Case Reports
Epidermoid Cyst of the Fourth Ventricle: A Case Report
中華放射醫誌　Chin J Radiol 1999；24(2)：77-80
Yuh-Feng Tsai1　Kou-Mou Huang1　Hon-Man Liu1　Yiu-Wah Li1　Ming-Ta Tsou2
Department of Medical Imaging1 and Pathology2, College of Medicine, National Taiwan University Hospital, Taipei, Taiwan

ABSTRACT
　Intracranial epidermoid cysts are uncommon benign congenital tumors and rarely occur in the fourth ventricle. In this article, we illustrate a typical presentation of fourth ventricular epidermoid cyst on magnetic resonance images. The clinical symptomatology, neuroimages, and management of intracranial epidermoid cysts are discussed.

Key words: Epidermoid cyst; Brain, ventricles; Brain, MR

INTRODUCTION
　It has been estimated that the epidermoid cysts (EC) constitute from 0.2-1.8% of all intracranial tumors [1-5]. Involvement in the fourth ventricle accounts for between 5% to 18% in most series [6,7]. The clinical symptoms of intracranial EC vary widely depending on the tumor sizes, locations, and involvement of the intracranial neurovasucular structures [1, 2, 7, 8].
　On magnetic resonance imaging (MRI), the EC usually appear inhomogeneous and hypointensive on T1-weighted image (T1WI), inhomogeneous and hyperintensive on T2-weighted image (T2WI), with no enhancement after contrast medium infusion [1, 2, 4-6, 9]. Nowadays, it has been well accepted that MRI provides better definition of the anatomical structures and better images of tumor extension than computed tomography (CT), thus permitting better pre-operative surgical planning [2, 4, 5, 9]

CASE REPORT
　A 37-year-old female patient had been bothered by mild headaches for ten years and intermittent vertigo for two years. Physical examination results were essentially normal except for bilateral nystagmus at lateral gaze (Burns-Cushing type) indicating the possibility of a cerebellopontine (CPA) lesion. No evidence of cranial nerve dysfunctions, gait ataxia or papilledema was found.
　Magnetic resonance imaging (MRI) was performed at another hospital prior to admission to our hospital and disclosed a lobulated cystic tumor within the fourth ventricle. On T1WI, the tumor appeared inhomogeneous with low signal intensity (SI) which was relatively hyperintensive as compared with that of the cerebrospinal fluid (CSF) (Fig. 1A, 1B), and no enhancement was seen after contrast medium infusion (Fig. 1C). Though mass effects on the dorsal aspect of the brainstem and on the vermis were significant, hydrocephalus was not evident (Fig. 1C). On T2WI, the tumor appeared inhomogeneous with high SI but without evidence of peritumoral edema (Fig. 1D, 1E).
　A surgical approach through the suboccipital craniotomy was performed thereafter. A pearly white cystic tumor, about 3x3x3 cm, which was soft, well-encapsulated, multi-lobulated, and typical of an epidermoid cyst, was found in the fourth ventricle with brainstem adhesion and foramen Magendie extension. Histopathologic test results confirmed the diagnosis of an epidermoid cyst (Fig. 2).



1a
1b
1c

1d
1e		  
Fig. 1. MRI at 1.0 T. Precontrast axial T1WI (Spin echo TR/TE/excitation = 500/15/1) at the level of the medulla oblongata a. and coronal TIWI b. show a heterogeneous low SI lesion, slight hyperintensity to that of CSF, in the 4th ventricle (large arrows). Bilateral tonsillar herniations (small arrows) are present. Postcontrast sagittal c. T1WI show no enhancement of the tumor after contrast infusion. There is a remarkable mass effect on the midbrain, the pons, and the medulla oblongata without evidence of hydrocephalus. Axial T2WI (TR/TE/excitations = 4000/110/2) at the level of the medulla oblongata d. and the pons e. demonstrate a lobulated mass in the 4th ventricle with inhomogeneous high SI. No evidence of peritumoral edema in the brainstem and the cerebellum was found.






2a
2b
Fig. 2. Photomicrography, a. H & E stain x 100 and b. H & E stain x 400. The tumor has a wall nearly identical to the epidermis (white arrows) and is filled with laminated strands of keratin (large black arrows). The granular layer (arrow heads) which contribute to the process of keratinization and the choroid plexus (small black arrows) outside the tumor are shown.



DISCUSSION
　Intracranial epidermoid cysts, also known as pearly tumors, result from ectopic inclusion of epithelial cells during closure of the neural tube between the third to fifth weeks of fetal development. They are benign, slow-growing tumors with preferred sites in the CPA (about 60%), subarachnoid cisterns, para-pituitary region and on the basal surface of the brain [1, 3, 6, 8]. Two peaks of incidence were recognized in the second and fifth decades of life without gender predilection [1]. Composed of an inner stratified epithelium and covered by an external capsule, the tumor grows through the accumulation of the desquamated epithelial cells and their breakdown products, keratin and cholesterin [2, 3, 6, 8, 10]. Like that of healthy human skin, it has been estimated the EC grow slowly with a linear growth rate of 1cm per year [7, 11, 12].
　Most patients with intracranial EC have a long clinical history of medical problems that can last for several years or even longer due to benign slow-growing nature of the cysts [1, 2]. This fact explains the considerable size of the cysts when found [5]. Although the clinical symptoms vary widely depending on the tumor location and extension, the predominant signs are cranial nerve palsy and cerebellar dysfunction [1, 2, 7]. In most patients with CPA epidermoids, the prevalent symptoms are headache, hypoacousia, gait ataxia, trigeminal neuragia, facial paresthesia, facial palsy, vertigo, and seizure [1, 2, 7, 8]. With parasellar or mesencephalic extension, visual disturbance or hemiparesis may occur [1, 2]. Headache, gait ataxia, and signs of local compression of the brainstem are commonly observed in the patients with fourth ventricular EC [1, 2, 7]. However, hydrocephalus is not a common feature because EC have loose capsules, allowing the entry of CSF between the interstices of the tumor capsules [7]. Temporary remission of symptoms, possibly due to emergence of the tumor from fourth ventricle into the foramina of fourth ventricle, have been reported [7].
　Neuroimaging plays an important role in the adequate diagnosis and detection of the location and extension of EC. Previously, pneumoencephalography and CT cisternography were applied for the diagnosis and evaluation of the EC, however both are invasive. On CT scans, EC typically are hypodense without enhancement after contrast medium infusion. Rarely, they show iso-dense or hyperdense (hyperdense epidermoid) [6, 12]. The components of hemosiderin and ferrocalcium within the tumors may explain their hyperdensity on CT [6]. CT also plays an important role in detecting calcifications. However, MRI stands out from other forms of imaging because it offers better anatomical definition that makes pre-operative planning more precise [2, 4, 5, 9]. Moreover, the relation of the cysts to healthy brain tissues, blood vessels, and the skull base can be evaluated. On MRI, the EC present some peculiar characteristics. Surrounded by thin and smooth rims, they usually show inhomogeneous low SI on T1WI and inhomogeneous high SI on T2WI [1, 2, 4-6, 9]. The reason the cholesterin-rich EC appear hypointensive instead of hyperintensive on T1WI is still puzzling. Ahmadi et al. reported that the protein concentration instead of the cholesterol or triglyceride concentration of intracranial cystic lesions is relative to the high SI on T1WI [14, 15]. On the other hand, Tampieri et al. and Vion-Dury et al. postulated that either the heterogeneity of the tumor contents (mixture of microcalcifications, keratin, cholesterin, epithelial debris, and CSF) or the chemical state of the cholesterin itself (in crystalline form) could explain the low SI of EC on T1WI [4, 5]. However, like that of craniopharyngioma, the cysts are sometimes hyperintensive on T1WI (white epidermoid). Horowitz et al. reported three case of white EC with high concentrations of triglycerides [16]. In addition, the cysts frequently insinuate themselves into the subarachnoid spaces and neurovascular structures with absence of peritumoral edema (related to the slow growth)　[4,5]. Therefore, the absence of peritumoral edema, the typical MR imaging, and the lack of hydrocephalus reminded us of the diagnosis of a fourth ventricular epidermoid in this patient.
　Controversy persists between the use of aggressive and conservative surgical approaches to treat the lesions. Many authors agree that complete tumor resection should not be attempted at the risk of injury to the neurovascular structures [2]. The spilling of tumor materials during surgery should be avoided, as it may cause aseptic menigitis, distant seeding, or death [1]. Surgical risks and recurrent rates are even higher for the fourth ventricular EC because of their frequent adhesion to the brainstem [1, 7]. Post-operative follow-up studies with MRI or CT cisternography are suggested. The MRI, though relatively less accurate in detecting smaller fragments of cholesterin when compared with CT cisternography, seems preferred by the patients and physicians because it is non-invasive [12]. Recently, neuroendoscopic approach to the intraventricular lesions has been applied which is believed to be safer and more effective [17]. Maybe, it will be the treatment of choice for patients with fourth ventricular tumors in the near future.

REFERENCES
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  10. Fein JM, Lipow K, Taati F, Lansem T. Epidermoid tumors of the cerebellopontine angle diagnostic value of computed tomographic metrizamide cisternography. Neurosurgery 1981; 9: 179-182
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  15. Ahmadi J, Destian S, Apuzzo ML, Segall HD, Zee CS. Cystic fluid in craniopharyngiomas: MR imaging and quantitative analysis. Radiology 1992; 182: 783-5
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  17. Gaab MR, Schroeder WS. Neuroendoscopic approach to intraventricular lesions. J Neurosurg 1998; 88: 496-505

第四腦室內的表皮樣囊腫：病例報告
蔡裕豐1　黃國茂1　廖漢文1　李瑤華1　鄒明達2
國立臺灣大學醫學院附設醫院　影像醫學部1　病理部2
　　顱內的表皮樣囊腫(epidermoid cyst)是一種不常見的先天性腫瘤。回顧文獻資料，發生在第四腦室內的比例甚低。此篇文章主要在報告一個第四腦室內的表皮樣囊腫，其典型的磁振造影表現，並且進一步探討顱內表皮樣囊腫的臨床表徵、影像表徵、以及臨床上的處理方式。

關鍵詞： 表皮樣囊腫；腦，腦室；腦，磁振造影
Received: December 30, 1998　　 Accepted: April 7, 1999
Correspondence to: Dr. Kou-Mou Huang
Department of Medical Imaging, College of Medicine, National Taiwan University Hospital
7, Chung-Shan South Road, Taipei, Taiwan, R.O.C.
FAX: 02-23224552