Lipomas are common soft tissue benign tumors, but are rare in the oral cavity, and especially on tongue. The incidence is very low [3,4,5,6,7,8,9,10,11]. Infrequently, lipomas can arise inside the muscle and are called ‘intramuscular(infiltrating) lipoma’. In 1853, muscular lipoma was first reported by Paget et al. described a lipoma infiltrating into the trapezius muscle. In 1946, the term of “infiltrating lipoma” was first defined in Regan’s article, referring to a lipoma infiltrating into muscle [37]. It is reported that this rare type of lipoma accounts for less than 1% of all lipomas [14]. The tongue lesion reported in our case was diagnosed as intramuscular lipoma. This case is unique because it is a very rare subtype of lipoma occurring in a very rare site.
In order to analyze the epidemiologic and clinical features of intramuscular lipoma developed on tongue, we have searched the literature published in English so far by Google Scholar and reviewed all case reports including our case (Table 1).
Since the first intramuscular lipoma on tongue was reported by Bennhoff et al. in 1978, the total of 24 cases have been reported in 23 literatures in English, including present case. Age distribution was shown from 37 to 75 years, and most of the patients over 50 years old (only 3 cases under 50 years). With regard to gender distribution, prevalence of the males was almost twice as high as that of females with 15 cases for male and 9 cases for female. As for the site of occurrence, the lateral side of the tongue was the majority with 16 cases including our case, followed by ventral surface with 4 cases, dorsal surface with 2 cases, and anterior tip with 2 case. The lesion size varied from 0.6 to 10 cm in maximum diameter with an average of 3.18 cm. Because the lesions are typically round or fusiform in shape, the size was analyzed based on the maximum diameter. In most cases, the patient had asymptomatic, painless swelling lesions, but in one case, painful swelling and difficulty in deglutition were present. In three cases, speech difficulties were found. Excision was performed as a treatment method in all cases. Except for six cases without information in the literature, there are no recurrences after surgery in all other cases during the average follow-up period of 17.29 months. Most of the features were not significantly different from simple lipoma of the oral cavity, but in gender distribution, intramuscular lipoma on the tongue was more prevalent in male, whereas simple lipoma was higher in female [8].
Clinically, intramuscular lipomas present as slow-growing, painless, rubbery, and swelling masses. Pain is an uncommon symptom. As the tumor grows in size, functional limitations can arise. Especially on tongue, it can cause discomfort on speech, deglutition, or mastication [12, 21, 25, 27, 29].
The exact etiology of intramuscular lipoma is still unclear. Trauma and chronic irritation have been suggested as probable causes, and also genetic factors and hormonal imbalance are suggested to be related [13, 38, 39].
Imaging such as the computed tomography (CT) scan or MRI plays an important role in making a diagnosis of intramuscular lipoma. In addition, preoperative imaging analysis is essential for defining the size, border, and location of the mass, and relationship with surrounding anatomical structures. Based on them, a surgical plan should be established. Imaging features on CT scan of intramuscular lipoma indicate a lesion situated within the muscle and thick and thin soft tissue density streaks inside a fat density lesion. MRI is also useful method for identifying lipomatous lesions. On MRI, the adipose tissue in the intramuscular lipomas appears as strikingly high intensity signal area on both T1- and T2-weighted images [12, 40, 41]. In our case, a preoperative diagnosis of lipoma was suggested on MRI features.
Intramuscular lipomas have typical histological features. They are histologically well-demarcated but unencapsulated, and have mature uni-vacuolated adipocytes irregularly infiltrating adjacent muscle fibers. No lipoblasts are identified. Also there is no increased mitosis, nuclear atypia, pleomorphysim, cellular hyperchromatism, and lipoblastic proliferation. They show the vasculature composed of thin-walled capillaries, which is occasionally not observed due to compression by surrounding adipocytes [12, 35, 41].
Meanwhile, what should be noted about intramuscular lipomas is that their clinical, histological and imaging characteristics are similar to well-differentiated liposarcomas, malignant tumors. This makes the differential diagnosis difficult [12, 42]. Therefore, careful clinical, histological, and imaging examinations are all necessary for accurate differential diagnosis of intramuscular lipomas, not just one examination alone. For the differential diagnosis in our case, clinical examination, MRI taking, and preoperative biopsy were all performed.
There are differences in imaging features between liposarcomas and intramuscular lipomas. On CT scans, liposarcomas, unlike intramuscular lipomas described above, are characterized by lesions spreading in the intra- and intermuscular layers and fat density lesions with a fair hazy amorphous areas and soft tissue density septa. And they are more oval shaped compared to intramuscular lipomas [40]. On MRI, liposarcomas tend to be larger than intramuscular lipomas; however, size alone is not a good predictive factor for malignancy. In contrast to intramuscular lipomas, liposarcomas are usually multilobular and have more and thicker septae with nodules [38]. The most distinctive histological point of liposarcomas is the presence of lipoblasts. And different from intramuscular lipoma, they exhibit features of increasing mitosis, mixoid differentiation, cellular pleomorphism, lipoblastic proliferation, and abundant vascularity [12, 24].
Due to their infiltrating tendency, the recurrence rate of intramuscular lipomas is known to be higher than that of other subtypes of lipoma. The recurrence rate after treatment ranges from 3 to 62.5% depending on the researchers [10, 13, 27, 40, 43]. However, in the literature we reviewed, there were no recurrences in all patients except for a few without information on recurrence (Table 1). All clinicians, including us, surgically treated patients with intramuscular lipomas on tongue and no local recurrence was noticed. Like other lipomatous tumors, the most main treatment of these lesions is surgical excision. Marginal excision of the well-circumscribed area and wide excision with free margin in the infiltrative areas are performed to prevent recurrence. But its condition infiltrating into the muscle fiber makes complete excision difficult [42, 44]. Conservative treatment is also practiced, but its role is very limited. Although there have been reports of animal studies that steroid injection into the lesion was effective, the efficacy in humans has not been proven [45].
When the lesion is large, surgical excision on the tongue lesion can result in the large defect, delayed healing, and patient discomfort. Primary suture was difficult and morphological deformation was predicted due to the large lesion. In this case, we opted for the enveloped flap design because the lesion was 4 cm large. In order to diminish the patient discomfort and reduce healing period, we used surgical excision with enveloped mucosal flap (Fig. 3b). Enveloped flap technique with checked margin free is advantageous in terms of surgical field of view, healing time and postoperative esthetics when extensively excising a large lesion. Extensive muscle resection and the intraoperative biopsy were performed to prevent recurrence. All adipose tissues of mucous membrane were removed and then sutured. The raw surface of tongue was minimized to reduce the healing period (Fig. 6).
