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3 DOI /s CASE REPORT Meralgia paresthetica due to a thigh hibernoma: a rare diagnosis case report Eurico Monteiro & João Torres & Manuel Gutierres & Sérgio Silva & Rui Pinto Received: 12 February 2013 / Accepted: 15 March 2013 # EFORT 2013 Introduction Meralgia paresthetica (MP) is a symptom complex that includes numbness, paresthesias, and neuropathic pain in the anterolateral cutaneous thigh and may be the result of an entrapment neuropathy or a neuroma of the lateral femoral cutaneous nerve (LFCN). The condition can be differentiated from other neurologic disorders by the typical exacerbating factors, the characteristic distribution of symptoms, and the use of electrodiagnostic studies like the electromyography (EMG) [1]. MP can develop spontaneously at any age, and the disease usually presents in the 30- to 40-year-old age group with an higher incidence than previously reported [2, 3]. Meralgia paresthetica can have many etiologies and can be subdivided into two main groups: spontaneous onset and iatrogenic. Spontaneous MP occurs without previous surgical intervention and can be subdivided into an idiopathic, metabolic, and/or mechanical type [4, 5]. Hibernoma is an unusual benign soft tissue tumor derived from a specialized form of brown fat [6]. Hibernomas have computed tomography and magnetic resonance imaging appearances similar to other fibrous and lipomatous tumors; therefore, histopathological analysis is always necessary for a correct diagnosis [7]. E. Monteiro (*) : J. Torres : M. Gutierres Serviço de Ortopedia e Traumatologia, Centro Hospitalar S. João EPE, Faculdade de Medicina da Universidade do Porto, Al. Hernâni Monteiro, 4200 Porto, Portugal elisboamonteiro@gmail.com S. Silva : R. Pinto Serviço de Ortopedia e Traumatologia, Centro Hospitalar S. João EPE, Al. Hernâni Monteiro, 4200 Porto, Portugal Although these tumors are always benign, they tend to grow to large sizes, sometimes causing compression of the neighboring structures, so that surgical excision is always recommended. Case report We report the case of a 57-year-old female, without any relevant medical history that was referred to the orthopedic outpatient clinic with MP of an insidious onset. She complained of numbness and persistent burning/stabbing pain on the anterolateral cutaneous region (the patient localized the complaints to the skin) of the left thigh. The patient did not improve with conservative treatment with nonsteroidal anti-inflammatory drugs (NSAIDs) or corticosteroids that she had been submitted to for 6 months. She performed an electromyographic study and a magnetic resonance imaging (MRI) examination which revealed conduction abnormalities of the LFCN and a large anterior mass on the anterolateral region of the left thigh (Figs. 1 and 2), respectively. Due to persistence of symptoms and the mass compression effect, she had the mass resected surgically by an anterior direct approach through the Heuter Interval in order to identify the LCFN and assess for its integrity, and that there was no concomitant injury to the LCFN like the existence of a neuroma. We performed a wide resection, and the mass was sent to the pathology department, and an histologic diagnosis of hibernoma was made. After surgery, the patient slowly recovered from her clinical picture of MP, with progressive recovery of sensitivity and resolution of pain and numbness. The EMG performed during the follow-up period was normal after 2-year follow-up, and no recurrence of the tumor was registered.

4 Fig. 1 MRI cut on the sagittal plane with a mass showing a similar signal, but not usually equal, to subcutaneous fat Discussion Hibernoma is an unusual benign soft tissue tumor derived from a specialized form of brown fat [6]. Brown adipocytes are multilocular fat cells that serve a vital role in nonshivering thermogenesis. Brown adipocytes contain a very large number of mitochondria and are innervated by the sympathetic nervous system. The literature reports hibernomas being commonly located in the thigh, shoulder, back, neck, chest, arm, and abdominal cavity/retroperitoneum [8]. Hibernomas usually present as slow growing, painless soft tissue masses. The only symptom they cause is from mass effect upon adjacent structures. In the literature, there have been conflicting reports regarding the female predominance of hibernomas. Hibernomas are commonly seen in the second through fifth decades with the classical teaching describing a female predominance for hibernomas [9]. Four histologic types of hibernomas have been described as follows: typical (82 %), myxoid (82 %), lipoma-like type (7 %), and spindle cell (2 %) [10]. The typical hibernoma Fig. 2 MRI cut on the transverse plane with a mass showing a similar signal, but not usually equal, to subcutaneous fat contains a varying mixture of brown and white fat cells. The myxoid type usually contains hibernoma cells floating in a loose acellular myxoid stroma. The lipoma-like type of hibernoma consists of few scattered hibernoma cells among a predominance of white fat cells. The spindle cell variant combines the features of a hibernoma with a spindle cell lipoma. Our case describes the typical hibernoma with mixed white and brown adipocytes. There have been no reported cases of malignant transformation or spread in the literature and no cases of recurrence when wide excision was performed, like it was done in the case we report. Hibernomas have characteristic, but not specific, imaging features. These lesions are often intramuscular and tend to insinuate between the fascial planes and wrap around adjacent structures without invasion. With intravenous contrast, hibernomas typically demonstrate diffuse enhancement. On T1- and T2-weighted images, hibernomas demonstrate increased signal intensity that is similar, but not usually equal, to subcutaneous fat. On fat-suppressed pulse sequences, hibernomas demonstrate partial signal loss consistent with fatty elements [11, 12]. The great majority of cases of MP have a favorable course and outcome, and 85 % of all patients will recover with conservative treatment [13]. There is only limited documentation regarding the interventional treatment of MP. When conservative treatment fails, an adequate study should be performed in order to exclude mechanical or metabolic causes of MP [4, 5]. In our case, there was a mechanical cause for MP surgery, and a wide excision was decided. When performing surgery, the entire nerve should be explored, and an adequate incision must be made to allow for the anatomic variations that have been reported and make sure that no concomitant pathology like neuromas are present. Wide incision recommended as the definitive diagnosis is ahistologicone,inordertoruleoutotherlipomatous neoformations [8]. After surgery, most patients have an excellent outcome with fully recovery from the MP, and the nerve conduction studies start to normalize recovery which usually begins somewhere between 6 months and 1 year. This time variable is based on the fact that axonal regeneration is length-dependent processes. The rate of axonal regeneration is classically described as occurring at a rate of 1 4 mm/day. In conclusion, conservative treatment is always the first choice of therapy. NSAIDs and infiltration with corticosteroids and local anesthetics can be considered. In the refractory cases, a thorough investigation should be performed to exclude all other causes of MP. Surgery should be reserved for cases of mass effect compression, and a wide excision should be performed as the definitive diagnosis which is made with a histologic study.

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