Contributors
J. Michael Schmidt, PhD, Assistant Professor of Neuropsychology (in Neurology) Neurological Intensive Care Unit, Columbia University College of Physicians and Surgeons,
1 Animal Models of Subarachnoid Hemorrhage
Gustavo Pradilla, MD, Resident
Quoc-Anh Thai, MD, Assistant Chief of Service, Instructor Department of Neurosurgery , Johns Hopkins University School of Medicine , Johns Hopkins Medical Institutions, Baltimore , Maryland , USA
Rafael J. Tamargo, MD, FACS Walter E. Dandy Professor a and Director b
a Departments of Neurosurgery, Otolaryngology, and Neck Surgery,
b Department of Cerebrovascular Neurosurgery, Johns Hopkins University School of Medicine, Johns Hopkins Medical Institutions, Baltimore, Maryland , USA
INTRODUCTION
Cerebral vasospasm is the delayed narrowing of cerebral arteries exposed to blood. Although vasospasm typically occurs after subarachnoid hemorrhage (SAH) from rupture of a cerebral aneurysm, it can also develop after trauma ( 1,2 ) and infections ( 3 ). In humans, vasospasm pres-ents as a biphasic phenomenon. Whereas acute vasospasm generally prespres-ents immediately after SAH and typically resolves within hours, chronic vasospasm occurs at 4 to 21 days and peaks 7 to 10 days after hemorrhage, with an overall angiographic incidence of 67% ( 4 ) and a clinical incidence of 37% ( 4 ). Chronic vasospasm causes delayed ischemic defi cits, stroke, and death.
The etiology of vasospasm remains unclear. Current hypotheses include endothelial dysfunction secondary to infl ammation of the arterial wall and transendothelial migration of macrophages and neutrophils ( 5,6 ), nitric oxide (NO) scavenging by such blood-degradation products as oxy-hemoglobin ( 7 ), depletion of NO secondary to NO synthase dysfunction ( 8 ), direct vasoconstriction due such to spasmogenic proteins as endothelin-1 ( 9 ), and dysregulation of electrolyte channels in the smooth muscle cell, such as K + ( 10 ) and Mg 2+ ( 11 ).
In 1949, Robertson at the Royal Melbourne Hospital in Melbourne, Australia, was the fi rst to describe delayed ischemia after SAH and to suggest that the ischemic changes could be related to temporary spasm of the supplying vessels ( 12 ). The fi rst angiographic description of cerebral vasospasm after SAH was reported in 1951 ( 13 ). Since then, this condition has been studied extensively in experimental models.
Studies on the pathophysiology of cerebral vasospasm in humans have been attempted using postmortem specimens ( 14–18 ). Delayed postmortem artifacts, however, have prevented adequate analyses of genomic and proteomic variables, as well as testing of physiologic responses.
To study cerebral vasospasm under more physiologic conditions, several experimental models have been developed. We present an overview of the different experimental models that have been used to date and comment on their technical and scientifi c characteristics.
IN VITRO MODELS OF VASOSPASM
In vitro models of vasospasm typically use an intracranial vessel that is harvested and either placed in a physiologic environment attached to a fi xation device for recording of tension and other variables, or prepared for extraction of endothelial and smooth muscle cells for culture.
The vessels can be harvested after exposure to blood in vivo or they can be harvested from a healthy animal for further experimental manipulation in vitro. Using these models, several pro-vasospastic agents have been characterized and some pharmacologic interventions have been proposed ( 19–24 ). Advantages of these in vitro models include a well-controlled environment, real-time observation of vascular tone and electrolyte changes, low cost, and an abundance of tissue for testing. Disadvantages include removal of the vessel from its natural environment, denervation of the arterial wall, absence of innate immunologic stimulus, and lack of prolonged injury and recovery periods. Due to these observations, the relevance of these models to human
,
cerebral vasospasm has been questioned ( 25 ), and the injection and clot-placement models in animals remain more suitable alternatives.
IN VIVO MODELS OF VASOSPASM
Animal models of SAH have been used extensively to induce vasospasm and include multiple species and diverse techniques. Careful consideration must be given to the selection of the species, because factors—such as time course of vasospasm, manifestations of delayed ischemic defi cits, responses to pharmacotherapy, anatomic composition of the arterial wall of cerebral vessels, and rates of clearing of subarachnoid blood, etc.—can differ from one species to another.
CREATION OF SAH AND INDUCTION OF VASOSPASM
Models of SAH should ideally consist of placement or injection of blood that surrounds a cerebral artery and results in consistent and reproducible, delayed vasospasm that lasts for several days, as confi rmed by angiographic or morphometric analysis. These models should be reproducible and cost effective, and they should use species closest to humans. Whole blood is preferable to induce vasospasm, because erythrocyte hemolysate has been shown to be less capable of generating a delayed, sustained response ( 26,27 ). To induce vasospasm, a number of techniques have been used that lead to the development of delayed-onset, sustained arte-rial narrowing. These techniques can be grouped into 3 general categories: (i) puncture of an artery (endovascularly or under direct vision), (ii) surgical exposure of an artery and placement of an autologous blood clot obtained from another vessel, and (iii) injection of blood obtained from a peripheral vessel into the subarachnoid space. A disappointing feature common to all animal models of SAH and vasospasm is the lack of vasospasm-related ischemic, neurologic defi cits ( 28 ), most likely secondary to an abundance of collateral blood fl ow in smaller verte-brates. However, in that most studies focus on the induction, prevention, and/or reversal of vessel constriction rather than on vasospasm-related ischemia, the absence of ischemic neurologic defi cits in experimental animals has limited signifi cance.
MONKEY MODELS
The fi rst use of monkeys for the study of vasospasm was reported in 1965 ( 29 ). In this study, a transoral approach to the skull base was used to expose the basilar and vertebral arteries, vaso-spasm was induced through application of autologous blood, and measurements were taken by in situ photographic analyses of the vessel calibers.
Currently, the most popular technique is the one described in 1982 in cynomolgus monkeys ( Macaca fascicularis ) ( 30 ). In this model, a preoperative angiogram is obtained, a right frontotem-poral craniectomy is performed, and the arachnoid cisterns encasing the internal carotid artery (ICA), anterior cerebral artery (ACA), and middle cerebral artery (MCA) are dissected. Arterial blood is withdrawn and allowed to clot, and the resulting clot is sectioned into fragments that are placed adjacent to the exposed vessels ( Fig. 1 ). A repeat angiogram is obtained 7 days after surgery, and vasospasm is determined by comparison with the preoperative angiogram ( Fig. 2 ).
Angiographic vessel narrowing ranges from 31% to 100% and typically occurs in all animals ( 30 ). Severe vasospasm, defi ned as a reduction >50% in arterial caliber, was present in only approximately 25% of the animals. The mortality reported by the authors was 10% ( 30 ). In our laboratory, however, we have had no mortality attributable to the model ( 32,33 ).
Advantages of this model include a well-defi ned course of angiographic vasospasm, devel-opment of distal vessel narrowing, histopathologic modifi cations in the exposed vessels, loss of autoregulatory mechanisms, disruption of cerebral blood fl ow (CBF), presence of a contralat-eral control, and absence of pharmacologic responses to standard vasodilators. Disadvantages of the model include high costs and risk of contamination with simian herpes virus. This model has been extensively used as described, or with minor modifi cations, to test several experimental therapies ( 34–43 ) and to analyze proposed pathogenetic mechanisms ( 11,42,44–52 ).
Figure 1 Monkey model of subarachnoid hemorrhage (SAH). ( A ) An artist’s illustration of the surgical technique for induction of SAH in monkey. ( B ) Cerebral angiograms of a cynomolgus monkey before ( top ) and after ( bottom ) SAH.
Other popular techniques used to induce vasospasm in monkeys are based on blood injection into intracranial cisterns, a technique fi rst described in 1968 that involves injection of 2 to 3 ml of blood into the cisterna magna ( 53 ) of African green monkeys. Modifi cations of this technique include injections into the subfrontal subarachnoid space ( 53 ) and prepontine cistern (via cervical laminectomy) ( 54 ). Standardization of blood volumes is critical to induce reproducible vasospasm with these methods. Disadvantages of this model, compared to clot placement techniques, include the lack of a contralateral control vessel and the high variability in the severity and course of the induced vasospasm. Models using rupture, puncture, or avul-sion of the intracranial vessels were used in the past ( 55–58 ), but the high mortality rates and the limited reproducibility of vasospasm have been discouraging.
RABBIT MODELS
The fi rst use of rabbits for the study of vasospasm was reported in 1969 ( 59 ). In this report, an occipital burr hole was placed and, under fl uoroscopic guidance, a catheter was directed into the subarachnoid space close to the orbit and inserted into the right carotid artery. The goal of the study was to analyze the electrocardiographic changes that occur after SAH; vasospasm was not evaluated.
In the most common rabbit model used today, the atlanto-occipital membrane is surgically exposed, cerebrospinal fl uid (CSF) is withdrawn, and 1.25 ml/kg of autologous arterial blood are injected into the surgically exposed cisterna magna, followed by placement of the animal head-down at 30° for 30 min to confi ne the blood to the intracranial cisterns ( Fig. 2 ) ( 28,60 ).
With this technique, peak vasospasm occurs 72 hr after SAH, and a 40% to 45% reduction in the diameter of the basilar artery is observed. A variation of this technique, using percutaneous injection of 1 ml/kg into the cisterna magna, produces similar results. A high correlation between angiographic and morphometric vasospasm has been observed in this model ( 61 ).
Advantages of this model include evaluation of an intracranial vessel, extensive histo-pathologic characterization, well-defi ned time course, and lower cost. Disadvantages include absence of reduction of CBF after single hemorrhage. Proponents of this model have used a double hemorrhage to induce more severe vasospasm ( 62,63 ). The need for double injection, however, has been questioned, because vasospasm is not signifi cantly greater when compared to the single-injection technique ( 64 ).
Numerous therapeutic agents have been tested, and those shown to prevent vasospasm include endothelin antagonists ( 65–67 ), calcium-channel blockers ( 68–70 ), nonsteroidal anti-infl ammatory drugs (NSAIDs) ( 71 ), monoclonal antibodies against CD11/CD18 ( 72 ) and intercellular adhesion molecule 1 (ICAM-1) ( 73 ), and NO donors ( 31 ). Reversal of established vasospasm in this model has been achieved by intracardiac infusion of papaverine, sodium nitroprusside, and adenosine ( 74 ), by local delivery of diethyl-triamine nitric oxide (DETA-NO) ( 75 ), and by various other means.
To induce ischemia after posthemorrhagic vasospasm in rabbits, bilateral common carotid artery (CCA) ligations were performed 2 weeks prior to a double-injection SAH. This maneuver caused cerebral infarction in only 15% of animals ( 76 ). Other, less common techniques include transorbital blood injection into the chiasmatic cistern, rupture of the MCA (puncture via cra-niotomy), mechanical compression, puncture of the MCA and the superior sagittal sinus, blood injection into the interpeduncular cistern, and transclival puncture of the basilar artery.
An extracranial model using the CCA has also been reported in rabbits ( 77 ). In this model, the CCA is encased in polyvinyl chloride cuffs and autologous blood is injected. Vasospasm develops 24 to 48 hr after hemorrhage and persists for approximately 6 days. This model has been used to show eicosanoid production after SAH and induction of vasospasm with the injection of human blood. Therapeutic approaches tested in this model include prophylactic laser treatment and transluminal angioplasty.
DOG MODELS
The fi rst dog model described for the study of vasospasm used a transoral/transclival approach to the chiasmatic cistern ( 78 ). In this study, only 42% of the animals experienced SAH after injection of 5 ml of arterial blood. Complications, such as intraventricular hemorrhages, meningitis, and subdural hematomas, developed, and the study showed that some of the animals developed symptomatic vasospasm. However, the authors did not perform lumen patency studies.
Figure 2 Rabbit model of subarachnoid hemorrhage (SAH). ( A ) An artist’s illustration of the surgical technique for induction of SAH in the rabbit . ( B ) Photograph of a macroscopic specimen showing a blood clot around the basilar artery of a rabbit after induction of SAH. ( C ) Microphotograph of a cross-section of the basilar artery of rabbit after SAH. Source: From Ref. 31.
The next reported dog model used a craniotomy to implant a strain-measuring device around the ICA and a thread to later avulse the ICA and induce SAH ( 79 ). With this technique, acute and chronic vasospasm of the ICA were documented, with peak vasospasm (20% decrease in lumen patency) occurring 4 to 6 days after hemorrhage. This model demonstrated that serial intra-arterial injections of serotonin fail to induce chronic vasospasm. A modifi cation of this technique was developed by avulsing the posterior communicating artery (PCoA) and measur-ing angiographic diameters ( 80 ). Vasospasm after avulsion of the PCoA was more severe than vasospasm after avulsion of the ICA and ranged from 25% to 40%. An acute phase developed 20 min after hemorrhage, followed by a delayed phase 24 hr later.
In the model that followed, 5 ml of blood was injected into the cisterna magna ( 81 ), which caused a decrease in the contractility of vasospastic arteries 7 days after hemorrhage.
This model has been used to study several experimental treatments, including papaverine ( 82 ). Standardization of the angiographic technique and use of a Trendelenberg position for 15 min after injection to encase the subarachnoid clot within the intracranial cisterns improved the reproducibility of this method. The modifi ed technique was reported to induce a 37%
decrease in basilar artery vasospasm 30 min and 48 hr after hemorrhage ( 83,84 ). Experimen-tal treatments tested in this fashion include Ca 2+ channel blockers ( 85 ), NO donors ( 86 ), an angiotensin-converting enzyme inhibitor ( 87 ), and several NSAIDs ( 88 ). Further studies of a single injection of blood into the cisterna magna have shown, however, that this technique does not produce sustained severe vasospasm, and that histopathologic and pharmacologic changes are not consistently present ( 16,89 ).
To address this problem, the use of multiple injections of blood into the cisterna magna in dogs was proposed ( 16 ). This technique induced angiographic vasospasm; however, histo-pathologic or ultrastructural changes were not found. A modifi cation of this technique led to the most popular model of SAH in dogs currently used. It consists of a standardized double injection of 4 ml of blood into the cisterna magna on the fi rst and third days of the study. With this method, angiographic vasospasm of the basilar artery was reported in 82% of animals 5 days after the fi rst injection ( 90,91 ). Further analyses showed histopathologic changes in the basilar artery that were associated with a decreased response to treatment with intra-arterial papaverine. Several experimental studies have been performed with this model. Advantages include a well-defi ned course of vasospasm that is comparable to that of humans, accessible percutaneous angiography, and histopathologic and pharmacologic changes. Disadvantages include limited monoclonal antibodies for analysis, elevated cost, and the need for a second injection.
CAT MODELS
SAH in cats was initially induced by electrical or mechanical stimulation, as well as by laceration of the basilar artery, and vasospasm was determined through diameter measurements under direct microscopic visualization ( 92–94 ) after a transclival approach. Vasospasm was also induced by lysed platelets, whole blood, hemolysate, serotonin, angiotensin, and norepinephrin ( 93,95,96 ) and was successfully prevented by treatment with chlorpromazine and papaverine ( 93,95 ). Whereas laceration of the basilar artery caused vasospasm that lasted for at least 100 min, mechanical spasm reverted within 15 min ( 93 ).
Blood injection into the cisterna magna causes angiographic vasospasm of the basilar artery at 4 hr and 1 to 7 days but fails to cause histopathologic changes in the smooth muscle cells ( 97 ). Nonetheless, this technique has been used to study CSF absorption after treatment with recombinant tissue plasminogen activator ( 98 ) to measure changes in intracranial pulse waves after SAH ( 99 ) and to study CBF post-SAH ( 100 ), despite reports of rapid clearing of blood from the subarachnoid space ( 101 ). Other techniques used include blood injection over the cerebral cortex ( 102 ), rupture of the MCA through puncture or incision ( 97,100 ), blood injection through a shunt from the abdominal aorta through the chiasmatic cistern ( 103 ), and avulsion of the MCA ( 104 ) or ICA ( 105 ). Cat models of vasospasm, however, have decreased in popularity in recent years due to the limited availability of biologic tools for protein analysis, scarce genetic information, and poor characterization of the onset and pro-gression of vasospasm.
RAT MODELS
Although rats have been used extensively to reproduce vasospasm, a number of issues have limited the applicability of the obtained fi ndings to the human disease, among them the lack of myointimal cells in intracranial vessels that might play a role in the intimal hyperplasia observed after vascular injury ( 106 ), high mortality rates, and early resolution of vasospasm.
Intracranial Models
The fi rst intracranial model consisted of transclival exposure of the basilar artery for either puncture with a microelectrode or clot placement, followed by measurements of vessel diam-eter under direct vision ( 107 ). One study used this technique to measure electrolytic changes in the basilar artery and subarachnoid clot and had a mortality of 26%, with peak vasospasm occurring 1 hr after puncture and maximal delayed spasm of 15% at 48 hr ( 108 ). Puncture of the basilar artery, however, resulted in variable amounts of SAH, and direct measurement of the basilar artery suffered from signifi cant interobserver variability.
The next model used transorbital blood injections into the chiasmatic cistern ( 109 ). A cath-eter was placed through a frontal burr hole and advanced around the hemisphere to the cistern to inject heparinized blood. This technique was used to test acute electrocardiographic changes, not vasospasm. Injection of heparinized blood could alter the development of vasospasm by preventing adequate clot formation, and the placement of a catheter “blindly” prevented local-ization of the SAH to one side and prevented the use of the contralateral side for control.
Models of blood injection into the cisterna magna used in other species were adapted for rats, following different methods. The fi rst method described consisted of placing a burr hole in the parietal region and inserting a cannula into the cisterna magna to inject blood and induce vasospasm of the basilar artery. This model was injected with 0.3 ml of either fresh autologous arterial blood or mock CSF, and CBF was determined by tracking labeled microspheres for 1 hr after injection. Rats with experimental SAH showed a 40% decrease in CBF, whereas those that received saline injection showed only a 15% decrease ( 110 ). An increase in the volume of blood injected (0.6 ml) resulted in a decrease in CBF 3 hr after SAH that returned to normal val-ues at 1, 2, 3, 7, and 14 days ( 111 ). In this model, variability of vasospasm was observed between Wistar and Sprague-Dawley rats. The second method consisted of a double injection, in which the posterior atlanto-occipital membrane was exposed, 0.1 ml of CSF was aspirated, mixed with 0.4 ml of venous blood, and 0.1 ml of the mixture was reinjected ( 112 ). In this model, corrosion casts of the cerebral arteries showed vasospasm that was not altered by nimodipine administration.
Endovascular perforation models were developed in rats and have become quite popu-lar. These models are generally referred to as the “Sheffi eld model” because they were initially described by researchers using Wistar rats in 1995 at the Royal Hallamshire Hospital in Sheffi eld, UK ( 113 ). This technique has also been described in Sprague-Dawley rats ( 114 ). The technique consists of inserting a pointed 3-0 monofi lament nylon suture into the ICA and advancing it until it perforates the ACA, which results in SAH in 89% of the animals and in intracerebral hemor-rhage in the remaining 11%. This model has a reported mortality of approximately 50%, and the severity of vasospasm varies signifi cantly; pharmacologic responses to delayed vasospasm and pathologic changes in the arterial wall do not occur ( 115 ). A study comparing injection into the cisterna magna with endovascular rupture through the ICA using a 3-0 or a 4-0 nylon suture showed that the 4-0 suture produced less SAH and resulted in lower peak intracranial pressure when compared to other groups and that CBF reductions were similar in all groups, with the injection group having a faster CBF recovery ( 116 ).
A more recent study was performed to compare the severity of vasospasm induced by either endovascular puncture with a 3-0 suture through the ICA, single injection of 0.3 ml of blood into the cisterna magna, or double injection of 0.3 ml (48-hr interval between injections) ( 117 ) in male Sprague-Dawley rats. Histopathologic examination and morphometric analy-sis were performed on the basilar artery and PCoA. The study showed that these techniques caused signifi cant vasospasm, with the double-hemorrhage model inducing the most severe vasospasm. Double hemorrhage, however, caused the highest mortality rate (57%) and had signifi cant variability in hemorrhage volumes when compared to the cisternal injection models.
Whereas vasospasm after endovascular perforation or single hemorrhage was more pronounced in the PCoA, vasospasm after double hemorrhage was more pronounced in the basilar artery.
The authors concluded that the double-hemorrhage model was the most suitable alternative for studying mechanistic and therapeutic approaches for vasospasm.
Extracranial Models
Another currently popular model utilizes the rat femoral artery. This model consists of exposing the femoral artery, isolating it in a silicon cuff, and fi lling the cuff with blood or blood components ( Fig. 3 ) ( 118 ). Peak morphometric vasospasm in this model occurs on day 7 and is accompanied by pathologic changes in the arterial wall. The major advantage of this model is the similarity of its course to that of human vasospasm. Other advantages are the availability of a contralateral control vessel and the controlled volume and localization of the hemorrhage. The main disad-vantage of this model is the use of a systemic vessel, which excludes CSF clearance, changes in intracranial pressure, and central nervous system–specifi c infl ammatory responses from the experimental variables. Several groups have shown that pharmacologic responses observed in this model correlate with those observed in other species ( 31,32,71,72,119–121 ) and that the pathologic changes observed are comparable to those seen after SAH in intracranial models and in humans.
MOUSE MODELS
Mouse models of vasospasm are similar to rat models and have been recently developed to take advantage of transgenic technology. The fi rst model used endovascular perforation of the ACA ( 122 ). In this model, a 5-0 monofi lament suture with a blunt end is advanced through the ICA up to the ACA until resistance is felt. The suture is advanced 5 mm further to perforate the ACA and then withdrawn. Acute mortality with this model is 28%. To determine vasospasm, animals are perfused with 10% formalin, followed by carbon mixed with 10% gelatin. The diameter of the MCA is then measured under a microscope. Peak vasospasm in this model occurs on day 3 and results in an approximately 20% decrease in MCA diameter compared to Figure 3 Rat femoral artery model of vasospasm. ( A ) Artist’s illustration of the surgical technique for induction of vasospasm in the femoral artery of a rat. ( B ) Microphotograph of a cross-section of the femoral artery of a rat after peri-adventitial blood deposition. Source: From Ref. 119.
controls. Studies performed with this model were designed to test the potential protective effect of overexpressed superoxide dismutase in transgenic mice.
Advantages of this model include a relatively low cost and the ability to use transgenic mice. Disadvantages include a high mortality rate, moderate vasospasm, lack of standardiza-tion of hemorrhagic volume, and interobserver variability when measuring MCA diameters.
A modifi ed technique for intracisternal injection has been reported ( 123 ) in which the femoral artery is cannulated and 60 mL of blood is withdrawn and reinjected into the cisterna magna. This technique carries a mortality of approximately 3%; acute (6–12 hr) and chronic (1–3 days) vasospasm are observed after transthoracic perfusion with 10% gelatin and 10%
India ink ( Fig. 4 ). The diameters of the basilar artery, ACA, and MCA are measured using a digital camera and stereomicroscope. A histopathologic analysis of cross-sections of the analyzed vessels in the current study showed arterial wall changes consistent with those seen in other models. This model appears to be reproducible, accessible, and comparable to other murine models, and it has the advantage of lower mortality. Disadvantages include mild-to-moderate sustained vasospasm with peaks at 3 days in the ACA (21%) and at 1.5 days in the MCA (15%) and basilar artery (15%).
OTHER MODELS Pig Models
A porcine model of vasospasm in pigs was developed that consists of placement of a catheter into the prepontine cistern, followed by two 12-ml injections of blood, with a 48-hr interval ( 124 ). Vasospasm was determined by angiography 48 hr after the fi rst injection in 4 out of the 6 animals studied. Arterial injury was observed in specimens collected between 7 and 24 days after the initial hemorrhage. Pigs were selected due to their tendency to develop spontaneous atherosclerosis with age.
Figure 4 Mouse model of subarachnoid hemorrhage (SAH). ( Left ) Vasospasm determined after India ink injec-tion. ( Right ) Evolution of the subarachnoid clot over time. Source: From Ref. 123.
A model that followed a clot placement technique via frontotemporal craniotomy was developed to determine the ability of specifi c blood fractions that contain hemoglobin to induce morphometric vasospasm of the MCA 10 days after hemorrhage ( 125 ). In this study, the fractions that contained hemoglobin induced morphometric vasospasm and ultrastructural changes that were comparable to those observed in the MCA of animals exposed to whole blood.
Goat Models
Blood was injected into the basal cisterns of goats through a parietotemporal catheter to study the role of endothelin-1 in posthemorrhagic vasospasm and to test the ability of nicardipine to prevent SAH and endothelin-1–induced vasospasm ( 126 ). The authors reported a decrease of 28% in CBF at 3 days after hemorrhage that resolved by day 7.
CONCLUSIONS
Currently, in vitro models of vasospasm remain inferior to in vivo models, and their use is limited to specifi c goals, such as physiologic studies of electron channels and screening of potentially favorable treatments. Among animal models, small species, such as mice, rats, and rabbits, because of their availability and low cost, are ideal for screening for novel therapeutics and test-ing physiologic variables, but monkeys, which are phylogenetically closest to humans, should be studied before experimental treatments are applied to humans.
Most animal models used to study vasospasm employ either blood injection/blood clot placement techniques or vessel puncture/avulsion techniques to induce SAH and vasospasm.
Injection/clot placement techniques are preferable. These techniques are easy to perform in most species, result in signifi cant and consistent vessel narrowing, and are associated with decreased animal morbidity and mortality. Vessel puncture/avulsion techniques are less appealing due to high mortality rates. The selection of one technique over another must be made according to the experimental question to be addressed.
The use of extracranial arteries to study cerebral vasospasm remains controversial. Intracra-nial vessels are favored because blood breakdown products remain in the CSF for longer periods than they do in the soft tissues, neural regulation of arterial tone differs between intracranial and extracranial vessels, and immunologic responses in the brain are modulated through cen-tral nervous system–specifi c pathways.
The most common endpoint in animal models of vasospasm is vessel narrowing, deter-mined by either morphometric or angiographic vessel diameter. Among all animal models, the rabbit and monkey models are currently the best models of SAH and vasospasm.
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