fistula and its internal openings.
• Fistulography involves X-ray imaging following the injection of a contrast dye, and can help to identify the fistula.
• Magnetic resonance imaging (MRI) is useful in visualizing the details of the anal fistula, and especially for complex or recurring cases.
• Computed tomography (CT) scan) can help to assess the extent of inflammation in the rectal area with detail, which is useful for patients with Crohn’s disease.

Types and Classification

Fistulae can be broadly categorized into two main types according to their location: low-level fistulae and high-level fistulae. Low-level fistula includes subcutaneous, submucous and low anal fistula whereas high-level fistula includes high anal and pelvic-rectal fistula.

Additionally, Park’s classification can be used to describe the type of the fistula as follows:

• superficial
• intersphincteric
• transphincteric
• suprasphinteric
• extrasphinteric.

Of these types, intersphincteric fistulas are  the most common, while  extrasphinteric fistulas are the least common.

These classifications assist the practitioner to communicate the characteristics of the fistula, and are helpful in determining the appropriate  treatment decisions for the individual patient.

References

• https://www.fascrs.org/patients/disease-condition/abscess-and-fistula-expanded-information
• http://emedicine.medscape.com/article/190234-overview#a5
• http://www.nhs.uk/Conditions/Anal-fistula/Pages/Diagnosis.aspx
• http://www.mayoclinic.org/diseases-conditions/anal-fistula/care-at-mayo-clinic/tests-diagnosis/con-20032352
• http://www.uptodate.com/contents/anorectal-fistula-clinical-manifestations-diagnosis-and-management-principles
• http://www.ncbi.nlm.nih.gov/books/NBK6943/