Multiloculated Caudal Fossa Cyst in a Dog

A 4 year and 8-month-old male entire Flatcoated Retriever was presented. The owner reports that he was the smallest of the litter and he always had a funny walk. Recently, the dog seemed less energetic. On clinical and neurological examination, the dog was bright, alert, and responsive. He showed a hypermetric gait, and intention tremor of head, trunk and limbs. The hopping response was delayed and then exaggerated with hyperflexion when tested on all four limbs. The spinal and cranial nerve reflexes were normal. There were no signs of hyperesthesia. The neurolocalization was cerebellum, and MRI was performed(see below)

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There is a large, well-defined extra-axial space-occupying lesion located on the left side of the cranial caudal fossa, filling more than 75% of its volume. It extends rostrally into the cranial fossa, reaching the level of the rostral colliculus (lateral to it) and medial to the left hippocampus.

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This lesion is slightly lobulated dorsally and it appears to have some incomplete septae. It is homogeneous and isointense in all sequences to CSF. On the FLAIR images, there is a thin and regular hyperintense wall, which may show very mild enhancement.

The lesion causes severe mass effect to the brain stem, cerebellum, III and IV ventricles, which are compressed and displaced to the right side. The vermis of the cerebellum appears to be well-developed. There is very mild perilesional oedema, mainly affecting the cerebellar vermis.

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The tentorium of the cerebellum is thinner, flattened, and displaced dorsally by the lesion described. There is also some thinning affecting the ventral occipital lobe and at the dorsal aspect, at the most caudal and dorsal aspect from the occipital protuberance.

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The MRI-based diagnosis was an extra-axial CSF-filled space-occupying lesion, compatible with an intra-arachnoid diverticulum.

The diagnostic differentials for cystic lesions of the caudal fossa other than these diverticula are epidermoid cysts, cerebellar hypoplasia/aplasia or Dandy Walker syndrome.

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Courtesy of Koen Santifort and Niklas Bergknut from Evidensia Dierenziekenhuis Hart van Brabant Waalwijk.