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Summary
Arachnoid cysts comprise a nervous system (CNS) disorder in which the arachnoid membrane covering of the central nervous system doesn't develop properly. The result is the formation of arachnoid cysts, cerebrospinal fluid-filled sacs. These can be located between the brain and the arachnoid membrane (called intracranial arachnoid cysts or "IACs") or between the spinal cord and this membrane (called spinal arachnoid cysts or "SACs").
Both SACs and IACs are considered uncommon in dogs and cats. They are only rarely diagnosed in dogs and still more rarely in cats. (Indeed in cats there are only four published cases confirming this disease entity.) The sparse arachnoid cyst findings in pets is almost certainly the result of the high expense associated with diagnosing nervous system lesions like these.
Consequently, most of what we know of this disease comes from human medicine. As such, we understand that this disease has a genetic origin. Affected animals are born with the condition.
Most pets diagnosed with this disease are not symptomatic, rather they are diagnosed as a result of an MRI or CT scan undertaken for a reason unrelated to the cyst in the dog itself. For that reason it's impossible to know the true incidence of this disease in the population of pets at large.
Symptoms and Identification
Symptomatic arachnoid cyst patients will exhibit signs consistent with the location of their lesion.
In spinal cases, ataxia (a wobbly gait), hypermetria (an exaggerated gait) and/or a loss of the ability to place the limbs normally (abnormal proprioception) can be observed. In humans, progressive back and leg pain and tingling or numbness in the legs or arms is most commonly reported with SAC.
In intracranial cases, however, seizures are the most common sign. Loss of vision and trouble with balance and walking are also possible. Weakness in two or four limbs and neck pain have also been reported. IAC-affected humans tend to complain of headaches as well.
Neurological examination, MRI, CT scans and special contrast studies of the spine (myelography) are typically employed to diagnose arachnoid cysts.
Affected Breeds
Small breed dogs, particularly those of brachycephalic (short-nosed) breeds seem to be predisposed, especially Pugs and Shih Tzus
Treatment
With arachnoid cysts, the possibility of treatment (and its efficacy) is all about location.
In the spinal versions, relieving the pressure exerted by the cyst can be 100% curative. This is achieved surgically in much the same way veterinary surgeons commonly treat disc disease in dogs. "Hemilaminectomy" with "cyst fenestration" (making an opening in the cyst wall) is the most common technique used to eliminate the damage caused by a cyst in the accessible spinal column. Not all spinal cysts, however, are so amenable.
With intracranial cysts things tend to get more complicated. Medical treatment for seizures can be attempted (with corticosteroids, diuretics and/or anticonvulsants), but surgical intervention can be far more successful when either "cyst fenestration" or "cystoperitoneal shunt placement" (a method used to drain the cyst fluid) is undertaken.
Veterinary Cost
Due to the expense of advanced imaging techniques and the possibility of neurosurgery, this is among the most expensive of veterinary genetic diseases to diagnose and treat. Diagnosis would likely come in at $1,500 to $5,000 while treatment, if surgically approached, would almost certainly cost between $3,000 and $10,000. A veterinary neurologist and/or surgeon is always required.
Prevention
Though inheritance modalities have not yet been established it's considered advisable to refrain from breeding affected dogs and their first degree relatives (parents and siblings).
References
Rengachary SS, Watanabe I. Ultrastructure and pathogenesis of intracranial arachnoid cysts. J Neuropath Exp Neurol 1981;40:61-83.
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.
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.
Kitagawa M, Kanayama K, Sakai T. Quadrigeminal cisterna arachnoid cyst diagnosed by MRI in five dogs. Aust Vet J 2003;81:340-343.
Duque C, Parent J, Brisson B, et al. Intracranial arachnoid cysts: are they clinically significant? J Vet Intern Med 2005;19:772-774.