EXPERIMENTAL HYDRANENCEPHALY IN THE OVINE FETUS

Hydranencephaly is defined as the replacement of a previously normal b rain, in whole or in part, by membranous fluid-filled sacs. The etiolo gy is not well understood, and the time course of development is unkno wn. Fifteen ovine fetuses were chronically cannulated and had both car otid arteries ligated at 100 days of gestation (term is 145-150 days). They were killed at 1 (n = 4), 2 (n = 6) and 4 (1? = 5) weeks post-su rgery, and the findings compared with those of 25 age-matched controls . By 2 weeks post-surgery the entire cerebral hemispheres and dienceph alon had been replaced by fluid closely resembling cerebrospinal fluid . The choroid plexus, pituitary and brain stem remained outwardly norm al, but the cerebellum showed signs of damage. Fetuses maintained norm al values for blood gases and hematocrit up to 4 weeks post-surgery, a nd grew normally. Light microscopy of the brain stem showed significan t losses of cell populations in the medulla by 4 weeks. Vascular casti ng and acute blood flow studies in an additional group of fetuses show ed that the entire brain was perfused via the vertebral-occipital anas tomosis immediately after acute bilateral carotid clamping, but that t he blood flow rate was insufficient to maintain adequate oxygen delive ry.