A 2-year-old 34-kg (74.8-lb) spayed female Golden Retriever was examined 3 months after the onset of progressive left facial involuntary muscle contractions. Initially, the muscular spasms affected the left periocular muscles and subsequently spread to include the left portion of the upper lip. One week prior to examination, the owner noticed that the dog would drop treats out of the left side of its mouth; however, the owner was unable to confirm that abnormalities were present when the dog was drinking water or eating food from its bowl. The muscle spasms only occurred when the dog was heavily sniffing or its face was touched or in close proximity to obstacles; no spasms were observed at rest. Muscle spasms were noted in the dog's left shoulder region, mostly when the area was stimulated by touch; spasms reportedly occurred occasionally when the dog would ambulate, but never when it was at rest. There was no history of trauma, and the dog had no signs of pain. On physical examination, the dog was overweight (body condition score, 6/9), but other findings were unremarkable. A neurologic examination was performed.
Etiologic diagnosis—Differential diagnoses considered for the 2-year-old Golden Retriever included infectious disease (viral [eg, distemper], parasitic [eg, Cuterebra larva migrans], fungal [eg, cryptococcosis], or bacterial), inflammatory disease (polyneuritis), congenital anomalies (arachnoid diverticulum), or neoplasia (lymphoma, histiocytic sarcoma, medulloblastoma, or meningioma) associated with the left portion of the brainstem. Plans for diagnostic testing included clinicopathologic evaluation (to assess general health status), MRI of the brain and analysis of a CSF sample (to detect infection, inflammatory disease, or neoplasia in the CNS), and electromyography of the affected muscles (to evaluate for neuromuscular abnormalities) followed by muscle and nerve biopsy procedures.
Diagnostic test findings—A CBC and serum biochemical profile had been performed by the referring veterinarian a month prior to the examination; no important abnormalities were detected. During anesthesia, MRI of the dog's brain was performeda and various image sequences were obtained as follows: T2-weighted in the sagittal plane; T2-weighted, T2*-gradient echo, fluid attenuated inversion recovery, T1-weighted, T1-weighted following IV gadolinium administration, diffusion-weighted, and apparent diffusion coefficient map in the transverse plane; and T1-weighted and T1-weighted (in the dorsal plane) following IV gadolinium administration.
Magnetic resonance imaging revealed a round fluid-filled cyst-like structure within the left caudal fossa, which had displaced and compressed the cerebellum to the right. The cyst appeared to have a thin peripheral margin and an extra-axial location in the region of the left fourth ventricular lateral recess. There was no evidence of associated obstructive hydrocephalus. The fluid within the cyst was isointense, compared with the brain ventricular system, on the T2 image series (Figures 1 and 2) and very mildly hyperintense, compared with the suppressed ventricular CSF fluid signal on the fluid attenuated inversion recovery image series. On the basis of the MRI findings, a congenital malformation, such as ependymal cyst, choroid plexus cyst, or atypical arachnoid diverticulum, was the most likely differential diagnosis. Less likely differential diagnoses included an epidermoid or dermoid cyst, neuroenteric cyst, or vascular hamartoma; cystic neoplasm (eg, cystic meningioma); or an infectious cyst-like structure (associated with coenurosis, echinococcosis, or cystocercosis). The infectious causes were considered unlikely because of the geographic location (ie, Pennsylvania) of the dog.
Sagittal T2-weighted image of the brain of a 2-year-old Golden Retriever that was examined 3 months after the onset of progressive left facial involuntary muscle contractions, which had appeared to subsequently affect the left side of the dog's mouth and its left shoulder region. Notice the fluid-filled cyst-like structure within the caudal fossa.
Citation: Journal of the American Veterinary Medical Association 247, 11; 10.2460/javma.247.11.1241
Empirical treatment with prednisone (1 mg/kg/d [0.45 mg/lb/d], PO) and omeprazole (1 mg/kg/d, PO) was initiated, and the dog was brought to the hospital 2 weeks after the MRI examination for surgical intervention. The dog was anesthetized, and a suboccipital craniectomy was performed to fenestrate the cyst. Marsupialization of the structure or placement of a shunt to divert the fluid from the cyst to the cisterna magna was also considered as an alternative surgical technique. Once exposed, the cyst was fenestrated and the fluid was collected for cytologic analyses. Also, 2 specimens representative of the cyst's wall were submitted for histologic examination. The gross appearance of the cyst's wall was similar to that of dural tissue.
The dog recovered uneventfully from anesthesia; after spending the night in the intensive care unit, the dog was discharged from the hospital. The neurologic examination findings at the time of hospital discharge were the same as they were prior to surgery.
Cytologic analysis of the cyst fluid revealed an absence of nucleated cells, confirming the presumptive diagnosis of a fluid-filled cyst. Histologic examination of the 2 cyst wall specimens revealed dense fibrous tissue with few small foci of mineral. The tissue had no natural borders with a few small blood vessels. No epithelial lining of the cyst wall was observed. The samples were compatible with meninges (dura mater).
Comments
Arachnoid cysts or diverticula are thought to derive from congenital splitting of the arachnoid layer of the meninges with accumulation of CSF within this potential space. The wall of such cysts is comprised of collagenous stroma lined by prominent meningeal cells; there is no solid component and no epithelial lining.1
To our knowledge, data regarding intracranial intra-arachnoid cysts in dogs are limited. In the few previous reports,2,3 the cysts were located in the quadrigeminal cistern and at the cerebellomedullary junction. The clinical signs of neurologic dysfunction in the affected dogs were mostly generalized seizures; one dog had paradoxical vestibular syndrome, reflecting the cerebellomedullary location of the cyst, and another had inflammatory brain disease.2 The reason that seizure activity is the most commonly observed clinical sign is not fully understood. Arachnoid diverticula are considered a developmental anomaly, and it is possible that the seizures and the presence of the cysts in those dogs were not related. It is also possible that the alteration in CSF dynamics and other cerebral cortical developmental anomalies were responsible for such clinical signs.2
Most of the dogs with intracranial arachnoid diverticula were < 7 years old.2 In 1 report,1 2 older dogs developed neurologic dysfunction as a result of hemorrhage into a quadrigeminal intracranial cyst. The clinical signs reported for both dogs were decreased mental status and tetra- or paraparesis following a traumatic episode. Both dogs underwent advanced diagnostic imaging followed by surgery, and a blood clot was removed from the lumen of the cyst. It was hypothesized that the late onset of clinical signs in these dogs resulted from the hemorrhage within the cyst.1
In the case described in the present report, the clinical signs were unique. The possibility that the facial spasms were focal seizures seemed unlikely given that the facial myoclonus was consistently inducible and never happened when the dog was at rest. The recent onset of dysphagia favored anatomic localization to the brainstem.
The findings of the histologic examination of cyst wall specimens confirmed the meningeal nature of the cyst's wall in the dog of the present report. The cyst wall was identified as dura mater, but a cellular structure resembling arachnoid villi was not observed. Intracranial dural cysts in people have been reported4,5 as a very rare form of congenital malformation. A few cases of intracranial cysts formed within 2 layers of normal dura in people have been reported.4,5 In 1 report4 of a 6-month-old child, clinical signs included enlargement of the head and delay of mental development. Results of cranial nerve examination, motor examination, and deep tendon reflex assessment were considered normal. The cyst was located in the posterior fossa, and the cyst's wall was delimitated by the anatomic structures of falx cerebri, tentorium cerebelli, and the dura beneath the child's skull.4 It is possible that the intracranial cyst of the dog described in the present report was contained within 2 layers of normal dura mater.
It is difficult to provide a prognosis for a dog with intracranial arachnoid diverticulum. In the previously reported cases, some dogs were euthanized at the owner's request because of the severity of clinical signs.3 One of the older dogs with an intra-arachnoid cyst with intracystic hemorrhage was alive 3.5 years after surgery, and neurologic examination findings were apparently normal. Brain MRI was repeated, and an intracranial cyst approximately the same size as the preoperative one was detected.1 The cyst did not appear to cause any clinical problem at the time of the second MRI examination.1
In general, the long-term prognosis for dogs with intracranial benign cystic structures seems to be correlated with the severity of clinical signs and owner's perception of the dog's quality of life. In the absence of worsening of clinical signs, serial follow-up MRI examinations can provide important information regarding the progression or regression of the disease or reoccurrence of the cystic structure following surgery.
Brivio MR355, 1.5 Tesla MRI unit, GE Healthcare, Milwaukee, Wis.
References
1. Vernau KM, Le Coutier RA, Sturges B, et al. Intracranial intra-arachnoid cyst with intracystic hemorrhage in two dogs. Vet Radiol Ultrasound 2002; 43: 449–454.
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. Wyss-Fluehmann G, Konar M, Jaggy A, et al. Cerebellar ependymal cyst in a dog. Vet Pathol 2008; 45: 910–913.
4. Inagaki T, Yamanouchi Y, Nishimura T, et al. Intracranial dural cyst. Childs Nerv Syst 1998;14 (1–2):69–73.
5. Haymaker W, Foster ME. Intracranial dural cyst. J Neurosurg 1994; 1: 211–218.
