Case Report: Achilles enthesitis as presenting clue to occult sacroiliitis in hidradenitis suppurativa

Hidradenitis suppurativa (HS) is increasingly recognized as a systemic inflammatory disease with an established overlap with spondyloarthritis. This case highlights how enthesitis can serve as a presenting clue to occult sacroiliitis in patients with HS, particularly when back pain is absent or unrecognized.

The authors report a 31-year-old man with Hurley stage II hidradenitis suppurativa who presented with Achilles enthesitis supported by point-of-care ultrasound. The patient denied any current or previous back pain lasting 3 months or longer, including inflammatory back pain and alternating buttock pain, prolonged morning stiffness, peripheral arthritis, dactylitis, uveitis, psoriasis, and gastrointestinal symptoms.

During workup for asymptomatic microscopic hematuria, computed tomography of the pelvis incidentally showed bilateral sacroiliac joint abnormalities that prompted dedicated imaging. Magnetic resonance imaging of the sacroiliac joints showed findings meeting the consensus definition of active sacroiliitis, with bilateral subchondral bone marrow edema and early structural changes.

The patient was human leukocyte antigen B27 positive, had elevated inflammatory markers, and had a first-degree family history of ankylosing spondylitis. Although the ASAS axial spondyloarthritis classification criteria were not fulfilled because the required history of back pain lasting 3 months or longer was absent, the patient fulfilled the ASAS peripheral spondyloarthritis classification criteria based on current Achilles enthesitis together with HLA-B27 positivity and sacroiliitis on imaging.

Converging clinical, imaging, immunogenetic, and familial features supported a clinical diagnosis of spondyloarthritis associated with hidradenitis suppurativa. Adalimumab was started using the licensed hidradenitis suppurativa regimen. At 6 months, calcaneal tenderness and skin lesions had improved, inflammatory markers had normalized, and follow-up imaging showed substantial reduction in bone marrow edema.

This case suggests that axial inflammation in hidradenitis suppurativa may be under-recognized when assessment relies exclusively on patient-reported back pain. Sacroiliac MRI should not be used routinely in all patients with hidradenitis suppurativa and enthesitis-like symptoms but may be considered after rheumatologic assessment when multiple clinical, laboratory, familial, or incidental imaging features collectively raise suspicion of spondyloarthritis.