1987 Volume 9 Issue 4 Pages 325-333
A model of brain stem ischemia has been developed in the dog by occluding the perforators of the posterior cerebral arteries bilaterally as far as the junctions with the posterior communicating arteries. The animals survived for 7 days with severe consciousness disturbance, tetraparesis, and oculomotor palsy. In acute models, which were sacrificed immediately after occlusion, carbon defects in the brain slices or avascular areas on microangiogram were mainly located in the midbrain, but also extended to the posterior portion of the thalamus rostrally and to the upper pons caudally. A wedge-shaped filling defect was seen on the coronal sections, its base being ventral and its apex pointing to the fourth ventricle. The anemic or partly hemorrhagic infarction was constantly found in chronic models which were kept alive for 7 days and was confirmed to correspond to the avascular areas mentioned above. Using this model, regional cerebral blood flow (rCBF), brain stem auditory evoked potential (BAEP), and electroencephalography (EEG) were measured until 6 hours after vascular occlusion.
Responses of rCBF to perforator occlusion was remarkable. rCBF in the ischemic region decreased from 52.5 ± 13.3 to 8.4 ± 2.5 m1/100 g/min 6 hours after occlusion. rCBF in the surrounding area of the ischemia was about 20 m1/100 g/min, compared with about 58 m1/100 g/min as control value. These results appeared to define the critical flow for infarction in the brain stem. When r-CBF fell below about 13 m1/100 g/min, local infarction resulted.
There was no changes in BAEPs in 8 out of 11 animals. The lesion in the midbrain of this model is not detected by BAEP and the accompanying events with the ischemia such as edema have little effects on the BAEP within 6 hours after occlusion.
In EEG, low voltage fast activity appeared in 7 out of 9 animals which were semicomatose state. This finding is also reported in clinical cases of brain stem strokes and is known as so-called β-coma. The ischemic lesion in the restricted midbrain tegmentum containing reticular formation is correlated with the appearance of desynchronization.
