Diagnosing Anal fistulas rest on identifying
- the external opening on the skin
- the internal opening to the abscess in the anal canal
- the path or tunnel of the fistula from one opening to another
Perianal fistulas often present as small holes or red bumps on the skin, and healthcare providers can find most external openings during a physical exam. During the exam, they will press on the skin where they suspect a fistula to determine if there is a drainage. Often the area is extremely painful to the touch the examination must be performed in the operating room under sedation. This is called examination under anesthesia (EUA). Finding the internal opening of the fistula can be more difficult. They may use a lighted scope, like an anoscope or proctoscope (a longer scope that can visualize your rectum) to view inside the rectal canal.
Knowing the complete path of an anal fistula is important for effective treatment. To determine the path (or tunnel), healthcare providers may use a probe inserted at one opening, traveling through the path and coming out at the other opening. This allows them to see where the fistula travels and if it goes through sphincter muscles.
There are also several imaging tests that can also attempt to determine this, including:
- Pelvic magnetic resonance imaging (Pelvic MRI): Sometimes also referred to as an MRI fistulogram, provides an image of the anal canal, sphincter muscles and the surrounding tissues. In this MRI, magnetic and electronic waves are used to create multiple 3D images which can be read by the healthcare provider to determine the openings and path of the fistula.
- Endoanal ultrasound: Takes images of the anal sphincter and surrounding areas to assess the area.
- Endoscopic ultrasound: This uses high-frequency sound waves to identify the fistula, the sphincter muscles and surrounding tissues.
- Fistulography: This is a traditional radiological technique used to define the anatomy of fistulas. The external opening of the fistula is injected with a water-soluble dye to define the fistulous path. An x-ray is then taken so that the path of the fistula can be seen.
- Computed tomography (CT) scanning: CT scanning is a specialized form of radiography in which multiple X-rays of the anus and rectum are taken at different angles and projections. Healthcare providers use these images to produce a two-dimensional image to see what is happening in the body.
- Anal endosonography, also known as endoanal ultrasonography (EUS) or transrectal ultrasonography (TRUS): In this procedure a special long tube is gently introduced into the rectum through the anus and gradually withdrawn through the anal canal. This produces an image with multiple layers to allow healthcare providers to see the area in 3-D.
What are the different types of anal fistulas?
Healthcare providers classify anal fistulas by where they are in relation to the anal sphincter muscles. These are the muscles that control bowel movements. There are specific names based on the fistula location.
- Intersphincteric anal fistula: Passes through the internal anal sphincter muscle and then comes out through the space between the internal and external sphincter muscles.
- Transsphincteric fistula: Travels through both layers of the anal sphincter muscles.
- Suprasphincteric fistula: Passes through the internal sphincter and then goes around the external sphincter.
- Extrasphincteric fistula: Goes around both sphincter muscles. This type is less common and usually originates from the rectum rather than the anus. This type does not begin as an abscess in an anal gland.
- Superficial anal fistula: Travels from the lower part of the anal canal, below the anal glands, through the skin nearby, bypassing the muscles. This type does not begin as an abscess in an anal gland.
Classifying Perianal Fistulas
Fistulas can be classified as simple or complex:
Simple perianal fistula: A low fistula, confined to the anal canal with a single external opening without abscess or stricture (abnormal narrowing of the anal canal). Simple perianal fistulas occur below the dentate line, and generally have no perianal complications.
Complex perianal fistula: high fistula, passes through or above muscle layer with single or multiple external openings with or without abscess. Complex perianal fistulas occur above the dentate line and may be associated with perianal abscesses, rectal stricture, inflammation of the lining of the rectum (proctitis) or connection with bladder or vagina.
Adapted from “Anal Fistulas: Symptoms and Diagnosis”- IFFGD Publication #145 by Satish Rao, MD, Harold Harrison, MD, Distinguished University Chair in Gastroenterology, Department of Gastroenterology/Hepatology, Augusta University, Augusta, Georgia
