British Journal of Radiology (2004) 77, 1016-1021
© 2004 British Institute of Radiology
doi: 10.1259/bjr/79074348
Neuroradiological findings of intracranial schwannomas not arising from the stems of cranial nerves
Boning Luo, MD, PhD1,
Gengxi Sun, MD1,
Bo Zhang, MD1,
Kangfu Liang, MD1,
Jianming Wen, MD2 and
Kunhao Fang, MD, PhD1
1 Department of Radiology, The First Affiliated Hospital of Sun Yat-sen University and 2 Department of Pathology, Medical College of Sun Yat-sen University, Guangzhou 510080, PR China
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Abstract
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Four cases of intracranial schwannomas not arising from the stems of cranial nerves are reported. All cases were confirmed by surgery and pathological examination. The related histogenesis and the CT and MRI findings are discussed, and the literature is reviewed. The CT and MRI findings of these tumours were analysed retrospectively and their pathological characteristics were reviewed. The tumours presented as heterogeneous hypointense to isointense mixed dense lesions with multiple areas of cystic degeneration and necrosis. Extensive cystic degeneration was the prominent characteristic on CT. The tumours showed hypointense to isointense signal on T1 weighted images and heterogeneous hyperintense signal on T2 weighted images. On contrast enhanced CT and MRI, the solid component and the wall of the tumours showed moderate to strong enhancement. Haematoxylin and eosin staining of the lesion demonstrated two histological patterns, Antoni type A and Antoni type B. Immunostaining showed intense immunoreactivity for S-100 protein. The CT and MRI findings of these tumours were similar to those of schwannomas arising from cranial nerves, although the former show a higher rate of cystic degeneration. Therefore, even when a mass is not in the usual cranial nerve region, if its imaging characteristics are similar to those of common schwannomas, the possibility of a schwannoma not arising from cranial nerve should be considered.
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Introduction
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Schwannomas are benign tumours accounting for approximately 8% of all intracranial tumours. They originate from Schwann cells, which are a neural crest derivative. The most common location is the VIIIth cranial nerve, followed by the Vth, IXth, Xth, and VIIth cranial nerves. Intracranial schwannomas not arising from cranial nerves are extremely rare. To date, there have only been a few reported cases [1–12]. In 1997, Huang [3] reviewed 45 collected cases and one personal case of intracranial schwannomas not arising from a cranial nerve. The neuroradiological findings have rarely been reported in depth. It appears that schwannomas not arising from cranial nerves are very uncommon and their correct diagnosis is difficult. However, correct analysis using modern neuroradiological studies should often facilitate the differential diagnosis, which guides correct surgical treatment of these benign tumours. We present four cases of intracranial schwannomas not arising from cranial nerves that were confirmed by surgery and pathological examination in our hospital between January 1997 and November 2002.
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Patients
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Case 1
A 40-year-old female was admitted for mental change lasting for 3 years, as well as headache accompanied by blurred vision of 2 months' duration. Slight bilateral papilloedema was found on ophthalmoscopic evaluation.
MRI showed a large (9.0 cm x 5.5 cm x 5.5 cm), well circumscribed extra-axial mass located at the left side of the cribriform plate and a large cyst within the left frontal lobe. The solid portion of the mass had invaded the cribriform plate and extended into the ethmoidal sinus inferiorly, which showed isontense to hypointense signal on fluid attenuated inversion recovery (FLAIR) weighted images (Figure 1
) and which enhanced markedly after Gd-DTPA administration (Figure 2). The pre-operative CT diagnosis was astrocytoma, but schwannoma was considered following MRI.
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Figure 1. Coronal fluid attenuated inversion recovery (FLAIR) image: the tumour was located left subfrontally and the cystic area demonstrated hyperintense signal.
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Figure 2. Contrast enhanced coronal image: tumour had extended into the ethmoidal sinus, and the intracerebral portion contained large areas of cystic degeneration.
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On low left frontal craniotomy, the well encapsulated cystic–solid mass arising from the olfactory groove was seen to extend into the ethmoidal sinus and left maxillary sinus.
The pathological diagnosis was schwannoma.
Case 2
A 42-year-old male noted the onset of headache for 6 years, associated with decreased vision in his right eye of 2 months' duration. The level of consciousness was normal.
CT and MRI revealed a large, well circumscribed, polylobulated mass (3.9 cm x 4.0 cm x 3.5 cm) located in the left temporal lobe, whose inferior boundary was ill defined in relation to the floor of the left middle cranial fossa (Figure 3). The solid portion enhanced greatly after contrast medium administration (Figure 4). Additional views revealed an associated large cyst located posteriorly and superiorly. There was some peritumoral oedema. The pre-operative diagnosis was glioma.
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Figure 3. Transverse T2 weighted image: the tumour in the right inferior temporal lobe was seen as mixed signal.
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Figure 4. Contrast enhanced transverse CT: the mass depicted nodular enhancement. The posterosuperior portion contained areas of cystic degeneration.
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On left temporal craniotomy, the cystic–solid mass was seen to have several connected nodules with a distinct border, and a cystic portion containing 30 ml of tan–yellow liquid. The mass did not show any connection to the cranial nerves.
Immunohistochemical examination confirmed the mass to be a schwannoma.
Case 3
A 36-year-old female presented with a 5-month history of headache accompanied by slightly decreased vision. In addition, the patient experienced right congruous hemianopia on admission. A funduscopic examination revealed papilloedema in the left eye.
Plain CT demonstrated a heterogeneously hypodense mass (4.0 cm x 5.0 cm x 6.0 cm) located on the right subfrontally and close to the floor of the right anterior cranial fossa (Figure 5). The mass showed inhomogeneous enhancement after contrast medium, with patchy unenhanced areas of degeneration and necrosis (Figure 6). There was minimal peritumoral oedema and no evidence of a dural tail. The initial radiological diagnosis was glioma.
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Figure 5. Non-contrast transverse CT: the mass in the right subfrontal lobe showed a hypodense to isodense mass with a distinct border.
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Figure 6. Contrast enhanced transverse CT: the mass was inhomogeneously enhanced after contrast medium.
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On right frontal craniotomy, the mass, with an intact capsule, was seen to arise from the right olfactory groove and was located predominantly in the frontal fossa, although a small portion of the mass extended into the left side, with moderate displacement of the falx to the left.
Histological examination revealed schwannoma.
Case 4
A 61-year-old male presented with a history of dizziness and intermittent left limb aesthesia for 2 weeks' duration. There were no neurological abnormalities observed at the time of admission and no signs of Von Recklinghausen's disease.
On MRI, the mass had a well defined margin and was in contact with the septum pellucidum. The inhomogeneous mass (1.8 cm x 1.7 cm x 1.6 cm) demonstrated hypointense to isointense signal on T1 weighted images (Figure 7) and hyperintense signal on T2 weighted images, and enhanced strongly after Gd-DTPA (Figure 8). The diagnosis was glioma.
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Figure 7. Transverse T1 weighted image: the inhomogeneous mass in the septum pellucidum was hypointense to isointense.
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Figure 8. Contrast enhanced coronal T1 weighted image: the tumour was significantly enhanced after contrast medium.
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On frontal craniotomy, the mass proved to be attached to the septum pellucidum and was supplied by the branches of the anterior cerebral arteries.
Immunostaining showed positive for S-100 protein and myelin, but negative for synaptophysin. Pathological examination after total resection confirmed schwannoma.
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Pathological findings
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All the tumours were similar microscopically and immunohistochemically. With haematoxylin and eosin stain, two patterns were seen in the four cases, Antoni Type A and Antoni Type B (Figure 9). The Antoni A areas consisted of compact intersecting bipolar spindle cells with elongated or oval nuclei and indistinct cell borders, arranged parallel in bundles or uncomplete whorls, and on occasion, well formed Verocay bodies. In less cellular areas there was a characteristic Antoni B pattern composed of spindle cells arranged loosely on a dense fibrocollagenous, focally hyalinized stroma. Immunostaining exhibited intense immunoreactivity for S-100 protein (Figure 10). Reticulin staining demonstrated a distinct basement membrane around each tumour cell. No lesion showed staining for epithelial membrane antigen. The pathological diagnosis was schwannoma of conventional type.
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Figure 9. Photomicrograph showing the type A pattern of a schwannoma, composed of compact spindle cells focally arranged in bundles (top) and the type B pattern area (bottom). Note the tumour cells arranged loosely on a dense fibrocollagenous focally hyalinized stroma (haematoxylin and eosin stain; original magnification x 100).
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Figure 10. Photomicrograph showing positive immunohistochemical staining for S-100 protein (original magnification x 400).
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Discussion
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The enigmatic histogenesis of schwannomas not arising from cranial nerves
Because the cerebral parenchyma and olfactory and optic nerves are generally free from Schwann cells, intracranial schwannomas arise only from the sheath of cranial nerves other than the olfactory and optic nerves. However, none of the four cases in our series showed any connection to the stems of cranial nerves. Of the cases reported to date, these tumours have been reported to arise from the intracerebral parenchyma and the floor of the skull. They are also primarily located in the midline and periventricular structures [2], and even arise from the temporal lobe and the floor of the anterior cranial fossa [3]. The histogenesis of intracranial schwannomas not arising from cranial nerves has been puzzling. Several theories have been proposed to explain the possible origin of these tumours. At present, there are two prevailing theories. One theory suggests a developmental origin, whereas the other suggests that they are more likely to be non-developmental in origin [3]. The former theory suggests that these tumours arise from aberrant Schwann cells in the central nervous system (CNS) parenchyma. Russel and Rubinstein [4] suggested that mesenchymal pial cells may transform into ectodermal Schwann cells, thus accounting for their presence in the CNS parenchyma. Feigin and Ogata [5] found peripheral type myelin in pathological conditions such as multiple sclerosis and cerebral infarctions, and presumed that peripheral-type myelin arose from Schwann cells that differentiated from connective tissue. In addition, they found that displaced neural crest cells may form foci of Schwann cells and then transform into myelin sheath during embryogenesis [5], which could explain the development of aberrant intracranial schwannomas. The young age of most patients reported in the literature (most below 25 years of age) and the tendency of the tumours to arise from midline structures suggested the possibility of a congenital basis for some schwannomas [3]. The occurrence of Schwannia differentiation observed in a complex neuroepithelial tumour may also indirectly support this hypothesis [8]. However, the mean age of the four cases in our series was 45 years (range 36–61 years), similar to that of common schwannomas.
The latter theory postulates that intracranial schwannomas not arising from the stems of cranial nerves originate from Schwann cells that are normally present on adjacent structures. Schwann cells exist within perivascular nerve plexuses around large arteries in the subarachnoid space. Similar nerve plexuses accompany intrinsic arteries of the spinal cord parenchyma [6]. Although the existence of these structures deep within cerebral tissue remains controversial, Schwann cells are clearly present in adrenergic nerve fibres innervating cerebral arteries. Additionally, Schwann cells are associated with meningeal branches of the trigeminal and glossopharyngeal nerves innervating the anterior cranial fossa and olfactory groove. This might be the anatomical basis for schwannomas [7]. Three of the four cases in our series occurred on the floor of skull, including the olfactory groove in two cases. We concluded that these intracranial schwannomas may arise from the meningeal branches of trigeminal nerves. However, during surgery, the tumour was seen to have extended through the meninges, and the extra-axial portion had become contiguous with the intracerebral portion, so it was not possible to discern the origin of these tumours. Several authors postulated that it may be difficult to distinguish schwannomas from cystic meningiomas based on clinical symptoms, neuroimaging or histological features (using routine haematoxylin and eosin staining) [3].
Neuroradiological imaging
In the reported cases, the schwannomas not arising from cranial nerves demonstrated hypointense to isointense mixed dense masses, with large areas of cystic degeneration and necrosis. The solid component of the tumours showed moderate to remarkable enhancement after contrast medium. The tumours showed hypointense signal on T1 weighted images and inhomogeneous hyperintense signal on T2 weighted images. The solid portion and the wall enhanced greatly after Gd-DTPA, and the border was distinct. The salient radiological feature in the reported cases was a very frequent cystic component. The CT and MRI findings in our four cases are similar to the features mentioned above, particularly in the three tumours located on the floor of the skull with large areas of cystic degeneration. These imaging features are similar to schwannomas arising from cranial nerves. Because this kind of tumour is extremely rare and the location is not typical, it is often misdiagnosed as another tumour, such as cystic meningioma or glioma. All four cases, except one (Case 4) arising from intracerebral parenchyma, occurred at the bottom of the brain without cranial nerve connection. In addition, the tumours outstripped the meninges and extended into the brain, making it difficult to discern the origin of the tumours on CT and MRI, adding to the difficulty in diagnosis. Schwannomas not associated with cranial nerves mentioned in some reports arose from the midline structures and tended to occur in children and adolescents [1]. However, except one case in our series occurring in the septum pellucidum, the tumours arose from the bottom of the brain without cranial nerves, and the mean age of the patients was 45 years, much older than other cases reported in the literature. Some reports thought that the cystic degeneration rate (54.5%) of intracerebral schwannomas was much higher than that of acoustic schwannomas (ranging from 9.6% to 20.5%) and believed that it was the characteristic of intracerebral schwannomas to form cysts in masses [1]. All our cases contained areas of cystic degeneration and necrosis, similar to the description in the literature. In fact, most of the larger schwannomas of cranial nerves reveal areas of cystic degeneration and necrosis. Schwannomas not associated with cranial nerves are similar. We believe that, apart from location, the imaging features of both moieties are similar. Schwannomas not arising from cranial nerves should be distinguished from meningiomas and gliomas. Meningiomas are extracerebral tumours, in which cystic degeneration is rare and they usually display a dural tail after contrast medium administration. Although we noticed that there was no enhanced meningeal tail sign in the two cases misdiagnosed as meningiomas in our series, we were not aware that schwannomas could arise away from the stems of cranial nerves, so they were misdiagnosed as meningiomas. Gliomas are intracerebral tumours. Although they often contain necrosis and cystic areas, the tumour outlines are ill defined. Furthermore, gliomas near the bottom of the brain are rare. Although the tumour in Case 4, which arose from the septum pellucidum with atypical location and signs, is very difficult to distinguish from central neurocytoma, central neurocytoma tends to arise from the region of the foramina of Monra, with extension into the ventricles. The diagnosis of Case 4 should be confirmed by pathological examination. In summary, intracranial schwannomas may, on rare occasions, be located in atypical locations away from the course of cranial nerves from which they normally arise. If a lesion displays the typical imaging features of a schwannoma, its atypical location should not preclude the inclusion of schwannoma in the differential diagnosis.
Received for publication December 1, 2003.
Revision received May 27, 2004.
Accepted for publication August 13, 2004.
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Abstract
Introduction
