A 55-year-old man was referred to Dermatology with a 6-month history of painful interdigital lesion on his right foot, associated with fluctuating inflammation. He had undergone several cycles of oral and topical antibiotic therapy without improvement. The patient worked as a sign maker and owned a dog. He did not report cutting hair or walking barefoot or wearing open footwear.

An 8-mm violaceous interdigital nodule was observed between the 4th and 5th toes of the patient's right foot, with a fistulous opening extending to the base of the fold (Fig. 1).

Fig. 1

Biopsy showed fibroinflammatory changes associated with a fistulous cystic lesion with a granular layer and acral-type squamous epithelium.

Ultrasound examination was performed using an 18-MHz linear probe in B-mode, revealing a heterogeneous hypoechoic area with hyperechoic foci within it (Fig. 2).

Fig. 2

What is your diagnosis?

Interdigital pilonidal sinus.

The patient returned for follow-up 2 months later. On this occasion, both clinically and dermatoscopically, a terminal hair could be observed in the biopsied area, which was removed with forceps (Fig. 3).

Fig. 3

Complete surgical excision of the lesion was proposed; however, given the absence of symptoms, conservative management was agreed upon for the time being.

Interdigital pilonidal sinus is an occupational dermatosis caused by the penetration of fragments of human or animal hair into the skin, which induces a chronic inflammatory response that, in an attempt to encapsulate and expel the foreign material, progresses to the formation of fistulas, sinuses, and cysts.1–3

It mainly affects hairdressers or barbers who attend predominantly male clients, whose hair is shorter, thicker, more rigid, and sharper, making it more likely to penetrate the epidermis compared with women's hair.4–6 Less frequently, it is also observed in professions associated with dog grooming, milking, or shearing.1,2 In our case, none of these risk factors were identified.

It usually occurs in the interdigital spaces of the hand, generally on the dominant side. On rare occasions it occurs on the foot and is usually a consequence of wearing open footwear at work.1 One of the distinguishing characteristics of interdigital pilonidal sinus is that it tends to occur in hairless areas and does not contain the patient's own hair, unlike pilonidal sinuses affecting the sacrococcygeal region, umbilicus, chest wall, anal canal, ear, and scalp.3

Clinical presentation usually consists of interdigital inflammation, often recurrent and associated with discharge; however, some patients remain asymptomatic.6

An occupational history generally confirms the diagnosis.6 Certain tests such as biopsy or ultrasound may be useful; ultrasound shows hyperechoic foci corresponding to hair shafts.7 Among the differential diagnoses to consider are fungal or mycobacterial infection, Orf nodule, and other traumatic causes.6

Conservative approaches in symptomatic patients, including hair removal from the sinus, drainage, and antibiotic administration, are usually not effective. In most cases, surgical excision of the affected tissue is curative.2,5

The most efficient way to prevent recurrence is protection through simple measures such as regular cleaning of the interdigital spaces, removal of loose hairs, the use of gloves, closed shoes, and barrier creams.5,6

We present a case of interdigital pilonidal sinus of the foot without an identifiable cause. Although it is considered an occupational dermatosis, it should be included in the differential diagnosis of interdigital lesions even in the absence of a history of a high-risk profession.

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We have no conflicts of interest.