Hidradenitis suppurativa (HS) tunnels are hollow, tube-like channels that burrow through the deeper layers of the skin, lined with a layer of cells that essentially makes them self-sustaining structures the body cannot easily reabsorb. They represent one of the most difficult features of advanced HS, connecting abscesses and inflamed nodules beneath the surface and draining pus, blood, or fluid through one or more openings in the skin. Older medical literature calls them sinus tracts or fistulas, but the international dermatology community has increasingly adopted “tunnels” as the preferred term. Understanding what these structures actually are, why they form, and what can be done about them matters because tunnels are a major driver of pain, scarring, and reduced quality of life in people living with HS.
What Makes a Tunnel Different From an Abscess or Nodule
An HS nodule is a solid, painful lump of inflammation under the skin. An abscess is a pocket of pus that may eventually rupture or be drained. Both of these can resolve, at least temporarily, with medication or the body’s own healing processes. A tunnel is structurally different. Its walls are lined with epithelium, the same type of tissue that lines the skin’s surface, which means the body treats it almost like a permanent passageway rather than a wound that needs to close. This epithelial lining is what sets tunnels apart from simple pockets of infection and is a key reason they persist even when the inflammation that initially created them calms down.
Research into tunnel tissue shows that hair follicle stem cells play a central role. When an HS lesion destroys the normal skin architecture, stem cells from damaged hair follicles seed into the disintegrating tissue and begin building new epithelial structures. Combined with ongoing pus production driven by massive numbers of neutrophils and bacteria, along with enzymes called matrix metalloproteinases that break down the tissue framework, these stem cells help create channels that become self-perpetuating tracts running through the dermis.1British Journal of Dermatology. Aetiology and pathogenesis of hidradenitis suppurativa Fibrosis and scarring accumulate around and within these tracts over time, making them increasingly entrenched in the tissue.2PubMed Central. Hidradenitis Suppurativa Tunnels: Unveiling a Unique Disease Entity
Where Tunnels Typically Develop
HS overwhelmingly targets skin folds and areas rich in apocrine glands: the armpits, groin, inner thighs, under the breasts, buttocks, and the area around the anus and genitals.3PubMed Central. Surgical Treatment in Hidradenitis Suppurativa Tunnels can form in any of these locations, and a single person may have tunnels in multiple body regions at once. The anogenital and perianal areas deserve special mention because tunnels there can be mistaken for Crohn’s disease fistulas or perianal abscesses from other causes, which sometimes leads to delayed or incorrect diagnosis.
Tunnels can also interconnect. A network of tunnels beneath the skin of one armpit, for example, may link several separate nodules and abscesses into a single system with multiple drainage points on the surface. This interconnection is one reason the disease feels so different from having a single boil: the inflammation is not confined to one spot but spreads beneath the skin in ways that are not always visible from the outside.
The Inflammatory Environment Inside Tunnels
Even though tunnel walls resemble the epidermis histologically, they are far from quiet tissue. Studies of tunnel samples show significantly higher levels of inflammatory immune cells, including T cells, dendritic cells, and neutrophils, compared to nearby non-tunnel HS skin or healthy skin. The tunnels also express elevated levels of several inflammatory signaling molecules, particularly IL-17A, IL-17C, IL-17F, IL-36α, and CXCL8.4Dermatology. Tunnels in Hidradenitis Suppurativa: Active Inflammatory Entities with Specific Molecular and Genetic Profiles – A Narrative Review This is relevant because it means tunnels are not just passive scars or empty tubes. They are active sources of ongoing inflammation, continuously sending out signals that recruit more immune cells and perpetuate the disease cycle.
This active inflammatory profile has direct implications for treatment. The fact that tunnels are rich in IL-17 signaling, for instance, helps explain why biologics that block that pathway have shown some ability to shrink tunnels, as discussed further below.
What Lives Inside HS Tunnels
The bacterial community inside tunnels is distinct from what lives on healthy skin. A systematic review of the tunnel microbiome found that tunnels harbor a predominantly anaerobic (oxygen-avoiding) bacterial population, with larger biofilms and more active bacteria than what is found on the skin surface. The usual commensal bacteria that keep healthy skin in balance are significantly reduced.5PubMed Central. The Microbiome in Hidradenitis Suppurativa Tunnels: A Systematic Review Specific bacteria frequently identified include Porphyromonas and Prevotella species, both anaerobes with a tendency to form biofilms on the tunnel walls.6Dermatology. Tunnels in Hidradenitis Suppurativa: Active Inflammatory Entities with Specific Molecular and Genetic Profiles – A Narrative Review
Biofilms are communities of bacteria encased in a protective matrix that makes them much harder to eliminate with antibiotics than free-floating bacteria. This is one reason oral or topical antibiotics can reduce symptoms in HS but rarely eliminate tunnels: the drugs may knock down some bacterial load, but the biofilm within the tunnel regenerates. The biofilm also contributes to the ongoing pus production that keeps the tunnel active and draining.
How Tunnels Are Detected and Measured
Tunnels are not always obvious from looking at the skin. A doctor may feel a cord-like structure under the surface during a physical exam, or they may see drainage openings and infer that a tunnel connects them. But palpation alone consistently underestimates how extensive the tunnels actually are. A comparative study measuring tunnel lengths found that when surgeons assessed tunnel size by feel, the median length was about 30 mm, whereas ultrasound measurements of the same tunnels yielded a median of 36 mm, roughly 10 mm longer on average.7PubMed Central. Preoperative Tunnel Measurement in Hidradenitis Suppurativa: Comparison of Palpation and Ultrasound That gap matters surgically: if a surgeon plans an operation based on what they can feel and the tunnel extends a centimeter beyond that, the procedure may not remove the full tract.
High-frequency ultrasound has become a valuable tool for mapping HS lesions before treatment. It can detect tunnels that are too deep to feel, distinguish tunnels from abscesses (which look different on ultrasound), and assess how much inflammation surrounds each structure. Conventional clinical examination frequently underestimates the subclinical extent of HS, and ultrasound helps refine the picture beyond what traditional staging systems capture.8PubMed. High-Frequency Ultrasound in Hidradenitis Suppurativa: From Lesion Detection to Iconographic Lesion-Level Management Ultra-high-frequency ultrasound can even visualize the keratinized epithelial lining of tunnels, appearing as characteristic bright bands on the scan, which correlates with what pathologists see under the microscope.9PubMed Central. Correlation Between Ultra-High Frequency Ultrasound (UHFUS) and Histological Features of Typical Lesions in Hidradenitis Suppurativa
For HS affecting the anogenital or perianal area, MRI can provide detailed imaging of deeper tissue planes that ultrasound may not fully reach. MRI is particularly useful when tunnels extend into the fat layers around the anus or pelvis, where distinguishing HS tunnels from Crohn’s-related fistulas or other conditions requires cross-sectional imaging.10PubMed Central. Magnetic Resonance Imaging of Hidradenitis Suppurativa: A Focus on the Anoperineal Location
Surgical Approaches to Tunnels
Because tunnels have a self-sustaining epithelial lining that medication struggles to eliminate, surgery is often the most definitive treatment for established tunnels. The specific technique depends on tunnel size, location, and how interconnected the network is.
Deroofing is one of the most widely used approaches for individual tunnels. A surgeon opens the roof of the tunnel with a scalpel, scrapes out its contents and lining with a curette, and leaves the floor intact. The wound heals from the bottom up (by secondary intention). This preserves as much healthy tissue as possible while removing the tunnel’s epithelial lining so it cannot simply re-form. A multicenter study comparing surgical deroofing to COâ‚‚ laser treatment found that about 80% of surgically deroofed tunnels were fully healed at six months, while 90% of laser-treated tunnels healed in the same period. Recurrence in the surgical group was low: only two patients had recurrence, at two and three months respectively.11PubMed Central. CO2 Laser Versus Surgical Deroofing for the Treatment of Hidradenitis Suppurativa Tunnels: A Comparative Multicentric, Retrospective Study
COâ‚‚ laser excision works similarly in concept: the laser vaporizes the tunnel lining tissue, sterilizing the wound bed in the process. The slight edge in healing rates seen in the study above may relate to the laser’s precision and its ability to seal small blood vessels as it cuts, reducing bleeding and potentially speeding healing. Both approaches share the same basic philosophy of destroying the epithelial lining so the tunnel cannot persist.
Wide excision is reserved for more severe or extensive disease where tunnels are numerous, interconnected, or have damaged a large area of skin beyond targeted repair. This involves cutting out the entire affected zone and either allowing it to heal on its own, closing it with stitches, or covering it with a skin graft or flap. Wide excision tends to produce the longest disease-free intervals, though recovery time is significantly longer and the cosmetic outcome depends on the closure technique used.12PubMed Central. Wound closure techniques after wide excision for hidradenitis suppurativa: a systematic review and meta-analysis
Can Medication Treat Tunnels Without Surgery
This is where expectations need careful calibration. Systemic medications, including biologics, can reduce tunnel size and drainage, but fully closing established tunnels with medication alone is uncommon. Systemic treatments tend to provide temporary relief rather than the prolonged disease-free periods that surgical excision achieves.13PubMed Central. Wound closure techniques after wide excision for hidradenitis suppurativa: a systematic review and meta-analysis
That said, biologics targeting key inflammatory pathways have shown measurable effects on tunnels. Brodalumab, which blocks the IL-17 receptor, was shown to decrease both the size and draining of tunnels in HS patients.14PubMed Central. Epithelialized tunnels are a source of inflammation in hidradenitis suppurativa This aligns with the finding that tunnels are enriched in IL-17 signaling. Adalimumab, a TNF-alpha inhibitor, is currently the most widely approved biologic for HS and can reduce flares and new tunnel formation, though its ability to close existing tunnels is more limited. The practical upshot is that biologics and surgery often work best in combination: medication to quiet the systemic inflammation and reduce new lesions, surgery to physically remove tunnels that drugs alone cannot eliminate.
Injections and Minimally Invasive Options
For patients who are not yet candidates for surgery or who want to manage acute flares, several in-office procedures target individual lesions. Ultrasound-guided corticosteroid injections deliver concentrated anti-inflammatory medication directly into the tunnel or surrounding inflamed tissue. In one study of over 400 inflammatory HS lesions, injecting triamcinolone acetonide resulted in a complete response in about 53% of draining fistulas at 12 weeks, compared to 35% of similar lesions that were not injected. The response was stronger for abscesses and nodules (roughly 72% and 81% respectively).15PubMed Central. Ultrasound-assisted intralesional corticosteroid infiltrations for patients with hidradenitis suppurativa Tunnels are harder to eliminate with injections than simpler lesion types, but corticosteroid shots can meaningfully reduce drainage and pain.
Newer minimally invasive approaches include cryoinsufflation (blowing cold gas into the tunnel to destroy the lining) and intralesional photodynamic therapy, which uses a light-sensitive chemical applied inside the tunnel followed by laser activation to kill bacteria and inflammatory tissue.16Current Dermatology Reports. In-office Procedures for Dermatologists Managing Hidradenitis Suppurativa These are still relatively new and the evidence base is smaller. Several case series have evaluated light-based intralesional therapies using photodynamic approaches or diode lasers, generally showing improvement in treated lesions, but the study sizes remain modest and direct comparisons with surgery are limited.17Dermatology. Intralesional Treatments in Hidradenitis Suppurativa: A Systematic Review These options may appeal to patients reluctant to undergo more invasive surgery, and they fill a treatment gap for smaller or less complex tunnels.
Wound Care After Tunnel Treatment
Whether a tunnel is deroofed, lasered, or widely excised, the resulting wound often heals by secondary intention, meaning it fills in from the bottom rather than being stitched closed. This process can take weeks to months depending on wound size and location. Proper wound care during this period is critical and is itself a part of HS management that does not always get the attention it deserves.
The evidence for any single optimal wound care protocol in HS remains limited. Experts emphasize a multidisciplinary approach that coordinates topical wound care with systemic therapy. Silver-based absorbable dressings are commonly used after deroofing procedures.18PubMed Central. CO2 Laser Versus Surgical Deroofing for the Treatment of Hidradenitis Suppurativa Tunnels: A Comparative Multicentric, Retrospective Study Other commonly used dressings include foam dressings, alginate dressings for heavily draining wounds, and hydrogel sheets for wounds that are drying out. For patients with active tunnels that are being managed conservatively rather than surgically, absorptive dressings and regular wound cleaning help control drainage and odor, which are among the most distressing daily symptoms for people living with HS.
When Tunnels Start in Younger Patients
HS is often described as a disease of adults, but symptoms frequently begin during or shortly after puberty. Adolescents can develop the full spectrum of HS lesions, including tunnels, particularly if diagnosis and treatment are delayed. The disease often begins after adrenarche (the early rise in adrenal hormones that precedes full puberty), and in some young patients it progresses to draining tunnels and permanent scarring, with significant effects on quality of life and mental health.19PubMed Central. Surgical Treatment in Hidradenitis Suppurativa Early recognition and treatment in younger patients is important precisely because preventing tunnel formation is far easier than treating tunnels once they have established their epithelial lining.
Diagnosing HS in teenagers can be tricky because early lesions, recurrent painful bumps in the groin or armpits, may be dismissed as ordinary boils or folliculitis. A pattern of recurrence in the same body areas, especially if there is a family history of HS, should raise suspicion and prompt referral to a dermatologist before the disease reaches the tunnel stage.
Why Tunnels Do Not Heal on Their Own
People often ask why these channels persist even during periods when inflammation seems lower. The answer lies in the epithelial lining. Normal wounds heal by contracting and closing from the edges inward. A tunnel lined with epithelium behaves more like a natural body cavity: the body sees the lining as “skin” and does not try to close it. The ongoing presence of biofilm-forming bacteria inside the tunnel perpetuates low-grade inflammation that recruits immune cells, which in turn produce enzymes that degrade the surrounding tissue and prevent normal wound closure.20British Journal of Dermatology. Aetiology and pathogenesis of hidradenitis suppurativa Fibrosis around the tunnel stiffens the surrounding tissue, locking the channel in place even further.
This is the fundamental reason that early-stage HS (nodules and abscesses without tunnels) is managed differently from advanced disease. Once tunnels form, the treatment toolkit shifts from primarily medical to a combination of medical and surgical. Every major clinical guideline for HS reflects this distinction, and it is why dermatologists push for early, aggressive treatment: keeping the disease in check before tunnels develop avoids a much harder problem down the road.
Imaging Advances and Personalized Surgical Planning
The gap between what a clinician can feel and what imaging reveals is pushing the field toward routine preoperative ultrasound for HS surgery. When tunnel length was systematically underestimated by about a centimeter on physical exam compared to ultrasound, as one study demonstrated,21PubMed Central. Preoperative Tunnel Measurement in Hidradenitis Suppurativa: Comparison of Palpation and Ultrasound the implication is that some surgeries may leave tunnel segments behind if guided by palpation alone. Ultrasound also helps identify occult (hidden) tunnels and inflammatory collections that were not clinically apparent, which may explain some cases of unexpectedly rapid recurrence after surgery.
Ultra-high-frequency ultrasound and MRI for perianal disease represent the leading edge of diagnostic precision. These tools are not yet universally available in every dermatology office, but their adoption is growing, particularly in specialized HS centers. For patients considering surgery, asking whether preoperative imaging is available is a reasonable step toward ensuring the procedure addresses the full extent of disease.