Hidradenitis Suppurativa Tunnelling: How Ultrasound Helps

Understanding what happens beneath the skin could make a big difference in how hidradenitis suppurativa is treated.

Hidradenitis suppurativa (HS) is a long-term inflammatory skin condition that causes painful lumps, abscesses and sometimes scarring. It commonly affects areas where skin rubs together, such as the armpits, groin, buttocks and under the breasts.

For many people living with HS, one of the most difficult problems is something called tunnelling.

Tunnels develop underneath the skin and can cause ongoing pain, swelling, leaking fluid and repeated flare-ups.

The challenge is that these tunnels are not always visible. What looks like a small lump on the surface may actually be part of a much larger problem beneath the skin.

This is why ultrasound is becoming an important tool in understanding and treating HS.

What is tunnelling in HS?

When HS causes repeated inflammation, the tissue beneath the skin can become damaged.

Over time, abnormal channels, known as tunnels or sinus tracts, can form underneath the skin.

These tunnels may connect several painful lumps or abscesses. They can also contain fluid and inflamed tissue.

Some tunnels are small, while others spread across a larger area and develop several branches.

Tunnels can cause:

Ongoing pain and tenderness.
Repeated swelling and inflammation.
Fluid or pus leaking from the skin.
Unpleasant odours.
Scarring and thickened skin.
Difficulty moving comfortably.

One of the frustrating things about HS is that a tunnel may continue to cause problems even after the skin appears to have healed.

For example, a painful lump may settle after treatment, but if an underlying tunnel remains, inflammation and symptoms can return.

This is one reason why some areas of HS keep flaring up in exactly the same place.

Why can’t doctors always see the full problem?

Doctors usually assess HS by looking at the skin, checking for painful lumps, abscesses, drainage and scarring.

Although this is important, examining the skin alone may not show everything that is happening underneath.

A small opening on the skin might lead to a much longer tunnel. Some tunnels do not have a visible opening at all.

Research has shown that ultrasound can identify tunnels that were missed during a normal skin examination.

In a 2025 study involving 122 people with HS, researchers found tiny tunnels, called microtunnels, in some patients whose disease had originally been classified as mild.

This suggests that HS may sometimes be more extensive than it first appears.

However, finding a small tunnel does not necessarily mean the disease will get worse. More research is needed to understand which early tunnels are likely to progress.

How does ultrasound help?

Ultrasound is a scan that uses sound waves to create images of structures inside the body.

It is commonly used during pregnancy, but specialised ultrasound can also examine the skin and the tissues underneath it.

In HS, high-frequency ultrasound allows a trained clinician to look beneath the surface and gain a better understanding of the disease.

An ultrasound scan can help answer several important questions.

1. Are there hidden tunnels?

Ultrasound can identify tunnels that cannot easily be seen or felt during a normal examination.

2. How far do the tunnels extend?

It can help show the length, depth and direction of tunnels, including whether they connect to other areas.

3. Is there still inflammation?

A special type of ultrasound called Doppler can detect blood flow around affected tissue. Increased blood flow may be a sign of active inflammation.

4. Is the lump an abscess or a tunnel?

These are different problems and may need different treatments. Ultrasound can help doctors tell them apart.

5. Is treatment working?

Ultrasound may help doctors track changes in inflammation, fluid collections and tunnels over time.

A tunnel doesn’t have to be leaking to be inflamed

One of the interesting findings from recent research is that a tunnel can still be inflamed even when it is not leaking fluid.

A 2026 study examined 312 HS tunnels in 84 patients.

Researchers found signs of increased blood flow around many of the tunnels, including some that were not actively draining.

Overall, Doppler ultrasound detected vascular activity in approximately 91% of the tunnels studied.

This matters because increased blood flow can be associated with inflammation.

It suggests that simply waiting for a tunnel to leak or become visibly swollen may not always give us the full picture.

For someone living with HS, this may help explain why an area continues to feel painful or uncomfortable even when there is no obvious discharge.

However, ultrasound findings need to be considered alongside symptoms and a clinical examination. Increased blood flow alone does not prove that a tunnel needs surgery or a particular medication.

How can ultrasound help doctors choose the right treatment?

Not all HS tunnels are the same, so they should not necessarily be treated in the same way.

Some areas have a lot of active inflammation. Others contain established tunnels and scar tissue that may not disappear with medication alone.

Ultrasound can provide more information to help doctors decide which treatment is most appropriate.

Treating inflammation with medication

If there is active inflammation, doctors may recommend medication to help control it.

Depending on how severe the HS is, treatments may include anti-inflammatory antibiotics, hormonal treatments for suitable patients or biologic medications.

Biologics are medicines that target specific parts of the immune system involved in inflammation.

Examples used for HS include adalimumab, secukinumab and bimekizumab.

These treatments can help reduce inflammation and flare-ups, but they may not completely remove tunnels that have already formed.

Treating tunnels with deroofing

When a tunnel keeps causing pain, swelling or discharge, a procedure called deroofing may be recommended.

Deroofing involves removing the skin covering the tunnel, opening it up so the affected tissue can heal.

Unlike simply draining an abscess, deroofing is designed to treat the tunnel itself.

Before the procedure, ultrasound can help identify where the tunnel starts, where it ends and whether it has any branches.

This can help the treating clinician plan the procedure and avoid missing parts of the tunnel.

Treating more extensive tunnels with surgery

Some people develop larger networks of tunnels that spread across a wider area.

In these cases, more extensive surgery may be needed to remove the affected tissue.

Ultrasound can help surgeons plan by showing how far the disease extends beneath the skin.

This may help reduce the chance of leaving behind affected tissue that was not visible during examination.

Research suggests that ultrasound-guided planning may improve surgical outcomes, although larger studies are still needed.

Combining medication and surgery

For some people, the best approach may be a combination of medication and surgery.

Medication can help control inflammation throughout the body, while surgery targets tunnels that continue to cause problems.

Research, including the SHARPS clinical trial, has shown that certain biologic treatments can be safely used alongside HS surgery in appropriately selected patients.

This highlights the importance of developing a treatment plan that addresses both inflammation and the tunnels themselves.

When might an ultrasound be useful?

An ultrasound assessment may be worth discussing with your treating doctor if:

The same area keeps flaring up.
A lump or painful area is not improving with treatment.
There is ongoing discharge from the same location.
There is scarring and a suspected tunnel underneath.
The pain seems worse than what is visible on the skin.
Deroofing or surgery is being considered.
Treatment is helping overall, but certain areas continue to cause problems.

Not everyone with HS needs an ultrasound. However, when tunnelling is suspected, the information may help guide treatment decisions.

It is important that HS ultrasound scans are performed and interpreted by clinicians with appropriate training, as not all ultrasound services specialise in skin conditions.

What does the latest research tell us?

International expert recommendations published in 2025–2026 recognise that ultrasound can provide valuable information when assessing HS.

Research shows that it can identify hidden tunnels, help assess inflammation and support surgical planning.

However, there are still questions that need answering.

For example, researchers are still investigating whether finding very early tunnels can help prevent more severe disease and whether regular ultrasound monitoring improves long-term outcomes.

For now, ultrasound is best understood as an additional tool that helps doctors make more informed decisions, rather than something that replaces normal clinical assessment.

Looking beyond the surface

One of the most important things to understand about hidradenitis suppurativa is that what we see on the skin does not always show us the full extent of the disease.

A small lump may have a tunnel underneath it.

An area that has stopped leaking may still have inflammation.

And a tunnel that continues to cause repeated flare-ups may need more than medication alone.

Ultrasound gives clinicians another way to understand what is happening beneath the skin.

By identifying hidden tunnels, measuring how far they extend and looking for signs of inflammation, ultrasound can help guide decisions about medication, deroofing and surgery.

The aim is to treat HS more effectively, reduce repeated flare-ups and improve quality of life.

Because when it comes to HS, understanding what is happening beneath the skin can be just as important as treating what we can see.

References and further reading

Wortsman X, et al. (2026). International consensus statement on the use of ultrasound in hidradenitis suppurativa. Journal of the European Academy of Dermatology and Venereology.
Jemec GBE, et al. (2026). European S2k guidelines: Epidemiology, diagnosis and clinical assessment.
Zouboulis CC, et al. (2025). European S2k guidelines: Treatment.
Martorell A, et al. (2026). Beyond Drainage: Clinical–Ultrasound Correlation Supports a Broader Definition of the Active Tunnel in Hidradenitis Suppurativa.
Michelucci A, et al. (2025). Enhancing Hidradenitis Suppurativa Assessment: The Role of Ultra-High Frequency Ultrasound in Detecting Microtunnels and Refining Disease Staging.
Bechara FG, et al. (2021). Efficacy and Safety of Adalimumab in Conjunction With Surgery: The SHARPS Randomized Clinical Trial.

Disclaimer: This article provides general health information and is not a substitute for individual medical advice. Treatment decisions should be made with a healthcare professional experienced in managing hidradenitis suppurativa.