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Original Report
August 2001

Arachnoiditis Ossificans: MR Imaging Features in Five Patients

Abstract

OBJECTIVE. The purpose of this article is to illustrate the appearance of arachnoiditis ossificans on MR imaging and discuss the implications this diagnosis has on treatment.
CONCLUSION. In patients with arachnoiditis ossificans, the MR imaging findings are of linear or masslike intrathecal lesions, which generally have some hyperintensity on T1- weighted sequences and are hyper- or hypointense on T2-weighted images, in the setting of arachnoiditis.

Introduction

A symptomatic ossified dural plaques of the spine are frequently found at surgery and autopsy but have little clinical significance [1]. However, intradural ossification associated with chronic arachnoiditis, termed “arachnoiditis ossificans,” has more ominous implications. Prior reports indicate that this type of spinal ossification is generally, although not invariably, associated with progressive neurologic deficits and that recognizing this entity has treatment implications.
The radiographic and CT appearances of arachnoiditis ossificans have been previously described [2,3,4,5,6]. With radiologists' declining use of CT and the corresponding rise in the use of MR imaging for the assessment of low back pain, knowledge of the MR imaging appearance of arachnoiditis ossificans has become important.
In this article, we present five patients with this unusual manifestation of arachnoiditis in the lumbar spine, review the radiographic and CT findings, and describe the appearance on MR imaging.

Materials and Methods

Five cases of arachnoiditis ossificans were retro-spectively reviewed; these cases were obtained from the University of Virginia in Charlottesville, VA, and the Foothills Hospital in Calgary, Alberta, Canada, over a 12-year period (1986-1998). The diagnosis was made when intrathecal ossification was observed in the setting of arachnoiditis. Clinical data were obtained if possible through clinic notes and patient interviews. All patients underwent conventional radiography and MR imaging. CT of the lumbar spine was available for correlation in four patients.
MR imaging consisted of standard fast spin-echo T1-weighted and T2-weighted MR imaging sequences in the axial and sagittal planes. T2-weighted gradient-echo sequences were attempted in one patient, but motion artifacts (mainly from the gastrointestinal tract) made these images non-diagnostic. IV gadolinium was not administered to any of the patients.

Results

Three of the patients were women and two were men, ranging in age from 44 to 67 years. All patients had a history of lumbar surgery; two had previously undergone myelography, whereas another had a remote history of major spinal trauma.
Recurrent lower back pain was the main presenting complaint of all the patients, and four complained of leg pain and weakness as well. One patient suffered from urinary incontinence and another, from urinary frequency. Neurologic examinations revealed normal or nonspecific findings in the patients in whom these findings were recorded.
All patients had evidence of arachnoiditis on MR imaging (Fig. 1A,1B,1C,1D), and myelography, when performed, showed clumped poorly defined nerve roots in the thecal sac. On MR images, superimposed on the findings of arachnoiditis, there were changes corresponding to ossification, which could be thin and linear (Figs. 2B and 2C) or globular and masslike (Fig. 3B). On T1-weighted sequences, this ossification was predominantly hyperintense (Fig. 2B) in three patients and hypointense in two (Fig. 3B). On T2-weighted imaging, the abnormality was less conspicuous and could be either hypointense (Fig. 2C) or hyperintense (Fig. 4C).
Fig. 1A. 53-year-old woman with burning pain in both legs and history of lumbar surgery 28 years earlier. Axial unenhanced CT scan of lumbar spine shows small focus of intrathecal ossification (arrow).
Fig. 1B. 53-year-old woman with burning pain in both legs and history of lumbar surgery 28 years earlier. Sagittal T1-weighted (B) and T2-weighted (C) MR images of lumbar spine reveal mixed-signal-intensity amorphous mass (arrows) in central canal. Individual nerve roots cannot be differentiated. These findings correspond to severe arachnoiditis.
Fig. 1C. 53-year-old woman with burning pain in both legs and history of lumbar surgery 28 years earlier. Sagittal T1-weighted (B) and T2-weighted (C) MR images of lumbar spine reveal mixed-signal-intensity amorphous mass (arrows) in central canal. Individual nerve roots cannot be differentiated. These findings correspond to severe arachnoiditis.
Fig. 1D. 53-year-old woman with burning pain in both legs and history of lumbar surgery 28 years earlier. Axial T1-weighted MR image shows small focus of superimposed signal hypointensity (arrow), which likely corresponds to ossification shown in A.
Fig. 2B. 67-year-old woman with lower back pain, leg weakness, and history of four prior back surgeries. Sagittal T1-weighted (B) and T2-weighted (C) MR images show clumped nerve roots indicating arachnoiditis. There is superimposed hyperintense (B) and hypointense (C) linear signal abnormality involving nerve roots, and possibly dura, consistent with calcification or ossification (arrows).
Fig. 3B. 55-year-old man with lower back pain and history of prior lumbar laminectomies. Axial T1-weighted MR image shows mixed-signal-intensity abnormality within central canal corresponding to ossification (arrowheads).
Fig. 2C. 67-year-old woman with lower back pain, leg weakness, and history of four prior back surgeries. Sagittal T1-weighted (B) and T2-weighted (C) MR images show clumped nerve roots indicating arachnoiditis. There is superimposed hyperintense (B) and hypointense (C) linear signal abnormality involving nerve roots, and possibly dura, consistent with calcification or ossification (arrows).
Fig. 4C. 44-year-old man who had undergone T11-L3 fusion 22 years earlier for L2 burst fracture presented with back pain radiating into right leg and urinary incontinence. Sagittal T1-weighted (B) and T2-weighted (C) MR images show intrathecal hyperintensity (arrows) that corresponds to arachnoiditis ossificans. Changes associated with L2 burst fracture and postoperative meningocele formation are also shown.
In four patients, the intrathecal ossification was confirmed on CT. In the patient in whom CT was not available, the ossification in the central canal was evident on conventional radiography (Fig. 2A).
Fig. 2A. 67-year-old woman with lower back pain, leg weakness, and history of four prior back surgeries. Anteroposterior conventional radiograph of lumbar spine shows extensive tubelike calcification in central canal (solid arrows). Note small amount of residual oil-based intrathecal contrast material (open arrow).
For one patient with arachnoiditis ossificans (Fig. 1A,1B,1C,1D), subsequent decompressive laminectomies, anterior fusion, and foraminotomies resulted in sufficient symptomatic relief. No further surgery was performed in the other four patients. In the patient who had suffered a remote burst fracture of the second lumbar vertebra (Fig. 4A,4B,4C) and had subsequently undergone spinal fusion, as his ossification increased over time, so did his leg weakness and urinary sphincter disturbance. The other patients were lost to clinical follow-up.
Fig. 4A. 44-year-old man who had undergone T11-L3 fusion 22 years earlier for L2 burst fracture presented with back pain radiating into right leg and urinary incontinence. Lateral conventional radiograph of lumbar spine shows old L2 burst fracture and linear calcification (arrowheads) in central canal.
Fig. 4B. 44-year-old man who had undergone T11-L3 fusion 22 years earlier for L2 burst fracture presented with back pain radiating into right leg and urinary incontinence. Sagittal T1-weighted (B) and T2-weighted (C) MR images show intrathecal hyperintensity (arrows) that corresponds to arachnoiditis ossificans. Changes associated with L2 burst fracture and postoperative meningocele formation are also shown.

Discussion

Small calcified plaques of the dura mater are frequently encountered at surgery and autopsy. Kaufman and Dunsmore [1] have emphasized that these patchy, thin, isolated asymptomatic calcifications should be distinguished from intrathecal ossification associated with chronic meningeal inflammation (or arachnoiditis), for which the term arachnoiditis ossificans should be reserved.
Arachnoiditis ossificans is frequently associated with a significant, often progressive, neurologic deficit [1,2,3,4, 7, 8]. Specifically, patients tend to present with symptoms of progressive compressive myelopathy. However, this presentation is variable, and clinical symptoms may be relatively mild or seemingly unrelated [5] as was shown in at least two of our five patients.
Prior trauma, surgery, subarachnoid hemorrhage, myelography (particularly using oil-based contrast agents), and spinal anesthesia have all been implicated as causes of arachnoiditis ossificans [1,2,3,4,5,6,7,8]. At least one of these was present in each of our patients.
Various mechanisms have been proposed for the development of the ossification including intradural hematoma, which organizes and ossifies; seeded bone fragments; and osseous metaplasia associated with chronic inflammation [1, 8]. The latter is probably the most likely cause, with arachnoiditis ossificans representing end-stage chronic arachnoiditis, as suggested by Kaufman and Dunsmore [1]. They found chronic fibroblastic proliferative change to the leptomeninges associated with the osseous metaplasia in all the cases they reviewed. However, a high prevalence of vascular abnormalities of the spinal cord was also seen in their series. They suggested that vascular shunting or pressure effects might contribute to the development of the disorder, possibly complicated by bleeding into the abnormal tissues. No associated vascular anomalies were detected in our patients.
Conventional radiographs rarely show the abnormality, and then only when it is extensive, as was seen in two of our five patients (Figs. 2A and 4A). Myelography may show the features of arachnoiditis, but the ossification can be overlooked because of obscuration by the contrast agent. Dennis et al. [4] indicated that myelography, in fact, might be misleading; their case report indicated that the myelogram suggested spinal stenosis rather than an intradural ossific mass.
Although unenhanced CT has been well shown to be exquisitely sensitive for the disorder [2,3,4,5,6, 8], it is being used less frequently for the routine evaluation of lower back pain. In all four of the patients in our series who underwent CT, the intrathecal ossification could be readily identified (Figs. 1A and 3A).
Fig. 3A. 55-year-old man with lower back pain and history of prior lumbar laminectomies. Axial unenhanced CT scan of lumbar spine reveals marked intradural ossification with nerve roots (arrow) passing through osseous mass.
All our patients had findings on MR imaging, albeit subtle in some, that were consistent with arachnoiditis ossificans. In all cases, MR images showed clumped nerve roots of the cauda equina indicative of arachnoiditis. The associated calcification or ossification had a variable appearance and was represented by superimposed linear or masslike signal abnormality, which was generally hyperintense on T1-weighted sequences and hypo- or hyperintense on T2-weighted images. Heterogeneity in the appearance is likely based on the stage of the calcification or ossification and differences in the calcium macromolecular environment [9]. Increased signal intensity on both T1-weighted and T2-weighted images may correspond to the development of bone marrow.
The importance in alerting the clinician to this condition lies in its implications for treatment. The literature is divided regarding surgical intervention in these patients, but in general, there is support that attempts to remove calcified plaques from the spinal cord or nerve roots should be avoided [2, 7]. Even if surgical removal of the intrathecal ossification seems technically feasible, the clinical result is generally poor and results in little, if any, symptomatic improvement [2, 7]. Better results may be expected with simple decompression of the spinal canal; Shiraishi et al. [6] reported two cases in which wheelchair-bound patients with arachnoiditis ossificans were able to walk after decompression laminectomies. These researchers stressed the need to decompress over the entire length of the ossified mass; therefore, the full extent of the ossified abnormality must be defined. This task would be best accomplished with unenhanced CT.
On MR imaging, differential diagnosis includes arachnoiditis with or without ossification. Retained oil-based intrathecal contrast material in the setting of arachnoiditis is also a consideration, and metastatic melanoma could conceivably give this appearance. CT should effectively differentiate between these possibilities if there is uncertainty.
In summary, arachnoiditis ossificans is an uncommon disorder, which most likely represents end-stage adhesive arachnoiditis. Most past reports indicate arachnoiditis ossificans is generally associated with neurologic deficits, but patients can also be relatively asymptomatic regardless of the degree of ossification. With CT being used less frequently in the routine assessment of low back pain, radiologists' ability to recognize the manifestations of this disorder on MR imaging has become important, because there are treatment implications. The MR imaging manifestations can be subtle, and if there is uncertainty with regard to the correct diagnosis, then a CT scan is useful to confirm the diagnosis. If extensive de-compressive laminectomy is being considered, we recommend unenhanced CT to evaluate the full extent of the ossified abnormality.

Footnote

Address correspondence to B. Frizzell.

References

1.
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Information & Authors

Information

Published In

American Journal of Roentgenology
Pages: 461 - 464
PubMed: 11461883

History

Submitted: July 9, 1999
Accepted: February 9, 2001

Authors

Affiliations

Bevan Frizzell
Department of Radiology, Foothills Hospital, 1403 29 St. N.W., Calgary, Alberta, T2N 2T9 Canada.
Phoebe Kaplan
Department of Radiology, University of Virginia, Box 170, Lee St., Charlottesville, VA 22908.
Robert Dussault
Department of Radiology, University of Virginia, Box 170, Lee St., Charlottesville, VA 22908.
Robert Sevick
Department of Radiology, Foothills Hospital, 1403 29 St. N.W., Calgary, Alberta, T2N 2T9 Canada.

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