Atypical presentation of cystic schwannoma of the sphenoid sinus: a nonsolitary mass with osseous, intracranial and cavernous sinus invasion

Although nearly half of all schwannomas involve the head and neck region, nasal and paranasal sinus presentations are quite rarely seen. Cystic schwannoma, characterized by cyst formation lined by S-100 protein positive membrane-like structures is very uncommonly seen in sphenoid sinus with only a single previously reported case. Here we report a young patient of cystic schwannoma of the paranasal sinuses having epicenter in the sphenoid sinus. The tumor had caused extensive erosion of the skull base and paranasal sinuses and extended intracranially that radiologically mimicked as infected mucocele causing loss of vision. This case denotes the aggressive behavior of such uncommon tumors.


Introduction
Schwannomas are slow growing benign tumors originating from the peripheral nerve sheaths. Approximately 25-50% of schwannomas occur in the head and neck region, but tumors originating from the nasal cavity or paranasal sinuses are rare, with just over 100 cases reported in literature [1]. Cystic schwannoma (CS), recognized by cyst formation lined by S-100 protein positive membrane-like structures in an otherwise ordinary schwannoma [2], is very unusually seen in paranasal sinuses. Till now, only a single case of CS of the sphenoid sinus has been described in literature [3]. Here we present an unusual case of CS of the paranasal sinuses having epicenter in the sphenoid sinus that masqueraded as an infected mucocele. The tumor had caused extensive erosion of skull base and paranasal sinuses and grown intracranially causing unilateral loss of vision in a young adult. This case is one noteworthy example of the aggressive behavior of such uncommon tumors. Postoperatively the patient improved well although the visual acuity remained same as preoperative state. He is currently on regular follow up since past 6 months.

Discussion
Schwannoma is a painless, benign slow growing tumor of the peripheral nerve sheath commonly associated with the cochlear nerve at the cerebellopontine angle. They rarely occur in the sinonasal cavities and in this location, they are typically described as solitary encapsulated lesions confined to the nasoethmoid region [4]. Schwannomas originate from Schwann cells of the peripheral nervous system. There have been various hypotheses proposed to explain the origin of intracranial schwannomas that do not arise Page number not for citation purposes 3 from cranial nerves. Schwann cell hyperplasia is the most commonly proposed theory, occurring within the perivascular nerve plexuses [5], the meningeal branch of the trigeminal nerve in the anterior cranial fossa, the anterior ethmoidal nerve around the cribriform plate, or the fila olfactoria, which are known to acquire Schwann cell sheaths that extend ~0.5 mm beyond the olfactory bulb [6]. The origin of schwannomas arising from the paranasal sinuses are presumed to be from the peripheral nerve sheaths of the branches of the ophthalmic and maxillary divisions of the trigeminal nerves and their ganglion and lack of any other nerve divisions near these sinuses. The sphenoid sinus is richly innervated by the posterior ethmoidal nerve, and we surmise that the current case might had its origin from this nerve. Cystic change or degeneration may be encountered in schwannomas frequently which is somehow a feature of so called "ancient schwannoma", which is also well known for atypical nuclear features of no particular prognostic significance.
But in the case of ancient schwannoma, cystic areas are smaller, multiple and are not lined by S-100 protein positive membrane-like structures [7]. CS are recognized by a typical clinical history of subtle complaints lasting for a long time, followed by acute progressive symptoms. This course is reminiscent of malignant transformation in a schwannoma [8]. In our patient, however, the complaints were not of very long duration (6 months), however the acute progressive nature of symptoms were suggested by his vision loss which occurred within a short period of time. It has been suggested that the accelerated clinical findings are not related to sudden tumor growth due to cell proliferation, but are due to expansion of the cyst [2]. It is believed that the clinical outcome of CS is worse than solid tumors [2]. However, it is difficult to assess this for cases occurring at the paranasal sinuses, since there are few cases of CS reported. For the present case total excision was the treatment of choice and the patient is in good health 6 months after the operation without any evidence of recurrence. Based on the current case, we can presume that maximum safe resection should be the primary goal of surgical intervention. However, long duration follow up is needed to better assess the clinical behavior of such uncommon tumors of paranasal sinus.

Conclusion
Our patient's schwannoma, presumed to originate from the posterior ethmoidal nerve at the sphenoid sinus, appears to be unique in its presentation as an erosive fronto-ethmoidal mucocele and loss of vision and a location which is very uncommonly seen.
The current case is one noteworthy example of mucocele formation secondary to the underlying schwannoma which may mimic features of the more commonly encountered lesions in this region that may cause diagnostic dilemmas and give intraoperative surprises. It remains to be seen if such unusual tumors can be studied in a more extensive way in their earlier juncture of development when the tumors are still small and if with the new radiological techniques like high-tesla MRI the nerves of their origin can be revealed and these tumors subclassified.

Competing interests
The authors declare no competing interest.