Epithelial inclusion cyst of the equine digital flexor tendon sheath: diagnosis by ultrasonography and magnetic resonance and successful treatment by tenoscopy

A six-year-old warmblood horse was presented with a longstanding frontlimb lameness with mild digital flexor tenosynovitis and swelling of the distomedial pastern. Ultrasonography and magnetic resonance revealed a dense mass lesion in the distal aspect of the digital flexor tendon sheath, with a partial lamellar architecture, absence of internal vascularization and adjacent smooth pressure osteolysis of the middle phalanx. After surgical excision, histopathology con- firmed an epithelial inclusion cyst. Epithelial inclusion cysts, also known as keratinizing or fol- licular cysts, are expansile benign mass-like lesions of aberrant epidermal tissue. In the horse, they are known to occur in cutaneous and several non-cutaneous tissues. In the digital flexor tendon sheath, they have rarely been described. Given their often chronic presentation in this location, they may appear as an atypical dense mass on imaging, uncommon for cystic lesions. Complete tenoscopic removal, even for larger masses, is achievable and considered curative with good prognosis for return.