The purpose of the present study is to clarify the effects of stimulation of the medullary reticular formation on brain edema and ICP in a cerebral contusion model. The study was divided into two experiments. Experiment 1 (32 cats). The effet of stimulation of the medullary reticular formation on tissue water permeability was assessed using normal brain and cold-induced edema. The cold injury was inflicted by applying a freezing probe of -50°C directly on the dura for 1 minute. The animals were divided into 4 groups. Group 1: intermittent electrical stimulation of the reticular formation of the medulla oblongata (P(10), L±(2.5), H(-9.0)) of a normal brain for 40 minutes; Group 2: cold lesion only; Group 3: same procedure as group 1 on cold-induced edema; Group 4: administration of Angiotensin II for 60 minutes. Water contents of the 4 groups were measured by a gravimetric technique from coronal slices across the lesion.
Experiment 2 (24 cats). The ICP and local cerebral blood volume (CBV) and blood pressure (BP) were continuously measured during stimulation of the medullary reticular formation in animals with cold-induced edema. The normal water content of the white matter was 67.7±0.9% g water/g tissue. Seventeen hours following cold-induced edema, the water content adjacent to the lesion increased to 71.8% and gradually decreased at areas further from the lesion. There were no significant changes in water content in the contralateral hemisphere. Medullary stimulation of brains with cold-induced edema produced a widespread significant increase in tissue water content in the injured hemisphere by 1.6 to 3.8%. In particular water contents adjacent to the lesion were significantly higher, by 1.3 to 1.8%, than those of group 4 with induced hypertension.
In experiment 2, the BP, ICP and CBV increased abruptly immediately after stimulation and the response patterns were divided into 2 groups. In 15 of 24 animals (62%), temporary elevation of the ICP ranging from 10 to 25mmHg was observed during stimulation with a concomitant increase in CBV and BP. In 9 animals (38%), progressive increase in ICP and CBV were produced by stimulation and even after cessation of stimulation, ICP progressively increased up to 60 to 100mmHg. Progressively increased ICPs were evoked with stimulation of the paramedian reticular formation of the medulla oblongata, that is, the region around the nucleus reticularis parvocellularis. Stimulation of the medullary reticular formation accelerated brain water permeability and increased CBV, resulting in acute intracranial hypertension in an injured brain.