Carotid sheath lipoma: first case report in the English literature

Lipomas are the most commonly encountered benign mesenchymal tumour, arising in any location where fat is normally present. Lipomas in the head and neck are rare in all age groups. Cases of vascular sheath lipomas in the femoral region have only been reported in adults. In children, vascular sheath lipomas have not been described to date. We report the first case of a carotid sheath lipoma in a seven-year-old boy. He had a non-tender soft mass with ill defined borders occupying the left upper part of the neck. Magnetic resonance imaging showed a mass at the bifurcation of the left common carotid artery without involving the same. The mass was hyperintense on T1 and T2 weighted sequences, suggestive of a lipoma. The lipoma was enucleated after incising the carotid sheath, safeguarding its contents. Histopathology confirmed it as a lipoma.

Lipomas are the most commonly encountered benign mesenchymal tumours, arising in any location where fat is normally present. 1,2 Occurrence in the head and neck is relatively rare. 3 It is most commonly found in the subcutaneous portion of the posterior aspect of the neck. Surgical excision here is challenging because of the proximity of vital structures in the neck. Knowledge of anatomy and meticulous surgical dissection are therefore essential. Until now, the only lipoma reported from a vascular sheath has been from the femoral sheath in an adult. 4 We describe the first case of a carotid sheath lipoma in a child.

Case history
A seven-year-old boy presented in the department having experienced a gradually increasing swelling in the left upper part of the neck for the previous two years. Clinical examination revealed a painless, non-tender soft mass with ill defined borders occupying the left upper part of the anterior neck. The surface was smooth and the overlying skin was normal. Ultrasonography was suggestive of venous malformation. Magnetic resonance imaging (MRI) showed a well circumscribed mass lesion of 3.8cm x 3.5cm x 2cm in the left submandibular region lying beneath the sternocleidomastoid muscle causing mild anterior displacement of the submandibular gland (Fig 1). The mass was hyperintense on T1 and T2 weighted sequences, suggestive of a lipoma. The mass was at the bifurcation of the left common carotid artery without involving the same. Superiorly, it extended up to the parapharyngeal space and inferiorly, up to the level of the third cervical vertebra.
The child was operated on through a left upper cervical transverse incision. The lipoma was dissected carefully. It was seen to arise from the carotid sheath near the bifurcation of the common carotid artery. The lipoma was enucleated after incising the carotid sheath and safeguarding its contents (Fig 2).
The histological findings grossly revealed a well encapsulated yellow-brown mass with a bosselated surface ( Fig  3). On microscopy, clusters of mature adipocytes separated by loose fibrovascular stroma were suggestive of a lipoma (Fig 4). The patient had a very good recovery with satisfying neck contour. Recurrence was not observed six months after surgery.

discussion
Lipomas are benign mesenchymal tumours that are histologically similar to mature adipose tissue but the presence of a fibrous capsule helps to differentiate them from simple fat aggregations. 5 Only 25% of lipomas arise from the head and neck. Intracranial lipomas are very rare congenital malformative lesions, comprising less than 0.1% of intracranial tumours. Lipomas of the corpus callosum are found in 1 in 2,500 to 1 in 25,000 autopsies. 6 Oral cavity lipomas comprise 1-5% of lipomas. Lipomas of the anterior neck are extremely rare; they can also originate in more atypical sites as long as adipose tissue is present. There is one report of an PArELKAr OAK KAPADnIS SAngHAvI JOSHI MunDADA SHETTy CaROtid sheath lipOma: fiRst Case RepORt in the english liteRatuRe intravascular femoral vein lipoma 7 and also a single case of a lipoma arising from the femoral vascular sheath, 4 both in an adult. However, a lipoma arising from the carotid sheath has not been described in the medical literature.
In 2002 Zhang and Lee studied fetal carotid sheaths. 8 They observed that the carotid sheath was attached to the subcutaneous fatty tissue without any clear demarcation by a fascia. The developing fatty tissue provides a thick sheet extending dorsolaterally from the adventitia of the common carotid artery to the subcutaneous area. Thus, the adventitia of common carotid artery contains fatty tissue. This fatty tissue sheet was sandwiched by the pretracheal lamina containing the omohyoid muscle and the pretracheal lamina covering the scalene. Some subcutaneous fatty tissue is therefore likely to be adjacent to the carotid sheath. 9 It is possible that the overgrowth of this tissue resulted in the lipoma in our patient.
Ultrasonography is useful as an initial imaging study in suspected neck lipomas. 10 Computed tomography (CT) or MRI are helpful in further assessment. 11 Compared with CT and MRI, ultrasonography is quick, easy and less costly. However, the soft tissue characterisation is less specific with ultrasonography. On CT, lipomas have typical characteristics of homogeneous masses with few septations. MRI can also accurately diagnose lipomas. Moreover, the margin of a lipoma is defined clearly by MRI as a black rim, enabling lipomas to be distinguished from surrounding adipose tissue, a distinction that cannot be made in CT. In the index case, although ultrasonography was suggestive of venous malformation, high resolution MRI was diagnostic of a lipoma.
Most lipomas are well encapsulated and easily dissected from surrounding tissues. They recur locally in 5% of cases. 12 The concept that lipomas can undergo malignant trans- Photomicroscopy showing clusters of mature adipocytes separated by loose fibrovascular stroma, which show focal aggregates of lymphocytes and lymphoid follicles formation to liposarcomas is no longer accepted. An original tumour is the most common explanation for the apparent 'transformation' of a lipoma to an atypical lipomatous neoplasm. 13 Histologically, these tumours are completely benign but they can have local invasive and infiltrative behaviour. 14 Regular follow-up appointments are recommended.

Conclusions
Carotid sheath lipomas are extremely rare in any age group and this is the first case that has been reported in the English literature.