Glomangiomyoma of the Knee: A Rare Juxtasynovial Presentation

Glomus tumors are benign tumors typically located in the subcutis or deep dermis of the subungual region of the fingers. Histologically, glomus tumors are divided into three subtypes, in descending order of frequency: solid glomus tumor, glomangioma and glomangiomyoma. We report a case of a symptomatic intracapsular juxtasynovial glomangiomyoma of the knee. To the best of our knowledge, this location of this uncommon histological subtype of glomus tumor has not been reported previously. Although the final diagnosis is made by histopathology, the radiologist should consider this rare lesion in the differential of highly vascularised synovial-based masses.


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. The lesion was closely related to the underlying synovium of the joint space. There was no invasion into adjacent bone or subcutaneous soft tissues. The lesion was isointense to muscle on T1-weighted image (WI) and hyperintense on fat-suppressed (FS) proton density (PD) T2-WI. Tortuous feeding vessels and intense homogeneous enhancement was noted on fat-suppressed T1-WI after gadolinium contrast administration.
Complete open surgical resection was performed and confirmed that the lesion was located inside the articular capsule and was attached to the underlying synovium. The postoperative follow-up was uneventful, and the patient was completely pain-free after eight weeks.
Histopathological examination of resection specimen revealed a proliferation of thick-walled blood vessels surrounded by a monotonous population of small cells with a round and regular nucleus. No cytonuclear atypia, atypical mitoses or other malignant characteristics were retained. Immunohistochemistery was strongly positive for smooth muscle actin (SMA) and weakly positive for synaptophysin. Markers for CD-34 and desmin were negative. Based on histological findings, a diagnosis of glomangiomyoma was made.

Discussion
Glomus tumors are typically small, usually less than 1 cm, and comprise only 1.6% of soft tissue tumors in the extremities. Histologically, glomus tumors are subdivided into solid glomus tumor, glomangioma or glomangiomyoma with a frequency of 75%, 20% and 5% respectively. In the solid type, the glomus cells predominate. Glomus cells are small, uniform cells containing eosinophilic cytoplasm and rounded nuclei. The glomangioma type also contains glomus cells, but has a greater vascularity. The least frequent glomangiomyoma type demonstrates glomus cells, vascular channels and smooth muscle cells. Glomangiomyoma are positive on immunohistochemy tests for SMA, myosin, vimentin, and desmin [2].
The typical presentation of subungual glomus tumors consists of a triad of paroxysmal pain, cold sensitivity and tenderness to palpation. The symptoms are disproportional to the small size of the tumor and may cause disuse syndromes due to the extreme pain [10]. However, the reported extradigital glomus tumors, in particular around the knee, usually do not present with typical clinical features of subungual glomus tumors [11].
MRI imaging is the preferred technique to localize and characterize glomus tumors. The lesion is seen as a solid, well-defined round or ovoid mass, and highly vascularised with prominent feeding vessels. The tumor is of low to intermediate signal on T1-WI, isointense compared to muscle. The lesion is of high signal on T2-WI and PD fatsuppressed WI. On contrast-enhanced T1-WI a marked and homogeneous enhancement is seen [12].
The differential consists of pigmented villonodular synovitis, tenosynovial giant cell tumor, melanoma, vascular malformation and other vascular neoplasms [12]. Pigmented villonodular synovitis and tenosynovial giant cell tumor may show a similar contrast-enhancement pattern but due to hemosiderin deposition, the lesion contains low-signal-intense foci on T1-and T2-WI, causing blooming artifacts on T2* sequences [13]. Synovial melanoma may show hyperintense foci on T1-WI due to the presence of melanin [14]. Vascular malformations involving the synovium show vascular channels with fluid-fluid levels, intermediate signal on T1-WI, high signal on T2-WI and diffuse contrast-enhancement. Often the lesions contain fatty components and the lesion may have both intra-and extraarticular extension [15]. Complete surgical resection of glomangiomyoma is the treatment of choice and results in prompt disappearance of symptoms. Other therapeutic options are sclerotherapy or embolization [11].

Conclusion
In conclusion, glomangiomyoma juxtasynovially located in the joint capsule is an extremely rare manifestation of glomus tumors and should be included in the differential diagnosis of hypervascular synovial-based tumors. However, histopathology is needed for definitive diagnosis.