Myxomas are benign mesenchymal tumours characterised by the production of mucinous material in a scant spindle cell matrix. ¹ Juxta-articular myxoma (JAM) is a rare form of myxoma in the vicinity of a joint. ² JAM has an association with trauma or joint arthrosis. It is may be locally aggressive and commonly recurs after surgical excision. ³

A 78-year-old man, with a history of colon carcinoma and gout, presented with a large mass in the thenar eminence and bilateral carpal tunnel syndrome. On clinical examination, the mass was fixed and had no inflammatory or infective features. Magnetic resonance imaging (MRI) was requested to assess the nature of the mass and exclude a neoplastic lesion.

JAM is most commonly associated with large joints, with a male preponderance. It is mostly found in the lower limbs, and is very unusual in the hand and wrist. ⁵ JAM involves periarticular tissues including subcutaneous fat, joint capsule, tendons and rarely skeletal muscle.

The aetiology of the lesion is uncertain; however, there is an association with previous trauma and osteoarthrosis of the underlying joint. ^{1, 6} The lesion itself is usually painless, but it may cause local mechanical symptoms. JAMs do not metastasise; however, as in this case, they can be very locally aggressive. Postoperative recurrence is seen in approximately 30% of JAMs within five years of excision. ^{3, 4} Other types of myxoma are intramuscular, cutaneous aggressive angiomyxoma, and myxoma of the nerve sheath (neurothekeoma). ³

Intramuscular myxoma is the most commonly encountered myxoma in clinical practice, occurring in the thigh, upper arm and buttock. ⁵ The lesion is characteristically a well-circumscribed oval mass that is cystic in appearance. Intramuscular myxomas are usually solitary but may be multiple and, in this scenario, are usually associated with monostotic or polyostotic fibrous dysplasia, which is known as Mazabraud's syndrome. ⁵

On ultrasound, the lesion may appear as a complex cystic mass (hypo- or anechoic with increased through-transmission and posterior acoustic enhancement) or may even appear solid. It is hypovascular. ¹ Computed tomography (CT) typically shows a lesion denser than water but less dense than muscle. ⁸ The lesion on MRI is iso- to hypointense on precontrast T1W, inhomogeneously hyperintense on T2W, and demonstrates mild to moderate diffuse or peripheral contrast enhancement. ⁸ Typical MRI findings also include mild perilesional oedema, owing to an incomplete fibrous capsule, and a split-fat sign as a result of adjacent muscle atrophy. ⁸

JAM and intramuscular myxoma share the same histological characteristics, but are differentiated by lesion location and the high recurrence rate of JAM. The differential diagnosis of a lesion with imaging findings of a soft-tissue mass that mimics a cyst includes synovial cyst, bursa, ganglion, neurogenic neoplasms, myxoid liposarcomas and myxoid malignant fibrous histiocytoma (MFH). ^{5, 7}

JAM is a benign mesenchymal soft-tissue tumour that is a variant of myxoma, associated with large joints and joint degeneration. The present case demonstrates a histologically proven case of JAM in an unusual location, with an unusually aggressive growth pattern and prominent local tissue infiltration.