Keywords
olfactory groove schwannoma - olfactory ensheathing cell tumor - anterior skull base
- neuroimaging - immunohistochemical - anosmia
Palavras-chave
schwannoma da goteira olfatória - tumor de células envoltórias olfativas - base do
crânio anterior - neuroimagem - imunohistoquímica - anosmia
Introduction
Accounting for ∼ 8% of the intracranial tumors, schwannomas are slow-growing, benign
tumors[1]
[2]
[3] that emerge from the nerve sheath.[4] The majority of the cases are located in the vestibular portion of the 8th cranial
nerve, and less commonly on the 5th, the 9th, the 10th and the 12th cranial nerves.[1]
[3] Extremely rare and with an enigmatic origin, schwannomas are found in the anterior
cranial fossa, specially in the olfactory groove.[2] We report the case of an olfactory groove schwannoma (OGS) along with a brief literature
review of the pathogenesis and origin of the schwanommas.
Case Report
A 32-year-old woman, heavily addicted to crack, was brought to the emergency department
after being physically assaulted. She presented with altered mental status, saying
inappropriate words (Glasgow coma score [GCS] = 13) and diffuse body hematomas, mainly
in the dorsal region. No focal neurological deficit was detected.
A computed tomography (CT) brain scan revealed an extra-axial hyperdense contrast-enhancing
lesion in the subfrontal sagittal region, measuring 5.0 × 4.8 × 4.5 cm, which caused
mass effect in the frontal lobes and displaced the cribriform plate downwards.
It was unlikely that the CT findings were related to the psychological disturbance
of the patient, since 24 hours after being admitted she recovered the mental status,
but complained of anosmia. Confusion and disorientation could have been caused either
by the effects of crack or the traumatic brain injury (TBI), with the association
of both also being a possibility regarding the peculiarity of the patient.
On further examination, a magnetic resonance imaging (MRI) exam evidenced a large
mass arising from the skull base, protruding into the nasal cavity and expanding between
the frontal lobes, displacing them laterally. A T1-weighted MRI with gadolinium enhancement
showed involvement of the olfactory groove and irregular contrast distribution inside
the tumor. A T2-weighted MRI exam evidenced the solid mass without invasion of the
frontal lobes, and the fluid attenuation inversion recovery (FLAIR) sequence demonstrated
a mild perilesional edema, despite the tumor volume ([Fig. 1A]–[E]).
Fig. 1 (A) Axial T1-weighted MRI with gadolinium enhancement, evidencing a huge mass, with
irregular contrast enhancement between the frontal lobes displacing them. (B) Sagittal T1-weighted MRI with gadolinium enhancement, showing the involvement of
the olfactory groove by the tumor with displacement of cribriform plate downwards.
(C) Coronal T1-weighted MRI with gadolinium enhancement, also showing the protrusion
of the mass through the nasal cavity. (D) Axial T2-weighted MRI, evidencing the solid and extra-axial mass, between the frontal
lobes, without invading them. (E) Axial FLAIR MRI, showing only mild edema, besides the great diameter of the lesion.
(F) Axial postoperative CT scan, evidencing total resection of the tumor. Abbreviations:
CT, computed tomography; FLAIR, fluid attenuation inversion recovery; MRI, magnetic
resonance imaging.
The patient was submitted to surgery, through a bifrontal craniotomy, ligation of
the anterior superior sagittal sinus and inter-hemispheric approach. The tumor was
fibroelastic, brownish and well-vascularized, attached to the olfactory groove. The
olfactory bulb was not identified during resection, and the tumor was completely excised.
An immediate postoperative CT scan confirmed the total removal of the tumor ([Fig. 1F]).
The resected tissue displayed all the histopathological schwannoma hallmarks, evidenced
by its two-phase aspect, composed of alternating cellular and hypocellular areas,
called Antoni A and Antoni B areas, respectively. Cells with twisted or wavy nuclei
and indistinct cytoplasmic borders. Palisading nuclei that form compact rows surrounding
fibrillary pink cytoplasm, called Verocay bodies. The cells had pleomorphic and hyperchromatic
nuclei, but mitotic figures were not evident ([Fig. 2 A], [B]). An immunohistochemical (IHC) staining test displayed positive tumor cells for
S-100 protein, but negative staining for epithelial membrane antigen (EMA) ([Fig. 2C]).
Fig. 2 (A) Photomicrograph showing the hallmarks of schwannoma. Biphasic pattern composed of
alternating cellular (Antoni A) and hypo cellular (Antoni B) areas. Cells have twisted
or wavy nuclei and indistinct cytoplasmic borders (H&E stain, magnification 100x).
(B) Palisading nuclei form compact rows surrounding fibrillary pink cytoplasm (Verocay
bodies). There are pleomorphic and hyperchromatic nuclei, but mitotic figures were
not evident (H&E stain, magnification 100x). (C) Immunohistochemical staining analysis shows that the tumor cells are positive for
S-100 protein (magnification 100x).
The postoperative period was uneventful, despite one episode of complex partial seizure,
and the preexisting anosmia. After 1 year, the patient remains free of disease and
had no other neurological deficits.
Discussion
To date, only 56 cases of OGS have been reported in the literature.[5] The ages of the affected patients vary, mainly, between 20 and 50 years old.[6] The clinical presentation of OGSs has a diverse range, including asymptomatic, anosmia/hyposmia
(the most common finding), seizures, headache,[1]
[6] visual deficits, cognitive disturbances, rhinorrhea,[1] motor and sensory dysfunction, and intracranial hypertension.[6]
Aware that optic and olfactory nerves, as part of the central nervous system (CNS),[7] do not have a Schwann cell sheath, and that intracranial schwannomas unrelated to
cranial nerve occur mainly in the midline of the anterior cranial fossa, there is
a divergence concerning the origin of OGSs and whether they are primarily intra or
extra-axial lesions.[1]
[3]
[6] Despite several hypothesis concerning their probable origin, the true original site
of OGSs remains controversial.[3]
[5]
Developmental theories suggest an intraparenchymal origin,[1] either by transformation of a mesenchymal pial cell into ectodermal Schwann cells
or by the migration of neural crest cells within the CNS.[6] Still, there is a theory suggesting the origin of Schwann cells from multipotent
mesenchymal cells after an injury.[2]
Non-developmental hypotheses believe that intracranial schwannomas are extra-axial
lesions arising from adjacent structures that present Schwann cells, namely, the meningeal
branches of the trigeminal, the anterior ethmoidal nerves and the nerve plexus of
dural vessels that innervate the anterior cranial fossa.[8]
[9]
Yasuda et al raised the question about the origin of Schwann cells in some olfactory
groove tumors, and attributed it to the olfactory ensheathing cells (OECs), suggesting,
in this way, another entity: olfactory ensheathing cell tumor (OECT).[6] Despite the similarities in radiological and microscopic assessment, they believed
that IHC staining could distinguish OGS from OECT, since Schwann cells, rather than
OECs, are positive for Leu7 (cluster of differentiation [CD] 57).[10] Within these criteria, nine cases of OECT have been proposed in the literature so
far.[9] On the other hand, Bohoun et al reported two cases of mass lesions in the anterior
cranial fossa called schwannoma-like tumors, due to the negative IHC staining for
Leu7 and positive IHC staining for Schwann/2E, which is a specific marker of Schwann
cells. Hence, according to them, this implies a mixed biological nature of both OGSs
and OECTs.[2] Schwann cells and OECs are next to each other by the cribriform plate of the ethmoid
bone. Schwann cells cover unmyelinated axons of the nerve fibers up to 0.5 mm before
the olfactory bulb, and OECs, likewise, until two external layers of the olfactory
bulb, to facilitate axonal growth.[11]
[12]
[13]
On MRI exams, these lesions appear as a well-circumscribed and encapsulated mass,
expressing low to intermediate signal intensity on T1-weighted images and high signal
intensity on T2-weighted images. The hyperintensity on T2-weighted images may either
be homogeneous or show a target sign with high signal intensity in the periphery of
the lesion.[6]
[8] These pathological findings correspond to a central fibrous tissue and an encircling
myxoid tissue.[6] A differential diagnosis with other tumors in the anterior cranial fossa must include
OECT,[9] olfactory groove meningioma (OGM),[4]
[9]
[14]
[15]
[16]
[17] low-grade astrocytoma,[9]
[14] olfactory neuroblastoma, and metastatic tumors.[9]
[16]
A histopathological examination, either in OGS or OECT, reveals a biphasic pattern,
consisting of a dense and hypercellular area among a sparse and hypocellular area,
called Antoni A and Antoni B areas, respectively.[6]
[8] Cells are narrow, elongated and wavy, with tapered ends scattered with spindle-shaped
cells (collagen fibers). Often seen, the nuclei of the tumor cells are elongated and
arranged regularly to form a palisading aspect, eventually surrounding acellular areas
(Verocay bodies).[6]
[8] Moreover, neoplastic cells display ill-defined cytoplasm, dense chromatin and frequently
degenerative nuclear atypia, but rare mitotic figures.[6]
Since OECTs mimic the clinical, radiological and histopathological features, besides
many structural, behavioral and IHC properties of OGSs,[6]
[9]
[12]
[14]
[18] the available IHC markers are not able to solve the puzzle regarding the differentiation
of these two similar entities. Up to now, both OECs and Schwann cells are positive
for S-100 and glial fibrillary acidic protein (GFAP), and negative for EMA.[9]
[12]
[13] Schwann/2E is considered a specific marker of Schwann cells,[19] and Leu7 may not be considered a tool to discriminate between OECTs and OGSs.[2]
Generalizing the knowledge of surgical strategies from OGMs to OGSs due to their similarities,
identical treatment goal and shortage of literature regarding these tumors, it is
noticed that the transcranial approach (TCA) is still preferred to the endoscopic
endonasal approach (EEA).[20] Selected cases with clinical or with acceptable postoperative anos- mia may benefit
from EEA,[20]
[21] specially when the size of the tumor is < 4.0 cm and visual improvement is the main
goal.[20] On the other hand, despite of acquiring a facial scar[20]
[21] and more evidence of frontal lobe injury on postoperative MRI assessement,[21] the TCA is not significantly influenced by tumor size and is superior to the EEA
in achieving the virtual cure by gross total resection (GTR) and to avoid cerebral
spinal fluid (CSF) leak.[20] There is no statistical difference between both methods concerning epilepsy, hydrocephalus,
infection, stroke, and perioperative mortality rates.[20]
Although subtotal resection is acceptable due to the slow-growing benign nature of
the tumor,[4]
[6] complete excision is the treatment of choice. Adjunctive therapy is an exceptional
option.[8]
[16] In addition, the prognosis of the patient after the total resection of the tumor
is favorable.[16]
Conclusion
Extremely rare tumors, OGSs occur less frequently than any other intracranial nerve
schwannoma. Differential diagnosis between OGSs and OECTs is unachievable with the
current diagnostic tools. However, the histopathological hallmarks of schwannomas
in the anterior cranial fossa after their complete resection showed to be curative.
To uncover the genuine origin and pathogenesis of OGSs, more studies are required.
