Introduction

Vasospasm is a common complication that follows aneurysmal subarachnoid hemorrhage (SAH). Ecker was first to point out the occurrence of arterial spasm following SAH [1]. Before him, Robertson had attributed ischemic brain lesions found on autopsy of patients with SAH to probable 'spasm of arteries' [2]. Despite growing literature, skepticism regarding the association between angiographic vasospasm and clinical findings persisted [3], until CM Fisher and colleagues published a synopsis on the matter in 1977 [4]. This seminal publication comprehensively described the deficits accompanying vasospasm and, most importantly, made the association between vasospasm and neurological deficits, also known as delayed ischemic deficits (DID).

The term vasospasm implies a reduction in the caliber of a vessel; however, in SAH it has multiple meanings. SAH-induced vasospasm is a complex entity due in part to a delayed and reversible vasculopathy, impaired autoregulatory function, and hypovolemia causing a regional reduction of cerebral perfusion to the point of causing ischemia [5, 6].

Radiographic evidence of vasospasm develops in 50% to 70% of patients with SAH, but only half of those experience symptoms of DID [7–12]. Proximal vessels, situated at the base of the brain, are preferentially affected; however, more distal arteries could also develop impaired vascular reactivity (autoregulation), further reducing cerebral blood flow [5, 13, 14]. A tendency toward spontaneous intravascular volume contraction can further compound the deleterious effect of a marginal cerebral blood flow (CBF) caused by vasoconstriction. These factors are probably in play in a subset of patients with DID who show no evidence of radiographic vasospasm.

Vasospasm adversely affects outcome in patients with SAH; it accounts for up to 23% of disability and deaths related to SAH [8, 9, 15–17]. However, given its predictable delayed onset between day 5 and 15 after bleeding, it is a potentially modifiable factor. Use of nimodipine, a calcium channel antagonist, and prompt recognition and treatment with hypervolemic hypertensive therapy (HHT) and endovascular interventions are likely responsible for the lower incidence of DID reported after their widespread use [17, 18]. They are by no means completely effective and additional treatments are needed. The ongoing elucidation of the pathophysiology of vasospasm is crucial, as it offers targets for novel therapeutic modalities.

Pathophysiology

The pathophysiology of vasospasm is far from being completely understood. Histologically, there are structural alterations in endothelial and smooth muscle cells in the arterial wall [19]. The presence of oxyhemoglobin in the subarachnoid space seems to be necessary to produce these changes [20–22]. The specific mechanisms leading to vasoconstriction, however, are unknown. In vitro, oxyhemoglobin stimulates the secretion of endothelin (ET)-1, a vasoconstrictor, inhibits the vasodilator nitric oxide (NO) and produces activated oxygen species [23–25]. These free radicals are believed to play a role in cell membrane lipid peroxidation, possibly mediating the structural changes in the vessel wall.

Whether inflammation is simply part of the multi-organ system dysfunction encountered in SAH [26] or contributes to the development of vasospasm is unsettled. The risk of vasospasm is increased in the presence of systemic inflammatory response syndrome [27]. Furthermore, cerebrospinal fluid (CSF) levels of interleukin-1β and -6 in patients with SAH are increased during the vasospasm period and in those in whom vasospasm and ischemia develop later [28]. Genetic and racial factors are likely important; studies of SAH from Japan revealed a higher incidence of vasospasm across different diagnostic methods [29]. Also, certain endothelial NO synthase (eNOS) gene polymorphisms seem to be associated with an increased risk of vasospasm [30].

Risk factors for vasospasm and DID are amount and duration of exposure to subarachnoid blood, thick blood collections in basal cisterns and fissures, and intraventricular blood [31–34]. Interestingly, however, endovascular coiling of the ruptured aneurysm, a procedure that does not involve a craniotomy and washing out of the subarachnoid blood, does not increase the risk of vasospasm in comparison to surgical clipping [35, 36]. Advanced age [37], race [29], poor neurological status on admission [17, 37, 38] and use of antifibrinolytic agents [16, 33, 39] are also associated with the development of DID. Factors less robustly linked to a higher incidence of DID are a longer duration of unconsciousness following the initial hemorrhage [40], history of hypertension [37, 41], smoking [42, 43], and excess weight [41].

Diagnosis of vasospasm

Clinical diagnosis

The diagnosis of vasospasm is primarily clinical. Vasospasm can be asymptomatic; however, when the net result of vasoconstriction, impaired autoregulation, and inadequate intravascular volume is a CBF below ischemic threshold, symptoms ensue. They typically develop subacutely, and because of the dynamic interplay between the inciting factors, they might fluctuate. Symptoms range from vague and non-specific, such as excess sleepiness, lethargy, and stupor, to a spectrum of localizing findings like hemiparesis or hemiplegia, abulia, language disturbances, visual fields deficits, gaze impairment, and cranial nerve palsies [4]. Although localizing, these signs are not diagnostic of any specific pathological process; therefore, alternative diagnoses, such as rebleeding, hydrocephalus, seizures and metabolic derangements, should be promptly excluded using radiographic, clinical and laboratory assessments. On the other hand, the neurological changes can be subtle or unapparent, as many individuals have an abnormal exam related to the initial hemorrhage. Detection of clinical signs of vasospasms is particularly difficult in poor grade patients because of the limited exam that is possible [44]. The frequent use of sedatives in SAH patients further complicates this task. Thus, the evaluation frequently includes transcranial doppler ultra-sonography (TCD) and angiography. Angiography can be both diagnostic and therapeutic (see below).

Cerebral angiography and transcranial doppler ultrasonography

Cerebral angiography is the gold standard for visualizing and studying cerebral arteries. The non-invasive nature of TCD, however, makes it an appealing method for monitoring for, and to help confirm, the clinical diagnosis of vasospasm. It detects elevation in mean CBF velocities, mainly in middle and internal cerebral arteries [45, 46]. Although it is almost as sensitive as angiography in detecting symptomatic vasospasm [47–49], inadequate insonation window in a proportion of patients, unacceptably high rate of false negatives [48], and failure to account for altered autoregulation during hemodynamic manipulation [13] limit its utility (Table 1).

Table 1 Detection of symptomatic vasospasm (mean flow velocity >120 cm/s) by transcranial doppler ultrasonography compared to clinical examination
Full size table

Emerging modalities

The ability of other imaging modalities, like perfusion computed tomography [50, 51], Xenon computed tomography [52, 53], diffusion weighted magnetic resonance imaging [54, 55], and single photon emission computed tomography (SPECT) [51, 56] in detecting vasospasm are under investigation. These imaging techniques could soon become routine in the diagnosis of vasospasm [57]. Unlike cerebral angiography and TCD, these techniques measure regional perfusion, not merely arterial diameter or flow velocities. Online micro-dialysis is another new technique currently being studied in vasospasm [58]. It involves measuring extracellular cerebral fluid levels of an array of substances like glucose, glutamate, lactate, and pyruvate.

Reducing the impact of vasospasm

The typical temporal course of vasospasm and its high incidence make prevention an attractive therapeutic approach. However, the process is a difficult one to study and despite investigation of a myriad of compounds, very few have made it to the clinical arena (Additional data file 1).

Nimodipine

Nimodipine is a dihydropyridine that blocks calcium influx through the L-type calcium channels. It is the most rigorously studied and only drug approved by the US Food and Drug Administration for use in treatment of vasospasm. It is safe [12, 59], cost-effective [60], and most importantly reduces the risk of poor outcome and secondary ischemia after aneurysmal SAH [7, 10–12, 61]. A major randomized controlled trial, the British aneurysm oral nimodipine trial, showed a significant reduction in the incidence of cerebral infarction and poor outcome at three months compared to placebo [12].

How nimodipine exerts its beneficial effects is not well understood and may involve neuronal as well as vascular factors, although, of note, it does not significantly reverse angiographic vasospasm [62]. Nimodipine is administered in a dose of 60 mg every 4 hours for 14–21 days after SAH. In Europe, nimodipine is also used as a continuous intravenous infusion, although this is often associated with hypotension.

Other calcium channel antagonists

Nicardipine [62–65] and diltiazem [62, 63, 66, 67] have both been studied, but only nicardipine in a controlled fashion. In a large randomized trial nicardipine decreased the incidence of DID, reduced the use of HHT and reduced angiographic vasospasm, yet it did not improve overall outcome at 3 months [62, 64, 65]. An unblinded small study of prophylactic, serial intrathecal nicardipine was conducted in 50 patients with SAH. This approach reduced the incidence of both angiographic and clinical vasospasm and improved good clinical outcome at 1 month by 15%. Adverse events were frequent; nine patients developed headache and two had meningitis [68].

Phase I and II safety studies of diltiazem in SAH demonstrated safety but no effect on vasospasm [67]. A recently published paper describing a series of 123 SAH patients treated with oral diltiazem instead of nimodipine reported a 19.5% incidence of DID [66]. Favorable outcome (Glasgow Outcome Scale of 4 or 5) was achieved in 75% of patients.

Tirilazad mesylate

Tirilazad, a non-glucocorticoid 21 amino-steroid free radical scavenger, was studied in several controlled trials [69–73] following promising results in primate vasospasm models [74–76]. It was well tolerated but had inconsistent effect on overall outcome across the different studies, possibly related to gender differences in drug metabolism and an interaction with phenytoin.

Prophylactic hypervolemia

In large prospective controlled studies, prophylactic volume expansion therapy failed to reduce the incidence of clinical or TCD-defined vasospasm, did not improve CBF, and had no effect on outcome [77–79]. In one of those studies, costs and complications were higher in the group treated with prophylactic hypervolemia [77]. A small retrospective cohort reported worsening outcome after discontinuing routine use of albumin to induce hypervolemia in SAH [80].

Lumbar drainage of CSF and intracisternal thrombolysis

The amount of blood in the subarachnoid space is a strong predictor for the development of vasospasm. Several interventions to facilitate the clearance of blood from the CSF following SAH have been studied. Cisternal irrigation by tissue plasminogen activator [81] was relatively safe [82, 83] but had no impact on incidence of angiographic vasospasm [84]. Intra-and post-operative cisternal irrigation with tissue plasminogen activator combined with continuous post-operative cisternal drainage was associated with a low incidence of vasospasm [85]. Intracisternal infusion of urokinase has also been studied in a small retrospective randomized, but not placebo-controlled trial [86, 87]. Incidence of vasospasm was significantly reduced and outcome improved.

Lumbar CSF drainage following SAH is another appealing technique to clear blood from the subarachnoid space. A non-randomized, controlled-cohort study enrolled 167 patients in whom CSF drainage reduced the incidence of clinical vasospasm, the use of angioplasty, and vasospasm-related infarction [88]. Larger placebo controlled studies are needed to determine if these interventions produce sustained clinical benefits.

Prophylactic transluminal balloon angioplasty

Following promising experimental results, a pilot study of prophylactic transluminal balloon angioplasty (TBA) was undertaken in a group of 13 patients with Fisher grade 3 SAH [89]. None of the patients developed DID. Recently, a multi-center randomized trial evaluated the use of prophylactic TBA in a larger group of patients [90]. The procedure showed no benefit, and was responsible for 3 deaths (4%) from vessel rupture, an incidence higher than the 1.1% reported in the literature [91].

Aggressive treatment of vasospasm

Given the limited impact of established and developing preventive measures, more aggressive interventions are often implemented. The threshold for instituting these interventions varies widely across centers. Some actively intervene in the setting of rising TCD velocities; others may treat angiographic vasospasm in asymptomatic patients, while some require a neurological deterioration before instituting aggressive measures. The ideal therapeutic combination would improve CBF, reverse or attenuate DID, and have low potential for adverse events. While this intervention has yet to be defined, varying combinations of medical and endovascular approaches are widely used to treat vasospasm.

Medical therapy

HHT, also described as hemodynamic augmentation, is the cornerstone of medical therapy for vasospasm. The varying nomenclature reflects the fact that it is unclear which specific intervention is most effective. Studies of CBF in SAH patients undergoing HHT have yielded varying results. While acute volume expansion in patients with symptomatic vasospasm increased CBF in areas of brain most vulnerable to ischemia on positron emission tomography (PET) [92], prophylactic hypervolemia did not produce such a response when SPECT [77] or 133Xe clearance [78] were used. HHT appears safe following endovascular coiling of aneurysm [93], and even in patients with prior cardiac disease [94].

In clinical practice, attempts to keep symptomatic patients hypervolemic using crystalloids or colloids should be made. Although exact criteria have been hard to establish, hypertension is induced using vasopressors until there is clinical improvement, a preset limit is reached, or adverse effects occur. Clinical improvement can be dramatic [94], but is an inconsistent finding across case series. Prospective controlled outcome studies of hemodynamic interventions are lacking. Yet, such clinical trials are unlikely to be completed given the widespread use of these interventions.

Endovascular therapy

Endovascular techniques frequently play a role in the aggressive treatment of vasospasm [95, 96]. They include TBA and intra-arterial infusion of vasodilators. Both methods have their unique associated risks and benefits and are usually undertaken after a trial of medical therapy except in patients with severe cardiac disease.

Transluminal balloon angioplasty

TBA is very effective at reversing angiographic spasm of large proximal vessels. It produces a sustained reversal of arterial narrowing, although clinical improvement is inconsistent [97–99]. The timing of TBA in regard to medical therapy is controversial. Some retrospective data suggest that early angioplasty (within 2 hours from onset of symptoms) is associated with sustained clinical improvement [100].

Age and poor neurological status are associated with poor outcome following TBA for symptomatic vasospasm [101]. The sustained effect of angioplasty may well be due to its ability to disrupt connective tissue, as has been seen in the media of cerebral arteries removed at autopsy from patients who underwent the procedure [102]. Major complications of TBA are encountered in about 5% of procedures [91] and include vessel rupture, occlusion, dissection, hemorrhagic infarction and hemorrhage from unsecured aneurysms [96].

Intra-arterial vasodilators

Papaverine is a potent smooth muscle relaxant; its use in SAH related vasospasm has been extensively studied. It is infused intra-arterially through a micro-catheter proximal to the vasospastic vessel. In most cases, its effect on angiographic vasospasm is immediate and dramatic [103–106] but reversal of clinical deficits is variable [91]. Papaverine has been shown to transiently improve regional CBF [103, 107]. The effect of papaverine on outcome is unknown. In one study, when compared to patients with similar characteristics and degree of vasospasm, patients who were treated with papaverine had similar outcome at three months [108].

In most centers, use of papaverine has been relegated to a secondary role or altogether abandoned because of its short-lived effect and a myriad of complications. The most serious are increased intracranial pressure [109], brainstem depression [110], worsening of vasospasm [111, 112], neurological deterioration with gray matter changes on MRI [113], and seizures [114].

This has led to growing use of intra-arterial nicardipine, verapamil, nimodipine, and milrinone as alternatives to papaverine. Nicardipine reverses angiographic vasospasm and significantly reduces mean peak systolic velocities in treated vessels, with no sustained effect on intracranial pressure or cardiovascular function [115]. Verapamil is reported to reduce angiographic spasm and produce clinical improvement in a third of cases without significant adverse events [116]. Nimodipine showed similar favorable results in two small retrospective series [117, 118]. Controlled clinical trials are lacking.

Future directions

A number of therapies are currently being developed and are at different stages of testing. They include magnesium sulfate (Mg++), statins, NO donors, and ET-1 antagonists.

Magnesium sulfate

Hypomagnesemia on admission occurs in 38% of individuals with SAH [119]. Whether it independently predicts the development of DID is controversial [119, 120]. The appeal of Mg++ in SAH stems from its biochemical properties as a physiological antagonist of calcium [121], ease of administration, low cost, the ability to measure and regulate concentration in body fluids [122, 123], and favorable safety profile.

There have been a number of encouraging reports on the effect of Mg++ in animal models of SAH related vasospasm [124–127]. In patients with stroke and SAH, administration of Mg++ is practical and safe [122, 123, 128–131]. In a pilot, randomized, double blind study comparing Mg++ to saline there was a trend toward less symptomatic vasospasm with Mg++ [129]. Yet a large controlled trial of continuous Mg++ infusion did not find conclusive effects on DID or outcome [132]. In a small, single-center trial Mg++ was similar to intravenous nimodipine in preventing DID [133]. On the other hand, Mg++ was of no added benefit in patients receiving prophylactic hypervolemia/hemodilution [134]. Interestingly, a TCD study showed no improvement in elevated mean flow velocities in middle cerebral arteries of patients with clinical vasospasm after receiving a bolus infusion of Mg++ [135].

Statins

Statins, or 3-hydroxy-3-methylglutaryl-CoA reductase inhibitors, appear to have a promising role in vasospasm prevention. The proposed mechanism of neuroprotection in vasospasm is related to induction of the NOS pathway, leading to dilation of cerebral vessels and improved CBF [136–138].

Two small randomized placebo-controlled, single-center studies investigated the safety and feasibility of statins in SAH. In one study, pravastatin reduced the incidence of TCD-defined vasospasm and shortened the duration of severe vasospasm [139]. Another randomized controlled trial used simvastatin in a smaller group of patients [140]. The incidence of TCD-defined vasospasm and DID was significantly reduced in the simvastatin group. The routine use of statins in SAH is awaiting larger, multi-center clinical trials showing clear reduction in DID and improvement in overall outcome.

Nitric oxide donors

NO is a free radical gas formed by the enzyme NOS from the substrate L-arginine. It was discovered in 1987 [141] and appears to have a crucial role in controlling cerebral vasomotor tone. Tonic release of NO is an important regulator of resting CBF; inhibition of NOS constricts cerebral arteries and decreases CBF [142–144].

Intraventricular administration of sodium nitroprusside, a NO donor, to patients with medically refractory vasospasm had variable effects on CBF and a high rate of adverse events [145]. Partial to complete reversal of angiographic vasospasm was seen in ten patients after sodium nitroprusside [146], and symptoms completely resolved in two. Vomiting was the most common adverse effect (in seven out of ten) and three patients had mild fluctuation in blood pressure. In three patients administered intrathecal sodium nitroprusside, clinical and angiographic improvement and excellent outcome with no systemic or neurological complications was reported [147].

Finally, transdermal nitroglycerin was tested in SAH. There were no differences in terms of DID and TCD velocities between the nitroglycerin group (nine patients) and the control group (eight patients). CBF, measured by perfusion computed tomography, was increased in the nitroglycerin group [148]. Large randomized and controlled trials of NO donors in SAH are in the planning stage.

Endothelin-1 antagonists

ET-1 was identified in 1988 [149]. It is a 21 amino acid peptide generated in the endothelium of blood vessels and has an important role in vascular tone regulation. ET-1 exerts its effects through two receptor subtypes, ETA and ETB. ETA receptors are found on vascular smooth muscle cells and mediate vasoconstriction of small and large blood vessels. ETB receptors, on the other hand, are found in brain, aorta, lung and kidney vascular endothelial cells where they modulate vasoconstriction in response to ET-1, through the production of vasodilator substances like prostacyclin and NO. They are also found on vascular smooth muscle cells where they can mediate vasoconstriction [150–153].

A phase IIa trial of clazosentan (an ETA antagonist) demonstrated reduction in the incidence and severity of angiographic vasospasm [154]. Adverse events were comparable to placebo. An ETA/B antagonist, TAK-044, was also tested in a phase II trial [155]. The drug was very well tolerated. Delayed ischemic deficits occurred in 29.5% of patients receiving active treatment and 36.6% of patients on placebo (risk reduction 0.8, 95% confidence interval of 0.61 to 1.06).

Most recently, clazosentan was tested in a controlled clinical trial enrolling 413 patients with SAH [156]. Moderate to severe angiographic spasm was significantly reduced, although there was no effect on outcome.

Other therapies

Enoxaparin, a low molecular weight heparin, was studied in a randomized clinical trial in SAH [157]. Although the incidence of DID and infarcts was reduced, the admission characteristics of the two groups were not well balanced.

Nicardipine prolonged-release implants (NPRIs) are placed in the subarachnoid space at the time of surgical clipping of aneurysm. Two case series describing the use of such implants are of interest [158, 159]. In one, Kasuya and colleagues report an incidence of DID of 6% when they were applied in 69 patients with thick subarachnoid clots [158]. Recently, a randomized double-blind trial of the implants showed a dramatic reduction in incidence of angiographic vasospasm and infarctions [160].

A randomized, controlled trial compared dapsone to placebo (n = 49) in Fisher grade 3 and 4 SAH [161]. It is thought to act as a glutamate receptor antagonist and reduced the incidence of DID (26.9% versus 63.6%, p = 0.01) and significantly improved outcome at discharge and three months (modified Rankin scale).

Conclusion

There is a great need for new preventive strategies and therapies to lessen the impact of vasospasm following SAH. Unfortunately, to date the available literature provides few definitive answers. A number of factors conspire to make the task of better defining treatment exceedingly challenging. They include the complex, incompletely understood mechanisms operating in SAH, the relatively low frequency of the disease, and most importantly, the large number of other factors that influence outcome is this population. To properly study interventions in SAH, very large multi-center, prospective, tightly controlled studies are needed; unfortunately, their design and execution remains a major challenge.

This lack of definitive answers leads to a wide variation in the specifics of managing patients with SAH. Yet in general, current management focuses on screening patients at risk for DID, implementing multiple preventive measures and more aggressive interventions in selected patients. A number of neuroprotective approaches as well as the use of multimodal treatment regimens [162] are under active development and hold promise in the treatment of vasospasm.

Note

This article is part of a review series on Stroke, edited by David Menon.

Other articles in the series can be found online at http://ccforum.com/articles/theme-series.asp?series=CC_Stroke