Cystic endometrial hyperplasia, pseudo-placentational endometrial hyperplasia, and other cystic conditions of the canine and feline uterus.

Cystic lesions in the uteri of bitches and queens arise from the uterine serosa, myometrium, or endometrium and include: serosal inclusion cysts, adenomyosis, endometrial polyps, cystic remnants of mesonephric ducts, and cysts associated with endometrial hyperplasia (both cystic glands and "pseudo-placentational" hyperplasia). Of these, "cystic endometrial hyperplasia (CEH)" is the most common and is frequently associated with pyometra. A second form of endometrial hyperplasia occurs in the bitch; although it was first described over 100 y ago, it is not widely recognized by clinicians or diagnostic pathologists. In this form, the endometrium proliferates in a highly organized manner, remodeling the uterine lining to closely resemble the histology of the endometrium at placentation sites in normal pregnancy. Although this lesion is very different from CEH, it is quite easy to induce in dogs during the luteal phase of their cycles and has been perhaps inappropriately proposed as modeling CEH. This lesion has been referred variously as "deciduoma", endometrial hyperplasia in pseudocyesis, and "maternal placental-like endometrial hyperplasia". An alternative name is suggested that is descriptive and draws attention to the difference between this lesion and CEH; the term pseudo-placentational endometrial hyperplasia (PEH) is proposed. The histopathology and pathogenesis of CEH and PEH are discussed. The objectives of this paper are to review the pathophysiology of cystic lesions of canine uterus, to demonstrate these using subgross photomicrographs taken from natural cases, and to present key diagnostic features of each.