Primary Mucoepidermoid Carcinoma of the Masticator Space Presenting with Myasthenia Gravis

Abstract

Introduction

Myasthenia gravis (MG) is a neuromuscular autoimmune disorder, commonly associated with thymomas in the context of paraneoplastic syndromes. There are limited reports describing MG in the setting of non-thymic malignancies. Mucoepidermoid carcinoma (MEC), a malignancy of the salivary glands, has been underrecognized as a trigger for paraneoplastic MG. We present a case of a woman with new-onset MG and concurrent MEC of the left masticator space, representing a rare instance of paraneoplastic MG.

Case Presentation

A 24-year-old woman presented with a 9-month history of left-sided hemifacial twitching and facial and neck weakness, which progressed to drooling and dysphagia. Examination was notable for bilateral ptosis and fatigable weakness of bulbar and proximal limb muscles. During hospitalization, she developed worsening respiratory insufficiency, necessitating intubation for airway protection. Given concern for myasthenic crisis, empiric plasma exchange was initiated, with significant clinical improvement. Computerized tomography (CT) imaging of the neck revealed a mass within the left masticator space, and core needle biopsy established the diagnosis of MEC. Chest CT demonstrated no evidence of thymoma. Serologic evaluation revealed positive acetylcholine receptor binding and blocking antibodies, confirming a diagnosis of MG.

Conclusion

The temporal relationship between MG symptoms and tumor discovery supports a paraneoplastic etiology in this patient. The occurrence of MG associated with non-thymic malignancies – particularly head and neck cancers – has only been reported twice. In MEC, the tumor may express antigens similar to those found in the neuromuscular junction in a process known as molecular mimicry. The immune system, in its attempt to target the tumor, inadvertently generates autoantibodies against these neuromuscular junction proteins, leading to the symptoms of MG. Whether resection of the tumor will lead to resolution of MG remains uncertain. Although no direct pathogenic link has been established between MEC and MG, this case suggests a need for increased clinical awareness and further investigation.

Introduction

Myasthenia gravis (MG) is an autoimmune disorder affecting the neuromuscular junction, commonly associated with thymomas in the context of paraneoplastic syndromes [1]. While the association between MG and thymic neoplasms is well established, there are only limited reports describing MG in the setting of non-thymic malignancies, particularly head and neck cancers [2, 3]. Mucoepidermoid carcinoma (MEC), a rare malignancy of the salivary glands, has been previously underrecognized as a trigger for paraneoplastic MG. We present a case of a young woman with new-onset MG and a concurrent diagnosis of MEC of the left masticator space, representing a rare instance of paraneoplastic MG.

Case Presentation

A 24-year-old woman with no known medical history presented with a 9-month history of progressive left-sided hemifacial twitching, blepharospasm, and facial and neck weakness, which eventually progressed to drooling and dysphagia. Neurological examination was notable for bilateral ptosis, cranial nerve III and IV palsies, facial diplegia, and fatigable weakness of bulbar and proximal limb muscles. Deep tendon reflexes were preserved. During hospitalization, she developed worsening secretions and respiratory insufficiency, ultimately requiring intubation for airway protection.

Computerized tomography imaging of the neck revealed a 3.3 × 2.7 × 3.2 cm heterogeneously enhancing mass within the left masticator space, exerting mass effect on the adjacent pterygoid muscle (Fig. 1). Core needle biopsy confirmed the diagnosis of MEC (Fig. 2). Given concern for myasthenic crisis, empiric plasma exchange was initiated, with significant clinical improvement. Chest computerized tomography demonstrated no evidence of thymoma. Serologic evaluation revealed positive acetylcholine receptor (AChR) binding and blocking antibodies, confirming a diagnosis of MG. The overall clinical picture was most consistent with paraneoplastic MG secondary to MEC.

Fig. 1.

Axial contrast-enhanced CT of the neck demonstrating a heterogeneous enhancing mass (green arrow) centered in the left masticator space, measuring approximately 3.3 × 2.7 × 3.2 cm. The mass causes localized abutment or involvement of the left pterygoid muscles and smooth osseous remodeling of the adjacent left maxilla. No osseous erosion of the left mandible is seen. Findings are consistent with a space-occupying lesion requiring histopathologic evaluation. CT, computerized tomography.

Fig. 2.

Histopathological report of the left masticator space mass. The biopsy revealed MEC with epidermoid, mucus, and intermediate (nondescript) cells embedded in sclerotic stroma. Mucicarmine stain highlighted mucus cells, and p63 was positive in malignant cells, while S100 and SOX10 were negative. The tumor was classified as intermediate grade; however, final grading was deferred due to limited biopsy size.

Discussion

This case highlights a rare and possibly underrecognized paraneoplastic association between MEC and MG. Although MG is classically associated with thymic pathology, its occurrence in the context of non-thymic malignancies has been increasingly documented, with some studies suggesting a higher than expected incidence of various cancers in patients with MG [2, 4]. The association with head and neck cancers, however, remains infrequently reported [3, 5, 6].

In the case of MEC, the tumor may express antigens that are similar to those found in the neuromuscular junction, such as the AChR or muscle-specific kinase (MuSK) [7]. This phenomenon, known as molecular mimicry, is a leading hypothesis for many paraneoplastic neurological syndromes [8]. The immune system, in its attempt to target the tumor, inadvertently generates autoantibodies against these structurally similar neuromuscular junction proteins, leading to the clinical manifestations of MG. While direct evidence of AChR expression on MEC cells is still a subject for research, this mechanism provides a plausible pathogenic link.

Whether resection of the tumor will lead to resolution of MG remains uncertain. In some paraneoplastic neurological syndromes, treatment of the underlying malignancy can lead to neurological improvement [9]. Although no direct pathogenic link has been established between MEC and AChR autoimmunity, this case suggests a need for increased clinical awareness and further investigation into this potential association.

Conclusion

Early recognition of paraneoplastic MG is critical as delayed diagnosis may result in respiratory compromise and poor outcomes. This case adds to the limited body of evidence suggesting that MEC can be a trigger for MG. Further studies are needed to clarify the incidence and pathophysiology of MG in association with non-thymic malignancies. Prompt diagnosis and management of both the neurologic and oncologic conditions are critical to prevent respiratory compromise and initiate appropriate oncologic therapy.

Statement of Ethics

This case report did not require ethics approval in accordance with local guidelines. The Institutional Review Board at Baylor College of Medicine determined that ethics approval was not required for case reports. Written informed consent was obtained from the patient for publication of the details of their medical case and accompanying clinical information. The CARE Checklist has been completed by the authors for this case report, attached as online supplementary material (for all online suppl. material, see https://doi.org/10.1159/000547406).

Conflict of Interest Statement

The authors have no conflicts of interest to declare.

Funding Sources

This study was not supported by any sponsor or funder.

Author Contributions

Sreya Rahman, BS, and Mitchell Boshkos, DO, contributed to the conception and design of the case report, performed the literature review, and drafted the manuscript. Mitchell Boshkos, DO, and Dharani Narendra, MD, critically revised the manuscript for important intellectual content. All authors approved the final version of the manuscript.

Funding Statement

This study was not supported by any sponsor or funder.

Data Availability Statement

All data generated or analyzed during this study are included in this article and its online supplementary material files. Further inquiries can be directed to the corresponding author.