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Original Report

OBJECTIVE

.

The purpose of this article is to illustrate the appearance of arachnoiditis os-

sificans 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.

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 arach-

noiditis, termed arachnoiditis ossificans, has

more ominous implications. Prior reports indi-

cate that this type

of spinal ossification is gen-erally, although not invariably, associated with

progressive neurologic deficits and that recog-

nizing this entity has treatment implications.

The radiographic and CT appearances of

arachnoiditis ossificans have been previously de-

scribed [26]. 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 onMR 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 (19861998). The diagnosis

was made when intrathecal ossification was observed

in the setting of arachnoiditis. Clinical data were ob-

tained if possible through clinic notes and patient in-

terviews. 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 attemptedin 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 pre-

senting complaint of all the patients, and fourcomplained of leg pain and weakness as well.

One patient suffered from urinary incontinence

and another, from urinary frequency. Neuro-

logic examinations revealed normal or nonspe-

cific findings in the patients in whom these

findings were recorded.

All patients had evidence of arachnoiditis on

MR imaging (Fig. 1), and myelography, when

performed, showed clumped poorly defined

Bevan Frizzell

1

Phoebe Kaplan

2

Robert Dussault

2

Robert Sevick

1

Received J uly 9, 1999; accepted after revisionFebruary 9, 2001.

1

Department of Radiology, Foothills Hospital, 1403 29 St.N.W., Calgary, Alberta, T2N 2T9 Canada. Addresscorrespondence to B. Frizzell.

2

Department of Radiology, University of Virginia, Box 170,Lee St., Charlottesville, VA 22908.

AJ R

2001;177:461464

0361803X/01/1772461

American Roentgen Ray Society

A

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nerve roots in the thecal sac. On MR images,

superimposed on the findings of arachnoiditis,

there were changes corresponding to ossifica-

tion, 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 lessconspicuous and could be either hypointense

(Fig. 2C) or hyperintense (Fig. 4C).

In four patients, the intrathecal ossifica-

tion was confirmed on CT. In the patient in

whom CT was not available, the ossification

in the central canal was evident on conven-

tional radiography (Fig. 2A).

For one patient with arachnoiditis ossificans

(Fig. 1), subsequent decompressive laminecto-

mies, anterior fusion, and foraminotomies re-

sulted in sufficient symptomatic relief. No

further surgery was performed in the other four

patients. In the patient who had suffered a re-

mote burst fracture of the second lumbar verte-

bra (Fig. 4) and had subsequently undergone

spinal fusion, as his ossification increased over

time, so did his leg weakness and urinary sphinc-

ter disturbance. The other patients were lost toclinical follow-up.

Discussion

Small calcified plaques of the dura mater

are frequently encountered at surgery and au-

topsy. Kaufman and Dunsmore [1] have em-

phasized that these patchy, thin, isolated

asymptomatic calcifications should be distin-

guished from intrathecal ossification associ-

ated with chronic meningeal inflammation

(or arachnoiditis), for which the term arach-

noiditis ossificans should be reserved.

Arachnoiditis ossificans is frequently associ-

ated with a significant, often progressive, neuro-

logic deficit [14, 7, 8]. Specifically, patients

tend to present with symptoms of progressive

compressive myelopathy. However, this presen-tation 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 hem-

orrhage, myelography (particularly using oil-

based contrast agents), and spinal anesthesia

have all been implicated as causes of arach-

noiditis ossificans [18]. At least one of these

was present in each of our patients.

CBA

Fig. 1.53-year-old woman with burning pain in both legs and history of lumbar surgery 28 years earlier.A,Axial unenhanced CT scan of lumbar spine shows small focus of intrathecal ossification (arrow).BandC,Sagittal T1-weighted (B) and T2-weighted (C) MR images of lumbar spine reveal mixed-signal-inten-sity amorphous mass (arrows) in central canal. Individual nerve roots cannot be differentiated. These find-ings correspond to severe arachnoiditis.D, Axial T1-weighted MR image shows small focus of superimposed signal hypointensity (arrow), whichlikely corresponds to ossification shown in A.

D

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Various mechanisms have been proposed for

the development of the ossification including in-

tradural hematoma, which organizes and ossi-

fies; seeded bone fragments; and osseous

metaplasia associated with chronic inflamma-

tion [1, 8]. The latter is probably the most likelycause, 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. How-

ever, a high prevalence of vascular abnormalities

of the spinal cord was also seen in their series.

They suggested that vascular shunting or pres-

sure effects might contribute to the development

of the disorder, possibly complicated by bleed-

ing into the abnormal tissues. No associated vas-

cular anomalies were detected in our patients.

Conventional radiographs rarely show the

abnormality, and then only when it is exten-sive, as was seen in two of our five patients

(Figs. 2A and 4A). Myelography may show

the features of arachnoiditis, but the ossifica-

tion can be overlooked because of obscura-

tion 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 disor-

der [26, 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 under-

went CT, the intrathecal ossification could bereadily identified (Figs. 1A and 3A).

All our patients had findings on MR imag-

ing, 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 abnor-

CBA

Fig. 2.67-year-old woman withlower back pain, leg weakness, andhistory of four prior back surgeries.A,Anteroposterior conventional radio-graph of lumbar spine shows extensivetubelike calcification in central canal(solid arrows). Note small amount ofresidual oil-based intrathecal contrastmaterial (open arrow).Band C,Sagittal T1-weighted (B) and

T2-weighted (C) MR images showclumped nerve roots indicating arach-noiditis. There is superimposed hyperin-tense (B) and hypointense (C) linearsignal abnormality involving nerveroots, and possibly dura, consistent withcalcification or ossification (arrows).

BA

Fig. 3.55-year-old man with lower back pain andhistory of prior lumbar laminectomies.A,Axial unenhanced CT scan of lumbar spine revealsmarked intradural ossification with nerve roots(arrow) passing through osseous mass.B,Axial T1-weighted MR image shows mixed-signal-intensity abnormality within central canal corre-sponding to ossification (arrowheads).

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mality, which was generally hyperintense on

T1-weighted sequences and hypo- or hyperin-

tense on T2-weighted images. Heterogeneity

in the appearance is likely based on the stageof the calcification or ossification and differ-

ences in the calcium macromolecular environ-

ment [9]. Increased signal intensity on both T1-

weighted and T2-weighted images may corre-

spond 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 inter-

vention 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 in-

trathecal ossification seems technically feasible,

the clinical result is generally poor and results inlittle, if any, symptomatic improvement [2, 7].

Better results may be expected with simple de-

compression 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 laminec-

tomies. These researchers stressed the need to

decompress over the entire length of the ossified

mass; therefore, the full extent of the ossified ab-

normality must be defined. This task would be

best accomplished with unenhanced CT.

On MR imaging, differential diagnosis includes

arachnoiditis with or without ossification. Re-tained 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 be-

tween these possibilities if there is uncertainty.

In summary, arachnoiditis ossificans is an un-

common disorder, which most likely represents

end-stage adhesive arachnoiditis. Most past re-

ports 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 themanifestations of this disorder on MR imaging

has become important, because there are treat-

ment implications. The MR imaging manifesta-

tions 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.

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CBA

Fig. 4.44-year-old man who had undergone T11L3 fusion 22 years earlier for L2 burst fracture presented with back pain radiating into right leg and urinary incontinence.A,Lateral conventional radiograph of lumbar spine shows old L2 burst fracture and linear calcification (arrowheads) in central canal.BandC,Sagittal T1-weighted (B) and T2-weighted (C) MR images show intrathecal hyperintensity (arrows) that corresponds to arachnoiditis ossificans. Changes associ-ated with L2 burst fracture and postoperative meningocele formation are also shown.