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
2 Dermoid 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.
image

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

HISTOLOGY

• Primary arachnoid cyst shows arachnoid lining the outer cystic wall (Figs. 5-2 and 5-3).
• The cystic wall is sometimes duplicated.
• In the cystic wall, no vascularity is observed.
• No significant inflammatory changes are seen.
• Secondary arachnoid cysts comprise fibrous connective tissue.
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Fig. 5-2 The outer wall of arachnoid cyst consists of delicate arachnoid membrane sparsely populated by meningothelial cells (hematoxylin–eosin stain, ×40).

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Fig. 5-3 High-power view of Fig. 5-2. A single layer of bland arachnoid cells is seen (hematoxylin–eosin stain, ×200).

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).
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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.

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

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

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

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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.
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