Neurosurg Focus 6 (5):Article 9, 1999 Extensive arachnoid ossification with associated syringomyelia presenting as thoracic Case report and review of the literature

Konstantin V. Slavin, M.D., Randall R. Nixon, M.D., Ph.D., Gary M. Nesbit, M.D., and Kim J. Burchiel, M.D., F.A.C.S. Departments of Neurological , Pathology, and Radiology, Oregon Health Sciences University, Portland, Oregon

The authors present the case of a patient in whom progressive thoracic myelopathy was caused by the extensive ossification of the arachnoid membrane and associated intramedullary syrinx. Based on their findings and the results of a literature search, they describe a pathological basis of this rare condition, discuss its incidence and symptomatology, and suggest a simple classification of various types of the arachnoid ossification. They also discuss magnetic resonance imaging features of arachnoid ossification and associated changes. Emphasis is placed on the particular value of plain computerized tomography, which is highly sensitive for detecting intraspinal calcifications and ossifications, in the diagnostic evaluation of patients whose clinical picture indicates progressive myelopathy. Key Words * * ossification * myelopathy * syringomyelia * thoracic spine

Of the various causes of , calcification and ossification of the arachnoid membrane are relatively rare. It appears that there are three distinct types of calcification, of which two carry some clinical significance due to the production of symptoms. The first type, with microscopic calcifications, is frequently encountered in asymptomatic patients during spine and in autopsy series of the general population. The second type involves formation of large isolated calcified and ossified plaques within, and adjacent to, the arachnoid membrane. This entity has been repeatedly described in patients presenting with signs of spinal cord compression, some of whom improved after undergoing surgical removal of the plaque(s). The third type is the least common, consisting of diffuse ossification of the arachnoid membrane at one or several segments of the spinal cord and may be associated with larger ossified masses such as those encountered in the second type. Because of the relative rarity of these conditions, it is not known whether all three types are the consequent stages of the same progressive disease. The development of modern imaging techniques raises the possibility that patients with this disease may be followed to determine the entity's natural history.

Unauthenticated | Downloaded 10/02/21 07:26 AM UTC In this report we describe a patient in whom diffuse arachnoid membrane ossification at the thoracic level was pathologically and radiologically verified, and we review the current literature on this subject. CASE REPORT This 54-year-old woman presented with a 9-year history of progressive spastic lower paraparesis and bilateral sensory loss in her legs that extended to the T-8 level. History. Without apparent reason, the patient began to experience symptoms of pain, tightness, and discomfort in the left side of her chest. She had not sustained any major , , systemic infection, or subarachnoid hemorrhage, and she was otherwise healthy. At the time of her initial neurological evaluation in 1991, she was found to have decreased left-sided sensation to pin-prick stimulation and temperature from the level of T-8 to the knee. Mild impairment of the superficial sensation on the right side of the chest wall was also noted. The patient was found to have a subtle decrease in strength and coordination in the left foot as compared with the right, with a positive Babinski sign on the left. At that time, magnetic resonance (MR) imaging of the thoracic spine revealed a large multiseptated syrinx that extended from T-7 to the conus medullaris at the L2­3 level (Fig. 1). No mass lesions, herniated discs, or other abnormalities were demonstrated, and no abnormal enhancement was observed anywhere within the spinal cord. There was no tonsillar herniation.

Fig. 1. Axial T1-weighted MR images. Left: Image obtained at the T-11 level revealing a hypointense intramedullary cavity (wide arrow) and bright hyperintense mass adjacent to the dorsolateral surface of the spinal cord (short arrow). Right: Image obtained at the T-8 level demonstrating large intramedullary syrinx and irregular hypointense rim (arrows) around the spinal cord suggestive of arachnoid ossification. (Study performed in June of 1992.) The patient gradually deteriorated over the next several years despite undergoing conservative treatment. Her sensory loss became more apparent on the right side, and the spastic paraparesis worsened to the point that she could not walk without holding onto something. The left foot drop was supported by external orthosis. She lost most of her lower-extremity and began using visual reference

Unauthenticated | Downloaded 10/02/21 07:26 AM UTC to maintain balance. Numbness in both legs was associated with a tingling sensation and feeling of tightness. She developed chronic constipation and difficulties initiating urination as well as episodes of urinary incontinence. Repeated MR images obtained on several occasions demonstrated no changes in the syrinx (Fig. 2).

Fig. 2. Sagittal T1 (left) and T2-weighted (right) MR images of the lower thoracic spine revealing a multiloculated intramedullary cavity extending from T8­12. The ossified lesion at T-11 posteriorly shown at Fig. 1 left appears hyperintense on T1 and hypointense on T2-weighted images (arrows). There is an abrupt lack of normal CSF signal at T7­8 posterior to the spinal cord (curved arrow). (Study performed in July of 1998.) In 1997, the patient developed a herpetic rash on the right side of her chest in the area corresponding to the upper edge of her sensory loss. Subsequently, she developed a postherpetic neuralgia with associated hyperesthesia and pain that partially responded to oral carbamazepine therapy. Presentation. The patient continued to work as a school teacher. In need of pain management, she was referred to us for consideration of surgical treatment of her intramedullary syrinx. She was presented with the available therapeutic options and chose to proceed with surgery. Operation. During surgical exploration, a T9­11 laminectomy was performed. After opening the dura, the spinal cord was found to be covered with a light gray hard shell, which appeared to be calcified arachnoid membrane. Using the microscope, we opened it and then carefully peeled it away from the substance of the spinal cord. This shell tracked ventrally on both sides and had to be sharply transected on the left and superiorly, and then pulled out in two irregularly shaped pieces that measured 14 X 8 mm and 6 X 8 mm, respectively. It appeared to be circumferentially encasing the spinal cord like an armor but did not follow the exiting nerve roots. In the superior (cephalad) aspect of the dural opening it continued under the dura, but in the caudal direction the spinal cord was visually free of the abnormal arachnoid. On gross inspection of the shell pieces a smooth outer surface facing the dura and an irregular

Unauthenticated | Downloaded 10/02/21 07:26 AM UTC rough surface facing the spinal cord were observed. An additional thicker ossified piece (3 X 3 X 5 mm), removed from the left dorsal surface of the spinal cord, left a deep groove on the surface of the cord. Immediately superior to that, a large vascular structure that resembled a venous malformation was visualized; it was coagulated and left in place. After that, a longitudinal midline myelotomy was performed, and the intramedullary was entered. The inner surface of the cyst was smooth and white, with no septations. The cyst continued caudally as a single compartment. Once the cyst was opened, the spinal cord collapsed, and the edges of the myelotomy were reapproximated. A K-tube was inserted into the cyst and tunneled under the skin into subcutaneous pocket. A duraplasty was performed with a dural substitute, and the incision closed by standard techniques. One week later, the syringoperitoneal shunt was completed. Histological Findings. Histological examination of the removed plaques revealed metaplastic woven and lamellar bone enclosed by dense fibrous tissue that was consistent with the arachnoidal location (Fig. 3). The bone fragments contained normal bone marrow constituents. No inflammation was present. A small fragment of spinal cord white dura mater was also examined and found to have no diagnostic abnormalities.

Fig. 3. Photomicrographs of the resected ossified sample of the thoracic arachnoid membrane. Upper: Polarized light. The plaque has the appearance of the normal bone with woven and lamellar elements and is surrounded by dense fibrous tissue representing the

Unauthenticated | Downloaded 10/02/21 07:26 AM UTC arachnoid. Normal bone marrow elements are seen inside the plaque. H&E; original magnification X 20. Postoperative Course. In the immediate postoperative period the patient developed worsening of her paraparesis, no change in sensation, and a slight improvement in the dysesthetic pain. She required intermittent bladder catheterization, but due to the effects of oral laxatives and stool softeners, her constipation improved. She was discharged to the rehabilitation facility, where the treatment goals of an interdisciplinary team were to improve her mobility, regain bladder control, and to reduce her postoperative pain. Postoperative computerized tomography (CT) scanning of the thoracic spine revealed an extensive ossification surrounding the spinal cord that extended from the uppermost slice at T-6 where it was predominantly posterolateral on the left. At the level of T7­8, the ossification became circumferential (Fig. 4 left) and then disappeared at the upper border of T-9, which corresponded to the upper end of the surgical exploration (Fig. 4 right).

Fig. 4. Axial CT scans at the levels of T-8 (left) and T-11 (right) after the patient underwent a T9­11 laminectomy. A circumferential ossification can be seen encasing the spinal cord at midthoracic level (arrows). At the level of the laminectomy the ossification has been removed. An MR image obtained 6 weeks after surgery revealed the syrinx had decreased in size as well as the appearance of a wide layer of (CSF) that had surrounded the spinal cord at the level at which the decompressive procedure and duraplasty had been performed (Fig. 5). A dynamic MR study of CSF flow demonstrated absence of the pulsatile flow at the level of T-6 posteriorly and at T-8, which corresponded to the area of circumferential encasement of the spinal cord by the arachnoid ossification.

Unauthenticated | Downloaded 10/02/21 07:26 AM UTC Fig. 5. Postoperative MR images. Left: Axial image obtained at the level of surgery demonstrating a wide layer of CSF around the spinal cord. Right: Sagittal T1-weighted image demonstrating a decrease in syrinx size. Follow-Up Examination. During the follow-up examination six months after surgery, the patient's paraparesis had improved slightly to the point that she was ambulating again with the aid of a walker, although she preferred to use a wheelchair. Examination of her sensory status revealed absent proprioception in both lower extremities, with patchy loss of pain and temperature sensation below the level of T-8, which was worse on the left than on the right side. DISCUSSION The arachnoid membrane frequently contains microscopic calcifications. In some cases such calcifications may become confluent and can be seen, during spinal explorations, as flaky patches inside an otherwise normal arachnoid layer. This is usually considered normal and is not associated with any symptomatology. On the other hand, the arachnoid membrane may occasionally ossify, and this bone formation is frequently diseased and, consequently, symptomatic. Both calcification and ossification of the arachnoid were extensively described in the past. In 1915 Cushing and Weed[8] presented a comprehensive review of the published literature and discussed their findings of such cases of calcification and ossification in cats, dogs, monkeys, and humans. They argued against Virchow's theory of ossification, postulating instead that the true bone formation in the arachnoid is a proliferative process rather than degenerative. Later, the incidence of arachnoidal calcification and ossification was analyzed in several autopsy-based studies .[13,17,24,41,48,49] In 1939 Herren [17] reported the results of an autopsy series of 25 cases in which grossly observed calcified plaques were found in 19 (76%) cases. Plaques were located predominantly over the dorsal surface of the spinal cord with the greatest predilection for the thoracic cord. In 1966 Knoblich and Olsen[24] investigated the incidence and characteristics of the plaques based on 217 consecutive autopsy examinations. Calcification and ossification were found in 43.3% of specimens. In the largest series of 470 autopsies,

Unauthenticated | Downloaded 10/02/21 07:26 AM UTC Soloviev[48,49] reported the presence of calcified and ossified plaques in 292 spinal cords (62.1%). Since that time, multiple reports have been published in which various clinical and pathological conditions that involve excessive deposition of calcium and formation of the new bone in the arachnoid have been described. The process is usually described as arachnoiditis ossificans (calcificans) and encompasses quite different entities that vary in the extent of pathological changes and clinical symptomatology. Based on existing literature descriptions, it is possible to divide all cases of arachnoidal ossification into three relatively distinct groups in which specific pathological and radiographic characteristics are demonstrated. Although a similar, but more detailed classification, was published earlier,[48] that article was never translated to English and, consequently, was never cited in the literature outside the Soviet Union. The first group (type 1) consists of minute calcifications and ossifications of the arachnoid membrane that are frequently encountered during surgical exploration and in autopsy procedures in asymptomatic persons. This calcification or ossification may be quite extensive and cause the arachnoid to become white and chalky in appearance.[3,7,16,18,24,38,41,42,47] It is known to be frequently associated with adhesive arachnoiditis. No specific treatment is required for this condition because the majority of patients never develop any neurological symptoms. The second group (type 2) includes cases in which the arachnoid membrane becomes ossified at a small segment, either as an isolated plaque or as a thicker piece of metaplastic bone. Type 2 ossification may compress the underlying spinal cord and nerve root, and the corresponding clinical picture suggests myelopathy and radiculopathy.[13,27­29,42,43,54,55,58,61] This condition is rare and has been described in associations with different causes, such as trauma, infection, hemorrhage, and arachnoiditis. Surgical removal of the ossified piece(s) was reported to achieve a high degree of success in halting or reversing neurological deterioration. The third group (type 3) is the rarest and is characterized by circumferential ossification of the arachnoid around the spinal cord [1,9,13,21,26,53] or cauda equina.[23,33,44,52,57] This type of ossification is almost always associated with a clinical picture of progressive neurological worsening; the cause of this condition and the value of surgery in its treatment are uncertain. The first clinical description of this type of ossification was given by Dandy[9] in 1925. He reported a patient in whom surgery was performed in 1919 and then again in 1924 for progressive neurological deterioration. A shell of the bone lying over the cord between the dura and pia was encountered at T9­L5 levels. With regard to the etiology of the arachnoid ossification, there are several theories, and each of them has its own supporting evidence. The first theory postulates that arachnoid ossification is the result of chronic inflammation. Based on this theory, Puusepp[42] in 1931 introduced the clinical term of "arachnoiditis ossificans." This theory is supported by the fact that ossification is frequently encountered in patients with adhesive arachnoiditis, and on some occasions the histological studies have shown formation of the bone along with signs of chronic active inflammation, such as the presence of neutrophils and fibroblasts.[31,58] The clinical presentation of some patients has been associated with inflammatory CSF changes.[36] The development of arachnoid ossification in patients with previous tuberculous meningitis, spinal anesthesia, myelograms, intrathecal injections of Depo-Medrol, and subarachnoid hemorrhages, all of which may be a cause of the chronic arachnoiditis, further supports this theory.[5,13,14,23,29,33,43,52,56­58,61]

Unauthenticated | Downloaded 10/02/21 07:26 AM UTC The second theory correlates formation of bone with the presence of arachnoidal granulations that have a propensity to calcify and ossify over time.[8,16,17,24] Histological evidence of such arachnoidal cell clusters was obtained during detailed studies of the spinal meninges earlier this century. The ossification, therefore, is considered the next stage in a continuous proliferative process that includes formation of microscopic and then macroscopic calcifications in the arachnoid membrane. This is supported by the fact that in many cases, including the one reported here, the ossified tissue does not have any signs of ongoing inflammation.[32,51,58,50] It also explains the relatively high incidence of calcifications and ossifications found in the autopsies of asymptomatic patients. According to this theory, all the etiological factors listed above, may facilitate the otherwise normal proliferative process inside the arachnoid membrane. Meningeal ossifications and calcifications are observed both in meningiomas [34,40] and in the arachnoid distant to the meningioma.[63] Based on this theory, many authors have recommended avoiding the term "arachnoiditis ossificans," because it is misleading, and using the term "arachnoid ossification" instead.[21,32,57] The term "arachnoidocalcinosis" has also been suggested.[50] According to the third theory, the arachnoid ossification is analogous to other cases of heterotopic ossification, frequently seen in patients with spinal cord injuries.[2,11] In these cases, the ossification usually involves soft tissue around major joints.[15] The exact cause of this process remains unknown, although microtrauma, chronic infection, genetic factors, and disturbance of calcium metabolism have been suggested as possible causative factors.[62] Arachnoid ossification has been reported in patients who experience posttraumatic paraplegia and paraparesis;[1,2,19,22] however, the majority of patients did not have a history of the spinal cord . Different types of arachnoid ossification are observed not only in humans but also in other animal species. Cushing and Weed[8] observed calcified deposits in the arachnoid membrane of cats, dogs, and monkeys; a case of symptomatic diffuse arachnoid calcification/ossification in a dog has also been reported.[45] The association between the arachnoid ossification and intramedullary syrinx, as we observed in our case, has been described in the past.[32,35,56] Here, again, the exact pathogenesis of the cavity formation inside the spinal cord is unknown. It has been suggested that focal disturbances of regional blood circulation, such as compression, stenosis or occlusion of the medullary vessels, may cause ischemic and gliotic changes in the dura mater of the spinal cord, eventually resulting in intramedullary cavitation.[12,25,32] Other possible causes may include tissue defects such as those caused by previous intramedullary hemorrhage, postmyelitic changes, and focal venous hypertension.[30] It is also possible that intramedullary syrinx forms as a result of the impairment of the circulation of the CSF.[12,30] This theory is supported by the absence of CSF flow at the level of spinal cord compression documented by cine­MR image flow studies in our patient. It is not clear whether the venous malformation sometimes observed around the area of arachnoid ossification develops from chronic venous compression, thereby representing a postcompression varicous dilation.[48] In the past, some of these vascular abnormalities were deemed responsible for causing the ossification itself as a possible source of minute subarachnoid hemorrhage that later resulted in arachnoidal inflammation and subsequent ossification.[13,21,42,53] Symptoms caused by arachnoid ossification are usually related to either external compression of the spinal cord by a piece of bone acting as a focal mass lesion[42] or by myelopathy secondary to circumferential compression of the cord by the ossified plaque or by the intramedullary syrinx

Unauthenticated | Downloaded 10/02/21 07:26 AM UTC formation.[35] In the former case, the symptoms may be more lateralized, whereas in the latter they resemble a usual course of diffuse myelopathy or syringomyelia. It is impossible, however, to render a diagnosis of arachnoid ossification on clinical grounds alone; however, it should be included in the differential diagnosis of progressive myelopathy or, in cases of caudal involvement, radiculopathy. The diagnosis of spinal arachnoid ossification is relatively straightforward if a plain CT scan of the spine is obtained.[1,2,10,20,22,23,37,43,46,51,54,56] Although many different intraspinal lesions (tumors, ligaments, discs, hematomas, tuberculomas, and bone fragments)[4,6,20,39] may be calcified or ossified, the CT picture of arachnoid ossification is quite typical, especially in cases of type 3 ossification, as was shown in our case. The circumferential high-density structure that follows the contours of the spinal cord or the cauda equina is pathognomonic for the diagnosis of arachnoid ossification.[1,10,37,44,46,54] Although intracranial meningeal ossification has been diagnosed by performing skull roentgenography,[28] plain spine x-ray films rarely demonstrate intraspinal ossification.[1] is helpful in detecting the level of radiographic block,[3,13,14,27,31] and it sometimes reveals multiple filling defects that correspond to the location of ossified arachnoid plaques.[26,54,58] Currently routine preoperative radiological study in the patients with spinal cord compression is commonly limited to the use of MR imaging which, unfortunately, is not particularly sensitive in detection of small calcifications or ossifications.[46,56] If, however, the newly formed metaplastic bone contains a true bone marrow, then MR imaging may demonstrate a bright, fatlike signal in the projection of arachnoid membrane, raising suspicion of a focal disease process in that area. Treatment of the arachnoid ossification deserves a separate discussion. Although Dandy[9] reported good clinical improvement in patients in whom total removal of the ossified arachnoid was performed, in subsequent reports other authors were less optimistic.[42,59] It is quite obvious that surgical removal of the compressing bone piece (type 2 ossification) may, to some extent, stop or even reverse the signs of the underlying compressive myelopathy. Resection of the entire circumference of the ossified arachnoid membrane (type 3) is rather problematic, especially if the main symptomatic lesion is the intramedullary syrinx. The situation may be further complicated by presence of tight adhesions between the ossification and the spinal cord dura mater or even ingrowing of the bone into the spinal cord.[24,26,27,52,60] In these cases, decompressive surgery of the cord with or without syrinx drainage may be considered a safer choice. In conclusion, we want to bring surgeons' attention to this relatively rare cause of myelopathy and radiculopathy and emphasize the usefulness of CT scanning, because of its higher sensitivity in detecting intrathecal calcification and ossification, in the diagnostic evaluation of patients with progressive myelopathy.

References 1. Barthelemy CR: Arachnoiditis ossificans. J Comput Assist Tomogr 6:809­811, 1982 2. Bell RB, Wallace CJ, Swanson HA, et al: Ossification of the lumbosacral dura and arachnoid following spinal cord trauma. Case report. Paraplegia 33:543­546, 1995 3. Bianchi-Maiocchi A: Un raro caso di calcificazioni diffuse dell'aracnoide spinale. (Aracnoidite calcare.) Archo Orthop 68:629­639, 1955

Unauthenticated | Downloaded 10/02/21 07:26 AM UTC 4. Bonstelle CT, Vines FS: Calcification in a cervical intraspinal neurilemmoma. Neuroradiology 10:231­233, 1976 5. Carta F, Canu C, Datti R, et al: Calcification and ossification of the spinal arachnoid after intrathecal administration of Depo-Medrol. Zentralbl Neurochir 48:256­261, 1987 6. Compton JS, Dorsch NWC: Intradural extramedullary tuberculoma of the cervical spine. Case report. J Neurosurg 60:200­203, 1984 7. Critchley M: The neurology of old age. Lancet 1:1119­1127, 1931 8. Cushing H, Weed LH: Studies on the cerebro-spinal fluid and its pathway. IX. Calcareous and osseous deposits in the arachnoidea. Bull Johns Hopkins Hosp 26:367­372, 1915 9. Dandy WE: The diagnosis and localization of spinal cord tumors. Ann Surg 81:223­354,1925 10. Dennis MD, Altschuler E, Glenn W, et al: Arachnoiditis ossificans. A case report diagnosed with computerized axial tomography. Spine 8:115­117, 1983 11. Ehni G: Comment on Lynch C, Moraes GP: Spinal arachnoiditis ossificans: case report. 12:323­324, 1983 12. Errea JM, Ara JR, Alberdi J, et al: [Syringomyelia due to arachnoiditis. Clinical-radiological description of 5 patients.] Neurologia 8:226­230, 1993 (Sp) 13. Gatzke LD, Dodge HW Jr, Dockerty MB: Arachnoiditis ossificans. Report of two cases. Proc Mayo Clinic 32:698­704, 1957 14. Gulati DR, Bhandari YS, Markand ON: Spinal arachnoiditis ossificans. Neurol India 13:196­198, 1965 15. Hardy AG, Dickson JW: Pathological ossification in traumatic paraplegia. J Bone Joint Surg (Br) 45:76­87, 1963 16. Hassler O: Calcifications in the spinal arachnoid in man. A microradiological and x-ray diffraction study. Acta Neuropathol 8:163­170, 1967 17. Herren RY: Occurrence and distribution of calcified plaques in the spinal arachnoid in man. Arch Neurol Psychiatry 41:1180­1186, 1939 18. Hiroshima K, Suzuki M, Namiki H: [Histopathological changes of the human spinal cord--with special reference to latent lesions found in spinal cords of the aged.] Nippon Seikeigeka Gakkai Zasshi 57:729­739, 1983 (Jpn) 19. Jarmundowicz W, Radek A: [Ossification in the area of the spinal cord arachnoid as the cause of a chronic pain syndrome after spinal fracture.] Neurol Neurochir Pol 22:78­81, 1988 (Pol) 20. Jaspan T, Preston BJ, Mulholland RC, et al: The CT appearances of arachnoiditis ossificans. Spine 15:148­151, 1990 21. Kaufman AB, Dunsmore RH: Clinicopathological considerations in spinal meningeal calcification and ossification. Neurology 21:1243­1248, 1971

Unauthenticated | Downloaded 10/02/21 07:26 AM UTC 22. Kinnunen J, Laasonen EM: Posttraumatic lumbar intraspinal extradural-intradural ossification. Neuroradiology 32:160­162, 1990 23. Kitagawa H, Kanamori M, Tatezaki S, et al: Multiple spinal ossified arachnoiditis. A case report. Spine 15:1236­1238, 1990 24. Knoblich R, Olsen BS: Calcified and ossified plaques of the spinal arachnoid membranes. J Neurosurg 25:275­279, 1966 25. Lubin AJ: Adhesive spinal arachnoiditis as a cause of intramedullary cavitation. Comparison with syringomyelia. Arch Neurol Psychoatry 44:409­420, 1940 26. Lynch C, Moraes GP: Spinal arachnoiditis ossificans: case report. Neurosurgery 12:321­324, 1983 27. McCullough GA: Arachnoid calcification producing spinal cord compression. J Neurol Neurosurg Psychiatry 38:1059­1062, 1975 28. McKendree CA, Imboden HM: Ossification of the meninges. Arch Neurol Psychiatry 6:529­540, 1921 29. Miles J, Bhandari YS: Ossifying spinal arachnoiditis. Neurochirurgia 14:184­188, 1971 30. Milhorat TH, Capocelli AL Jr, Anzil AP, et al: Pathological basis of spinal cord cavitation in syringomyelia: analysis of 105 autopsy cases. J Neurosurg 82:802­812, 1995 31. Morello A, Nastasi G, Mangione G: [Arachnoiditis ossificans and osseus metaplasia of the arachnoid.] Acta Neurol 22:787­795, 1967 (Ital) 32. Nagpal RD, Gokhale SD, Parikh VR: Ossification of spinal arachnoid with unrelated syringomyelia. Case report. J Neurosurg 42:222­225, 1975 33. Nainkin L: Arachnoiditis ossificans. Report of a case. Spine 3:83­86, 1978 34. Nakayama N, Isu T, Asaoka K, et al: [Two cases of ossified spinal meningioma.] No Shinkei Geka 24:351­355, 1996 (Jpn) 35. Nelson J: Intramedullary cavitation resulting from adhesive spinal arachnoiditis. Arch Neurol Psychiatry 50:1­7, 1943 36. Neretin VI, Kir'iakov VA, Ignat'eva EI: [Clinico-morphologic manifestations of ossifying cerebrospinal arachnoiditis.] Zh Nevropatol Psikhiatr 89:19­21, 1989 (Rus) 37. Ng P, Lorentz I, Soo YS: Arachnoiditis ossificans of the cauda equina demonstrated on computed tomography scanogram. A case report. Spine 21:2504­2507, 1996 38. Nizzoli V, Testa C: A case of calcification in the spinal arachnoid giving rise to spinal cord compression. J Neurol Sci 7:381­384, 1968 39. O'Rourke DM, Simeone FA: Subacute onset of thoracic myelopathy secondary to compressive posterior osseous anomaly. Case report. Spine 16:990­991, 1991 40. Pearl BL, Boyd HR: Roentgenographically visible calcifications in spinal meningeoma. AJR

Unauthenticated | Downloaded 10/02/21 07:26 AM UTC 120:32­45, 1974 41. Pomerance A: Spinal arachnoiditis ossificans. J Path Bact 87:421­423, 1964 42. Puusepp L: Surgical intervention in four cases of compression caused by osseous deposits in the arachnoidea of the spinal cord (archnoiditis ossificans). J Nerv Mental Dis 73:1­19, 1931 43. Quiles M, Marchisello PJ, Tsairis P: Lumbar adhesive arachnoiditis. Etiologic and pathologic aspects. Spine 3:45­50, 1978 44. Sefczek RJ, Deeb ZL: Case report: computed tomography findings in spinal arachnoiditis ossificans. J Comput Tomogr 7:315­318, 1983 45. Sherwood BF, LeMay JC, Vakilzadeh U: Spinal arachnoid calcification: diagnosis. J Am Vet Med Assoc 156:1917­1918, 1970 46. Shiraishi T, Crock HV, Reynolds A: Spinal arachnoiditis ossificans. Observations on its investigation and treatment. Eur Spine J 4:60­63, 1995 47. Slager UT: Arachnoiditis ossificans. Report of a case and review of the subject. Arch Pathol 70:322­327, 1960 48. Soloviev VN: [Changes in the spinal cord in so-called spinal ossificating arachnoiditis.] Arkh Patol 35:48­54, 1973 (Rus) 49. Soloviev VN: [The clinical picture and course of so-called spinal ossifying arachnoiditis.] Zh Nevropatol Psikhiatr 74:1141­1148, 1974 (Rus) 50. Soloviev VN: [Several notes concerning the etiology and naming of so-called ossifying arachnoiditis.] Zh Nevropatol Psikhiatr 78:768­771, 1978 (Rus) 51. Tanaka K, Nishiura I, Koyama T: [Arachnoiditis ossificans after repeated myelographies and spinal operations--a case report and review of the literature.] No Shinkei Geka 15:89­93, 1987 (Jpn) 52. Tetsworth KD, Ferguson RL: Arachnoiditis ossificans of the cauda equina. A case report. Spine 11:765­766, 1986 53. Tomasello F, D'Avanzo R, Albanese V, et al: Spinal ossifying arachnoiditis. Case report. J Neurosurg Sci 29:335­340, 1985 54. Toribatake Y, Baba H, Maezawa Y, et al: Symptomatic arachnoiditis ossificans of the thoracic spine. Case report. Paraplegia 33:224­227, 1995 55. Tsuruta T, Kato K, Matsuike H, et al: [Case of arachnoiditis ossificans.] Seikei Geka 20:1571­1575, 1969 (Jpn) 56. Van Paesschen W, Van den Kerchove M, Appel B, et al: Arachnoiditis ossificans with after cranial tuberculous meningitis. Neurology 40:714­716, 1990 57. Varughese G: Lumbosacral intradural periradicular ossification. Case report. J Neurosurg 49:132­137, 1978

Unauthenticated | Downloaded 10/02/21 07:26 AM UTC 58. Whittle IR, Dorsch NW, Segelov JN: Symptomatic arachnoiditis ossificans. Report of two cases. Acta Neurochir 65:207­216, 1982 59. Whittle IR, Segelov JN: Spinal arachnoiditis ossificans. Neurosurgery 13:737, 1983 60. Wilkinson HA, Shuman N: Results of the surgical lysis of lumbar adhesive arachnoiditis. Neurosurgery 4:401­409, 1979 61. Wise BL, Smith M: Spinal arachnoiditis ossificans. Arch Neurol 13:391­394, 1965 62. Wittenberg RH, Peschke U, Botel U: Heterotopic ossification after spinal cord injury. Epidemiology and risk factors. J Bone Joint Surg (Br) 74:215­218, 1992 63. Yamashita M, Fukui M, Kitamura K: Intracerebral meningioma with disseminated arachnoidal ossification. Surg Neurol 9:95­98, 1978

Manuscript received March 11, 1999. Accepted in final form March 26, 1999. Address reprint requests to: Konstantin V. Slavin, M.D., Department of Neurological Surgery, Oregon Health Sciences University, 3181 SW Sam Jackson Park Road, L 472, Portland, Oregon 97201­3098. email: [email protected].

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