Intradural Extramedullary Cystic Lesions

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Chapter 5 Intradural Extramedullary Cystic Lesions

Jae-Soo Koh, Ung-Kyu Chang, Se-Hoon Kim, Terri Haddix

1 Intradural Arachnoid Cyst

3 Epidermoid Cyst

4 Neurenteric Cyst

INTRODUCTION

Cystic masses in the intradural extramedullary space are benign tumors that are commonly encountered. Arachnoid cysts are composed of duplication of the arachnoid membrane and resultant cerebrospinal fluid (CSF) collection. Their occurrence may be congenital or acquired. Dermoid cysts arise congenitally from inclusion of ectodermal elements at the time of neural groove closure, or as an acquired lesion after the introduction of dermal elements into the spinal subarachnoid space as a result of lumbar puncture. Epidermoid cysts develop with a similar mechanism. Neurenteric cysts are uncommon malformed lesions lined with epithelium of endodermal origin, which result from ectoderm–endoderm adhesion.

INTRADURAL ARACHNOID CYST

EPIDEMIOLOGY

• Arachnoid cysts are the most commonly found cystic lesions in the spinal intradural space.

• They are divided into acquired cyst and congenital cyst. Acquired arachnoid cysts are more common and can be the result of spinal cord trauma, postsurgical arachnoiditis, meningeal infection, and other insults that cause inflammation and subarachnoid adhesions.¹

• Congenital or idiopathic spinal arachnoid cysts are diagnosed when there is no antecedent history of trauma or other etiologic factors and the intraoperative macroscopic and microscopic findings show a cyst wall composed of normal or slightly thickened arachnoid tissue.

• Various theories have been proposed to explain the formation of arachnoid cysts.² Perret et al³ proposed that they arise from diverticula in the septum posticum. However, reports of cysts ventral to the spinal cord suggest that there must be another origin.⁴ Incarceration of arachnoid granulations may produce CSF that becomes entrapped in arachnoid diverticula. These sequestered pockets of fluid lead to further disruption of normal pulsatile CSF flow and thus are capable of expanding or producing associated syringomyelia.^5,⁶

DISTRIBUTION

• The regional distribution of the spinal arachnoid cyst is presented in Fig. 5-1.² Most of them are located in the thoracic region (80%), as compared with 15% in the cervical spine and 5% in the lumbar spine.

• Most of the arachnoid cyst is located dorsal to the spinal cord. In a review of the six published series that specified the arachnoid cysts’ location, 12 of 69 (17%) of the cysts were anterior to the spinal cord.^5,⁷

• They may be single or multiple. Men and women are equally affected and usually in the third through fifth decades of life.

Fig. 5-1 Regional distribution of the spinal arachnoid cyst. The thoracic spine level is the most commonly involved segment.

RADIOLOGY

• The primary magnetic resonance imaging (MRI) finding is a spinal space–occupying mass, nonenhancing extramedullary loculated CSF collection.

• T1- and T2-weighted signals of arachnoid cyst are identical to those of CSF. There is no enhancement with gadolinium use.

• Intradural arachnoid cysts are poorly delineated on MRI because the signal intensities of both the cysts and the surrounding CSF space are more or less the same.

• Widening of the posterior CSF space on MRI, with anterior displacement and thinning of the spinal cord, may be the only sign of intradural arachnoid cyst (Figs. 5-4 and 5-5).⁸

• MRI results have been reported showing segmental clear-cut dilatation of the posterior CSF space, at one or two vertebral levels, with focal thinning of the neural structure, thus providing a definite diagnosis of intradural cysts.⁸

• However, because of the chronicity of this disorder, intradural arachnoid cysts may distend very slowly and elongate over multiple vertebral levels.

• Inhomogeneous areas of hypointense T2-weighted signal with respect to the surrounding CSF may be seen. This phenomenon is consistently interpreted as artifacts from aortic pulsations into CSF (see Figs. 5-4 and 5-5).⁹

• Anterior cysts are more likely to cause weakness and myelopathy, whereas dorsal cysts present more commonly with neuropathic pain and numbness (Figs. 5-6 and 5-7).

• Grossly the cyst is transparent and the cystic wall is very thin (Fig. 5-8).

Fig. 5-4 T2 sagittal MR of intradural arachnoid cyst. The enlargement of dorsal cystic lesion is observed and the spinal cord is displaced to the anterior side at the cervicothoracic junction. The signal is isointense with CSF. In the cystic lesion, a focal low signal spot is seen.

Fig. 5-5 T2 axial MR of intradural arachnoid cyst. High signal cyst mass compressed the spinal cord from the dorsal side.

Fig. 5-6 T2 sagittal image of ventral arachnoid cyst. The enlargement of ventral subarachnoid space is observed.

Fig. 5-7 T2 axial MR of ventral arachnoid cyst. The cystic mass occupies the ventral half of the intradural space.

Fig. 5-8 Intraoperative view of ventral arachnoid cyst. After dural opening, the cystic mass is seen over the spinal cord. With a small catheter, cyst aspiration is tried.

DERMOID CYST

EPIDEMIOLOGY

• Dermoid cysts are slow growing, congenital benign tumors of the central nervous system.

• They arise from inclusion of ectodermal elements at the time of neural groove closure, which normally occurs between the third and fifth weeks of embryonic life.

• They might also occur after the introduction of dermal elements into the spinal subarachnoid space as a result of lumbar puncture, surgery, or trauma.¹⁰

• Intraspinal dermoid cysts are commonly associated with mesodermal malformations.

• Combined malformation is often observed when the lesion involves the vertebrae (e.g., hemivertebrae, absent vertebrae, fused vertebrae, butterfly vertebrae, or midline bony spurs) and is associated with dermal sinuses, myelomeningocele, syringomyelia, and repeated lumbar punctures.¹¹

• Dysembryogenic tumors are, on rare occasions, found in adult patients without dysraphism. They may be intradural (25%) or extradural (75%).

• Dermoid tumors account for 0.1–0.7% of all spinal tumors with a cranial to spinal ratio of 6:1, and they are 13–17% of all primary spinal cord tumors between 1–15years of age.¹²

DISTRIBUTION

• Dermoid cysts represent less than 1% of intraspinal tumors and might be intramedullary, intradural-extramedullary, or extradural.¹¹ The extradural location is the most common site (75%).

• They occur predominantly in the lumbosacral region (60%), involving the cauda equina and conus medullaris, and are quite rare in the upper thoracic (10%) and cervical regions (5%).¹³

• Dermoid cysts generally present at an average age of 15 years with a female predominance.

HISTOLOGY

• On the operative finding, the mass is severely adhered to the nerve root.

• Microscopic examination shows stratified squamous epithelium with hair follicles, sebaceous glands, and sweat glands (Fig. 5-9).

• Grossly, the mass is usually yellow and lobulated (Fig. 5-10).

Fig. 5-9 Dermoid cyst with keratin-filled cyst and skin adnexal tissue, including sebaceous glands and sweat glands (hematoxylin–eosin stain, ×40).

Fig. 5-10 Photograph of the specimen showing multiple, lobulated, yellowish masses and hair-like materials.

RADIOLOGY

• MR findings of dermoid tumors show a varied appearance depending on the fat content of the tumor. They are commonly visualized as heterogeneous lesions with very high signal intensity on T1-weighted images (T1WI) and low signal intensity on T2-weighted images (T2WI).¹⁰

• The inhomogeneous signal on long relaxation time/long echo time images from the more solid core of the mass reflects the mixture of the intact and partially degenerated materials of the hair, glandular elements, and more solid cholesterol.¹⁰

• Fat droplets in the subarachnoid spaces appear hyperintense on T1WI and hypointense on T2WI.

EPIDERMOID CYST

EPIDEMIOLOGY

• Epidermoid cysts represent less than 1% of all intraspinal tumors.¹⁴

• The origin of spinal epidermoids can be congenital or acquired.

• Forty percent of epidermal cysts are acquired.

• The more common is an anomalous implantation of ectodermal cells during closure of the neural tube between the third and fifth week of embryonic life.¹⁴

• Another cause is the displacement of epithelial tissue secondary to previous lumbar puncture. The incidence of this phenomenon has decreased significantly in the last 25 years, but sporadic case reports still exist.¹⁵

• Growth of epidermoids and dermoids was confirmed after direct skin implantation along the neuraxis of newborn rats.¹⁵

• The time duration between the lumbar puncture and the onset of the symptoms related to the epidermoid tumor ranges from 2–23 years.

DISTRIBUTION

• Acquired lesions are found most commonly in the intradural, extramedullary space of the lumbosacral region.

• Congenital lesions are evenly distributed throughout the spine. They are associated with the dermal sinus in 20% of cases.

HISTOLOGY

• A well-demarcated plane is noted between the tumor and the spinal cord. Grossly, the lesion is flaky and pearly white.

• Intraoperative spillage of the cyst content (e.g., keratin debris) during removal may lead to a prolonged chemical meningitis and subsequent arachnoiditis. Great concern should be taken not to contaminate the intradural space with cyst content.

• The histologic examination reveals multiple layers of stratified squamous epithelium and anucleated squames in the cyst (Fig. 5-11). Rupture of cyst evolves foreign body type granulomatous inflammation (Fig. 5-12).

Fig. 5-11 Epidermal cyst lined by squamous epithelium; granular layer is evident (hematoxylin–eosin, ×100).

Fig. 5-12 Cyst filled with keratin debris. The cyst is ruptured and evolves foreign body reaction at surrounding connective tissue (hematoxylin–eosin stain, ×40).

RADIOLOGY

• Radiographic characteristics of epidermoids typically show a displacement of nerve roots and the spinal cord in the subdural space.

• T1- and T2-weighted signal on MRI usually varies from hypointense to isointense but may occasionally have increased intensity on T2WI (Figs. 5-13 to 5-18).¹⁶

• This variation in signal characteristics, which might be related to the chemical state of cholesterol or the relative composition of cholesterol and keratin, makes the preoperative diagnosis in the spinal lesions difficult.

• Differential diagnosis is considered with nerve sheath tumor, meningioma, ependymoma, paraganglioma, and intradural lipoma.

Fig. 5-13 T2 sagittal MR of epidermoid cyst. High signal cystic mass is outlined with low signal margin at L3–4 level.

Fig. 5-14 T1 sagittal MR of epidermoid cyst. Mixed signal of iso-intensity and hypointensity is seen in the tumor mass.

Fig. 5-15 Enhanced T1 sagittal MR of epidermoid cyst. With contrast, the outline is brightened and the iso-signal portion is enhanced. The low signal portion is not enhanced.

Fig. 5-16 T2 axial MR of epidermoid cyst. Round, high signal mass is located in the intradural space. The cauda equina is compressed and displaced to the anterior side.

Fig. 5-17 T1 axial MR of epidermoid cyst. The signal of the mass is mixed. The isointense portion is located in the posterior side of the cyst.

Fig. 5-18 Enhanced T1 axial MR of epidermoid cyst. The cyst wall and posterior portion are enhanced.

NEURENTERIC CYST

EPIDEMIOLOGY

• Neurenteric (NE) cyst is an uncommon developmental lesion lined with epithelium of endodermal origin. This malformation is the result of ectoderm–endoderm adhesion.¹⁷

• Approximately one-half of the patients with NE cyst are known to have spinal anomalies, including anterior spina bifida, Klippel–Feil anomaly, other segmentation and fusion anomalies, spinal canal enlargement, diastematomyelia, and posterior spina bifida.^18,¹⁹

• Their occurrences have been reported by other authors but are rare, comprising 0.7–1.3% of all spinal cord tumors and 16% of cysts.¹⁶ About 150 cases with histologic verification have been reported in the literature.¹⁷

• These cysts occur in men 1.5–3 times as often as in women and usually present in the second and third decades of life.

DISTRIBUTION

Intradural NE cysts are usually located ventral to the spinal cord and are extramedullary lesions.^18,¹⁹ It also is well known that they are commonly located in cervical, cervicothoracic, and thoracic spine regions.¹⁹

HISTOLOGY

• On operation field, the consistency of the tumor mass is jelly-like (Fig. 5-19).²⁰

• A simple epithelium rests on a delicate fibrovascular tissue.

• A columnar epithelium is lined along the cystic wall (Fig. 5-20, A).

• Submucosal glands of serous and mucinous types may be seen in the specimen.

• Ciliated columnar cells and mucin-containing columnar cells are seen (Fig. 5-20, B).

Fig. 5-19 Intraoperative photograph of intramedullary neurenteric cyst. When the bulged dura is opened, a bluish cyst wall appears under the transparent pia mater (A). After posterior midline myelotomy, jelly-like contents are seen (B).

Fig. 5-20 Photomicrograph of the intramedullary neurenteric cyst of the spinal cord. The cyst is lined by non-ciliated cuboidal and columnar epithelial cells. The cells contain apical mucin vacuoles that show a strong positive reaction for periodic acid–Schiff (PAS) stains. The wall of the cyst is composed of loose fibrous tissue (A, hematoxylin–eosin stain, ×100; B, PAS, ×400).

RADIOLOGY

• Most intraspinal NE cysts are intradural, extramedullary cystic lesions. Rarely, extraspinal extension through the dural defect has been reported. Intramedullary location also has been reported.

• MR findings are low signal intensity on T1WI and high signal intensity on T2WI. They look like cerebrospinal fluid. NE cysts do not show gadolinium enhancement (Fig. 5-21).

• Differential diagnosis should be considered for cystic intradural extramedullary tumors such as neurilemmoma, epidermoid, or dermoid cysts.

• It is an important differential point that NE cysts have the tendency of ventral location compared with the other cystic tumors.

Fig. 5-21 MR images of C6–7 intramedullary neurentric cyst, T2 sagittal (A), T2 axial (B), and postcontrast sagittal (C) images. The cystic mass is located ventral to the spinal cord and is seen to compress it. There is no enhancing part of the cystic lesion. Bony and soft tissue abnormalities are not seen.

Adapted from J Korean Neurosurg Soc 2004;36:249–253.

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