Multimodal MRI characterization of experimental subarachnoid hemorrhage

Highlights

• Subarachnoid hemorrhage (SAH) results in transient vasospasm and venous abnormality.

• SAH transiently reduces basal cerebral blood flow and its response to hypercapnia.

• SAH causes transient diffusion change but no ischemic brain injury.

• Both SAH and control rats show reduced open-field activities.

• There are however no differences in functional deficits between groups.

Abstract

Subarachnoid hemorrhage (SAH) is associated with significant morbidity and mortality. We implemented an in-scanner rat model of mild SAH in which blood or vehicle was injected into the cistern magna, and applied multimodal MRI to study the brain prior to, immediately after (5 min to 4 h), and upto 7 days after SAH. Vehicle injection did not change arterial lumen diameter, apparent diffusion coefficient (ADC), T₂, venous signal, vascular reactivity to hypercapnia, or foot-fault scores, but mildly reduce cerebral blood flow (CBF) up to 4 h, and open-field activity up to 7 days post injection. By contrast, blood injection caused: (i) vasospasm 30 min after SAH but not thereafter, (ii) venous abnormalities at 3 h and 2 days, delayed relative to vasospasm, (iii) reduced basal CBF and to hypercapnia 1–4 h but not thereafter, (iv) reduced ADC immediately after SAH but no ADC and T₂ changes on days 2 and 7, and (v) reduced open-field activities in both SAH and vehicle animals, but no significant differences in open-field activities and foot-fault tests between groups. Mild SAH exhibited transient and mild hemodynamic disturbances and diffusion changes, but did not show apparent ischemic brain injury nor functional deficits.

Introduction

Subarachnoid hemorrhage (SAH) is a neurologic emergency associated with significant morbidity and mortality representing the deadliest type of acute stroke (Suarez et al., 2006). The hallmarks of SAH include increased intracranial pressure (ICP), hypoperfusion, and delayed cerebral ischemia (with or without vasospasm) (Eide and Sorteberg, 2006, Ansar and Edvinsson, 2009, Westermaier et al., 2011, Kelly et al., 2013, Danura et al., 2015). The pathophysiology of acute SAH, particularly in the milder forms, is poorly understood because it is challenging to study acute SAH in a systematic and controlled manner in humans. Animal models of SAH are important to unravel the underlying pathophysiological mechanisms that could inform clinical SAH conditions. Two common animal models of SAH are arterial perforation typically via the internal carotid artery, and blood injection typically into the cistern magna. The values of the blood injection model are that it models the effects of bleeding in the subarachnoid space, the degree of injury can be controlled, and it has relatively high survival rates (Prunell et al., 2002, Reilly et al., 2004, Vatter et al., 2006). The disadvantage is that it does not mimic the processes surrounding aneurysmal rupture (Sehba and Pluta, 2013).

Magnetic resonance imaging (MRI) provides non-invasive structural, physiological and functional imaging data of the whole brain in a longitudinal fashion. MRI studies of SAH have reported vasospasm using magnetic resonance angiography (MRA) (van den Bergh et al., 2005), reduced cerebral blood flow (CBF) (Guresir et al., 2010, Guresir et al., 2013), reduced cerebrovascular reserve by hypercapnic challenge (Reinprecht et al., 2005, da Costa et al., 2014), ischemic brain injury using diffusion-weighted MRI (Busch et al., 1998, Piepgras et al., 2001), and cerebral edema or infarction using T₂ MRI (van den Bergh et al., 2005, Okubo et al., 2013, Tiebosch et al., 2013). However, these changes in the blood-injection model to the cistern magna have not been widely explored, especially in its hyperacute phase. Moreover, the effects of SAH on the veins have not been reported to our knowledge. We suspect that SAH could affect venules and veins resulting in changes that could contribute to the pathophysiology of SAH. This is supported by the fact that vascular oxidative stress and tissue hypoxia after SAH increase thrombogenicity, tissue inflammation, and neurodegenerative changes (Ostergaard et al., 2013). In addition, the release of vasoactive agents after SAH are likely to interfere with both arterioles and venules tone. MR venography could offer a unique non-invasive means to visualize changes in the veins associated with SAH.

The goal of our study was to implement an in-scanner rat model of mild SAH in which blood is injected into the cistern magna, and to apply multimodal MRI to investigate SAH pathophysiology in the hyperacute to subacute phase. The in-scanner SAH model enabled MRI measurements before and immediately after SAH in the same animals. We chose to investigate a relatively mild SAH model to achieve good survival rate for longitudinal studies. MRA, MR venography (MRV), CBF, and cerebrovascular reserve (by measuring CBF response to hypercapnia), diffusion and T₂ were evaluated. Functional measures using the open-field and foot-fault tests were also performed. Comparisons were made with the vehicle group injected with artificial cerebrospinal fluid.