Serial Magnetic Resonance Imaging of Rat Brain After Induction of Renal Hypertension

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Abstract
Background and Purpose —Hypertension is a major risk factor for ischemic and hemorrhagic stroke and may also cause more chronic and subtle brain injury. Progressive brain changes in a rat model of renal hypertension have been assessed to better understand the pathogenesis of hypertensive brain damage. Methods —Young adult rats were made hypertensive by partial occlusion of both renal arteries. MR images of brain were obtained weekly, and histopathological outcome was assessed. A separate group of rats was used to measure brain specific gravity and Evans blue dye content as an indicator of extravasation. Results —Rats developed maximal mean systolic blood pressures of 173 to >300 mm Hg, reaching a plateau in 6 to 8 weeks. Rats whose mean systolic pressure never exceeded 210 mm Hg never had brain lesions, while rats whose mean systolic pressure exceeded 276 mm Hg consistently developed brain lesions. Brain T2 values increased with increasing blood pressure. Lesions seen on MRI corresponded to those seen histologically. MRI also demonstrated transient brain expansion, probably due to diffusely increased water content, and rarely demonstrated focal cortical edema, which had no histological correlate. These transient phenomena, as well as hemorrhagic and ischemic infarcts, occurred mainly during the phase of climbing blood pressure and early stages of stable hypertension. Conclusions —Serial MRI reveals aspects of hypertensive brain disease that cannot be studied by histological examination alone. The observed phenomena are likely related to loss of autoregulation and/or blood-brain barrier integrity. Breach of blood vessel integrity is less likely once the vessels become accustomed to high pressures.