A surgical model of middle cerebral artery occlusive stroke in the sheep.

Abstract

Background: Stroke is an acute neurological injury secondary to vascular pathology, and is the second biggest killer of Australians and the leading cause of adult disability. The rationale of current therapy for occlusive stroke is rapid reperfusion of the ischaemic brain to limit the size of the injury. However, there are no standard neuroprotective therapies that have proven to be beneficial in clinical stroke, despite in excess of 1000 novel drugs showing promise in preclinical rodent studies. The consistent failure of clinical translation in rodent models suggests that they are perhaps not the best choice to simulate the intracranial pathophysiological changes that occur following human cerebral ischaemia, and that a better representative animal model with similar neuroanatomical features is required. Small ruminants such as the sheep have proven to be valuable in traumatic brain injury models, and a surgical model of permanent middle cerebral artery occlusion (MCAO) has recently been developed in the sheep. However, the existing model has a number of shortcomings and is in need of further characterisation before its widespread use in preclinical testing. The aim of this study was therefore to characterise the pathophysiological and radiological response to both temporary and permanent MCAO using a sheep model. Methods: Several different studies were performed. In the first to determine the feasibility of the project, 18 adult male and female Merino sheep were randomised to sham surgery (n=6), permanent MCAO (n=6) or 2 h temporary MCAO (n=6), and animals had intracranial pressure (ICP) and regional brain tissue oxygen (PbtO₂) monitored for 4 h. 6 further animals had magnetic resonance imaging (MRI) after permanent (n=3) or temporary (n=3) MCAO. In the second study, 10 adult Merino sheep were randomised to sham surgery (n=5) or temporary MCAO (n=5), with continuous monitoring of PbtO₂ to determine the relationship between duration of temporary MCAO and the development of regional hypoxia. In the third study, 28 adult female Merino sheep were randomised to sham surgery (n=6), permanent MCAO (n=10) or temporary MCAO (n=12), and monitored for 24 h under light general anaesthesia. MRI was performed in 12 animals (permanent MCAO n=6, temporary MCAO n=6). Stroke volume was calculated after staining fresh brains with 2,3,5-triphenyltetrazolium chloride (TTC). Results: The first study demonstrated the feasibility of performing surgical MCAO, with significantly larger ischaemic lesion areas on histology and MRI following permanent versus temporary occlusion. The second study demonstrated that PbtO₂ fell from a mean baseline of 45.0 +/- 14.1mmHg to a predefined hypoxic threshold of 15mmHg after 42.4 +/- 11.2 minutes of temporary MCAO, at a rate of 1.3mmHg/min. The third study showed a significantly elevated ICP, infarct volumes of 27.4 +/- 6.4%, evidence of space occupying cerebral oedema on MRI and a 30% mortality rate following permanent MCAO monitored for 24 h. Conclusions: A surgical model of temporary and permanent proximal MCAO stroke has been developed in the sheep. The response of the sheep brain to cerebral ischaemia shares many features with the human brain, particularly following permanent proximal occlusion and the development of space occupying cerebral oedema. The sheep as a representative model of human occlusive stroke appears highly promising for use in preclinical testing, for drugs that demonstrate efficacy in the sheep model may be more likely to successfully translate to clinical stroke.