In more severe cases of the condition, hidradenitis suppurativa (HS) sometimes manifests with “tunnels” or inflamed sinus tracts under the skin. They’re often filled with pus or blood and may be painful.

Hidradenitis suppurativa (HS) is typically characterized by painful bumps, boils, and sometimes tunnels under the skin.

Also called fistulas, these tunnels typically occur in areas where there are sweat glands and where the skin often rubs together, such as the armpits, the groin, the buttocks area, or under or between the breasts.

Though bumps and boils can often be successfully treated with topical or oral medication, the tunnels may be more challenging to treat and may require surgical removal. Here’s what else to know about the condition and how to treat it.

Though experts don’t know exactly why or how hidradenitis suppurativa (HS) manifests, they know that it is a chronic inflammatory condition that affects the sweat glands and hair follicles.

HS onsets due to blocked hair follicles in areas where there’s friction, the result of factors like excess oil production, the buildup of dead skin, and bacterial overgrowth. Many experts think that the blocked hair follicles may trigger an inflammatory response in the body, which thus creates bumps and boils.

As the swelling worsens, these bumps and boils can turn into tunnels beneath the skin as the HS lesions repeatedly open and heal during flare-ups.

That said, in a 2022 study, researchers found that the tunnels themselves may actually worsen the disease’s inflammation. While previously scientists thought that they were simply the end result of inflammation, they now think they may actively worsen existing HS.

For that reason, experts believe that treating the rash or skin alone isn’t enough to stop HS symptoms. Actively treating the tunnels and underlying inflammation, whether through surgery or medication, may help finally slow the inflammatory process.

HS tunnels look like red or discolored, raised tracts of skin. In people with darker skin tones, they can appear purple, dark brown, or gray. They tend to crop up in clusters in areas where the skin rubs together, such as the armpits or buttocks. They may be as short as a couple of centimeters or as long as several inches in length.

The tunnels are typically surrounded by other common HS manifestations like pustules and blisters. Over time, multiple tunnels will form near each other and connect, which can cause increasing pain, irritation, and, potentially, long-term skin damage.

The tunnels tend to be soft and tender to the touch and may be filled with pus or blood. They may contain small holes where fluid leaks out, causing chronic drainage. The tunnels tend to grow longer and larger with time, especially without treatment. The skin surrounding the tunnels is also typically very inflamed.

Over time, they tend to lead to significant rope-like scarring both within the tunnels and the surrounding skin.

Many patients describe HS tunnels as painful, with pain that may worsen as the tunnels become longer, deeper, and more inflamed.

In the early stages, however, the tunnels may simply be tender and a bit painful to the touch. But as they worsen, many people turn to prescription-strength medication to help manage the pain. Without treatment, the pain and discomfort may significantly disrupt one’s day-to-day quality of life.

Before a tunnel forms, you may also notice a lump in your skin and feel some discomfort. This spot may continue to swell and become painful as the HS tunnel begins to form.

Though HS currently doesn’t yet have a known cure, there are various ways to manage it, including:

  • Lifestyle changes: Before anything else, doctors often recommend quitting smoking (if you smoke) and reaching a healthy weight. In a 2019 study, researchers noted that smoking regularly and having a higher body weight are the main risk factors for recurring tunnels.
  • Medication: Tunnels may be soothed with saline injections, biologic therapies, or corticosteroid injections. A 2022 study found that injections of the biologic brodalumab, which blocks inflammatory pathways, caused them to shrink significantly and drain less pus.
  • Surgery: To remove the tunnels completely, surgical intervention is often required. In the earlier stages of tunnel development, deroofing may be sufficient to remove the tunnels, which involves taking the top or “roofs” of the tunnels off surgically and removing the inflamed debris so that they can heal. In more severe cases, the tunnels may need to be totally excised. After surgical removal, treating the underlying inflammation may help prevent their recurrence.

Ultimately, treating HS requires an individualized treatment plan, so if possible, talk with your doctor about what works best for your needs.

Healthline has also compiled a list of natural treatments that may help soothe symptoms at home.

Without intervention, HS tunnels tend to be painful and may significantly interfere with patients’ quality of life. Treatments like saline injections, biologic therapies, and in more severe cases, surgery may be necessary to remove the tunnels.

Addressing the underlying inflammation by quitting smoking and maintaining a healthy weight may help prevent the tunnels from recurring. But since HS treatments require a case-by-case approach, talk with a medical professional about what’s right for you.