Surgical treatment of a syringomyelia associated with an idiopathic arachnoid malformation disclosed by preoperative MRI

Spine

Surgical Treatment of a Syringomyelia Associated with an Idiopathic Arachnoid Malformation Disclosed by Preoperative MRI Michae¨l Bruneau, M.D.,* Thierry Duprez, M.D.,† Denis Rommel, M.D.,* and Christian Raftopoulos, M.D., Ph.D.* *Department of Neurosurgery, and †Department of Radiology and Medical Imaging. Cliniques Universitaires Saint-Luc, Universite´ Catholique de Louvain, Brussels, Belgium

Bruneau M, Duprez T, Rommel D, Raftopoulos C. Surgical treatment of a syringomyelia associated with an idiopathic arachnoid malformation disclosed by preoperative MRI. Surg Neurol 2004; 62:552–5. BACKGROUND

We describe the very rare condition of an idiopathic spinal arachnoid malformation associated with syringomyelia (SM) and depicted on preoperative magnetic resonance imaging (MRI) whose features were confirmed at surgery. CASE DESCRIPTION

A 34-year-old female suffered from progressive gait impairment because of lower limb palsy and sensory disturbances. MRI demonstrated a bulging membrane at the T6 level that was transversely stretched between the dorsal aspect of the spinal cord and the posterior dura mater. At this level, the spinal cord appeared atrophic and pushed anteriorly against the dura with enlargement of the posterior subarachnoid spaces (SAS) and focal collapse of an associated panmedullar SM. Surgery consisted in releasing the arachnoid malformation and opening the inferior segment of the syringomyelic cavity. Pathological examination revealed a fibro-sclerotic tissue with cellular areas of meningo-endothelial cells. Postoperative neurological status progressively improved but slightly. Three-months and 1 year postoperatively, MRI showed the collapse of the whole SM and restoration of cerebrospinal fluid (CSF) flow at the treated T6 level. CONCLUSION

Spinal arachnoid malformations associated with SM are very rare and have never been described up to now on MRI. Surgical removal of the causative malformation allows spinal cord decompression and prevents the recurrence of the SM by restoring normal CSF circulation. © 2004 Elsevier Inc. All rights reserved. KEY WORDS

Arachnoid malformation, idiopathic syringomyelia, occult arachnoid abnormalities. Address reprint requests to: Raftopoulos Christian, M.D., Ph.D., Cliniques Universitaires Saint-Luc, Service de Neurochirurgie, Avenue Hippocrate, 10, 1200 Brussels, Belgium Received November 23, 2003; accepted December 31, 2003. 0090-3019/04/$–see front matter doi:10.1016/j.surneu.2003.12.016

yringomyelia (SM) is an intramedullar cavity containing cerebrospinal fluid (CSF). If we consider the causal mechanism, we differentiate 3 types of intramedullar cavities: atrophic, neoplasic, and resulting from CSF circulation disturbances [6]. Based on MRI-histopathological correlates and on CSF circulation, sub-types of communicating and noncommunicating SM can be distinguished. Primary parenchymal cavitations are mainly because of spinal cord trauma and ischemic or hemorrhagic medullar strokes. Noncommunicating SM can be central canal dilation, primary intraparenchymal medullar cavities, or both. CSF obstacles responsible for SM can be Chiari malformation, acquired tonsillar herniation, extramedullary compression (basilar invagination, spondylosis, discal hernia, tumors, and cysts), spinal arachnoiditis (traumatic or infectious) or tethered cord. Idiopathic SM refers to the remainder cases in which the causative mechanism is still undetermined after comprehensive clinical and radiologic work-up [4]. We report a very rare case of spinal arachnoid malformation associated with a panmedullar SM. Magnetic resonance imaging (MRI) enabled preoperative diagnosis that has never been reported before, and peroperative findings completely matched the MRI features.

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Case Report HISTORY AND EXAMINATION A 34-year-old female, without any past medical or traumatic history, complained of progressive gait disturbance for the past 18 months. Clinical examination revealed a paresis of the right foot and its lateral peroneal muscles, graded at 4/5. Hypoesthe© 2004 Elsevier Inc. All rights reserved. 360 Park Avenue South, New York, NY 10010 –1710

Syringomyelia and Arachnoid Malformation

Preoperative MRI. (A) Mid-sagittal T2-weighted fast spin echo (FSE) image. Syringomyelia is observed from C3 to T11. At T6 level, syringomyelia stenosis is related to a membrane bulging upwards. (B) Magnified view of previous image. The arachnoid malformation (arrow) extended from the spinal cord to the dura mater, obstructing the perimedullar SAS. Cord is enlarged down to the membrane, Below the membrane, cord cavity is collapsed, posterior SAS are enlarged and cord is displaced anteriorly. (C) Transverse T2-weighted FSE, immediately below the transverse membrane. The spinal cord is atrophic and applied against the anterior dura. The posterior SAS are enlarged.

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sia to prick pain suspended from T5 to T10 and a global hypoesthesia of the left lower limb were also noted. MRI revealed a SM extending from C3 to T11 (Figure 1). Focal narrowing of the cord was observed at T6 level together with the presence of a bulging membrane bridging the dorsal aspect of the cord and the posterior dura. Anterior displacement of the cord, enlargement of the posterior SAS and collapse of the cavity were present below the membrane. CSF-flow related artifacts were not observed within enlarged posterior SAS. OPERATION At surgery, after T6-T7 laminectomy and dura opening, the thoracic spinal cord visible through the arachnoid appeared focally compressed by a circular arachnoid malformation consisting of a thick white arachnoid membrane extending circumferentially from the dura to the pia mater and maintaining ventrally the spinal cord (Figure 2). CSF flow blockage was complete by this diaphragm-like arach-

Operative views. (A) The arachnoid malformation is a thickened diaphragm-like arachnoid membrane obstructing the normal perimedullar space. (B) after release of the arachnoid malformation, re-inflation of the constricted spinal cord is allowed.

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noid malformation that moved up and down with CSF pulsations. Surgery consisted in the removal of the malformation from the dura and then from the pia mater. After complete removal of the malformation, the spinal cord inflated and CSF flow was restored around the spinal cord. Surgical exploration did not reveal any defect of the anterior dura mater, which could have been associated with a spinal cord herniation. At the end of the procedure and because the inferior part of the SM was visible through a transparent wall, the cavity was opened by a short incision of a few millimeters. Pathologic examination of the lesion revealed a fibro-sclerotic tissue with cellular areas of meningo-endothelial cells. The sample was free of any inflammatory or tumoral process. POSTOPERATIVE COURSE Three months after surgery, the clinical examination was stable. Follow-up MRI demonstrated disappearance of the membrane, collapse of the whole SM, and reappearance of flow-related artifacts throughout the spinal SAS (Figure 3B). After 1 year, neurologic deficits had slightly improved and the

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Postoperative MR monitoring: comparative mid-sagittal T2-weighted FSE images. (A) preoperative status. See comment in Figure 1. (B) Three months postoperative. The surgical approach is observed. Syringomyelia has shrunk but small cavities are still present. CSF space is restored within the subarachnoid compartment and hypointense CSF flow-related artifacts are well detected upward and below the operated level. (C) One-year postoperative. Shrinkage of the cavity has increased and flow artifacts are present.

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SM was even more collapsed with persistent CSFflow artifacts on MRI (Figure 3C).

Discussion This is the first time that a spinal arachnoid malformation potentially responsible for a SM is identified by MRI. The malformation was well defined on T2weighted images and appeared as a bulging membrane extending from the dura to the spinal cord at a level where the spinal cord was atrophic and anteriorly displaced (Figure 1). A syringomyelic cavity was observed above and below the constricting arachnoid malformation. The presence of a medullar stenosis at the middle segment of the syrinx was a major indirect sign leading to the suspicion of an associated perimedullar malformation. Up to now, MRI has failed to detect the primary abnormality, and therefore, authors have advocated myelography for patients with idiopathic SM to disclose occult arachnoid webs, pouches, or cysts [4]. Nevertheless, even using optimized technique of computed tomography (CT) myelography, arachnoid webs can be missed [4,8]. Preoperative identification of such malformation is of major interest, by directing the surgical procedure toward the offending pathology.

In the literature, Mallucci et al stressed the importance of occult arachnoid webs, pouches or cysts at the origin of idiopathic SM and identified 4 cases of arachnoid webs [4]. Paramore reported 2 cases of SM secondary to obstruction of CSF flow by an arachnoid web, raising the hypothesis that web could be a variant of arachnoid cyst responsible for a spinal cord compression. Arachnoid cysts are pockets of CSF adjacent to normal CSF spaces [8]. Two etiologic mechanisms have been hypothesized: first, from the septum posticum, a well developed septum between the posterior dura and the posterior spinal cord, or second, from trapped arachnoid granulations, leading to sequestered CSF secretion [1,9]. Nevertheless, none of these theories have been proven. In this current case, no cyst was identified. In absence of prior trauma, surgery, inflammation, or infection, the arachnoid abnormality was considered congenital. The malformation appeared as a diaphragm around the spinal cord and consisted of a thick arachnoid membrane of normal appearance. The diaphragm-like malformation led to spinal cord tethering with patent blockage of CSF flow that likely led to the enlargement of the dorsal SAS and subsequent spinal cord compression. The presence of a perimedullar obstacle precludes dampening of

gomyelia and Arachnoid Malformation

CSF pressure waves, either by the lumbar sacral sac or by the encephalic SAS, and therefore, CSF can be forced into the spinal cord parenchyma through perivascular and interstitial spaces [5–7, 10]. This mechanism might be responsible for the generation and maintenance of a SM by an increased extracellular CSF flow [2,3]. The main purpose of surgical treatment in such condition is stabilization of the patient’s neurologic status. Clinical recovery does not necessarily parallel the radiologic evidence of syrinx resolution [4]. After identification by MRI, the appropriate treatment is the liberation of the offending malformation to release the spinal cord compression and to re-establish CSF circulation. This procedure may by itself allow the collapse of the intramedullar syrinx. However, in our case, the immediate postoperative shrinking could be alternatively explained by the opening of the syrinx. However, the absence of recurrence, even better cavity resolution and reappearance of persisting CSF flow artifacts 1 year after surgery strongly suggest the major role played by the arachnoid malformation removal. On the contrary, shunting syrinxes are associated with a high rate of recurrence, poor longterm results and sometimes a worsening of the neurologic status [2,4].

Conclusion We have reported a very rare case of a diaphragmlike spinal arachnoid malformation associated with a panmedullar SM and diagnosed for the first time by MRI. Resection of the malformation yielded stable shrinkage of the associated SM by restoring the patency of the perimedullar CSF pathways. REFERENCES 1. Fortuna A, La Torre E, Ciappetta P. Arachnoid diverticula: a unitary approach to spinal cysts communicating with the subarachnoid space. Acta Neurochir 1977;39:259 –68. 2. Klekamp J, Batzdorf U, Samii M, et al. Treatment of syringomyelia associated with arachnoid scarring caused by arachnoiditis or trauma. J Neurosurg 1997; 86:233–40. 3. Li KC, Chui MC. Conventional and CT metrizamide myelography in Arnold-Chiari malformation and syringomyelia. AJNR 1987;8:11–7. 4. Mallucci CL, Stacey RJ. Idiopathic syringomyelia and

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the importance of occult arachnoid webs, pouches or cysts. Br J Neurosurg 1997;11:306 –9. Milhorat TH, Nobandegani F, Miller JI, et al. Noncommunicating syringomyelia following occlusion of central canal in rats: experimental model and histological findings. J Neurosurg 1993;78:274 –9. Milhorat TH. Classification of syringomyelia. Neurosurg Focus 2000;8:Article 1. Oldfield EH, Muraszko K, Shawker TH, et al. Pathophysiology of syringomyelia associated with Chiari I malformation of the cerebellar tonsils. J Neurosurg 1994;80:3–15. Paramore CG. Dorsal arachnoid web with spinal cord compression: variant of an arachnoid cyst? Report of two cases. J Neurosurg (Spine 2) 2000;93:287–90. Perret G, Green D, Keller J. Diagnosis and treatment of intradural arachnoid cysts of the thoracic spine. Radiology 1962;79:424 –9. Raftopoulos C, Sanchez A, Matos C, et al. Hydrosyringomyelia-Chiari I complex. Prospective evaluation of a modified foramen magnum decompression procedure: preliminary results. Surg Neurol 1993;39:163–9.

COMMENTARY

Although the exact mechanism is debated, syringomyelia is most often a result of abnormal CSF flow at the cranio-cervical junction or in the spinal subarachnoid space that causes fluid to accumulate in the center of the spinal cord. Identification of the lesion obstructing flow, such as impacted tonsils at the foramen magnum in the Chiari I malformation or subarachnoid adhesions after spinal trauma, is essential as it allows treatment to be directed at the offending pathology. Surgical correction of the obstruction frequently results in collapse of the syrinx and thus avoids the need for placement of syrinx to subarachnoid or pleural shunt. The authors clearly describe and illustrate a diaphragm-like arachnoid membrane obstructing spinal fluid flow in the thoracic spinal canal causing cord compression and syringomyelia. The syrinx collapsed after lysis of the arachnoid malformation and fenestration of the syrinx through a transparent section of spinal cord. Although it cannot be determined which of the treatments, or possibly both, resulted in collapse of the syrinx, the patient’s positive response supports the authors treatment approach. John J. Oro´, M.D. Department of Neurosurgery University of Missouri-Columbia Columbia, Missouri