Article ID: 2016-0002-CR
Cutaneous metaplastic synovial cyst (CMSC) is a rare tumor typically representing a solitary, well-circumscribed and cystic mass, which is not connected to the joint. Synovial cysts have been predominantly reported by orthopedists or pathologists and the presence of CMSC has not been well recognized by dermatologists. Herein, we reported CMSC developing in a 68-year-old woman receiving systemic corticosteroid for the treatment of eosinophilic granulomatosis with polyangitis and attempt to delineate clinical characteristics of this unusual neoplasm by reviewing the literature, especially focusing on dermatological descriptions. In histology, the cystic wall was lined by the layers of flattened synovial cell-like cells and connective tissues, mimicking the synovial membrane. Positive immunoreactivity of the lining cells against vimentin was detected, but not for cytokeratin, CEA, CD68 and S-100 were detected. The pathogenesis of CMSC remains unclear, but has been tightly linked to direct traumatic stimuli or relative tissue fragility, which potentially accounts for CMSC development in our case. Most CMSCs reported by dermatologists located on the extremities, while those described by other specialists tended to distribute more globally. Preoperative diagnoses were either epidermal cyst or suture/foreign body granuloma. Incomplete surgical excision of usual synovial cyst may lead to local recurrence, which has been reported in oral and maxillofacial surgery, but not in dermatology. This could be explained by technical difficulty of surgical excision due to anatomical location. Dermatologists need to be aware of CMSC, which needs to be included as differential diagnosis of subcutaneous cyst.
Cutaneous metaplastic synovial cyst (CMSC) is a rare subcutaneous tumor that typically presents a solitary, well-circumscribed and tender cystic mass.1 In histology, the cystic cavity is surrounded by synovial-like cells resembling the cells observed in hyperplastic synovium.2 Villous-like structures often protrude into the lumen of the cyst.2 It usually appears at the site of preceding surgery or trauma and is not related to joint or synovial structures.3 CMSC has not been considered as a true cyst becasue it lacks definite epithelial lining.1 Synovial cyst has been predominantly discussed in the field of orthopedic medicine or pathology. To date, as few as 30 CMSC cases, including our case, have been reported in the literature. Nineteen cases were described by dermatologists. Here, we report a case of solitary CMSC on the left lower thigh in a 68-year-old Japanese female and attempted to delineate clinical characteristics by reviewing the literature from the dermatological point of view.
A 68-year-old Japanese female presented with a 1-year history of a solitary mass on anterior surface of left lower leg. She had a 7-year medical history of eosinophilic granulomatosis with polyangitis, which had been well controlled by systemic corticosteroid. On physical examination, a solitary, well-circumscribed, 35-mm-diameter tense cystic mass was noted (Fig. 1a). The mass was slightly painful and almost freely movable over underlying tissue and from the overlying skin. Laboratory examinations were within normal limits. Ultrasonography detected a well-defined and hypoechoic structure in the subcutaneous tissue (Fig. 1b). The lesion connected to the dermis and surrounded by subcutaneous fat tissue was surgically removed. In histology, a cystic structure without distinct epithelial lining was found in the subcutaneous tissue (Fig. 1c). The cystic wall was lined by the layers of flattened synovial cell-like cells and connective tissues, mimicking the synovial membrane (Fig. 1d). The lining cells partially protruded into the cystic cavity. In the surrounded tissue, various structures, hyalinized connective tissue, fibroblastic cells, or inflammatory cells, were observed. Immunohistochemical analysis revealed that the lining cells were positive for vimentin (Fig. 1e), but negative for cytokeratin14 (Fig. 1f), CEA, CD68, and S-100 (data not shown). Based on these findings, the diagnosis of cutaneous metaplastic synovial cyst was made.
(a) A solitary, well-circumscribed, 35-mm-diameter cystic mass was noted on left lower leg. (b) Ultrasonography revealed a hypoechoic structure in the subcutaneous tissue. (c) A cystic structure without epithelial lining was noted (hematoxylin–eosin [HE], original magnification ×40). (d) The cystic wall consisted of flattened cells resembled synovial cells and connective tissues (HE, ×400). (e) Positive immunoreactivity against vimentin (black arrowhead) was detected in the lining cells (×400). (f) Negative immunoreactivity against cytokeratin14 was observed (×400).
In 1953, Selye first reported experimental induction of a cyst with hyperplastic synovial-like structure at the site of artificial trauma in rat.4 Since then, cystic tumors with similar histopathological characteristics, which were likely triggered by external insults, have been reported mostly in the field of orthopedics as synovial cysts.5 Synovial cysts develop mainly in bones or joints after orthopedic surgery, while CMSCs occur in the skin. Gonzalez et al. reported the first case as synovial metaplasia of the skin for the first time in 1987.1 To the best of our knowledge, only 30 CMSC cases have been reported in the literature (Table.1). Approximately, 60% of the cases were described by dermatologists. It could develop at any age and the mean age at the diagnosis was 41.5 years (range 7 to 82 years). No sex difference was noted. Clinically, cases cystic mass sized from 4 to 30 mm in diameter and were sometimes painful (n =12). The most common locations were the limbs (n =14) and especially occurred in palmoplantar (n =9). Most CMSCs reported by dermatologists located on the extremities, while those described by other specialists tended to distribute more globally. Associated conditions included Ehlers-Danlos syndrome,3,10 rheumatoid arthritis,11,13 basal cell carcinoma.8 Characteristically, 17 cases (59%) developed at the scar of operation or trauma, and time of evolution after operation or trauma was a few weeks to years (range 3 weeks to 2 years), however, our case did not have such history. Preoperative diagnoses by dermatologist were either epidermal cyst or suture/foreign body granuloma. CMSC could also be diagnosed as a mucous cyst by oral and maxillofacial surgery oral and maxillofacial surgeons.
| Author | Age/Sex | Site | Pre-existing lesion |
Associated condition |
Time of evolution | Size (mm) | clinical diagnosis |
|---|---|---|---|---|---|---|---|
| Gonzalez1 | 82 F | Chest | Surgical scar | ― | 10mon | 4 | periprosthetic infection in scar |
| 27 F | Abdomen | Surgical scar | ― | 9mon | 6 | suture granuloma | |
| 77 M | Abdomen | Surgical scar | ― | 3y | 7 | suture granuloma | |
| Stern6* | 7 F | Scalp | Surgical scar | ― | N/A | N/A | suture granuloma |
| Gomez7 | 28 M | Arm | Biopsy wound | ― | N/A | N/A | N/A |
| Bhawan8* | 40 M | Finger | ― | ― | N/A | N/A | Leiomyoma vs. eccrine poroma |
| 40 F | Palm | Puncture wound | ― | N/A | N/A | foreign body granuloma | |
| 20 M | Abdomen | Surgical scar | ― | N/A | N/A | Suture granuloma | |
| 67 M | Face | ― | BCC | N/A | N/A | BCC | |
| Beham9 | 15 F | Knee | trauma | ― | 3mon | 8 | N/A |
| 45 F | Hand | trauma | ― | several month | N/A | N/A | |
| Nieto10* | 15 F | Elbow | ― | EDS | N/A | 17 × 12 | molluscoid pseudotumor |
| Singh11* | 72 M | Buttock | ― | RA | N/A | N/A | epidermal inclusion cyst |
| Lin5* | 61 F | Buttock | Surgical scar | atherome | 3week | 10 | inflammatory epidermal cyst |
| Yang12* | 80 F | Buttock | ― | ― | N/A | N/A | traumatic panniculitis |
| Choonhakarn13* | 55 F | Toe | ― | RA | N/A | N/A | N/A |
| 46 M | Thumb | ― | RA | N/A | N/A | N/A | |
| Goiriz14* | 64 F | Wrist | trauma | ― | 6mon | 7 | N/A |
| 63 F | Finger | ― | Arthrosis | 3mon | 4 | N/A | |
| 51 M | Foot | ― | ― | 1y | 10 | N/A | |
| 36 F | Abdomen | Surgical scar | ― | 1y | 5 | N/A | |
| 60 M | Abdomen | Surgical scar | ― | 1y | 10 | N/A | |
| Ramdial15 | 23 M | Chest | Surgical scar | gynecomastia | 2y | 6 × 4x4 | gynecomastia |
| Chakravarthy16 | 34 M | Neck | Surgical scar | several month | N/A | epidermoid cyst | |
| Guala3 | 7 M | Knee | ― | EDS | N/A | 20 | N/A |
| Inchingolo17 | 26 F | Face | Injection of hyaluronic acid |
― | N/A | 5 | injection-site reaction |
| Kim2* | 51 F | Foot | ― | ― | N/A | 6.2 × 13.1 | N/A |
| Shim18* | 18 M | Face | Scar of acne | ― | 6mon | 30 | epidermal cyst |
| Kermani19 | 29 M | Face | Surgical scar | ― | 8year | 20 × 40 | CMSC |
| Our case* | 68 F | Lower leg | ― | EGPA | 1year | 15 | lymphangioma |
* Described by dermatologists. BCC: Basal cell carcinoma, EDS: Ehlers-Danlos syndrome, RA: Rhumatoid arthritis, EGPA: Eosinophilic granulomatosis with polyangitis, N/A: not available
Normal synovial tissue is usually crimped into folds and has two main layers. The outer layer, or subintima, contains blood and lymphatic vessels. The inner layer, or intima, consists of a sheet of flat synovial cells. The lining cell of CMSC resembles hyperplastic synovial membrane with partial hyalinization. The cyst wall has villous like structures toward the cystic cavity. Synovial-like cells in CMSC morphologically resemble true synovial cells and fibroblasts. They can hardly be distinguished by those unfamiliar to the synovial tissue, including dermatologists. Positive immunoreactivity of lining cells against vimentin, but not for cytokeratin, CEA, and S-100,3 together with the absence of CD68, a synovium marker, expression supports the diagnosis.
The pathogenesis of CMSC has also been linked to relative tissue fragility secondary to Ehlers-Danlos syndrome3,10 and rheumatoid arthritis.11,13 Experimentally, normal synovium was formed by local mechanical disruption of the connective tissues in vivo.20 Synovium-like cells of MSC could be differentiated from mesenchymal cells during wound healing process. A history of trauma was not noted in our case, however a long-term systemic corticosteroid administration may increase tissue fragility and lead to CMSC formation.
Local recurrence of common synovial cyst has been noted. In addition, the synovial tissues may evolve ectopically after the disruption of connective tissue during surgery. Interestingly, local recurrence has been reported in oral and maxillofacial surgery, but not in dermatology. This could be explained by technical difficulty of surgical excision due to anatomical location. Still, close follow-up may be necessary in cases with underling diseases who are prone to develop CMSC similar to our case.
We would like to thank Ms. Yoshimi Yamazaki for technical assistance with immunohistochemical study.
The authors have no conflicts of interest to report.
