Beyond Drainage: Clinical–Ultrasound Correlation Supports a Broader Definition of the Active Tunnel in Hidradenitis Suppurativa

INTRODUCTION: Current severity classification systems for hidradenitis suppurativa (HS) define tunnel activity almost exclusively by the presence of purulent drainage. In daily clinical practice, however, non-draining tunnels frequently exhibit other signs suggestive of ongoing inflammation, raising uncertainty about their true activity status and potentially delaying therapeutic intervention. Consequently, a substantial proportion of inflamed yet non-draining tunnels is overlooked by current staging frameworks. METHODS: Gold) was used to assess vascular flow within tunnel walls as an objective imaging surrogate of inflammation. Clinical signs pain (spontaneous or on palpation), erythema, induration and purulent drainage, were recorded independently prior to ultrasound examination and correlated with Doppler findings. RESULTS: Colour Doppler ultrasound detected vascular activity in 91% (284/312) of tunnels. Purulent drainage correlated with Doppler positivity in 100% of cases. Among non-draining tunnels, pain associated with erythema or pain associated with induration showed a clinically meaningful correlation with Doppler activity. Dermal tunnels (type A), despite being universally non-draining, exhibited Doppler positivity in the majority of cases and showed the highest prevalence of pain-erythema combinations (88%). CONCLUSIONS: These findings suggest that an active tunnel in HS may be defined as one presenting purulent drainage or pain associated with erythema or induration. This Doppler-informed definition may allow earlier recognition of inflammatory tunnels, particularly non-draining dermal forms, which could represent a critical therapeutic window, a hypothesis requiring prospective longitudinal confirmation.