Elevated Intracranial Pressure and Cerebral Edema following Permanent MCA Occlusion in an Ovine Model

Abstract

Introduction: Malignant middle cerebral artery (MCA) stroke has a disproportionately high mortality due to the rapid development of refractory space-occupying cerebral edema. Animal models are essential in developing successful anti-edema therapies; however to date poor clinical translation has been associated with the predominately used rodent models. As such, large animal gyrencephalic models of stroke are urgently needed. The aim of the study was to characterize the intracranial pressure (ICP) response to MCA occlusion in our recently developed ovine stroke model.

Materials and methods: 30 adult female Merino sheep (n = 8-12/gp) were randomized to sham surgery, temporary or permanent proximal MCA occlusion. ICP and brain tissue oxygen were monitored for 24 hours under general anesthesia. MRI, infarct volume with triphenyltetrazolium chloride (TTC) staining and histology were performed.

Results: No increase in ICP, radiological evidence of ischemia within the MCA territory but without space-occupying edema, and TTC infarct volumes of 7.9+/-5.1% were seen with temporary MCAO. Permanent MCAO resulted in significantly elevated ICP, accompanied by 30% mortality, radiological evidence of space-occupying cerebral edema and TTC infarct volumes of 27.4+/-6.4%.

Conclusions: Permanent proximal MCAO in the sheep results in space-occupying cerebral edema, raised ICP and mortality similar to human malignant MCA stroke. This animal model may prove useful for pre-clinical testing of anti-edema therapies that have shown promise in rodent studies.

Fig 1. Mean 24 hour ICP and PbtO₂ following MCAO.

Mean ICP following sham surgery, temporary MCAO or permanent MCAO (A). Mean ICP following permanent MCAO, animals that died within the 24-hour monitoring period versus animals that survived (B). Mean PbtO₂ following temporary and permanent MCAO or sham surgery (C). ICP, intracranial pressure; MCAO, middle cerebral artery occlusion; PbtO₂, partial pressure of brain tissue oxygen, n = 8–12/gp.

Fig 2. MRI findings at 24 hours following stroke: MRA and DWI.

Temporary MCAO MRA demonstrates reperfusion in the right MCA territory (A), and diffusion deficit on DWI in the right MCA cortex (B). Permanent MCAO MRA shows no flow beyond the right proximal MCA (C), and a larger diffusion deficit involving the whole MCA territory including subcortical structures (D). DWI, diffusion weighted imaging; MCA, middle cerebral artery; MCAO, middle cerebral artery occlusion; MRA, magnetic resonance angiography, n = 6/gp.

Fig 3. MRI findings at 24 hours following stroke: T1 and T2 weighted imaging.

Temporary MCAO demonstrates cerebral edema in a right MCA distribution similar to the diffusion deficit in Fig 2B on T2 coronal imaging (A), and no mass effect or midline shift on T2 axial imaging (B). Sagittal T1 sequences show preserved basal cisterns and posterior fossa CSF spaces (C). T2 coronal (D) and axial (E) imaging after permanent MCAO show cerebral edema distributed as for the diffusion deficit in Fig 2D, with associated mass effect and midline shift. Sagittal T1 imaging demonstrates effacement of the basal cisterns and cisterna magna, tonsillar herniation and brainstem compression (F). MCA, middle cerebral artery; MCAO, middle cerebral artery occlusion, n = 6/gp.

Fig 4. TTC at 24 hours, coronal stack.

Unstained brain tissue represents cerebral ischemia. There is no evidence of ischemia in sham animals in the left column (A), small cortical ischemia in temporary MCAO animals in the center column (B), and large MCA territory ischemia in permanent MCAO animals in the right column (C). MCA, middle cerebral artery; MCAO, middle cerebral artery occlusion; TTC, 2,3,5-triphenyltetrazolium chloride, n = 5 shams, n = 11 transient MCAO, n = 8 permanent MCAO.

Fig 5. Histopathology, coronal section.

Section level with the origin of the MCA. H&E for sham surgery (A), temporary MCAO (B) and permanent MCAO (C). Albumin immunostaining for sham (D), temporary MCAO (E) and permanent MCAO (F). Weil stain for sham (G), temporary MCAO (H) and permanent MCAO (I). MCA, middle cerebral artery; MCAO, middle cerebral artery occlusion, n = 8 shams, n = 12 transient MCAO, n = 10 permanent MCAO.

Fig 6. Histopathology for H&E.

The infarct was evident as a region of tissue pallor, charcterised by extensive cell injury/death and tissue vacuolation (A). The was selective cell sparing and cell injury/loss within the penumbral tissue (B). n = 8 shams, n = 12 transient MCAO, n = 10 permanent MCAO.

Fig 7. TUNEL staining.

TUNEL immunoreactivity, indicative of cell death, was observed throughout the infarcted tissue. The region of TUNEL immunoreactivity was larger and more widespread following permanent (B) than transient MCAO (A). TUNEL positive cells (C) were observed throughout the infarcted tissue, typically seen in areas of tissue vacuolation and extensive cell injury (D). The number of TUNEL positive cells (E) and total area of TUNEL immunoreactivity (F) was significantly higher following permanent than transient MCAO. n = 3/gp.