IMAGING DIAGNOSIS: ERRONEOUS LOCALIZATION OF SPINAL

ARACHNOID CYST

RITA GONcALVES, GAWAIN HAMMOND, JACQUES PENDERIS

Signalment

THREE-YEAR-OLD, neutered female, Staffordshire Bull

Terrier.

History and Physical Findings

The dog had a 6week history of spinal pain, fecal in-

continence, and mild pelvic limb ataxia. The fecal incon-

tinence was characterized by defecation of normal stool

without posturing, most commonly during sleep. There

were conscious and unconscious proprioceptive deficits in

both pelvic limbs with normal muscle tone and segmental

spinal reflexes. These findings were consistent with a lesion

affecting the T3–L3 spinal cord segments. Considerations

included disk disease, neoplasia, inflammatory disease, ar-

achnoid cyst, or congenital malformation.

Imaging

Radiographically, there was fusion of the laminae with

malformation of the articular processes and spondylosis at

T13–L1. Ioversol� was injected into the lumbar cistern and

the needle left in place to allow introduction of additional

contrast medium. There was a bulbous dilation of the dor-

sal aspect of the subarachnoid space between the caudal

portion of T12 and the cranial portion of L2 causing com-

pression of the spinal cord (Fig. 1A). There was obstruc-

tion to flow of contrast medium cranial to the point of the

dilation; thus the injection pressure was increased. Cranial

to this dorsal compression, the dorsal aspect of the contrast

medium column was absent over T10–T11, with a second

and more subtle dilation of the dorsal aspect of the sub-

arachnoid space over T8–T9.

Transverse computed tomography (CT) images of the

spine were acquired immediately following myelography,

with the dog in dorsal recumbencyw using a 2.7mm slice

thickness (ST) (Fig. 1B–E). There was an enlargement of

the dorsal aspect of the subarachnoid space from the mid

portion of T12 to the cranial aspect of L2. There was a

subtle filling defect in the dorsal aspect of the subarachnoid

space at T10. These findings were interpreted as a dorsal

spinal arachnoid cyst.

Following identification of the dorsal filling defect at

T10, magnetic resonance (MR) imaging was performed

using a 1.5-Tesla systemz and an extremity coil. The fol-

lowing pulse sequences were used: T1-weighted pre- and

postcontrasty and T2-weighted. Images were acquired in

sagittal and transverse planes. The repetition time [TR] and

echo time [TE] used for sagittal images were as follows:

turbo spin echo (TSE) T1 (TR¼ 605, TE¼ 12) and TSE T2

(TR¼ 2300, TE¼ 120) with a 2.75mm ST. The following

settings were used for the transverse images: TSE T1

(TR¼ 725, TE¼ 14) and TSE T2 (TR¼ 5594, TE¼ 125)

with a 4mm ST.

On T2-weighted images (Fig. 2), the dorsal aspect of the

subarachnoid space was distorted by two areas that were

hyperintense to the spinal cord, one at the level of T10

(Fig. 2C) and the other at T12 (Fig. 2E). The same areas

were hypointense on T1-weighted images and the signal

intensity was equivalent to cerebrospinal fluid (CSF).

These findings supported the diagnosis of spinal arachnoid

cyst. In addition to the spinal arachnoid cyst, spinal cord

T2-hyperintensity consistent with syringohydromyelia was

also present between T3 and L2 (Fig. 2B, D, and F); this

was most severe from T11 to L2 (Fig. 2F). After the ad-

ministration of contrast medium, no abnormal enhance-

ment was identified.

MR imaging allowed identification of the true location

of the cyst, which had been incorrect on myelography,

most likely due to the injection of the myelographic

contrast medium under pressure, causing collapse of the

presumed syringohydromyelia caudal to the arachnoid

cyst.

Diagnosis/Outcome

A dorsal laminectomy was performed from T10 to T13.

A subdural fluid accumulation was identified over T10–

T12. Durotomy with wide excision of the dura mater and

cyst wall was performed and the cyst lining was debrided.

Address correspondence and reprint requests to Dr. Jacques Penderis atthe above address. E-mail: j.penderis@vet.gla.ac.uk

Received September 11, 2007; accepted for publication February 17,2008.

doi: 10.1111/j.1740-8261.2008.00408.x

From the Division of Companion Animal Sciences, Institute ofComparative Medicine, Faculty of Veterinary Medicine, University ofGlasgow, Glasgow, G61 1QH, UK.

�Optiray, 300mgI/ml, Mallinckrodt Medical Imaging, Dublin, Ireland.wExcel 2400, twin flash multidetector CT, Elscint-Marconi, Haifa,

Israel.

zGyroscan ACS NT, Philips Medical Systems, Eindhoven, the Neth-erlands.y0.1mmol/kg gadopentetate dimeglumine, Berlex, Montville, New

Jersey.

460

The abnormal material was not submitted for histopatho-

logic examination but the surgical appearance was similar

to that of an arachnoid cyst, although an inflammatory

process such as subarachnoiditis cannot be excluded. The

dog improved and was discharged 7 days after surgery.

Twelve months after surgery, the spinal pain had resolved

and the pelvic limb ataxia was less severe but there was no

change in the fecal incontinence.

Fig. 1. Lateral myelogram (A) and transverse CT-myelogram images (B–E), with the level of the transverse images (B–E) represented by the dashed arrowsin A. There is a bulbous dilation of the dorsal aspect of the subarachnoid space causing marked compression of the spinal cord between the caudal portion ofT12 and the cranial portion of L2 in A. Cranial to this dorsal compression, loss of the dorsal aspect of the contrast medium column is evident over T10–T12. Inthe CT images, there is progressive enlargement of the dorsal aspect of the subarachnoid space from the mid portion of T12 to the cranial aspect of L2 (D, E).The enlargement of the dorsal aspect of the subarachnoid space in D corresponds to an area of loss of the dorsal aspect of the contrast medium column in A andprobably represents delayed filling of the spinal arachnoid cyst. A subtle filling defect is also present within the contrast medium in the dorsal aspect of thesubarachnoid space at the level of the T10 vertebra (B), most likely representing an additional component of the spinal arachnoid cyst. Left-sided spondylosis ispresent at T13–L1 and there is fusion of the laminae, articular processes and pedicles from L1 to L3.

Fig. 2. Sagittal (A) and transverse (B–F) T2–weighted magnetic resonance images of the spine from C7 to L5. The level of the transverse images B–F arerepresented by the dashed arrows in the sagittal image (A). The dorsal aspect of the subarachnoid space is distorted by two areas that are hyperintense to thespinal cord, one at T10 (C) and the other at T12 (E), consistent with a spinal arachnoid cyst. In addition, there is T2 hyperintensity in the spinal cord consistentwith syringohydromyelia between T3 and L2 (B, D, F). This presumed syringohydromyelia is most severe from T11 to L2, occupying the majority of the spinalcord diameter in this area (F).

461ARACHNOID CYSTVol. 49, No. 5

Discussion

The etiology of spinal arachnoid cysts in dogs is unclear

but congenital malformation of the arachnoid membrane

or acquired malformation following traumatic or inflam-

matory injury have been proposed.1–7 As arachnoid cysts

represent accumulation of CSF within a diverticulum of the

arachnoid mater, the term arachnoid cyst is not actually

appropriate as they lack an epithelial lining. These lesions

are more correctly described as arachnoid diverticulae, cav-

itations, or pseudocysts.1,2,7 However, the term arachnoid

cyst has become accepted to describe these lesions.

Myelographically, an arachnoid cyst is characterized by

a tear-drop-shaped bulbous expansion of the subarachnoid

space causing attenuation of the spinal cord.6,8,9 Large

cysts may appear as filling defects during myelography,

suggesting a lack of communication between the arachnoid

cyst and the subarachnoid space.10–12 This lack of filling

may be due to a one-way valve that opens periodically with

fluctuations in CSF pressure or to the arachnoid cyst open-

ing being in a position such that the contrast medium can

only enter the cyst from one direction.7,10 In these patients,

administration of contrast medium via the cisterna magna

or imaging 12–24h after the administration of the contrast

medium may allow identification of these cysts.11,13,14

In our patient, the myelographic localization of the ar-

achnoid cyst was erroneous, possibly due to the injection of

contrast medium under pressure, causing collapse of the

presumed syringohydromyelia caudal to the arachnoid

cyst, thereby mimicking the appearance of an arachnoid

cyst and leading to an incorrect anatomic localization at

T13–L1 vs. the true location at T10 and T12.

In humans, before MR neuroimaging became routine,

identification of syringohydromyelia was achieved by noting

the ‘‘collapsing cord sign’’ on myelography or delayed diffu-

sion of contrast medium into the cystic area on CT myelo-

graphy.15 For identification of the ‘‘collapsing cord sign,’’

patients were radiographed in different body positions to

demonstrate collapse of the elevated spinal cord segment

and distension of the dependent spinal cord segment, sug-

gesting that relatively low pressures may result in collapse of

spinal cord segments affected by syringohydromyelia.

As a lower concentration of contrast medium is neces-

sary for visualization of arachnoid cysts using CT, this

modality can facilitate diagnosis of arachnoid cysts in pa-

tients where the cyst appears as a filling defect on myelo-

graphy.10 Delayed CT myelography is often recommended

in humans to help visualize slow filling arachnoid cysts.11,16

In our patient, CT myelography allowed identification of

the arachnoid cyst at T12, which was only evident as a

filling defect in the dorsal aspect of the subarachnoid space

on myelography. However, this was confounded by the

larger dilation of the subarachnoid space immediately cau-

dal to this, resulting from the injection of the contrast me-

dium collapsing the presumed syringohydromyelia. CT

myelography failed to allow identification of the arachnoid

cyst at T10. Delayed CT myelography may have helped to

allow visualization of the arachnoid cyst at T10, but that

was not performed in our patient.

In MR images of dogs with spinal arachnoid cysts, the

cysts typically appear as easily identifiable enlargements of

the subarachnoid space that are isointense to CSF. The

advantages of using MR imaging over myelography or CT

myelography include identification of noncommunicating

cysts and identification of associated spinal cord abnor-

malities.6,9,10,13 In our patient, MR imaging allowed iden-

tification of the true location of the cyst at T10 and T12 as

well as the presence of syringohydromyelia. Identifying

syringohydromyelia is important as it can affect prognosis

and treatment options.11,13 Syringohydromyelia can occur

cranial, caudal, or ventral to an arachnoid cyst, with the

most common location being caudal.10,14,17 Syringohyd-

romyelia is thought to arise due to altered CSF flow within

the spinal cord subarachnoid space, occurring secondary to

the arachnoid cyst.14

If MR imaging had not been performed in this patient,

decompressive surgery would have been performed at the

wrong location.

REFERENCES

1. Parker AJ, Adams WM, Zackery JF. Spinal arachnoid cysts in the

dog. J Am Anim Hosp Assoc 1983;19:1001–1008.2. Dyce J, Herrtage ME, Houlton JEF, Palmer AC. Canine spinal

arachnoid cysts. J Small Anim Pract 1991;32:433–437.3. Hardie RJ, Linn KA, Rendano VT. Spinal meningeal cyst in a dog:

a case report and literature review. J Am Anim Hosp Assoc 1996;32:

477–480.4. Bagley RS, Silver GM, Seguin B, Lincoln JD, Britt LG. Scoliosis and

associated cystic spinal cord lesion in a dog. J Am Vet Med Assoc 1997;

211:573–575.5. Frykman OF. Spinal arachnoid cyst in four dogs: diagnosis, surgical

treatment and follow-up results. J Small Anim Pract 1999;40:544–549.6. Galloway AM, Curtis NC, Sommerlad SF, Watt PR. Correlative

imaging findings in seven dogs and one cat with spinal arachnoid cysts. Vet

Radiol Ultrasound 1999;4:445–452.

7. Nabors MW, Pait TG, Byrd EB, et al. Updated assessment and

current classification of spinal meningeal cysts. J Neurosurg 1988;68:

366–377.8. Cambridge AJ, Bagley RS, Britt LG, Silver GM. Radiographic

diagnosis: arachnoid cyst in a dog. Vet Radiol Ultrasound 1997;38:434–436.9. Gnirs K, Ruel Y, Blot S, et al. Spinal subarachnoid cysts in 13 dogs.

Vet Radiol Ultrasound 2003;44:402–408.10. Skeen TM, Olby NJ, Munana KR, Sharp NJ. Spinal arachnoid cysts

in 17 dogs. J Am Anim Hosp Assoc 2003;39:271–282.11. Sklar E, Quencer RM, Green BA, Montalvo BM, Post MJD. Ac-

quired spinal subarachnoid cysts: evaluation with MRI, CT myelography,

and intraoperative sonography. Am J Neuroradiol 1989;10:1097–1104.12. Kendall BE, Valentine AR, Keis B. Spinal arachnoid cysts: clinical

and radiological correlation with prognosis. Neuroradiology 1982;22:

225–234.

462 GONc� ALVES, HAMMOND, AND PENDERIS 2008

13. Jurina K, Grevel V. Spinal arachnoid pseudocysts in 10 rottweilers.

J Small Anim Pract 2004;45:9–15.14. Holly LT, Batzdorf U. Syringomyelia associated with intradural

arachnoid cysts. J Neursurg Spine 2006;5:111–116.15. Kan S, Fox AJ, Vinuela F, Debrun G. Spinal cord size in

syringomyelia: change with position on metrizamide myelography. Radio-

logy 1983;146:409–414.

16. Krings T, Lukas R, Reul J, et al. Diagnostic and therapeuticmanagement of spinal arachnoid cysts. Acta Neurochir 2001;143:227–235.

17. Webb AA. Intradural spinal arachnoid cyst in a dog. Can Vet J1999;40:588–589.

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