Incidentally Found Olfactory Schwannoma: A Rare Case Report

Abstract

Background:

Olfactory nerve schwannomas are sporadic, benign neurogenic tumors arising from Schwann cells within the olfactory pathway. Their clinical presentation is typically nonspecific, and diagnosis often requires careful radiologic and histopathologic evaluation.

Case presentation:

We describe a 19-year-old male patient with an olfactory schwannoma that was incidentally discovered during evaluation for head trauma. The patient was asymptomatic, with preserved olfactory function.

Management and results:

The patient underwent complete surgical excision via bifrontal craniotomy. Immunohistochemical staining demonstrated positivity for S100, CD34, and Bcl-2, and negativity for GFAP and EMA, confirming the diagnosis of schwannoma. The postoperative course was uneventful, with full preservation of olfactory nerve function, and the patient was discharged in stable condition on postoperative day 7.

Conclusion:

This case highlights the clinical significance of recognizing olfactory schwannomas, which may be discovered incidentally. Comprehensive imaging and histopathological evaluation are crucial for accurate diagnosis. Early detection and total resection contribute to favorable neurological outcomes in asymptomatic patients.

Introduction

Schwannomas are benign, slow-growing nerve sheath tumors that typically arise from Schwann cells of the central or peripheral nerves. They account for approximately 8% of all intracranial tumors, with most cases originating from the vestibular division of the eighth cranial nerve. Rarely, schwannomas have been reported to arise from the olfactory or optic nerves, which normally lack a myelin sheath. To date, approximately 77 cases of olfactory nerve schwannomas have been reported in the literature.^1,2 The signs and symptoms of olfactory nerve schwannomas may vary among individuals, and their slow progression can contribute to delayed detection and diagnosis.

Due to their rarity and nonspecific presentation, olfactory nerve schwannomas are often difficult to diagnose preoperatively. Symptoms may be subtle or even absent, resulting in delayed or incidental detection during imaging performed for unrelated conditions. Imaging studies, such as magnetic resonance imaging (MRI) and computed tomography (CT), are essential for identifying the lesion and determining its precise location within the olfactory region. In some cases, a tissue biopsy may be performed to confirm the diagnosis. A thorough neurological examination may also be conducted to assess the patient’s sensory function, including olfaction, and to identify any associated neurological deficits.^3,4 In some cases, a tissue biopsy or surgical resection is required for definitive diagnosis, followed by histopathological and immunohistochemical analyses using markers such as calretinin, CD56, CD34, Bcl-2, GFAP, EMA, and S-100.^5-7

Given the diagnostic challenges and extreme rarity of this entity,^1,2 each newly identified or incidentally discovered case contributes valuable information to the current understanding of olfactory schwannomas. Reporting such cases enhances recognition of their imaging features and histopathological characteristics, and helps clinicians avoid misdiagnosis and inappropriate management.^3,4

Case Presentation

The patient was a 19-year-old man who sustained multiple traumas after falling from a height. He was fully conscious at the time of the fall and had a clear recollection of the events. A CT scan revealed a fracture of the radius in the right hand, while the skull showed no evidence of injury or intracranial bleeding. However, a solid-cystic heterogeneous mass with multiple foci of calcification in the frontal region bilaterally, crossing the midline and extending to the frontal horn of the right lateral ventricle and the suprasellar region, was incidentally identified during routine trauma imaging, with no associated neurological or olfactory symptoms. Contrast-enhanced MRI and CT angiography of the brain confirmed the presence of an extra-axial solid-cystic mass in the anterior cranial fossa, demonstrating enhancement and calcified foci in its solid component, with cystic extension into the right lateral ventricle (Figure 1). The patient’s history revealed no prior evidence of a mass. Additionally, there was no personal or family history of neurological disorders. Neurological examinations were unremarkable, and formal olfactory testing yielded normal results, confirming the absence of anosmia.

Figure 1.

Radiological images of the incidentally discovered solid-cystic mass containing calcified foci: (A) axial non-contrast CT scan showing the frontal mass, (B) axial contrast-enhanced MRI demonstrating heterogeneous enhancement, (C) coronal MRI view showing cystic extension into the right lateral ventricle, and (D) CT angiography showing displacement of anterior cerebral arteries without vascular invasion.

After localizing the mass, the patient underwent a bifrontal craniotomy. Prior to durotomy, the superior sagittal sinus was ligated. Following tumor exposure and debulking, the mass, which was attached to the anterior cerebral artery (ACA) and frontopolar region, was carefully dissected and removed. The tumor measured 5 × 5 × 2 cm and was accompanied by an arachnoid cyst measuring 0.2 × 0.6 × 2 cm. After tumor removal, the floor of the right olfactory groove was cauterized, and the cystic component was separated from the lamina terminalis region and biopsied for immunohistochemical (IHC) analysis. Histopathological examination revealed neoplastic tissue comprising areas of low cellularity with a spongy appearance and regions with densely packed palisading cells (Antoni A pattern), along with numerous thin-walled and hyalinized vessels and areas of calcification. Immunohistochemical studies demonstrated positivity for S100, CD34, and Bcl-2, and negativity for GFAP and EMA (Figure 2), confirming the schwannoma nature of the tumor. A panel of these markers is recommended for accurate diagnosis. Postoperatively, the function of the left olfactory nerve remained normal, and the patient was discharged on postoperative day 7 in good general condition. At the 6-month follow-up, MRI revealed no recurrence, and the patient’s olfactory function remained intact.

Figure 2.

Histopathological investigations (A and B) showed neoplastic tissue consisting of spongy areas with few cells (red arrow; Antoni B) and areas with numerous palisading cells (black arrow; Antoni A), along with vessels showing hyalinized walls and focal calcifications (thick purple arrow; H&E staining). Immunohistochemical staining (C) shows that the tumor cells are positive for S-100, CD34, and Bcl-2, confirming the schwannomatous origin of the lesion (magnification 100×).

Discussion

In the present report, we describe a case of an asymptomatic olfactory nerve schwannoma incidentally discovered in a young man, highlighting its rarity and distinct clinical presentation. Schwannoma is a type of benign nerve tumor that typically arises in regions of the nervous system where Schwann cells are present. Since only the olfactory and optic nerves lack Schwann cells, schwannomas originating in these locations are extremely rare. ⁸ However, to the best of our knowledge, to date, only 77 cases of olfactory nerve schwannoma have been reported, and the most recent cases are summarized in this section.

In this case, histopathological analysis revealed an Antoni cellular pattern (types A and B), consistent with tumors occurring in the olfactory groove, including olfactory schwannoma and olfactory ensheathing cell tumor. Immunohistochemical results showed positivity for S100, CD34, and Bcl-2, and negativity for GFAP and EMA, confirming the diagnosis of schwannoma. Schwannomas originate from multipotent neural crest cells, which typically express S100 but not EMA. GFAP is useful for distinguishing glial from non-glial tumors. ⁶ Additional markers, such as CD34 and Bcl-2, which are frequently positive in spindle cell neoplasms, ⁹ may provide insights into tumor differentiation and potential molecular characteristics.

Theoretically, the origin of olfactory nerve schwannoma is debated. Some propose that the olfactory bulb lacks Schwann cells and that olfactory schwannomas could be a result of mesenchymal pial cells transforming into ectodermal Schwann cells, or due to the movement or displacement of neural crest cells. ¹⁰ Alternatively, these tumors may originate from intracranial peripheral structures such as the perivascular plexus, the anterior ethmoidal nerve, or branches of the trigeminal nerve. ² Some researchers consider the presence of schwannoma in the olfactory groove to be related to the zero cranial nerve. ¹¹ Some have reported that the olfactory filia, which connect the olfactory mucosa with the olfactory bulbs, do contain Schwann cells and hence “olfactory schwannomas” are reported, although they do not originate from the olfactory nerve itself. ⁸ However, according to previous studies, there is a consensus about the treatment approach, such that surgery is reported as the best option, with a good prognosis.^1,12

Previous reports of olfactory schwannoma can be categorized based on the clinical presentation into symptomatic and asymptomatic cases.

Symptomatic Cases

Besharatirad et al reported a 30-year-old man with a 1-year history of anosmia. MRI revealed a 6.5 × 5.9 × 4.7 cm anterior frontal lesion attached to the cribriform plate, initially suspected to be esthesioneuroblastoma. Surgical resection confirmed olfactory schwannoma with positive S100 and CD34 and negative GFAP. ¹ Yoon et al described a 50-year-old male presenting with headache, diplopia, and nausea; anosmia was also noted. MRI revealed a 4.3 × 6.3 × 4.7 cm tumor adhered to the falx cerebri, which was completely resected. IHC results showed strong S100 positivity and EMA negativity. ⁶ Figueroa-Ángel and Neurosurgery reported a 46-year-old woman with tinnitus, dizziness, and headache. MRI and CT showed an extra-axial homogeneous tumor in the right anterior cranial cavity. Surgical resection and histopathology confirmed schwannoma with S100-positive and EMA-negative staining. ¹² Shao et al presented a 21-year-old man with headache and blurred vision; MRI showed a 6.1 × 5.2 ×4 cm left frontal cystic mass. Bifrontal craniotomy was performed, and IHC revealed S100 and CD34 positivity and EMA and GFAP negativity. ⁷ Kim et al described a 49-year-old female with headache, nausea, and vomiting. MRI showed a 3 cm hypo-intense mass in the left frontal base attached to the cribriform plate. Complete resection confirmed schwannoma by S100 positivity. ²

Asymptomatic/Incidental Cases

Taha et al reported a 56-year-old male whose olfactory schwannoma was incidentally discovered after a road traffic accident. Tumor excision revealed an Antoni A pattern. The authors noted that the lack of IHC studies limited characterization for future management. ³

Unlike most previously reported cases, the present patient was completely asymptomatic, with the tumor incidentally detected. This distinction may relate to his younger age, smaller tumor size, slower progression, or limited invasion of surrounding structures.

The treatment of olfactory schwannoma for symptomatic cases is always surgical and has a good prognosis. However, further research on the pathogenesis and origin of olfactory groove schwannomas is needed to better understand this unique entity and improve its management.

Specialists should always keep such rare cases in mind so that they can use the best treatment and diagnosis for their patients. Also, differentiating schwannoma from other tumors of the olfactory groove is essential for future treatments.

This report describes a single case, which limits the generalizability of the findings. The tumor was incidentally discovered, and therefore longitudinal data on its natural course are unavailable. Additionally, the follow-up period of 6 months is relatively short, and longer-term monitoring is needed to fully assess recurrence risk and outcomes; unfortunately, longer follow-up was not possible in this case.