Intramuscular lipoma of the temporalis muscle extending to the infratemporal fossa: surgical pitfalls and short literature review

Abstract

Lipomas are benign tumours of mesenchymal origin, representing one of the most common tumours of the body. They are often observed between the fourth and the sixth decade of life and in 13% of the cases they occur in the head and neck region. In case of symptoms, surgical removal is the treatment of choice; when the formation involves the temporal region, the surgical approach is often challenging due to the presence of the neurovascular structures, such as the temporalis branch of the facial nerve and their potential extension to the nearby structures under the zygomatic arch to either the infratemporal fossa or the buccal region. Recurrence can occur frequently only if there is incomplete removal of lipoma. In this paper, the authors discuss surgical pitfalls of a very rare case of a large-sized symptomatic lipoma extended to the infratemporal and pterygomaxillary fossa, surgically removed via trans-zygomatic hemicoronal approach.

Background

Lipomas are the most common slow growing benign tumours of mesenchymal origin occurring in any part of the body where fat is represented. They are often observed between 40 and 60 years being more frequent in male obese individuals. Although most commonly seen on the trunk and lower limbs, lipomas are rare in the head and neck, accounting for 1%–4.4% of all benign tumours in the head and neck,¹ occurring especially in the posterior neck region and very rarely in the temporal region.^{2 3 4} On clinical examination, lipomas are non-tender, soft, mobile masses usually asymptomatic. Most subcutaneous lipomas may be suspected with a high degree of accuracy on clinical examination only. Ultrasound or MRI assessment is mandatory in case of infiltrating masses, diagnostic confirmation or surgical planning. Ultrasonography plays a major role in the study of lipomatous tumours and ultrasound features of the masses may vary from roundish well-defined margins to widespread ill-defined masses. The overall echogenicity is mostly hyperechoic compared with muscle. MRI is very useful for differential diagnosis within lipomas and other soft tissue tumours. It is also an excellent imaging modality to distinguish among lipomatous masses. The fatty tissue demonstrates high signal intensity on both T1-weighted and T2-weighted images. Fat-suppressed sequences demonstrate signal suppression similar to normal fat. Tumour margins can be well defined whereas capsule may not be distinguished from the surrounding muscle even if present. CT scan, finally, might be useful if ossification is present and if myositis ossificans or other calcified tumours are suspected. Differential diagnoses include hematoma, muscle herniation, ganglion cyst, heterotopic ossification, angiolipoma, hemangioma, fibrous myositis, primary muscular disease with fatty infiltration, liposarcoma, fibrosarcoma and other soft tissue masses.^5–7

Case presentation

A 79-year-old woman presented with a lump on the left temporal region (figure 1) and complaining about persistent headache and limitation of mouth opening without any sign or symptom of temporomandibular joint dysfunction. At the clinical examination, a palpable soft mobile swelling of the left temporal area was visible. Neither signs nor symptoms of facial nerve involvement were noted.

Figure 1.

A lump on patient’s left temporal region was present at the first visit.

Investigations

To better define features and extension of the mass the patient underwent MRI, which revealed the presence of a single and homogeneous lipomatous tumour sized 6 × 4 cm in the left temporal fossa extending between the outer side of the zygomatic arch and the pterygoid muscle (figure 2).

Figure 2.

The MRI showed a high-intensity 6 × 4 cm roundish structure, located in the infratemporal fossa.

Treatment

Thus surgical removal of the tumour via trans-zygomatic hemicoronal approach was planned. After the incision, an accurate dissection towards the zygomatic arch was performed; once skeletonised, an osteotomy of the zygomatic arch was performed. The dissection proceeded below the temporal muscle, which was detached from the superior temporal line and flipped inferiorly (figure 3). After this procedure it was possible to reach through gentle dissection the infratemporal fossa, in which the presence of a mass of soft consistency was evident (figure 4). The tumour was then completely excised from the surrounding structures; it measured approximately 6 cm in diameter and was sent for histological examination.

Figure 3.

The temporal muscle was detached from the superior temporal line and flipped inferiorly to gain access to the infratemporal fossa.

Figure 4.

The lipomatous formation was visible underneath the temporal muscle.

The zygomatic arch was repositioned, fixed with n.2 1.5 mm straight plates and screws.

The surgical procedure lasted 2.5 hours and was free from complications; the patient was dismissed after 2 days.

In the postoperative course, the patient appreciated improvement in the opening of the mouth.

Histopathological response confirmed the clinico-radiological diagnosis of intramuscular lipoma.

Outcome and follow-up

At the 6 months follow-up, the patient fully recovered aesthetic and functionality.

Discussion

Lipomas are common tumours mostly occurring in areas of the body where fat is present more.⁶ These tumours are usually slow growing and asymptomatic unless they cause mass effect on the adjacent neurovascular structures; benign lipomas are classified as classic lipoma, angiolipoma, chondroid lipoma, myxolipoma and spindle cell/pleomorphic lipoma.⁸ Rarely, a lipoma may infiltrate the adjacent muscle and is called an infiltrating lipoma, which may be of two types; the common intermuscular variety and the rarer intramuscular form. Infiltrating lipomas have a high rate of recurrence.^{6 9} Lipomas are more common in males in the age group of 40–60 years. Rarely, lipomas may be associated with inherited disorders such as Gardner’s syndrome, Madelung’s disease and familial multiple lipomatosis.¹

Conservative treatment has a limited role in the treatment of symptomatic lipomas of the infratemporal fossa, being surgical excision the treatment of choice. Recurrence can occur frequently only if there is incomplete removal of lipoma and malignant transformation of intramuscular lipomas even if not definitively proven, cannot be excluded. That is why minimal but sufficient exposure of the tumour is mandatory along with minimising the risk to vessels and nerves damage.¹⁰

Removal of lipomas extending to the infratemporal fossa is challenging due to the anatomical complexity of the region. The complex lateral skull base and its anatomical structures is affected in many ways by masses and for its removal approaches may vary. To the purpose, a basic anatomical knowledge of the infratemporal fossa is mandatory to choose the suitable approach.^{10 11}

The infratemporal space is located within the zygoma, the mandibular ramus, the lateral pterygoid plate, the tympanic plate with mastoid and styloid processes and the posterior maxillary wall. Anteriorly, infratemporal fossa joins the maxillary tuberosity, pterygomaxillary fossa and the inferior orbital fissure, whereas posteriorly extends to the infratemporal surface of the sphenoidal greater wing and the inferior border of the medial pterygoid muscle. Within the infratemporal space there are the muscles of mastication (including the temporalis, masseter, lateral and medial pterygoid muscles), vessels (including maxillary artery with veins and the pterygoid plexus of veins), nerves (including the mandibular, lingual and otic ganglia) and the temporomandibular joint.¹²

In this case a trans-zygomatic approach consisting by vertical sectioning of zygomatic arch was used thus allowing to easily reach and totally remove the lipomatous mass. The appropriate technique was selected according to age, gender and biological characteristics of pathology with the purpose of complete mass removal of pathology and carefully considering postoperative outcome. The procedure/approach that has been adopted in this case was advantageous allowing an accurate evaluation of the lipoma site and a much secure approach to maxillary vessels and pterygoid venous plexus. The patient was then discharged within 2 days, after recovering mandibular functions.

Although frequently occurring in any part of the body, lipoma of the infratemporal fossa is very rare. It is a benign lesion whose removal is mandatory in case of symptoms complaining such as limited mouth opening or sensitive alteration of the facial district. Despite the localisation, a well-performed surgical approach of the infratemporal fossa results as safe and effective. To the purpose, the hemicoronal surgical approach gives advantages in terms of surgical field exposition, safe mass removal and preservation of maxillary vessels and pterygoid venous plexus. In conclusion, when facing a lipomatous mass of the infratemporal fossa, we suggest a hemicoronal approach with zygomatic arch osteotomy to safely remove tumours of the temporal fossa without damaging vascular and nervous structures.

Learning points.

• Lipoma represents a rare but important entity in the infratemporal fossa.

• Due to the complexity of the region, anatomical knowledge and radiological assessment are paramount.

• Surgery must be programmed step-by-step and performed with great attention to spare neurovascular structures.

Ethics statements

Patient consent for publication

Consent obtained directly from patient(s).