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Neurochirurgie
Volume 59, n° 2
pages 75-80 (avril 2013)
Doi : 10.1016/j.neuchi.2013.02.003
Received : 29 September 2012 ;  accepted : 21 February 2013
Anterior fossa schwannoma mimicking an olfactory groove meningioma: Case report and literature review
Les schwannomes de la fosse antérieure : un diagnostic différentiel des méningiomes de la gouttière olfactive. À propos d’un cas et revue de la littérature
 

F. Sauvaget , P. François , M. Ben Ismail , C. Thomas , S. Velut
 Department of Neurosurgery, François-Rabelais University, 10, boulevard Tonnellé, 37044 Tours, France 

Corresponding author.
Abstract

Intracranial schwannomas not associated with cranial nerves account for less than 1% of surgically treated schwannomas of the central and peripheral nervous system. With only 45 cases reported to date, subfrontal schwannomas are very rare tumors, leaving the issue of their origin controversial. A 66-year-old woman presented with a 1-year history of progressive headaches. Clinical examination revealed hypoesthesia of the nasal tip. CT-scan and MRI studies revealed a large subfrontal tumor thought preoperatively to be a meningioma. Intraoperatively, a large extra-axial tumor arising from the floor of the right frontal fossa was encountered. Histopathology identified the tumor as a schwannoma. This current case gives strong clinical presumption of an origin from the anterior ethmoidal nerve. We reviewed the literature in order to establish the epidemiology of these tumors, from which there appear to be divergent profiles depending on tumor origin and histology. Despite close similarities with olfactory groove meningiomas, patient history and radiological findings provide substantial evidence for differential diagnosis.

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Résumé

Les schwannomes intracrâniens non associés aux paires crâniennes représentent moins de 1 % des schwannomes opérés. Avec seulement 45 cas reportés, les schwannomes de la fosse antérieure constituent un groupe rare de tumeur, dont l’origine prête à controverse. Nous reportons le cas d’une patiente de 66ans aux antécédents de céphalées depuis un an. L’examen clinique ne retrouvait qu’une hypoesthésie unilatérale de la pointe du nez. L’imagerie TDM et IRM ont permis d’identifier une volumineuse tumeur sous-frontale envahissant l’ethmoïde, évoquant un méningiome olfactif. L’analyse anatomopathologique de la pièce opératoire concluait cependant en un schwannome bénin. Ce cas évoque fortement une tumeur originaire du nerf ethmoïdal antérieur. Nous avons réalisé une revue de la littérature dans l’objectif d’établir une épidémiologie de ces tumeurs, qui semblent présenter des profils divergents. En dépit d’une grande similarité avec les méningiomes de la gouttière olfactive, l’histoire clinique et l’imagerie préopératoire offrent de nombreux éléments de différentiation.

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Keywords : Olfactory groove, Schwannoma, Nasociliary nerve

Mots clés : Gouttière olfactive, Schwannome, Nerf nasociliaire

Abbreviations : CD, CT-scan, CP, MRI-scan, OEC, OECT, OG, OGS, SBD, SR


Introduction

Schwannomas are benign, slowly growing nerve sheath tumors. They account for about 8% of all intracranial tumors and can arise from any nerve containing Schwann cells. The most common location is the vestibular portion of the VIIIth cranial nerve and, less commonly, the Vth, IXth, Xth, and VIIth cranial nerves. The occurrence of a schwannoma not related to cranial nerves is exceedingly rare [1], the most common location being the anterior cranial fossa. To date, only 45 cases without neurofibromatosis disease have been described. Recent histological findings concerning the olfactory groove differentiate two kinds of ensheating cells tumor: typical schwannomas (OGS) and olfactory ensheating cells tumors (OECT) [2].

We describe a case of an OGS in a 66-year-old Caucasian woman, preoperatively thought to be an anterior fossa meningioma. Due to their unusual frequency, we review the literature in order to establish the epidemiological profile and pathogenesis of this kind of tumor.

Case report

A 66-year-old right-handed Caucasian female having no medical history was admitted to our department for progressive headaches. The neurological examination showed a sensitive defect on the right side of the nasal tip. She had neither anosmia nor gustative disorder. The endonasal examination was normal. There were no skin stigmata of neurofibromatosis, or anomaly of the fundus.

CT-scan revealed a low density mass without calcifications, localized in the basis of the right frontal cranial fossa, eroding the cribriform plate and invading the posterior cells of the ethmoid sinus. A Gadolinium enhanced MR-scan showed a large, homogeneously enhanced extra-cerebral lesion arising from the floor of the right anterior cranial fossa. A small portion of the tumor crossed the midline, deflecting off the falx medially and inferiorly. We observed minimal peritumoral edema and no evidence of a dural tail sign. Preoperatively we evoked a left olfactory groove meningioma (Fig. 1).



Fig. 1


Fig. 1. 

A. B. Preoperative T1 coronal MRI, showing the ethmoidal sinus extension of the tumor. C. Preoperative T1 weighted free gadolinium sagittal MRI. D. Immediate postoperative view on T1w MR-scan; complete removal.

A. B. IRM préopératoire T1 en coupe coronale, montrant l’extension tumorale au sinus ethmoïdal. C. IRM préopératoire T1 en coupe sagittale. D. IRM T1 postopératoire précoce ne retrouvant pas de reliquat.

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Surgery was performed through a right frontal craniotomy. The tumor had a solid hypovascular yellowish-white colored aspect arising from the right olfactory groove. It elevated the right frontal lobe. We were unable to identify the homolateral olfactory bulb. The removal of the intracranial portion of the tumor allowed us to see the contralateral olfactive tract. Despite the invasion of the ethmoid sinus and the bone erosion, the nasal cavity was respected.

A postoperative rapidly regressive cerebral edema occurred. Postoperative evolution was good with no new neurological deficit and complete smelling preservation.

Microscopic examination showed bipolar spindle shaped cells spread into a fibrous texture, evoking the Antoni pattern A. Because of the unusual location, immunostaining was performed. Tumor cells stained highly positive for S100 protein and CD-57 (Leu7) and negative for epithelial membrane antigen, AE1 and AE3 cytokeratin and P63. Ki67 labeling index was negative. The histological examination confirmed the diagnosis of benign schwannoma (Fig. 2).



Fig. 2


Fig. 2. 

Photomicrographs of the tumor specimen, immunohistochemical staining (×400). Left: protein S100 staining, right: EMA staining.

Microphotographie histologique de la tumeur après immunofixation (×400). Gauche : protéine S100, droite : EMA.

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Discussion

Intracranial schwannomas account for approximately 8% of all the intracranial tumors. Most of them arise from the vestibular branch of vestibulocochlear nerve (80–90%). They can also involve trigeminal nerve (8%), facial nerve (1.9%) and, less frequently, other cranial nerves. Despite the high frequency of head and neck schwannomas (35–45%), they involve paranasal sinuses in less than 2% of the cases [3, 4].

Similarly, schwannomas represent less than 1% of the intraorbital tumor [5]. Intracerebral schwannomas not associated with cranial nerves account for less than 2% of surgically treated schwannomas of the central nervous system [4].

The most frequent location of these rare tumors is the anterior skull base. The origin of these tumors is still a controversial issue [6].

We found 45 cases of schwannomas, and six cases of olfactory ensheating tumors in the world literature (Table 1).

Based on the reported origins, 18 schwannomas were from the cribriform Plate (CP), 13 from the olfactory groove (OG) without more precision, and 11 were more related to the meningeal structures: the skull base dura (SBD) and the falx (Table 2).

Clinically, patients frequently report a history of headache (50%), with the most common reported symptoms being seizure (43%), hyposmia (43%) and visual disorders (17%). The median age is 33 years. The male/female sex-ratio (SR) is about 1.5 (27 males for 19 females). Based on the site of attachment, we divide OGS into two types: schwannomas of the olfactory site (OG or CP) and schwannomas that arise from non-olfactory sites (SBD). A few articles have differentiated OG and CP.1 However this distinction seems too subtle. It is likely that most authors have used these two words to describe the same region.

Recent articles in the literature emphasize the difference between OECT and schwannomas. Six cases of OECT have been described since 2006, with a mean age of 32yrs and a sex-ratio (M/F) of 1. olfactory groove (OG) related schwannomas have a mean age of 36yrs and a SR (M/F) of 0.9. These data are similar to literature findings for trigeminal schwannomas [53]. CP related schwannomas have a mean age of 34yrs and a SR (M/F) of 3.5. OG and CP represent both olfactory related schwannomas, with a total of 28 cases, a mean age of 35yrs and a SR (M/F) of 1.5.

In contrast, SBD schwannomas have a mean age of 25years and a SR (M/F) of 2. Prevalence of young patients presenting schwannomas is commonly seen in phakomatosis and has also been seen for intraparenchymal schwannomas [39].

There is an extracranial component in 58% of OGS, 50% of OECT, and 33% of SBD schwannomas. We did not find any correlation between the initial preoperative symptoms and other elements, such as the site of attachment or the age at the diagnosis. The olfactory tracts were not detected in 28 cases, involved in six cases and free in seven cases. Olfaction was impaired in 50%, but not assessed in many articles. The radiological aspect of the lesion was solid in 54% of the cases and heterogeneous after contrast injection in 63% of the cases. A solid aspect with a homogeneous enhancement, which is likely to be confused with a meningioma, is found in 50% of the cases.

Intracranial schwannomas are histological paradoxes when they are not directly related to cranial nerves [12]. Theories to explain their birth opposed developmental origin against adjacent neural structure origin [54]. Developmental theories advocate these lesions primarily arising from ectopic Schwann cells in the central nervous system. These theories also suggest an origin from multipotential mesenchymal cells or displacement of neural crest cells, forming focus of Schwann cells within brain parenchyma [55]. Olfactory bulbs and tracts lack Schwann cells. Fila olfactoria contain a specific type of sheath cells known as olfactory ensheating cells (OEC) that express S100 protein, but are negative for CD-57 testing (whereas Schwann cells express CD-57). OEC embryologically derives from olfactory placodes, whereas Schwann cells originate from the neural crests [56, 57, 58].

Six recent articles have emphasized that OEC tumors, which seem to originate from the fila olfactoria, may be mistaken for schwannomas. Negative immunostaining of the CD-57 receptor characterizes olfactory ensheating cells.

Schwann cells are normally present within the perivascular nerve plexuses around large arteries in the sub-arachnoid space and in adrenergic nerve fibers of cerebral arteries. They are found, in addition, in the innervation of the dura either from the meningeal ramus of the anterior ethmoidal nerve, which innervate the anterior fossa and the olfactory groove, or from the middle meningeal nerve. Adjacent extracranial nervous structures can give birth to neoplasms that are susceptible to intracranial invasion. Interestingly, in our case, the patient showed a nasal tip hypoesthesia, which may be a sign of anterior ethmoidal nerve impairment.

Although schwannomas are not typically osteolytic, they can expand into foramina and grow through several compartments. It is noticeable that the nasociliary nerve, which displays consequent caliber and contains many myelinated fibers, occupy the entire compartment involved by anterior subfrontal schwannoma in the literature.

Meningioma, esthesioneuroblastoma, squamous cell carcinoma, adenocarcinoma, lymphoma and metastasis should be considered in differential diagnosis in adults harboring an anterior skull base mass with a sino-nasal involvement. It obviates the need for more aggressive craniofacial resection. Esthesioneuroblastoma are very rare sino-nasal malignant tumors, which can easily mimic benign tumor in the beginning of their evolution. Therefore, one has to be very careful regarding the discovery of a tumor of the olfactory groove presenting an extracranial component. The presence of bone scalloping and the absence of bone sclerosis and a dura tail may help in differentiating it from a meningioma. A recent report has suggested that a T2*-weighted MRI sequence may help in distinguishing meningioma from schwannoma [59]. Presence of multiple foci of low signal intensities (related to microbleeds) in T2*-weighted MRI seems to point towards a schwannoma. Clinical findings are very similar except for a high increase of seizure in the case of schwannoma (>40% in OGS versus only 10–20% in OG meningiomas) [60, 61].

Preoperative findings are summarized in Table 3.

We suggest surgical resection by pterional approaches for small lesions (<25mm) and frontal approaches for wider lesions with impaired olfaction [62]. Minimally invasive approaches are a current challenge as in olfactory and planum sphenoidal meningioma. Keyhole approaches with an endoscopic device through supraorbital approaches are promising techniques for small tumors [21, 63, 64]. In our review, complications encountered are mostly frontal lobe edema, CSF leakage, and postoperative olfactory function impairment. Hyposmia is related to the local involution or disconnection of the olfactory bulb. However, in many cases, both olfactory tracts were not seen despite the persistence of the sense of smell. The olfactory system is known to show a great capacity for regeneration. As olfaction disability is still a frequent morbidity [6], repairing the olfactory tract might be a new challenge. There is a lack of evidence in the world literature about the efficiency of olfactory nerve repair in human. Olfactory tracts have shown surprising recovery ability in experimental models [65, 66] where nerve reconstruction was attempted by direct neural anastomosis with or without scaffold.

Conclusion

OGS are rare benign slow-growing tumors. A literature review allows the distinction between at least two epidemiological profiles related to different sites of attachment: OGS related to intracerebral or intradural Schwann cells that involves predominantly young male patients and OGS related to anterior ethmoidal nerve affecting older patients. It is unknown whether this difference is the consequence of a Schwann cell phenotypic divergence, a genetic or developmental predisposal, or a recruitment bias. We must take care not to overstate the significance of these observations considering the small number of this tumor.

OECT share with OGS several clinical, radiological, and histological similarities that may be responsible for underdiagnosis of OECT, which are the true olfactive schwannomas.

Due to their easy removal and good outcome, OGS must be seen as peripheral schwannomas developing into intracranial space and differing from other intracranial trigeminal schwannomas. A pterional approach is preferred for smaller lesions whereas frontal craniotomy is indicated in greater lesions. Olfactory repair and minimally invasive approaches might be tomorrow’s challenges.

Disclosure of interest

The authors declare that they have no conflicts of interest concerning this article.

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