In : Braund ' s

This chapter reviews the major structural malformations and developmental defects encountered in dogs and cats and their related effects on the central nervous system. Note that congenital deafness and congenital vestibular disorders are discussed with deafness and vestibular disease in the chapter on peripheral nerve disorders (See Myopathies and Neuropathies). Arachnoid Cysts Arachnoid cysts, also known as intra-arachnoid cysts, meningeal cysts, leptomeningeal cysts, and arachnoid diverticula, have been reported with increasing frequency over the past few years in dogs and cats [1-13]. It has been suggested that spinal arachnoid cysts in dogs most closely resemble type III spinal meningeal cysts in people [297]. As far as breed predilection is concerned, Rottweilers are often cited cited (in one report, 8/14 dogs were Rottweilers [297]) and the condition has been observed in related Schipperkes [7] and Shih Tzu littermates [10], suggesting an inherited etiology. The cysts are characterized as cerebrospinal fluid (CSF)-filled, dorsal midline, intradural, extramedullary cavitational lesions associated with coarse arachnoid trabeculation, that result in spinal cord compression. Occasionally, cysts located in ventral and dorsolateral locations in the cervical spinal region have been observed [14]. The cystic cavities are reportedly separated from the compressed spinal cord by an intact pia mater. In one series of affected dogs, numerous blood vessels with enlarged perivascular spaces were noted caudal to the cyst [4]. A cranial opening to the cysts, presumably continuous with the subarachnoid space and allowing flow of CSF, has been noted [7]. Usually there is no evidence of inflammation within the meninges or tissues lining the cysts, however, a mixed cellular inflammatory reaction and reactive connective tissue proliferation was reported in the cyst wall surgically removed in one case [7]. In this dog, the cyst wall was composed of pia-arachnoid meningothelial cells [7]. Many animals are less than 1 year of age, but cysts have been reported in animals up to 12 years of age. Onset of signs between 11 and 24 months was reported in one series of cases [7]. Arachnoid cysts have been observed mainly in rostral cervical and caudal thoracic/thoracolumbar sites; however, multiple cysts in the caudal cervical area of three Rottweiler dogs have been observed [14]. Cysts may also be multilobed or bilobed [297]. The pathogenesis is unknown [15] although congenital spinal dysraphism due to failure of fusion of the neural crest has been suggested [4], and indeed, in people, primary arachnoid cysts are regarded as a …