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Compendium April 2009 (Vol 31, No 4)

Intracranial Arachnoid Cysts in Dogs

by Curtis W. Dewey, DVM, MS, DACVIM (Neurology), DACVS, Peter V. Scrivani, DVM, Ursula Krotscheck, DVM, DACVS, Sofia Cerda-Gonzalez, DVM, DACVIM (Neurology), Kerry Smith Bailey, DVM, DACVIM (Neurology), Dominic J. Marino, DVM, DACVS

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    Abstract

    Intracranial arachnoid cyst (IAC) is an infrequently reported developmental disorder seen primarily in small-breed dogs. It usually occurs in the caudal fossa, in the region of the quadrigeminal cistern. Although still considered uncommon, IAC is being recognized more frequently in veterinary medicine, coinciding with the increased availability of magnetic resonance imaging. In this article, clinical information from previously reported cases of canine IAC is combined with additional case information from our hospitals. Similar to IAC in people, it is thought that canine IAC is often an incidental finding. When IAC is responsible for neurologic disease in dogs, generalized seizures and cerebellovestibular dysfunction are the most common clinical presentations. Medical therapy of IAC focuses on management of increased intracranial pressure and seizures, 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.

    Intracranial arachnoid cyst (IAC), also called intracranial intraarachnoid cyst and quadrigeminal cyst, is a developmental brain disorder in which cerebrospinal fluid (CSF) is thought to accumulate within a split of the arachnoid membrane.1 Although IACs have been reported to occur in several locations in humans, all reported canine cases have been in the caudal fossa.2-11 Because IAC is typically associated with the quadrigeminal cistern in dogs, these accumulations of fluid are often called quadrigeminal cysts in this species and have a characteristic appearance on magnetic resonance imaging (MRI) scans4-6,10,11 (FIGURE 1). IACs account for 1% of all intracranial masses in people and are often considered incidental findings.12-14

    There are 10 clinical reports of IAC in dogs in the veterinary literature.2-11 This review combines these reported cases with three additional cases from our hospitals to present information regarding 56 dogs with IAC. Most reported cases of IAC in dogs are in small breeds, with a predominance of brachycephalic animals.2-11 Shih tzus may be overrepresented.11,15 Male sex also appears to be a predisposing factor. Clinical signs attributable to IAC in dogs are most often related to cerebral or cerebellar compression by the cyst; generalized seizures and central vestibular dysfunction are most commonly noted.15 Similar to human IACs, a large proportion of reported IACs in dogs were suspected to be incidental.4-6,11 Medical and surgical options are available to treat IAC in dogs.

    Pathogenesis

    IACs are believed to represent a developmental abnormality caused by an aberrant split in the arachnoid membrane during embryogenesis.1,12 The developing neural tube is surrounded by a loose layer of mesenchymal tissue called the perimedullary mesh; this tissue eventually becomes the pia and arachnoid layers of the meninges. In normal development, pulsatile CSF flow from the choroid plexuses is thought to divide the perimedullary mesh into the pia and arachnoid layers, effectively creating the subarachnoid space. It is postulated that some aberration of CSF flow from the choroid plexuses during this stage of development forces a separation within the forming arachnoid layer, eventually leading to the creation of an IAC.1,12 The intraarachnoid location of IACs has been demonstrated via light and electron microscopy in people.1 Depending on whether these cysts communicate with the subarachnoid space or the ventricular system, they are sometimes referred to as communicating or noncommunicating.1

    The mechanisms by which an IAC continues to expand with fluid are unknown, but several theories have been proposed.1,12 There is evidence that arachnoid cells lining the IAC may have secretory capacity.1,12,16 Fluid may also move into the cyst via an osmotic pressure gradient. However, considering that the fluid within the IAC is nearly identical to CSF, this theory is unlikely.1,12 In addition, there have been documented cases in people in which small slits exist between the IAC and the subarachnoid space; these slits act as one-way valves, diverting CSF into the cyst during systole and preventing its return to the subarachnoid space during diastole.1,12,17

    Clinical Features

    Most reported dogs with IAC have been small breeds, and many had brachycephalic conformation. The following information was obtained by combining the IAC cases reported in the literature with three additional cases from our hospitals. The breed distribution of these 56 dogs is listed in BOX 1 . Approximately 66% of the dogs (37 of 56) were male.2-11

    There is a wide age range at clinical presentation for dogs with IAC (2 months to 12 years), with an approximate average age of 4 years. The most common clinical signs (BOX 2) seen with IAC are reflective of forebrain or central vestibular (cerebellovestibular) dysfunction. Other reported clinical signs include neck pain, paraparesis, and tetraparesis.2-11

    Diagnosis

    Diagnosis of IAC in dogs is typically made via computed tomography (FIGURE 2) or, preferably, MRI.2-5,7-10 IACs may also be visualized using ultrasound (via the foramen magnum, temporal window, or persistent bregmatic fontanelle), especially in younger dogs.6 The characteristic ultrasonographic appearance of IAC is a large, fluid-filled structure, isoechoic with the CSF spaces (e.g., lateral ventricle) and located between the occipital lobe of the cerebrum and rostral lobe of the cerebellum.6

    MRI provides the best detail for diagnosis of IAC and is most likely to provide information regarding the presence and nature of concurrent disease states. The typical appearance of IAC on MRI is a well-demarcated, cystic-appearing structure that is hypointense on T1-weighted images, hyperintense on T2-weighted images, and non-contrast enhancing with intravenous gadolinium administration and that suppresses on FLAIR (fluid attenuation inversion recovery) images2-5,10,11 (FIGURE 3). Because IAC may be an incidental finding, it is important to rule out concurrent inflammatory disease with a CSF examination. In the absence of an additional brain disorder, the CSF is typically normal in dogs with IAC.

    In our opinion, it is often difficult or impossible to discern whether a large IAC in the presence of another brain disorder is purely an incidental finding. We have seen a number of patients with relatively small dilations of the quadrigeminal cistern (FIGURE 4), which may represent a variant of normal structure or may be evidence of nascent IACs that may be of no clinical significance. Conversely, very large IACs have been described both as sole disease entities and as suspected incidental findings in patients with other intracranial disease processes. Anecdotally, we have also observed similar cystic structures in the brain that do not appear to be associated with the quadrigeminal cistern (FIGURE 5); considering that IACs occur in multiple locations in the human brain, this finding may simply imply that the term quadrigeminal cyst is too restrictive in describing IAC in dogs.12 Since the presence of a large, fluid-filled structure within the cranial vault likely decreases intracranial compliance, some IACs may be contributory rather than incidental findings. In other words, when there is evidence of a large IAC and another disease (e.g., granulomatous meningoencephalomyelitis, hydrocephalus) in the same patient, the optimal response to treatment may require treating both conditions.

    In addition, the combined presence of hydrocephalus and IAC in a patient does not necessarily make one of the disorders (IAC) incidental, nor does this combination ensure that surgically treating one disorder will address the other. Hydrocephalus can be secondary to an IAC, developing because of mechanical obstruction of normal CSF flow by the expanding cyst (i.e., obstructive hydrocephalus).10,18,19 It is unlikely that an IAC can be distinguished as communicating or noncommunicating based on standard MRI sequences; such a distinction would likely require either contrast studies or phase-contrast (cine) MRI. In one report of two dogs with IAC, intracystic hemorrhage, which was suspected to have contributed to the development of neurologic dysfunction, was verified at surgery.3

    In a recent study, the degree of brain compression by an IAC was calculated from MRI scans of dogs with the disorder, and these measurements were correlated with the presence or absence of clinical dysfunction. It was found that dogs with >14% compression of the occipital lobe always displayed clinical signs and that dogs with compression of both the cerebellum and the occipital lobe were significantly more likely to display clinical signs than dogs with compression of only one region or dogs with no apparent brain compression.11

    Treatment and Prognosis

    Because IAC in dogs, as in people, is often considered an incidental finding, it is frequently assumed that treatment of the suspected primary disorder (e.g., inflammatory brain disease, hydrocephalus) is indicated and the IAC is of little or no clinical consequence. As mentioned above, we question whether this approach is appropriate. In cases in which the IAC is considered the primary disease condition, medical therapy is aimed at reducing brain edema and increased intracranial pressure associated with the IAC, as well as controlling seizure activity if present. Medical treatment for IAC is identical to that described for congenital hydrocephalus (e.g., corticosteroids, diuretics, anticonvulsants if indicated), and dose recommendations for various therapies are summarized in BOX 3 .20,21 Dogs with IAC tend to respond initially to medical therapy, but the response may be temporary.

    Surgical management of IAC in people is typically achieved via either cyst fenestration (i.e., making an opening into the cyst wall) or cystoperitoneal shunt (CPS) placement.14,18,19,22-24 Proponents of fenestration cite a high surgical success rate and avoidance of shunt-related complications as reasons for this surgical choice; proponents of CPS report high success rates and avoidance of cyst reexpansion as reasons for the use of this technique.13,14,18,19,22-26 Both fenestration and CPS (FIGURE 6) procedures have been reported in dogs with IAC.2,3,5,7,10 IAC was considered the primary disease in five reported fenestration cases. Three patients were reimaged after surgery; two of the three dogs had evidence of cyst persistence on MRI. However, only one dog required reoperation.2,3,7 Successful CPS of dogs with IAC has also been reported.10 The cyst did not reform in any of the shunted cases.

    The success rate for surgical management of IAC appears to be high in people and dogs, and whether fenestration or CPS is the preferred procedure remains controversial for both species.10 Because of the paucity of reports of surgically managed dogs with IAC as well as the suspected high incidence of this abnormality being an incidental finding, the actual surgical success rate for IAC treatment in dogs should be regarded as unknown at this time. Hopefully, as information regarding medical and surgical treatment of clinically significant IAC in dogs accumulates, the understanding of the natural course of this disorder and the effectiveness of medical and surgical therapies to manage it will improve.

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    Dr. Dewey discloses that he has received financial support from Boehringer-Ingelheim.

    1. Rengachary SS, Watanabe I. Ultrastructure and pathogenesis of intracranial arachnoid cysts. J Neuropath Exp Neurol 1981;40:61-83.

    2. Vernau KM, Kortz GD, Koblik PD, et al. Magnetic resonance imaging and computed tomography characteristics of intracranial intra-arachnoid cysts in 6 dogs. Vet Radiol Ultrasound 1997;38:171-176.

    3. Vernau KM, LeCouteur RA, Sturges BK, et al. Intracranial intra-arachnoid cyst with intracystic hemorrhage in two dogs. Vet Radiol Ultrasound 2002;43:449-454.

    4. Kitagawa M, Kanayama K, Sakai T. Quadrigeminal cisterna arachnoid cyst diagnosed by MRI in five dogs. Aust Vet J 2003;81:340-343.

    5. Duque C, Parent J, Brisson B, et al. Intracranial arachnoid cysts: are they clinically significant? J Vet Intern Med 2005;19:772-774.

    6. Saito M, Olby NJ, Spaulding K. Identification of arachnoid cysts in the quadrigeminal cistern using ultrasonography. Vet Radiol Ultrasound 2001;42:435-439.

    7. Platt SR. What is your diagnosis? J Small Anim Pract 2002;43:469-470.

    8. Nagae H, Oomura T, Kato Y, et al. A disorder resembling arachnoid cyst in a dog. J Jpn Vet Neurol 1995;2:9-14.

    9. Orima H, Fujita M, Hara Y, et al. A case of the dog with arachnoid cyst. Jpn J Vet Imag 1998;10:49-51.

    10. 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.

    11. Matiasek LA, Platt SR, Shaw S, Dennis R. Clinical and magnetic resonance imaging characteristics of quadrigeminal cysts in dogs. J Vet Intern Med 2007;21:1021-1026.

    12. Cincu R, Agrawal A, Eiras J. Intracranial arachnoid cysts: current concepts and treatment alternatives. Clin Neurol Neurosurg 2007;109:837-843.

    13. Erdincler P, Kaynar MY, Bozkus H, et al. Posterior fossa arachnoid cysts. Br J Neurosurg 1999;13:10-17.

    14. Kandenwein JA, Richter HP, Borm W. Surgical therapy of symptomatic arachnoid cysts—an outcome analysis. Acta Neurochir (Wien) 2004;146:1317-1322.

    15. Dewey CW. Encephalopathies: disorders of the brain. In: Dewey CW, ed. A Practical Guide to Canine and Feline Neurology. 2nd ed. Ames, Iowa: Wiley-Blackwell; 2008:115-120.

    16. Go KG, Houthoff HJ, Blaauw EH, et al. Arachnoid cyst of the sylvian fissure: evidence of fluid secretion. J Neurosurg 1984;60:803-813.

    17. Santamarta D, Aguas J, Ferrer E. The natural history of arachnoid cysts: endoscopic and cine-mode MRI evidence of a slit-valve mechanism. Minim Invasive Neurosurg 1995;38:133-137.

    18. Raffel C, McComb JG. To shunt or fenestrate: which is the best surgical treatment for arachnoid cysts in pediatric patients? Neurosurgery 1988;23:338-342.

    19. Locatelli D, Bonfanti N, Sfogliarini R, et al. Arachnoid cysts: diagnosis and treatments. Childs Nerv Syst 1987;3:121-124.

    20. Coates JR, Axlund TW, Dewey CW, Smith J. Hydrocephalus in dogs and cats. Compend Contin Educ Pract Vet 2006;28:136-146.

    21. Dewey CW. Anticonvulsant therapy in dogs and cats. Vet Clin North Am Small Anim Pract 2006;36:1107-1127.

    22. Ciricillo SF, Cogen PH, Harsh GT, et al. Intracranial arachnoid cysts in children: a comparison of the effects of fenestration and shunting. J Neurosurg 1991;74:230-235.

    23. Kaplan BK, Mickle JP, Parkhurst R. Cystoperitoneal shunting for congenital arachnoid cysts. Childs Brain 1984;11:304-311.

    24. Stein SC. Intracranial developmental cysts in children: treatment by cystoperitoneal shunting. Neurosurgery 1981;8:647-650.

    25. Kim SK, Cho BK, Chung YN, et al. Shunt dependency in shunted arachnoid cyst: a reason to avoid shunting. Pediatr Neurosurg 2002;37:178-185.

    26. Gangemi M, Maiuri F, Colella G, et al. Endoscopic treatment of quadrigeminal cistern arachnoid cysts. Minim Invasive Neurosurg 2005;48:289-292.

    References »

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