| Abstract|| |
Intramuscular lipoma is an uncommon variant of lipoma that occurs inside muscle tissue. Pathogenesis is thought to be related to neoplastic activity of mesenchymal stem cells, but there still is not an established theory. This entity can be found in almost all areas of the body, mostly within or in connection with skeletal muscle tissue. We have encountered an atypical intramuscular lipoma located in the epidermal layer of the right thigh of an otherwise healthy 35-year-old female. The superficial, protruding skin mass resembled a sebaceous nevus or skin malignancy but was histologically composed of well-defined adipocytes between a mixture of muscle fibers, corresponding with the diagnosis of intramuscular lipoma. Intramuscular lipoma of the skin has never been reported before; thus, the authors suggest the classification “cutaneous” intramuscular lipoma to describe lesions located in the dermal or epidermal layer.
Keywords: Cutaneous lipoma, intramuscular lipoma, pedunculated lipoma
|How to cite this article:|
Seo BF, Choi JS, Shim HS. Cutaneous intramuscular lipoma: A new subtype of intramuscular lipoma. Indian J Pathol Microbiol 2018;61:425-7
| Introduction|| |
Adipocytic tumors represent the largest single group of mesenchymal tumors, with a high prevalence of lipomas. Several subtypes of lipomas exist depending on location; intermuscular, intramuscular, synovial, parosteal, intraosseous, lumbosacral, and thymolipoma. Intramuscular lipomas arise inside muscle tissue and account for <1% of all lipomas. While they are rarely found in hand and foot, they have been reported to occur in almost any anatomical area of the body. Most are circumscribed, round, fusiform tumors surrounded by muscle fibers, although some have large extramuscular components. They are further classified as infiltrative, well-defined/noninfiltrative, and mixed based on the histological analysis.
Although the presentation of intramuscular lipomas may be diverse, the vast majority are found within or in connection with anatomically located muscle. The authors have encountered and treated a case of intramuscular lipoma presented as a protruding skin lesion, clinically resembling a sebaceous nevus or verruca.
| Case Report|| |
A 35-year-old female patient presented with a soft, nontender protruding mass on the posterior aspect of her right thigh. She first noticed this lesion 6 months prior, after which it had shown rapid growth. She recalled no history of trauma in this area. Physical examination revealed a brownish, pedunculated mass with a thick stalk located inferior to the right gluteal fold, with a dimension of 3 cm × 3 cm [Figure 1]a. Computed topography was performed to evaluate the depth and characteristics of the lesion. Images showed an oval-shaped mass with relatively homogeneous low attenuation and no contrast uptake [Figure 1]b. Excisional biopsy was performed under local anesthesia. The mass was approached through a spindle-shaped skin incision which encompassed the entire lesion and the root and was carefully separated from the surrounding subcutaneous tissue [Figure 1]c. Frozen sections revealed a benign lesion with no margin invasion.
|Figure 1: (a) Preoperative view of the 3 cm × 3 cm sized pedunculated mass in the right posterior thigh. (b) Axial section of the right thigh seen in preoperative computed tomography images. The mass is shown to be a well demarcated oval-shaped lesion with fat density, indicated with a white arrow. (c) The pale pink, oval-shaped specimen measuring 4 cm × 2.3 cm|
Click here to view
Histologically, a mixture of univacuolated mature adipocytes scattered irregularly between striated muscle fibers was found, corresponding to the diagnosis of infiltrating intramuscular lipoma [Figure 2]a and [Figure 2]b. There were no findings of mitosis or cellular atypia.
|Figure 2: Histological appearance of the infiltrative intramuscular lipoma. (a) Mature univacuolated adipocytes of fairly uniform size irregularly infiltrate between muscle fibers (H and E, ×40). (b) Well-differentiated mature adipocytes are seen scattered between striated muscle fibers (H and E, ×100)|
Click here to view
The excision wound healed well without hematoma or infection. The follow-up was done for 12 months after the surgery, and there was no evidence of recurrence.
| Discussion|| |
Lipomas are the most common mesenchymal tumor, reported in around 2.1/1000 people. They may be found on most areas of the body with several subtypes depending on location; intermuscular, intramuscular, synovial, parosteal, intraosseous, lumbosacral, and thymolipoma, or depending on tissue elements; angiolipoma, myolipoma, chondroid lipoma, spindle cell/pleomorphic lipoma, fibrolipoma, myxolipoma, and osteolipoma.
Intramuscular lipomas are mature benign adipose tissue tumors involving skeletal muscle and may be circumscribed or infiltrative. They account for around 1.8% of all primary tumors of adipose tissue, and may be found in any age group, the majority reported between the ages of 40 and 70 years. Gender predilection is not clearly established. Topographical distribution differs among studies, but most are found in large muscles, therefore usually in the trunk, head and neck, and extremities, usually sparing the hand and foot.
Early descriptions of this intramuscular subtype of lipoma recognized the presence of lipomatous tissue inside or between skeletal muscles. Paget reported a lipoma infiltrating into the trapezius muscle in 1853, after which Regan et al. introduced the term infiltrating lipoma. Greenberg et al. found that such “infiltrating lipoma” may be either intermuscular or intramuscular based on their location, and subsequent studies revealed that either type may have infiltrative, noninfiltrative, or mixed patterns of growth.
Clinically, the most common presentation of intramuscular lipoma is a deep-seated, nontender swelling of relatively soft consistency. Symptoms may occur with growth. Pain is thought to be caused by compression of adjacent peripheral nerves. It may cause restriction of motion, and has been reported to cause impingement syndrome when found in the supraspinatus. The cutaneous, pedunculated location of the lesion in this report is highly atypical. The mass was not round or fusiform, and resembled verruca or sebaceous nevi, requiring differentiation from benign or malignant entities of ectoderm origin.
The intramuscular location and infiltrative pattern of intramuscular lipoma obligate the surgeon to differentiate this benign lesion with a soft-tissue sarcoma, especially a liposarcoma. Radiologic findings contribute in differentiation; however, histological analysis is the most accurate method of diagnosis. The most pathognomonic feature is interdigitation of skeletal muscle and fat in a characteristic striated appearance. These findings are in contrast to findings of liposarcoma, which are usually multilobular and have heterogeneous portions of tissue with signal intensity different from muscle.
Although it is most likely that intramuscular lipomas, as with other lipoma subtypes, probably originate from neoplastic growth of multipotent mesenchymal cells, the mechanism or prognostic value of the degree of infiltration is not fully understood. While trauma, chronic irritation, endocrine, metabolic, or genetic factors have been proposed as the initiating cause of uncontrolled growth, the pathogenesis of infiltration is yet to be elucidated. Mori et al. have reported that type-selective muscle fiber atrophy or degenerative changes found on immunohistochemical analysis have been found in 70% of intramuscular lipomas, such patterns of muscle fiber atrophy were even detected in peripheral muscle tissue where tumor involvement was not prominent, suggesting that myogenic or neurogenic disorders may be related to the cause of this entity. The cutaneous, protruding intramuscular lipoma is difficult to explain with this hypothesis. Alternative postulations may be related to ectopic muscular tissue. Ectopic skeletal muscle based in this area may have been exposed to constant pressure in the gluteal area that predisposed this tissue to neoplastic growth. Growth in extensions of the gluteus maximus, biceps femoris, or semitendinosus musculature is also possibilities. Reports of other cases of intramuscular lipoma found in superficial tissue may aid our comprehension of the pathogenesis of this entity.
Intramuscular lipomas may differ in presentation but are usually unvaryingly found in connection to anatomical skeletal muscle. While the vast majority of intramuscular lipomas are found in deep seat106029ed in large muscle groups, there have been reports of this entity situated in atypical locations: Kalmar and Doobay reported one found in the flexor hallucis brevis muscle, Vincent et al. documented a midline nasal intramuscular lipoma in a 3-month-old infant, Han et al. found an lipoma in the splenius muscle compressing the lesser occipital nerve and causing occipital neuralgia.,, However, all of these lesions were found in locations in which muscle is normally located. The unexpected presentation seen in this case expands the clinical landscape of this entity, suggesting an additional subtype of intramuscular lipoma that can be found in the layer of ectodermal tissue which has never been reported before. Therefore, the authors suggest the use of the term “cutaneous intramuscular lipoma” when describing this type of lesion. Classification of a “cutaneous intramuscular lipoma” will enable the surgeon and pathologist to consider this entity as a definite diagnosis when encountering such a superficial soft-tissue lesion.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
| References|| |
Mai KT, Yazdi HM, Collins JP. Vascular myxolipoma (“angiomyxolipoma”) of the spermatic cord. Am J Surg Pathol 1996;20:1145-8.
Fletcher CD, Martin-Bates E. Intramuscular and intermuscular lipoma: Neglected diagnoses. Histopathology 1988;12:275-87.
Silistreli OK, Durmuş EU, Ulusal BG, Oztan Y, Görgü M. What should be the treatment modality in giant cutaneous lipomas? Review of the literature and report of 4 cases. Br J Plast Surg 2005;58:394-8.
Regan JM, Bickel WH, Broders AC. Infiltrating benign lipomas of the extremities. West J Surg Obstet Gynecol 1946;54:87-93.
Greenberg SD, Isensee C, Gonzalez-Angulo A, Wallace SA. Infiltrating lipomas of the thigh. Am J Clin Pathol 1963;39:66-72.
Lee YH, Jung JM, Baek GH, Chung MS. Intramuscular lipoma in thenar or hypothenar muscles. Hand Surg 2004;9:49-54.
Mori K, Chano T, Matsumoto K, Ishizawa M, Matsusue Y, Okabe H, et al.
Type-selective muscular degeneration promotes infiltrative growth of intramuscular lipoma. BMC Musculoskelet Disord 2004;5:20.
Kalmar G, Doobay N. Intramuscular lipoma of the flexor hallucis brevis muscle. A case report. J Am Podiatr Med Assoc 2017;107:80-4.
Vincent J, Baker P, Grischkan J, Fernandez Faith E. Subcutaneous midline nasal mass in an infant due to an intramuscular lipoma. Pediatr Dermatol 2017;34:e135-6.
Han HH, Kim HS, Rhie JW, Moon SH. Intramuscular lipoma-induced occipital neuralgia on the lesser occipital nerve. J Craniofac Surg 2016;27:e350-2.
Department of Plastic and Reconstructive Surgery, St. Vincent's Hospital, The Catholic University of Korea, 91-6 Joongbu-Daero, Paldal-Gu, Suwon City
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2]