Porencephaly with an optic organ abnormality in a beagle dog.

A female TOYO beagle dog showed porencephaly and visual organ abnormalities. At necropsy, there was a cavity filled with cerebrospinal fluid in the right cerebral hemisphere and an adhesion area between the cerebral cortex and the skull, which was partially thickened. Additionally, the right optic nerve showed a slight decrease in diameter. Histopathological examination revealed increased glial fibers and collagen fibers, hemosiderin deposition, and an increased number of microglia in the adhesion area, along with a marked reduction of the cerebral parenchyma. In the right eyeball, the retina and optic nerve showed focal atrophy in the nerve fiber layer and inner granular layer to full retinal atrophy and hypoplasia of the myelinated nerve fibers, respectively. Electron microscopic examination revealed hypoplasia of the myelin sheath of nerve fibers in the right optic nerve. This is an extremely rare case of porencephaly and congenital optic nerve hypoplasia, along with independent retinal thinning.