CREDITS CE Article 1
Intracranial Arachnoid Cysts in Dogs
❯❯ Curtis W. Dewey, DVM, Abstract: Intracranial arachnoid cyst (IAC) is an infrequently reported developmental
MS, DACVIM (Neurology), disorder seen primarily in small-breed dogs. It usually occurs in the caudal fossa, in the
DACVSa region of the quadrigeminal cistern. Although still considered uncommon, IAC is being
❯❯ Peter V. Scrivani, DVM, recognized more frequently in veterinary medicine, coinciding with the increased avail-
ability of magnetic resonance imaging. In this article, clinical information from previously
❯❯ Ursula Krotscheck, DVM,
DACVS reported cases of canine IAC is combined with additional case information from our
❯❯ Soﬁa Cerda-Gonzalez, hospitals. Similar to IAC in people, it is thought that canine IAC is often an incidental
DVM, DACVIM (Neurology) ﬁnding. When IAC is responsible for neurologic disease in dogs, generalized seizures
Cornell University and cerebellovestibular dysfunction are the most common clinical presentations. Medi-
cal therapy of IAC focuses on management of increased intracranial pressure and sei-
❯❯ Kerry Smith Bailey, DVM, zures, if the latter are part of the clinical complaints. Surgical therapy of IAC involves
either cyst fenestration or shunting the excess ﬂuid to the peritoneal cavity.
Oradell Animal Hospital
Paramus, New Jersey
ntracranial arachnoid cyst (IAC), also caudal fossa.2–11 Because IAC is typically
❯❯ Dominic J. Marino, DVM, called intracranial intraarachnoid cyst associated with the quadrigeminal cistern
DACVS and quadrigeminal cyst, is a develop- in dogs, these accumulations of ﬂuid are
Long Island Veterinary mental brain disorder in which cerebro- often called quadrigeminal cysts in this
Specialists spinal ﬂuid (CSF) is thought to accumulate species and have a characteristic appear-
Plainview, New York within a split of the arachnoid membrane.1 ance on magnetic resonance imaging (MRI)
Although IACs have been reported to scans4–6,10,11 (FIGURE 1). IACs account for 1%
occur in several locations in humans, all of all intracranial masses in people and are
reported canine cases have been in the often considered incidental ﬁndings.12–14
At a Glance FIGURE 1
Medical Therapy Options
Treatment and Prognosis
Midsagittal T2-weighted image of a dog’s brain demonstrating the characteristic appear-
Dr. Dewey discloses that he has ance of an IAC in this species. The asterisk is in the center of the cyst. (Reprinted with permission
received ﬁnancial support from from Dewey CW. Encephalopathies: disorders of the brain. In: Dewey CW, ed. A Practical Guide to
Boehringer-Ingelheim. Canine and Feline Neurology. 2nd ed. Ames, Iowa: Wiley-Blackwell; 2008:115-120.)
160 Compendium: Continuing Education for Veterinarians® | April 2009 | CompendiumVet.com
Intracranial Arachnoid Cysts in Dogs CE
Dr. Dewey is an associate
professor of neurology and
neurosurgery and chief of
the section of neurology at
CompendiumVet.com | April 2009 | Compendium: Continuing Education for Veterinarians® 161
CE Intracranial Arachnoid Cysts in Dogs
There are 10 clinical reports of IAC in tissue called the perimedullary mesh; this
dogs in the veterinary literature. 2–11 This tissue eventually becomes the pia and arach-
review combines these reported cases with noid layers of the meninges. In normal devel-
three additional cases from our hospitals to opment, pulsatile CSF ﬂow from the choroid
present information regarding 56 dogs with plexuses is thought to divide the perimedul-
IAC. Most reported cases of IAC in dogs lary mesh into the pia and arachnoid layers,
are in small breeds, with a predominance effectively creating the subarachnoid space.
of brachycephalic animals. 2–11 Shih tzus It is postulated that some aberration of CSF
may be overrepresented.11,15 Male sex also ﬂow from the choroid plexuses during this
appears to be a predisposing factor. Clinical stage of development forces a separation
signs attributable to IAC in dogs are most within the forming arachnoid layer, eventu-
often related to cerebral or cerebellar com- ally leading to the creation of an IAC.1,12 The
pression by the cyst; generalized seizures intraarachnoid location of IACs has been
and central vestibular dysfunction are most demonstrated via light and electron micros-
commonly noted.15 Similar to human IACs, copy in people.1 Depending on whether
a large proportion of reported IACs in dogs these cysts communicate with the subarach-
were suspected to be incidental.4–6,11 Medical noid space or the ventricular system, they are
and surgical options are available to treat sometimes referred to as communicating or
IAC in dogs. noncommunicating.1
QuickNotes The mechanisms by which an IAC con-
When there is evi- Pathogenesis tinues to expand with fluid are unknown,
IACs are believed to represent a developmen- but several theories have been proposed.1,12
dence of a large
tal abnormality caused by an aberrant split There is evidence that arachnoid cells lining
IAC and another in the arachnoid membrane during embryo- the IAC may have secretory capacity.1,12,16
disease (e.g., genesis.1,12 The developing neural tube is sur- Fluid may also move into the cyst via an
granulomatous rounded by a loose layer of mesenchymal osmotic pressure gradient. However, consid-
meningoencepha- ering that the fluid within the IAC is nearly
lomyelitis, hydro- BOX 1
identical to CSF, this theory is unlikely.1,12 In
cephalus) in the addition, there have been documented cases
same patient, the Breed Distribution of Reported in people in which small slits exist between
optimal response Intracranial Arachnoid Cysts the IAC and the subarachnoid space; these
slits act as one-way valves, diverting CSF
to treatment may in Dogs into the cyst during systole and preventing
require treating its return to the subarachnoid space during
both conditions. Shih tzu . . . . . . . . . . . . . . . . . . . . . . . . . 15 dogs diastole.1,12,17
Maltese . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Pug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 Clinical Features
Lhasa apso . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 Most reported dogs with IAC have been small
Cavalier King Charles spaniel . . . . . . . . . . . .4 breeds, and many had brachycephalic confor-
Yorkshire terrier . . . . . . . . . . . . . . . . . . . . . . . .4 mation. The following information was obtained
Chihuahua. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 by combining the IAC cases reported in the
Staffordshire bull terrier . . . . . . . . . . . . . . . . .3
literature with three additional cases from our
Bulldog. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
hospitals. The breed distribution of these 56
Pekingese . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
dogs is listed in BOX 1. Approximately 66% of
West Highland white terrier . . . . . . . . . . . . . .2
the dogs (37 of 56) were male.2–11
Bichon frise . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
There is a wide age range at clinical pre-
Pomeranian . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
sentation for dogs with IAC (2 months to 12
Cairn terrier. . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
years), with an approximate average age of 4
Jack Russell (Parson Russell) terrier . . . . . .1
Terrier mix. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 years. The most common clinical signs (BOX 2)
Beagle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 seen with IAC are reﬂective of forebrain or
Miniature schnauzer. . . . . . . . . . . . . . . . . . . . .1 central vestibular (cerebellovestibular) dysfunc-
German shorthaired pointer . . . . . . . . . . . . .1 tion. Other reported clinical signs include neck
pain, paraparesis, and tetraparesis.2–11
162 Compendium: Continuing Education for Veterinarians® | April 2009 | CompendiumVet.com
CE Intracranial Arachnoid Cysts in Dogs
Diagnosis presence of another brain disorder is purely
Diagnosis of IAC in dogs is typically made via an incidental ﬁ nding. We have seen a number
computed tomography (FIGURE 2) or, prefer- of patients with relatively small dilations of
ably, MRI.2–5,7–10 IACs may also be visualized the quadrigeminal cistern (FIGURE 4), which
using ultrasound (via the foramen magnum, may represent a variant of normal structure
temporal window, or persistent bregmatic or may be evidence of nascent IACs that may
fontanelle), especially in younger dogs.6 The be of no clinical signiﬁcance. Conversely, very
characteristic ultrasonographic appearance of large IACs have been described both as sole
IAC is a large, ﬂuid-ﬁlled structure, isoechoic disease entities and as suspected incidental
with the CSF spaces (e.g., lateral ventricle) and ﬁndings in patients with other intracranial
located between the occipital lobe of the cere- disease processes. Anecdotally, we have also
brum and rostral lobe of the cerebellum.6 observed similar cystic structures in the brain
MRI provides the best detail for diagnosis of that do not appear to be associated with the
IAC and is most likely to provide information quadrigeminal cistern (FIGURE 5); consider-
regarding the presence and nature of concurrent ing that IACs occur in multiple locations in
disease states. The typical appearance of IAC the human brain, this ﬁ nding may simply
on MRI is a well-demarcated, cystic-appearing imply that the term quadrigeminal cyst is too
structure that is hypointense on T1-weighted restrictive in describing IAC in dogs.12 Since
images, hyperintense on T2-weighted images, the presence of a large, ﬂuid-ﬁ lled struc-
and non-contrast enhancing with intrave- ture within the cranial vault likely decreases
nous gadolinium administration and that sup- intracranial compliance, some IACs may be
presses on FLAIR (ﬂuid attenuation inversion contributory rather than incidental ﬁndings.
recovery) images2–5,10,11 (FIGURE 3). Because IAC In other words, when there is evidence of a
may be an incidental ﬁnding, it is important large IAC and another disease (e.g., granu-
to rule out concurrent inﬂammatory disease lomatous meningoencephalomyelitis, hydro-
with a CSF examination. In the absence of an cephalus) in the same patient, the optimal
additional brain disorder, the CSF is typically response to treatment may require treating
normal in dogs with IAC. both conditions.
In our opinion, it is often difﬁcult or impos- In addition, the combined presence of
sible to discern whether a large IAC in the hydrocephalus and IAC in a patient does
Transaxial Computed Tomography Images
Preoperative (A) and postoperative (B) transaxial computed tomography images
from a dog with an IAC treated with cystoperitoneal shunting. In A, the arrowheads are point-
ing to the ventral aspect of the cyst and the outlined arrowheads are pointing to the dorsal aspect
of the cyst. In B, the arrow is pointing to the rostral aspect of the shunt. (Reprinted with permission
from Dewey CW, Krotscheck U, Bailey KS, Marino DJ. Craniotomy with cystoperitoneal shunting for treat-
ment of intracranial arachnoid cysts in dogs. Vet Surg 2007;36:416-422.)
164 Compendium: Continuing Education for Veterinarians® | April 2009 | CompendiumVet.com
Intracranial Arachnoid Cysts in Dogs CE
not necessarily make one of the disorders Treatment and Prognosis
(IAC) incidental, nor does this combination Because IAC in dogs, as in people, is often
ensure that surgically treating one disorder considered an incidental finding, it is fre-
will address the other. Hydrocephalus can be quently assumed that treatment of the sus-
secondary to an IAC, developing because of pected primary disorder (e.g., inﬂammatory
mechanical obstruction of normal CSF ﬂow brain disease, hydrocephalus) is indicated
by the expanding cyst (i.e., obstructive hydro- and the IAC is of little or no clinical con-
cephalus).10,18,19 It is unlikely that an IAC can be sequence. As mentioned above, we ques-
distinguished as communicating or noncom- tion whether this approach is appropriate. In
municating based on standard MRI sequences; cases in which the IAC is considered the pri-
such a distinction would likely require either mary disease condition, medical therapy is
contrast studies or phase-contrast (cine) MRI. aimed at reducing brain edema and increased
In one report of two dogs with IAC, intracys- intracranial pressure associated with the IAC,
tic hemorrhage, which was suspected to have as well as controlling seizure activity if pres-
contributed to the development of neurologic ent. Medical treatment for IAC is identical to
dysfunction, was veriﬁed at surgery.3 that described for congenital hydrocephalus
In a recent study, the degree of brain com- (e.g., corticosteroids, diuretics, anticonvul-
pression by an IAC was calculated from MRI sants if indicated), and dose recommenda-
scans of dogs with the disorder, and these tions for various therapies are summarized
measurements were correlated with the pres- in BOX 3.20,21 Dogs with IAC tend to respond QuickNotes
ence or absence of clinical dysfunction. It initially to medical therapy, but the response
In cases in which
was found that dogs with >14% compression may be temporary.
of the occipital lobe always displayed clini-
the IAC is consid-
cal signs and that dogs with compression of ered the primary
BOX 3 disease condition,
both the cerebellum and the occipital lobe
were signiﬁcantly more likely to display clini- Medical Therapy Options medical therapy is
cal signs than dogs with compression of only
for Dogs With Intracranial aimed at reducing
one region or dogs with no apparent brain brain edema and
compression.11 Arachnoid Cysts
nial pressure asso-
Prednisone: 0.25–0.50 mg/kg PO q12h ciated with the IAC.
Clinical Signs Reported Diuretics
in Dogs With Intracranial Furosemide: 0.5–4.0 mg/kg PO q12–24h
Acetazolamide: 10 mg/kg PO q6–8h
Proton pump inhibitors
Abnormal mentation Omeprazole: 10 mg/kg PO q24h (dogs
<20 kg); 20 mg/kg PO q24h (dogs >20 kg)
Ataxia Anticonvulsant drugs
Head tilt Phenobarbital: 3–5 mg/kg PO q12h
Strabismus Potassium bromide: 35 mg/kg PO divided
Paraparesis Gabapentin: 10 mg/kg PO q8h
Felbamate: 15 mg/kg PO q8h
Zonisamide: 5 mg/kg PO q12h (if not on
Tetraparesis phenobarbital; 10 mg/kg if on phenobarbital)
Visual deﬁcits Levetiracetam: 20 mg/kg PO q8h
Neck pain Pregabalin: 3 to 4 mg/kg PO q8–12h*
*Because IAC is often considered an incidental ﬁnding, some of *Dewey CW, Cerda-Gonzalez S, Levine JM, et al. Pregabalin
these reported clinical signs may be attributable to concurrent therapy for refractory idiopathic epilepsy in dogs. J Vet Intern
intracranial diseases. Med 2008;22:765.
CompendiumVet.com | April 2009 | Compendium: Continuing Education for Veterinarians® 165
CE Intracranial Arachnoid Cysts in Dogs
MRI provides the
best detail for diag-
nosis of IAC and
is most likely to
regarding the pres-
ence and nature of
Midsagittal (A), dorsal (B), and transaxial (C) T1-weighted MRI scans
demonstrating an IAC in a dog. The arrows are pointing to the cyst in all images.
FIGURE 4 FIGURE 5
Midsagittal T1-weighted brain MRI scan of Midsagittal T1-weighted brain MRI scan of
a dog with a small dilation of the quadrigeminal a dog, demonstrating a cystic structure (white
cistern (white arrow). The black arrow indicates arrow) associated with the third ventricle, rather
the dorsal aspect (tectrum) of the midbrain. than the quadrigeminal cistern (black arrow).
166 Compendium: Continuing Education for Veterinarians® | April 2009 | CompendiumVet.com
Intracranial Arachnoid Cysts in Dogs CE
Cystoperitoneal Shunt Placement
Placement of the rostral end of a cystoperitoneal shunt in a dog (A)
and a postoperative ventrodorsal radiograph (B) demonstrating location of
the shunting device (arrowheads). (Reprinted with permission from Dewey CW,
Krotscheck U, Bailey KS, Marino DJ. Craniotomy with cystoperitoneal shunting for
treatment of intracranial arachnoid cysts in dogs. Vet Surg 2007;36:416-422.)
Surgical management of IAC in people is reported.10 The cyst did not reform in any of
typically achieved via either cyst fenestration the shunted cases.
(i.e., making an opening into the cyst wall) or The success rate for surgical management
cystoperitoneal shunt (CPS) placement.14,18,19,22–24 of IAC appears to be high in people and dogs,
Proponents of fenestration cite a high surgical and whether fenestration or CPS is the pre-
success rate and avoidance of shunt-related ferred procedure remains controversial for both
complications as reasons for this surgical choice; species.10 Because of the paucity of reports of
proponents of CPS report high success rates surgically managed dogs with IAC as well as
and avoidance of cyst reexpansion as reasons the suspected high incidence of this abnor-
for the use of this technique.13,14,18,19,22–26 Both mality being an incidental ﬁnding, the actual
fenestration and CPS (FIGURE 6) procedures surgical success rate for IAC treatment in dogs
have been reported in dogs with IAC.2,3,5,7,10 should be regarded as unknown at this time.
IAC was considered the primary disease in Hopefully, as information regarding medical
ﬁve reported fenestration cases. Three patients and surgical treatment of clinically signiﬁcant
were reimaged after surgery; two of the three IAC in dogs accumulates, the understanding
dogs had evidence of cyst persistence on MRI. of the natural course of this disorder and the
However, only one dog required reoperation.2,3,7 effectiveness of medical and surgical therapies
Successful CPS of dogs with IAC has also been to manage it will improve.
1. Rengachary SS, Watanabe I. Ultrastructure and pathogenesis of 5. Duque C, Parent J, Brisson B, et al. Intracranial arachnoid cysts:
intracranial arachnoid cysts. J Neuropath Exp Neurol 1981;40:61-83. are they clinically signiﬁcant? J Vet Intern Med 2005;19:772-774.
2. Vernau KM, Kortz GD, Koblik PD, et al. Magnetic resonance im- 6. Saito M, Olby NJ, Spaulding K. Identiﬁcation of arachnoid cysts
aging and computed tomography characteristics of intracranial intra- in the quadrigeminal cistern using ultrasonography. Vet Radiol Ul-
arachnoid cysts in 6 dogs. Vet Radiol Ultrasound 1997;38:171-176. trasound 2001;42:435-439.
3. Vernau KM, LeCouteur RA, Sturges BK, et al. Intracranial intra- 7. Platt SR. What is your diagnosis? J Small Anim Pract 2002;43:469-470.
arachnoid cyst with intracystic hemorrhage in two dogs. Vet Radiol 8. Nagae H, Oomura T, Kato Y, et al. A disorder resembling arach-
Ultrasound 2002;43:449-454. noid cyst in a dog. J Jpn Vet Neurol 1995;2:9-14.
4. Kitagawa M, Kanayama K, Sakai T. Quadrigeminal cisterna arach- 9. Orima H, Fujita M, Hara Y, et al. A case of the dog with arachnoid
noid cyst diagnosed by MRI in ﬁve dogs. Aust Vet J 2003;81:340-343. cyst. Jpn J Vet Imag 1998;10:49-51.
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10. Dewey CW, Krotscheck U, Bailey KS, Marino DJ. Craniotomy with 18. Raffel C, McComb JG. To shunt or fenestrate: which is the best sur-
cystoperitoneal shunting for treatment of intracranial arachnoid cysts gical treatment for arachnoid cysts in pediatric patients? Neurosurgery
in dogs. Vet Surg 2007;36:416-422. 1988;23:338-342.
11. Matiasek LA, Platt SR, Shaw S, Dennis R. Clinical and magnetic 19. Locatelli D, Bonfanti N, Sfogliarini R, et al. Arachnoid cysts: di-
resonance imaging characteristics of quadrigeminal cysts in dogs. J agnosis and treatments. Childs Nerv Syst 1987;3:121-124.
Vet Intern Med 2007;21:1021-1026. 20. Coates JR, Axlund TW, Dewey CW, Smith J. Hydrocephalus in
12. Cincu R, Agrawal A, Eiras J. Intracranial arachnoid cysts: cur- dogs and cats. Compend Contin Educ Pract Vet 2006;28:136-146.
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1. In dogs, all reported cases of IAC have 4. Which is most characteristic of the compression by the cyst was measured,
been in the _________ fossa. typical signalment for dog with an IAC? dogs were found to be most likely to
a. rostral a. 10-year-old female spayed German exhibit clinical dysfunction if
b. middle shepherd a. the cyst compressed more than 14% of
c. caudal b. 4-year-old male castrated shih tzu the occipital lobe of the cerebrum.
d. none of the above c. 2-year-old female greyhound b. both the occipital lobe of the cerebrum
d. There is no typical signalment. and the cerebellum were compressed by
2. Formation of IACs in dogs is believed the cyst.
to be due to 5. Clinical signs associated with IAC in c. a and b
a. a split in the arachnoid meningeal dogs include d. none of the above
layer during embryogenesis. a. abnormal mentation.
b. failure of the neuroectoderm and b. generalized seizures. 9. Medical therapy for IAC in dogs is
nonneural ectoderm to separate c. cerebellar dysfunction. directed at
during embryogenesis. d. all of the above a. decreasing brain edema associated with
c. compensatory ﬂuid accumulation the cyst.
following an in utero brain infarction 6. The preferred imaging modality for b. controlling seizure activity if present.
(stroke). diagnosis of IAC in dogs is c. minimizing increases in intracranial
d. failure of the cerebellar vermis a. ultrasound. pressure.
to form correctly during b. scintigraphy. d. all of the above
embryogenesis. c. magnetic resonance imaging.
d. computed tomography. 10. Which statement regarding surgical
3. Proposed theories to explain progressive management of IAC is false?
expansion of IACs in dogs include 7. The characteristic MRI appearance a. Both cyst fenestration and
a. active secretion by the arachnoid of an IAC in a dog is a large, well- cystoperitoneal shunting procedures
cells lining the cyst cavity. demarcated, cyst-like structure that is have been described in dogs.
b. movement of ﬂuid into the cyst cavity a. hypointense on T1-weighted images. b. Cystoperitoneal shunt (CPS) placement
along an osmotic pressure gradient. b. hyperintense on T2-weighted images. has been shown to be superior to fenes-
c. movement of ﬂuid into the cyst from c. contrast-enhancing and hyperintense tration in dogs and humans with IAC.
the neighboring subarachnoid space on FLAIR images. c. The success rate for surgical
via slit-like openings (one-way valves) d. a and b management of IAC appears to be high
into the cyst lumen. in people and dogs.
d. all of the above 8. In a study of IAC cases in which brain d. a and b
168 Compendium: Continuing Education for Veterinarians® | April 2009 | CompendiumVet.com