Despite the intensity of the research in the field of SAH and the development of therapeutic strategies such as endovascular and pharmacological treatment options increasing over the past twenty years, mortality remains high within the first week of hospital admission and reaches about 50% during the following 6 months24,25. This current preclinical model by daily double injection of homologous arterial blood into the cisterna magna has been recognized for its validity and its association with a low mortality rate. Indeed, among SAH rodent models, a wide range of mortality rates has been reported: 0-16% mortality with single blood injection into cisterna magn26,27,28,29,30,31,32,33,34,35, 10-33% mortality with blood injection into prechiasmatic cister20,27,36,37, 16-66% mortality in the model by endovascular perforation38,39,40,41,42,43,44,45 and 0-43% with the model by double blood injection into cisterna magna34,35,46,47,48. The low mortality rate in the model (9% or 27%, depending on the age of mice) can result from the weak amount of blood injected, the slow-duration of injection, and the tilting of the animal avoiding localized pressure on the brainstem, compared with other double injection models. In SAH patients, the window of CVS occurrence is classically detected at D4-D10 post-hemorrhage. However, in animals, the time to onset and the duration of CVS are less studied and may vary between HSA models, likely depending on experimental protocols and animal species21.
In this context, the model here resembles clinical SAH physiopathology in terms of SAH-associated CVS. In general, in endovascular perforation model, CVS occurs in the MCA and BA after 1 hour in rats40 and after 3 days in mice17. In the model by blood injection into prechiasmatic cistern, CVS occurrence was shown between two49 and eight days37 in rats. In the model of double injection into cisterna magna in rats, CVS develops between 10 minutes29 and 3 days31. We are the first to describe kinetic of appearance of CVS in a mouse model of SAH by double injection, establishing CVS in main cerebral arteries (ACA, MCA and BA) since 3 days and being sustained until the 10th day post-SAH22, close to what is observed in SAH patients. This last model could be defined as a skilled model of SAH, severe enough without mortality, allowing investigation of mechanisms and therapeutics targeting CVS.
However, this mouse SAH model may also present some limits. The first point is the lack of vessel wall rupture, as possibly reproduced in the collagenase-induced SAH model, through destruction/digestion of blood vessel basal lamina50. As for the occurrence of macroscopic CVS, reduced cerebral blood flow (CBF) in some brain territories is not systematically correlated with neurological outcome, thus CBF should be evaluated in this proposed model of SAH. Previous rat studies using Laser Doppler Flowmetry in a SAH model of double injection demonstrated the CBF acute decrease to 30–52% from baseline after the first injection, with a return to baseline after 2 to 3 days post-injection51,52,53. In agreement, it has been shown by MRI a decrease of CBF of 33-50% at D3 and 27–44% at D5 after SAH induction in rat double injection models54,55. The double injection into the cisterna magna allows for a predictable distribution of blood along the subarachnoid space, resulting especially in blood clots around the posterior circulation, but can introduce variations in physiological parameters. To avoid intracranial pressure (ICP) from rising with the volume of injected blood entering into the spinal canal, both leading to confounding functional impairments56, the choice to remove an equivalent volume of cerebrospinal fluid could be done, as previously done in other models30,51. In the model here, sham mice received an equivalent volume of aCSF or physiological 0.9% NaCl, depending on the experiment, obviously leading to a rising of ICP. Thus, an acute increase of ICP results in an increase from 18 mmHg to 120 mmHg27,48,53 in the single injection of blood in the cisterna magna model, from 46 to 107 mmHg27,37,49 in the prechiasmatic cistern blood injection model, and from 27 to 110 mmHg 39,40,53,57,58 in the endovascular perforation model. In contrast, the double blood injection into cisterna magna was associated with a smaller ICP increase from 60 to 67 mmHg48,53. Moreover, the action of removing CSF would also alter ICP and modify CSF. In the SAH model here, the decision was to not remove CSF before blood injection but to accompany surgery by a procedure consisting in hilting the animal head from 30°. The aim is to attenuate ICP by allowing the blood distribution into the anterior circulation, an important and necessary step to mimic human SAH physiopathology. In SAH patients, a sharp rise in ICP is detected and is associated with a transient global cerebral ischemia59, likely contributing to a sustained impairment of autoregulation and early neuronal cell loss60. However, after the first event post-SAH, an early external ventricular drainage is often adopted for concerned SAH patients, to avoid brain swelling and hydrocephalus61. Here, the double injection SAH model may not be severe at the first bleeding event to provoke the ICP-dependent consequences observed in patients, but likely reproduce a sustained and mild enhanced ICP for days post-SAH.
In addition, another uncontrolled parameter in the SAH model here was the potential variations of the mean arterial blood pressure (MABP) induced by excessively rapid blood injection procedure27. Indeed, MABP typically acutely rises after experimental SAH to preserve cerebral perfusion pressure and thereafter, falls to baseline. In the SAH model here, we injected blood or aCSF (~ 10 µL/min) at a low rate to avoid these MABP variations. Regarding the neurobiological events in this model mimicking those observed in humans, we previously showed that the double blood injection model of SAH induces long-lasting CVS, microthrombosis formation and cerebral brain damage including defect in potential paravascular diffusion from day 3 to day 10 post-SAH22. However, recent data describing that CSD is involved in SAH-associated DCI13 strongly support the pursuit of this type of investigations in the mouse model of double injection. This should enable scientific breakthroughs on the beneficial impact of new therapies targeting CSD.
To conclude, the model of double injection of whole arterial blood into the cisterna magna is a mastered model that allows an easy way to mimic the human SAH physiopathology including CVS, microthrombosis, vascular inflammation, neurological deficits and mortality rate. It represents a validated model for testing novel therapeutic approaches to treat SAH-associated morbi-mortality.