AJR 2004; 182:191-194
© American Roentgen Ray Society
Intraspinal Posterior Epidural Cysts Associated with Baastrup's Disease: Report of 10 Patients
Clement K. H. Chen1,2,
LeeRen Yeh1,2,
Donald Resnick3,
Ping-Hong Lai1,2,
Huei-Lung Liang1,2,
Huay-Ben Pan1,2 and
Chien-Fang Yang1,2
1 Department of Radiology, Kaohsiung Veterans General Hospital, 386 Ta-Chung 1st
Rd., Kaohsiung 813, Taiwan.
2 National Yang-Ming University, School of Medicine, 155, Sec. 2, Li-Nong St.,
Shih-Pai Rd., PeiTou, Taipei 112, Taiwan.
3 Department of Radiology, Veterans Affairs Medical Center and University of
California San Diego, 3350 La Jolla Village Dr., San Diego, CA 92161.
Received January 28, 2003;
accepted after revision July 23, 2003.
Address correspondence to C. K. H. Chen
(khchen{at}isca.vghks.gov.tw).
Abstract
OBJECTIVE. This study was performed to describe the association of
posterocentral epidural cysts with interspinous bursal fluid in Baastrup's
disease using MRI and interspinous bursography.
CONCLUSION. Interspinous bursal fluid in Baastrup's disease can
extend into the posterocentral epidural space and cause various degrees of
central canal stenosis. Bursography alone or combined with CT allows
documentation of abnormal communicating channels between the interspinous
bursa and epidural cyst.
Introduction
Baastrup's disease is a well-documented disease that is characterized by
close approximation and contact of adjacent spinous processes (kissing spine)
with resultant enlargement, flattening, and reactive sclerosis of apposing
interspinous surfaces [1]. It
can manifest clinically as midline localized lumbar tenderness and pain on
back extension that can be relieved by flexion, local anesthetic injection,
and excision of part of the spinous processes
[2]. We report 10 cases of
intraspinal posterior epidural cysts associated with Baastrup's disease that
resulted in varying degrees of posterior central compression of the thecal
sac. Interspinous bursography combined with CT confirmed that the interspinous
bursitis extended to the intraspinal cystic mass in five patients. To our
knowledge, these findings have not been reported previously.
Materials and Methods
Between December 1999 and October 2002, 10 cases of intraspinal
posterocentral epidural cysts were found to be associated with Baastrup's
disease during interpretation of 1,148 MRIs of the lumbar spine. The patients
include nine men and one woman, whose ages ranged from 43 to 77 years (average
age, 67 years). According to the medical records, they were referred for MRI
evaluation of lower back pain (nine patients), radiculopathy in one or both
legs (eight patients), or signs of spinal stenosis (five patients). MRIs were
obtained with a 1.5-T imager (Signa, General Electric Medical Systems,
Milwaukee, WI). Our routine MRI protocol for the lumbar spine consisted of
coronal fat-suppressed fast spin-echo T2-weighted sequences (TR
range/TEeff range, 4,400–4,550/67–90), sagittal
spin-echo T1-weighted sequences (400–416/14–16), fast spin-echo
T2-weighted sequences (3,300–3,800/98–110), oblique axial
sequences (angled along respective intervertebral disks), spin-echo
T1-weighted sequences (400–616/13–16), and fat-suppressed fast
spin-echo T2-weighted sequences (3,500–8,000/81–96). The diagnosis
of Baastrup's disease was based on the presence of high T2 signal (similar to
that of fluid) in the narrow interspinous space and flattening and sclerosis
of the apposing surfaces of the spinous processes on sagittal MRIs or frontal
and lateral radiographs (available in all 10 patients), or both.
Interspinous bursography and CT were performed in five patients within days
after the MRI studies. Under fluoroscopy, with the patient lying on the right
side, a 21-gauge spinal needle was inserted from a direct posterior approach
until its tip was located at the midpoint of the involved interspinous space.
The amount of nonionic contrast material (iohexol, Omnipaque, Nycomed
Amersham, Princeton, NJ) that was injected depended on the capacity of the
cystic lesions seen under fluoroscopy and ranged from 5 to 15 mL. Helical CT
with coronal and sagittal reformations was performed using a Somatom Plus 4
scanner (Siemens, Erlangen, Germany) with the following parameters: table
speed, 3 mm/sec; collimation, 2 mm; pitch, 1.5; and reconstruction interval, 2
mm.
Results
On sagittal and axial MRIs, the posterocentral epidural cysts were
contiguous with the interspinous space. Varying degrees of posterocentral
compression of the thecal sac were observed (Figs.
1A,
1B,
2A,
3A, and
3B).
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Fig. 1A. —75-year-old man with lower back pain. Sagittal spin-echo
T1-weighted image (TR/TE, 416/16) shows Baastrup's disease at L4–L5 disk
level with approximation of spinous processes and flattening of their apposing
surfaces. Small posterocentral epidural cystic mass (arrow) is
contiguous to interspinous soft tissue.
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Fig. 1B. —75-year-old man with lower back pain. Axial fat-suppressed
fast spin-echo T2-weighted image (6,000/96) confirms posterocentral epidural
cystic mass (arrow) that has led to mild compression of thecal sac.
No significant facet arthrosis or effusion or evidence of communicating
channel between cyst and facet joints is present.
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Fig. 2A. —73-year-old man with lower back pain radiating to both legs.
Sagittal fast spin-echo T2-weighted image (TR/TE, 3,500/105) shows evidence of
Baastrup's disease and central canal stenosis resulting from anterior disk
bulging and posterior epidural lesion (arrow) at L4–L5
level.
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Fig. 3A. —60-year-old man with lower back pain radiating to both legs.
Sagittal fast spin-echo T2-weighted image (TR/TE, 3,500/110) shows
posterocentral epidural cystic mass (straight arrow) at L2–L3
level and Baastrup's disease with interspinous bursal fluid (curved
arrow). Subjacent spondylolisthesis between L3 and L4 is also seen.
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Fig. 3B. —60-year-old man with lower back pain radiating to both legs.
Axial fat-suppressed fast spin-echo T2-weighted image (7,500/96) shows evident
compression of thecal sac by posterocentral epidural cyst
(arrow).
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During bursography, the contrast material flowed directly and smoothly from
the interspinous bursa into the posterior epidural cysts (Figs.
2B and
2C). These connections were
apparent during fluoroscopic monitoring as well as on axial or reformatted CT
images (Figs. 1C,
2B,
2D,
3C, and
3D). Concurrent opacification
of a single facet joint was noted in two patients and opacification of the
facet joints on both sides (Figs.
3C and
3D) in two other patients;
opacification of an adjacent defect of the pars interarticularis was observed
in one patient (Fig. 3C).
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Fig. 2B. —73-year-old man with lower back pain radiating to both legs.
Interspinous bursogram, lateral projection, shows bursal fluid extending
directly to posterior epidural cyst (open arrow). Tip of spinal
needle (solid arrow) was placed at midpoint of L4–L5
interspinous space.
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Fig. 1C. —75-year-old man with lower back pain. Sagittally reformatted
CT image obtained after interspinous bursogram shows contrast material
(arrowheads) in interspinous space extending to small posterior
epidural cyst. Note air (arrow) that was inadvertently injected and
accumulated in cyst.
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Fig. 2D. —73-year-old man with lower back pain radiating to both legs.
Axial CT image shows bursal fluid extending directly to posterior epidural
cyst (arrow). Small opacified communicating channel is evident
between cyst and interspinous bursal fluid. No significant facet arthrosis or
effusion is present.
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Fig. 3C. —60-year-old man with lower back pain radiating to both legs.
Interspinous bursograms reveal opacification of interspinous bursa (curved
arrow) and fluid extending to posterior epidural cyst (straight
arrow). Concurrent opacification of L3 spondylolysis (arrowhead)
is also present. Spondylolysis at L3 level is more evident on lateral
radiograph and MRI of lumbar spine.
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Fig. 3D. —60-year-old man with lower back pain radiating to both legs.
Axial CT image shows contrast material opacification of interspinous bursa
extending to posterior epidural cyst (open arrow). Small opacified
communicating channel is evident between cyst and interspinous bursal fluid.
Resultant moderate to severe central canal stenosis is evident. Concurrent
opacification of bilateral facet joint spaces (solid arrows) at
L2–L3 spinal level is also visible.
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Concurrent degenerative disk disease and variable degrees of spinal
stenosis were noted in all patients, spondylolisthesis at or below the level
of the cyst (Figs. 3A and
3C) in six patients, and
posterior protrusion or extrusion of an intervertebral disk in two patients.
Significant arthrosis of the adjacent facet joints with joint effusion was
found in three patients.
Six patients underwent spinal decompression surgery, including laminectomy,
posterolateral fusion, and posterior instrumentation. Postoperative clinical
follow-up recorded improvement in four patients. During retrospective review
of the operation notes, we noted that only two posterior epidural cysts were
meticulously dissected during surgery. The histologic assessment in these
cases indicated a synovial cyst in one and focal granulation tissue and
fibrosis in another.
Discussion
Baastrup's disease is well documented, although concurrent spinal lesions
(e.g., degenerative disk disease, spinal stenosis, and spondylolisthesis) are
common [3]. An excellent review
of the history of Baastrup's disease is that of Bywaters and Evans
[2] in 1982. In 1929,
Brailsford [4] described
arthritic joints between the spinous processes on radiologic assessment and
noted that "such patients have pain in the back when standing erect
which is relieved by bending forward." In 1933, Baastrup
[5] described in detail the
clinical and radiographic features of the syndrome. Radiographically, the
disorder is characterized by close approximation and contact of adjacent
spinous processes (kissing spines) and resultant enlargement, flattening, and
reactive sclerosis of apposing interspinous surfaces. Baastrup's disease
manifests clinically as localized midline lumbar tenderness and pain on spine
extension that can be relieved by spinal flexion, local anesthetic injection
[6], and excision of part of
the involved spinous processes
[2].
Histologically, a cavity or adventitious bursa has been described in the
interspinous space. In 1825, Mayer of Bonn
[7] noted such fluid-filled
cavities and designated them "diarthroses interspinosae." Rissanen
[8] reported that these
cavities did not appear before the age of 10 years but were present in adults
with a frequency that increased with age. Bywaters and Evans
[2] documented the occurrence
of a synovial membrane with a thin layer of sparse lining cells around the
cavity and confirmed the presence of an adventitious bursa that related to
repeated shearing movement between adjacent spinous processes.
In a study of lumbar facet joint arthrography, Sarazin et al.
[9] reported vertical
communications (i.e., opacification of the upper or lower ipsilateral facet
joint), horizontal communications (i.e., opacification of the contralateral
facet joint), and opacification of a defect in the pars interarticularis that
may occur via the retrodural or interspinous space. In cases of advanced
Baastrup's disease, communication between the interspinous space and both
facet joints leads to a classic butterfly appearance.
These patterns of communication were also observed during interspinous
bursography in our patients. This observation indicates that extensive
degeneration in the soft tissue of retrodural and interspinous spaces may
occur and form communicating pathways. When the fluid collection in the
interspinous space is large, it may extend directly to the ligamentum flavum
and even to the posterior epidural space in a manner similar to the formation
of a synovial cyst in the presence of a joint effusion that produces high
intraarticular pressure.
In our patients, the presence of direct continuity on sagittal MRIs and a
direct opacified communicating channel on CT bursograms between the epidural
cysts and the interspinous bursae (Figs.
2D and
3D), the absence of
significant facet arthrosis and effusion, the absence of a visible
communicating channel between the cysts and facet joints on MRIs (Figs.
1B,
2D, and
3B), and the absence of
retrograde contrast material opacification of facet space in at least one
patient allow us to postulate that the posterior epidural cyst may be the
direct anterior extension of the interspinous bursal fluid. The latter may
also communicate with the facet joint, as reported by Sarazin et al.
[9]. We cannot, however,
exclude the possibility of posterior midline extension of the synovial cyst
arising from the facet joints in some patients.
MRI of the lumbar spine can clearly depict Baastrup's disease, interspinous
bursal fluid, and an associated posterocentral epidural cyst. Interspinous
bursography is not really necessary for preoperative evaluation of spinal
stenosis for these patients. For this report, we performed bursography to
document the direct communication between the bursa and the epidural cyst. In
our opinion, however, bursography with steroid injection may be helpful in
conservative treatment of epidural cysts because it is helpful in treating
synovial cysts of the facet joints.
Our study has some limitations. Potential selection bias is unavoidable
because only 10 cases of MRI manifestation of posterocentral epidural cysts
were selected for evaluation. Correlation of clinical findings with the
location and size of the epidural cysts was not possible. The number of
patients was small, and few patients underwent bursography. Surgical
observations and histologic data were limited.
In conclusion, Baastrup's disease is associated with interspinous bursal
fluid. Fluid in the bursa can extend into the posterocentral epidural space
and cause central spinal stenosis with posterior compression of the thecal
sac. Bursography alone or combined with CT allows documentation of abnormal
communicating channels between the interspinous bursa and epidural cyst.
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Abstract
Introduction
