NEUROSURGICAL FOCUS Neurosurg Focus 47 (1):E20, 2019 In vivo cerebral aneurysm models John W. Thompson, PhD,1,3 Omar Elwardany, MD,1,3 David J. McCarthy, MS,1,3 Dallas L. Sheinberg, BS,1,3 Carlos M. Alvarez, MD,1,3 Ahmed Nada, MD,1,3 Brian M. Snelling, MD,1,3,4 Stephanie H. Chen, MD,1,3 Samir Sur, MD,1,3 and Robert M. Starke, MD1–3 Departments of 1Neurological Surgery and 2Radiology, University of Miami; 3The University of Miami Cerebrovascular Initiative, University of Miami; and 4Marcus Neuroscience Institute, Boca Raton Regional Hospital, Boca Raton, Florida Cerebral aneurysm rupture is a devastating event resulting in subarachnoid hemorrhage and is associated with signifi- cant morbidity and death. Up to 50% of individuals do not survive aneurysm rupture, with the majority of survivors suf- fering some degree of neurological deficit. Therefore, prior to aneurysm rupture, a large number of diagnosed patients are treated either microsurgically via clipping or endovascularly to prevent aneurysm filling. With the advancement of endovascular surgical techniques and devices, endovascular treatment of cerebral aneurysms is becoming the first-line therapy at many hospitals. Despite this fact, a large number of endovascularly treated patients will have aneurysm re- canalization and progression and will require retreatment. The lack of approved pharmacological interventions for cere- bral aneurysms and the need for retreatment have led to a growing interest in understanding the molecular, cellular, and physiological determinants of cerebral aneurysm pathogenesis, maturation, and rupture. To this end, the use of animal cerebral aneurysm models has contributed significantly to our current understanding of cerebral aneurysm biology and to the development of and training in endovascular devices. This review summarizes the small and large animal models of cerebral aneurysm that are being used to explore the pathophysiology of cerebral aneurysms, as well as the develop- ment of novel endovascular devices for aneurysm treatment. https://thejns.org/doi/abs/10.3171/2019.4.FOCUS19219 KEYWORDS aneurysm; animal; model; in vivo; mice; rabbit; porcine; canine NRUPTURED cerebral aneurysms (CAs) are common pathology of CAs. Equally vital to our understanding of in the general population, with an estimated prev- CA biology and treatment has been the use of CA animal alence ranging from 2% to 6%.68 If left untreated, models, which attempt to replicate the morphological, his- Uaneurysms can progress and spontaneously rupture, pro- tological, and hemodynamic features observed in human ducing a subarachnoid hemorrhage and resulting in sig- CAs. These animal models provide a method for inves- nificant morbidity and death. The pathophysiology of CA tigating aneurysm formation, growth, and rupture while formation and rupture is not fully defined, but risk factors also providing a means of testing new treatment modali- have been identified including increasing age, female sex, ties. CA models have been developed in numerous species hypertension, excessive alcohol intake, and smoking.16,34,68 including mice, rats, rabbits, swine, sheep, canines, and Studies have suggested that hemodynamic stress is a criti- primates, with each model having advantages and limita- cal factor in CA pathogenesis17 leading to endothelial dys- tions such that the model selection depends on the purpose function, inflammatory cell infiltration, and arterial wall of the study. This review explores some of the more com- remodeling.6–7 Vascular smooth-muscle cells undergo a monly used models of CAs and compares the advantages phenotypic switch, which exacerbates inflammation by ex- and disadvantages of each system. pressing inflammatory and matrix remodeling proteins,50 ultimately culminating in histological changes character- ized by disruption of the internal elastic lamina, extracel- Small Animal CA Models lular matrix digestion, thinning of the media, cell loss, and The theory behind CA formation in rats and mice is aneurysm formation. that weakening of the cerebral blood vessels combined Molecular and histological analysis of human CA spec- with hemodynamic stress will induce CA formation. Nu- imens has revealed significant information regarding the merous rat and mouse models of CA formation exist and ABBREVIATIONS BAPN = b-aminopropionitrile; CA = cerebral aneurysm; CCA = common carotid artery; DOCA = deoxycorticosterone acetate. SUBMITTED March 1, 2019. ACCEPTED April 9, 2019. INCLUDE WHEN CITING DOI: 10.3171/2019.4.FOCUS19219. ©AANS 2019, except where prohibited by US copyright law Neurosurg Focus Volume 47 • July 2019 1 Unauthenticated | Downloaded 09/25/21 09:34 PM UTC Thompson et al. FIG. 1. Cerebral aneurysm formation in rodents, hemodynamic stress, and vessel wall weakening. The procedures used for CA formations in rats and mice vary primarily in the method of inducing hypertension, increasing the flow rate, and weakening the ves- sel wall. Hypertension can be induced by a combination of a high salt diet, unilateral nephrectomy or bilateral ligation of the posterior branches of the renal arteries (not shown), and subcutaneous placement of DOCA pellets or angiotensin II–filled microosmotic pump (not shown). Increases in flow rate are accomplished by ligation of the left CCA, which causes a compensatory increase in flow rate in the contralateral internal carotid artery. Vessel wall weakening is accomplished by feeding a diet containing 0.12% BAPN, a lysyl oxidase inhibitor, or by a single stereotactic injection of elastase. Copyright Robert Starke. Published with permission. primarily differ in the mechanisms of vessel wall weaken- at the bifurcation of the right anterior cerebral artery and ing and hemodynamic stress induction (Fig. 1). the olfactory artery. Histological analysis revealed frag- mented elastic laminin and media thinning suggestive of Hemodynamic Stress and Vessel Wall Weakening aneurysm formation in 78% of the treated mice. However, Hemodynamic stress in the cerebral vasculature can be the CAs formed by this method are small with a few mi- increased by hypertension and/or an increase in flow rate. croaneurysms observable by light microscopy, while other Using the combination of hypertension and flow rate to aneurysms require electron microscopy for visualization. induce hemodynamic stress, Hashimoto et al. created the This method of CA formation suffers from slow aneurysm first rodent CA model in rats.31 During a series of surger- formation. Other adaptations to this protocol include liga- ies, hemodynamic stress was increased by ligation of the tion of the left renal artery, unilateral nephrectomy, and bilateral ligation of the posterior branches of the renal ar- left common carotid artery (CCA) while hypertension was 3–6,8,10 induced by unilateral nephrectomy, followed by subcuta- teries during the same surgery. neous injections of deoxycorticosterone acetate (DOCA) and the addition of 1% sodium chloride to the drinking Elastase and Angiotensin II water. Vessel walls were weakened by feeding the rats Early stages of aneurysm formation are associated with chow containing 0.12% b-aminopropionitrile (BAPN), a elastic lamina degeneration, which may contribute to an- lysyl oxidase inhibitor, which prevents collagen and elas- eurysm progression and rupture. Given this histological tin cross-linking, leading to increased vessel fragility and finding, Nuki et al.54 stereotactically injected elastase into a greater likelihood of aneurysm formation. Morimoto et the cerebrospinal fluid of the right basal cistern. To induce al. later adapted this method for CA formation in mice and hypertension, angiotensin II was continuously adminis- included bilateral ligation of the posterior branches of the tered via a subcutaneously placed microosmotic pump. renal arteries.51 Four months following surgery, CAs were CA formation was achieved in 77% of the mice within 2 observed at various stages of formation, located primarily weeks of treatment. Histologically, the aneurysms demon- 2 Neurosurg Focus Volume 47 • July 2019 Unauthenticated | Downloaded 09/25/21 09:34 PM UTC Thompson et al. strated degeneration of the media layer and elastic lamina tigation of aneurysm biology at a molecular, cellular, and and infiltration of inflammatory cells. physiological level. Excluding surgically created saccular aneurysms, rodent CA models do not require direct vessel Intracranial Aneurysm Rupture Model manipulation and have an intracranial location. This leads The spontaneous aneurysm rupture model was in- to the question of what constitutes an aneurysm. In the troduced by Makino et al.,41 who used a combination of early studies, aneurysm formation produced small micro- elastase treatment to weaken cerebral blood vessels and aneurysms that were rarely visible and only detectable by hypertension. With this method, in a series of surger- light or electron microscopy or histological alterations of ies, hypertension is induced by unilateral nephrectomy, the vessel wall. Some scientists do not believe these “mi- implantation of a DOCA-salt pellet, and the addition of croaneurysms” recapitulate human CA disease. In con- 1% NaCl to the drinking water. During the same surgery trast, elastase treatment results in clear and defined out- ward bulging of the vessel walls of the circle of Willis and as the DOCA-salt pellet implantation, the mice receive 61 a single injection of elastase into the right basal cistern. its major branches. Starke et al. defined an aneurysm as a bulge in the vessel