The hidden tunnels of anatomy: how an infection leaves a permanent path

Science · Published 23 October 2026 Swipe more like this →

In short: An anal fistula is more than just a surface irritation; it is a physical tunnel carved through tissue. Formed by the body's own attempt to drain infection, these abnormal connections can persist indefinitely without intervention, creating a chronic cycle of inflammation and pain.

Most anal fistulas originate from an infected anal gland. When this gland becomes infected, it creates an abscess—a pocket of pus trapped under the skin. To relieve the pressure of this collection, the body (or a medical professional) provides a drainage path. Once that pus clears, the path remains as a small tunnel connecting the internal anal canal or gland to a hole in the external skin. The resulting tunnel can be difficult to see without a mirror. It often presents as an opening on the skin that may ooze blood or stool. Chronic inflammation from these injuries eventually erodes into nearby tissues.

The anatomy of these tunnels is complex because they often involve the sphincter muscles, which are the rings of muscle responsible for controlling stool release. Because of this, surgeons classify fistulas based on how much of the sphincter muscle the tunnel involves. This classification is critical because it determines the surgical approach and helps manage the risk of complications like fecal incontinence following treatment. Diagnosis usually involves a rectal examination where a doctor inserts a gloved finger to check for signs. Specialists might also use proctoscopy with a lighted telescope or perform imaging like ultrasound, MRI, and CT scans.

While these conditions are most common in adults around age 40, they can appear in younger people—particularly those with Crohn's disease or other inflammatory bowel diseases. Risk factors also include previous abscesses, trauma to the area, radiation for anal cancer, or general infections. Because fistulas typically do not heal on their own and can lead to recurring abscesses, surgery is recommended as the primary treatment in most cases. Symptoms often include a constant throbbing pain that intensifies during coughing or movement. Patients may also experience skin irritation, swelling, redness, and sometimes a high temperature if an abscess is present.

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