Perineural Spread of High-Grade Mucoepidermoid Malignancy Leading to Bells and Lateral Rectus Palsy

INTRODUCTION

Mucoepidermoid carcinoma (MEC) is the second most common salivary gland malignancy, following adenoid cystic carcinoma. It accounts for 30% of all salivary gland malignancies. In the literature, it is established that there is a female predilection.^[1] Most perineural spreads occur through retrograde spread via the foramen rotundum, which can lead to hearing issues. This case report describes how a salivary gland malignancy can give rise to Lagophthalmos due to rapid perineural spread into the base of the skull and the infraorbital canal through the foramen ovale, without apparent clinical swelling. This case report will help clinicians suspect perineural spread of a salivary gland malignancy, even in the absence of noticeable clinical features.

CASE REPORT

An elderly woman in her late 60s presented to the Maxillofacial Outpatient Department (OPD) with difficulty in the mouth for the past 6 months. The patient's medical history revealed she was anaemic and on oral iron supplements. A detailed history of the present complaint revealed the following aspects: the patient experienced the onset of a burning sensation in the mouth 8 months ago. Over the past six months, she began experiencing difficulty opening her mouth, which was insidious in onset and gradually progressed, followed by reduced sensation on the left side of her face and a diminution of vision in her left eye in subsequent months. She developed difficulty in lateral gaze for the past four months. Additionally, she noticed mild swelling on the left side of her face for the past one month. Upon examination, the patient exhibited mild swelling, approximately 3×5 cm in size, in the left masseter region, which appeared firm, and a mouth opening of 17 mm [Figures 1 and 2]. No palpable lymph nodes were detected. Eye movements were restricted on the left side during medial and lateral gaze. Sensation below the left eye was absent, and classical features of lower motor neuron palsy were evident on the left half of the face.

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Figure 1:

Showing restricted mouth opening.

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Figure 2:

Showing mild swelling measuring approximately 3×5 cm in the left masseter region.

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Intra-oral examination revealed pale, blanched mucosa near the left retro-trigone area. Ophthalmic examination revealed diplopia in left gaze and lagophthalmos, along with decreased vision at distances of up to 3 feet. Corneal sensations were reduced, and the conjunctiva appeared congested. CT scans with contrast revealed an irregularly heterogeneously enhancing tumour mass measuring 3.9 × 5 × 4.8 cm.

The tumour was observed to medially involve the ipsilateral pterygoid muscle and extend to the submucosa of the oropharynx at the tonsillar pillar. Laterally, it infiltrated the masseter muscle and extended to the submucosa. Anteriorly, the lesion extended through the buccal space lateral to the left hemimandible up to the symphysis menti, and the body of the mandible was eroded, while posteriorly, it extended into the substance of the parotid gland and foramen ovale. The cavernous sinus and adjacent dura also showed thickening, and the lesion involved the infraorbital canal, and mild proptosis was present, suggesting intra-orbital extension [Figures 3a and 3b].

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Figure 3:

(a) Figure showing the swelling over the left side of face, measuring approximately 3×5 cm in the left masseter region, (b) Contrast CT scan showing infiltration of the tumour to the cranial base through the foramen ovale (red circle). CT: Computed tomography.

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The diagnosis of MEC was ascertained by fine needle aspiration cytology (FNAC) of the lesion. The specimen obtained during the superficial parotidectomy procedure was subjected to IHC markers. IHC was strongly positive for p63 and mucicarmine stains in the lumen and cytoplasm, which confirmed the high-grade MEC diagnosis. Since tumour node metastasis (TNM) staging of the patient was T4bNxMx, she underwent a superficial parotidectomy to debulk the lesion [Figure 4], which was followed by the standard radiation therapy with a total dose of 60 g administered over 30 appointments.

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Figure 4:

Illustrating superficial parotidectomy.

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Outcome & Follow-Up: The administered treatment showed some success in slowing the rapid progression of the tumor; however, it had a limited impact on reversing the loss of sensation, Bell's palsy, and lateral rectus palsy. There was an improvement in mouth opening, reaching 25 mm, and the patient's condition has remained stable for the past 3 months.

DISCUSSION

According to a comprehensive literature review, salivary gland tumours comprise approximately 3–4% of all reported tumours, with the most common structure involved being the parotid gland in around 80% of cases. Minor salivary gland tumours constitute about 10–15% of all salivary gland tumours and are predominantly malignant.^[2–4] The prevalence is between 1.5 - 4 per 100,000 individuals.^[5] MEC is the most prevalent type among the various malignancies of the salivary glands, accounting for 34% of all salivary gland tumors. This type of carcinoma occurs more in women, with the highest incidence during the mid-40s and 50s.^[6] MEC originates from excretory duct reserve cells and is characterised by three types of cells: mucous cells, epidermoid cells, and poorly differentiated intermediate cells. For classification, the armed forces institute of pathology (AFIP) system, along with the modified Haley system and the Brandwein system, are widely used to classify MEC into low, middle, and high grades based on cytomorphologic and architectural criteria, as well as perineural and angiolymphatic invasion.^[7–9] The grading of MEC plays a pivotal role in predicting patient prognosis. Research by Chen et al. demonstrated that high-grade MEC exhibited a worse survival rate, more distant metastases, and greater lymph node involvement than the other two grades.^[10] High-grade MEC is known to have a varied histopathological expression, a highly aggressive nature, and a propensity for infiltration and distant metastasis.^[11] Therefore, secondary complications may arise, as observed in this particular case. The patient sought treatment due to a progressive decrease in vision followed by restricted mouth opening. Surgical excision is the primary treatment for salivary gland tumours, and surgery alone is typically required in cases of low-grade carcinoma if complete excision is feasible. Post-operatively, radiation therapy (RT) is also given in case of high-grade neoplasms and with ill-defined margins. RT alone is frequently employed when cancers are only partially resectable due to inadequate margins, extra-glandular extension, perineural invasion, and lymph node metastases. Patients solely treated with RT have shown a poor prognosis, as noted in the experience of Mendenhall et al.^[5] Perineural infiltration with factors such as tumour stage (T) and cases where margins are involved both have a worse prognosis and can manifest clinically in around 30% of patients.^[12] Perineural spread may lead to neurological impairment and related complications, such as facial or orbital palsy,^[13] before the tumour becomes clinically evident, as demonstrated in this case, where the patient experienced sensory loss and restricted eye movements eight and six months, respectively, before the appearance of a clinically evident swelling. Therefore, these clinical findings corroborate the loss of infraorbital (V CN) sensation, lateral rectus palsy (VI CN), and Bell's palsy (VII CN). This case report highlights the importance of rare clinical manifestations of a commonly known salivary gland malignancy, about which clinicians should be aware.

CONCLUSION

MEC primarily affects the parotid gland and is more common in females, with a peak incidence during the fifth decade of life, underscoring the need for screening in individuals at risk. Lagophthalmos (lateral rectus palsy) should prompt the clinician to suspect perineural invasion of the tumour, even in the absence of noticeable clinical swelling, and the clinician should ensure a proper workup in such cases. High-grade MEC of the oral region is highly invasive and can exhibit rapid and extensive spread to the infratemporal fossa and cranial base, even in the absence of apparent clinical manifestations.