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Dennis J. Rivet and Ralph G. Dacey

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Gregory J. Zipfel and Ralph G. Dacey

Over the past decade, several factors have led to a dramatic change in the manner in which patients with unruptured intracranial aneurysms are diagnosed and treated. These factors include the increased use of noninvasive imaging modalities for the diagnosis of intracranial aneurysms, publication of new natural history data detailing the hemorrhage risks associated with unruptured intracranial aneurysms, and the broad application of endovascular therapy for their treatment. With these new technologies and new natural history data has come considerable uncertainty about the optimal treatment strategy for patients with unruptured intracranial aneurysms. In this light, it seems prudent to review periodically and examine critically all recent data pertaining to the natural history and treatment of unruptured intracranial aneurysms, in an effort to provide a scientific update on which management recommendations can be based. This review article represents the authors' attempt at such an update, and it is their hope that members of the community of neurovascular surgeons might find this information helpful during their continuing efforts to provide optimal care for their patients with unruptured intracranial aneurysms.

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Masakazu Takayasu and Ralph G. Dacey Jr.

✓ An isolated cerebral arteriole preparation was used to test the hypothesis that a temporary reduction in transmural pressure causes a subsequent vasodilation mediated by mechanisms intrinsic to the vessel wall. Thirty-five cerebral vessels of 44.7 ± 1.4 µm (± standard error of the mean) mean diameter were cannulated in vitro and pressurized at a transmural pressure of 60 mm Hg; after an equilibration period the vessels developed spontaneous tone. When transmural pressure was decreased to 0 mm Hg for a period of 4 minutes then returned to 60 mm Hg, vessels dilated to 155.1% ± 6.8% of control diameter before gradually redeveloping spontaneous tone in 5.5 ± 0.7 minutes. Varying the duration of the period during which transmural pressure was at 0 mm Hg had no significant effect on the degree of vasodilation. Conversely, varying the level of decreased transmural pressure between 0 and 20 mm Hg significantly affected both the magnitude of vasodilation and the time course of spontaneous tone recovery. These findings indicate that a temporary period of decreased transmural pressure may result in a loss of spontaneous tone in the resistance vessels of the cerebral microcirculation. Mechanisms intrinsic to the vessel wall may play a significant role in the early stage of post-reperfusion hyperemia. Such mechanisms could also be implicated in other hyperemic phenomena affecting the cerebral circulation, such as the rapid increase in intracranial pressure after subarachnoid hemorrhage, the development of the normal perfusion pressure breakthrough phenomenon, and the initiation of intracranial pressure plateau waves.

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Dennis G. Vollmer, Masakazu Takayasu and Ralph G. Dacey Jr.

✓ The reactivity of rabbit basilar artery and penetrating arteriolar microvessels was studied in vitro using an isometric-tension measurement technique and an isolated perfused arteriole preparation, respectively. Comparisons were made between reactivities of normal vessels and those obtained from animals subjected to experimental subarachnoid hemorrhage (SAH) 3 days prior to examination. Subarachnoid hemorrhage produced significant increases in basilar artery contraction in response to increasing concentrations of serotonin (5-hydroxytryptamine) (10−9 to 10−5 M) and prostaglandin F (10−9 to 10−5 M) when compared to normal arteries. In addition, SAH attenuated the relaxing effect of acetylcholine following serotonin-induced contraction and of adenosine triphosphate after KCl-induced basilar artery contractions. In contrast to the changes observed in large arteries, cerebral microvessels did not demonstrate significant differences in spontaneous tone or in reactivity to a number of vasoactive stimuli including application of calcium, serotonin, and acetylcholine. On the other hand, small but significant changes in arteriolar responsiveness to changes in extraluminal pH and to application of KCl were noted.

Findings from this study suggest that intracerebral resistance vessels of the cerebral microcirculation are not greatly affected by the presence of subarachnoid clot, in contrast to the large arteries in the basal subarachnoid space. The small changes that do occur are qualitatively different from those observed for large arteries. These findings are consistent with the observation of significant therapeutic benefit with the use of calcium channel blockers without changes in angiographically visible vasospasm in large vessels. It is likely, therefore, that calcium antagonists may act to decrease total cerebrovascular resistance at the level of the relatively unaffected microcirculation after SAH without changing large vessel diameter.

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Koichiro Ogura, Masakazu Takayasu and Ralph G. Dacey Jr

✓ The effects of hypothermia and hyperthermia on the cerebral microcirculation were studied using isolated perfused intracerebral (parenchymal) arterioles obtained from rats. In a temperature-dependent manner, hypothermia (20.0° to 35.0°C) dilated the spontaneous tone developed by the arterioles and also diminished their contractile response to potassium and prostaglandin F. In contrast, hyperthermia (40.0° to 45.0°C) induced a biphasic response consisting of initial vasoconstriction and secondary vasodilation. Exposure of the vessels to 45.0°C for 30 minutes irreversibly abolished the spontaneous tone and responsiveness of the arterioles when the temperature of the preparation was returned to 37.5°C. In calcium-free solutions, however, the arteriolar diameter was not affected within a temperature range of 20.0° to 45°C. Furthermore, arterioles that had been in a calcium-free solution during exposure to 45°C temperature recovered their viability at 37.5°C. These results suggest that changes in ambient temperature alter calcium-induced contraction in arteriolar smooth muscle, and that the irreversible effects of hyperthermia on the arterioles are dependent upon extracellular calcium. These studies indicate that alterations in brain temperature may affect the pathogenesis of cerebral ischemia by mechanisms that are in part independent of parenchymal metabolism.

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Hans H. Dietrich and Ralph G. Dacey Jr.

✓ The relationship between cell membrane potential, vessel diameter, and pH in small cerebral arterioles is not completely understood. This study involved direct, simultaneous measurement of cell membrane potential and vessel diameter at various extracellular pH levels. Arterioles ranging from 44 to 91 µm in diameter were isolated, transferred to a temperature-controlled microscope chamber, which was used as an organ bath, and observed through an inverted videomicroscope. Two vessel cannulation procedures were used: a single-sided cannulation with the other side occluded, and a double-sided and perfused cannulation. After cannulation, the vessels were pressurized to 60 mm Hg intraluminally and the bath temperature was raised to 37°C. Cell membrane potentials of vessel wall cells were obtained after the bath temperature reached 37°C with the vessels partly constricted and again after spontaneous tone (constriction) of the healthy vessels had developed.

The effect of extraluminal pH on cell membrane potentials was studied by changing the bath pH from 7.3 to either 7.65 or 6.8 in the single-sided cannulation. The average cell membrane potential for vessels at 37°C, with 60 mm Hg of intraluminal pressure and pH 7.3, was −37.5 mV. The cell membrane potential depolarized to −30.9 mV at pH 7.65 and hyperpolarized to −58.4 mV at pH 6.8, with a slope of 25.8 mV/pH unit. The effect of depolarizing extracellular potassium ions on the cell membrane potential was examined by perfusing two vessels with modified Ringer's solution containing 70 mM KCl. This perfusion method decreased the vessel diameter by 48% and depolarized the observed cell membrane potential from −41.9 to −19.8 mV, with a slope of −0.42 mV per percentage diameter change.

These data provide the first measurements of membrane potentials of isolated penetrating arteriole wall cells in vitro. The results indicate that the cell membrane potential relates linearly to the vessel diameter. This new technique opens the possibility for studying vessel response to stimuli under controlled conditions and regulatory mechanisms such as the propagation of vasomotor responses.

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Kent D. Yundt, Ralph G. Dacey Jr. and Michael N. Diringer

✓ The authors reviewed clinical and financial data for all patients treated for nontraumatic subarachnoid hemorrhage (SAH) and unruptured cerebral aneurysms at their institution between June 1993 and December 1994. This study sought to identify specific areas of high resource utilization that may be amenable to reduction of expenditures without compromising quality of care. Detailed hospital financial data were correlated with clinical grade and course. Areas of high resource use were identified based on patient charges and category-specific loaded hospital cost. Patients were divided into four groups: Group 1, surgically treated unruptured aneurysms (28 patients); Group 2, acute SAH (42 patients); Group 3, SAH with vasospasm (32 patients); and Group 4, SAH with negative angiogram (10 patients). Total cost per patient (mean ± standard deviation in thousands of U.S. dollars) was highest for Group 3 (38.4 ± 21.3; vs. Group 1, 12.7 ± 8.8; Group 2, 22.6 ± 20.9; and Group 4, 25.0 ± 33.5) and correlated with hospital length of stay, Hunt and Hess grade, and Fisher grade. Areas of highest hospital cost were not always reflected in patient charges. The three areas of highest cost accounted for 48.5% of the total cost and were: 1) intensive care unit (ICU) room (Group 1, 2.5 ± 3.5; Group 2, 7.0 ± 9.2; Group 3, 11.0 ± 7.8; and Group 4, 7.9 ± 14.1); 2) arteriography (Group 1, 1.7 ± 1.2; Group 2, 2.1 ± 1.5; Group 3, 4.1 ± 2.1; and Group 4, 2.2 ± 0.7); and 3) ICU medicosurgical supplies (Group 1, 1.7 ± 0.8; Group 2, 2.0 ± 1.5; Group 3, 3.7 ± 1.7; and Group 4, 2.0 ± 3.0). It is concluded that cost containment strategies should be based on cost rather than charge and novel approaches will be required to reduce the cost of treating patients with SAH. Such approaches might include preventing vasospasm, reducing ICU stay, selective use of arteriography, and reducing the cost of supplies.

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Yasukazu Kajita, Hans H. Dietrich and Ralph G. Dacey Jr.

✓ After subarachnoid hemorrhage (SAH), cerebral arteries display impaired vasomotor control, resulting in decreased regional cerebral blood flow. Recently, propagation of vasomotor responses has been recognized as an important regulatory mechanism in microcirculation. In this study, the authors tested the hypothesis that oxyhemoglobin (OxyHb) inhibits the vasodilatory effect of chemical mediators such as adenosine and adenine nucleotides at a local and/or propagated site.

Penetrating intracerebral arterioles were surgically isolated from the middle cerebral arteries of rat brains, cannulated, and observed videomicroscopically in an organ bath under an inverted microscope. The effects of 10−5 M OxyHb on vasoactive responses to adenosine, adenosine diphosphate (ADP), and adenosine triphosphate (ATP) were examined. The drugs were extraluminally applied either to the bath (10−10−10−3 M) or, using pressure microejection (pipette concentration 10−2 M), locally.

The ATP and ADP initially constricted and then significantly dilated the vessels after both extraluminal application and microapplication. Furthermore, local microstimulation by these drugs produced conducted vasodilation. Adenosine elicited significant vasodilation after both extraluminal and local stimulation. Again, conducted vasodilation was observed. The vasomotor responses that were induced by a maximum local stimulation corresponded in magnitude to those observed at bath concentrations of 10−5 to 10−4 M of the same drug.

Pretreatment with OxyHb constricted arterioles to an average of 87% of control and blunted extraluminally induced dilation at low concentrations (10−10−10−8) of ATP and ADP, but did not affect vasodilation induced by 10−4 M or greater concentrations of ATP, ADP, or adenosine. Although the local response to local microstimulation was unaltered, propagated vasodilation as a response to ATP, ADP, and adenosine was significantly attenuated by OxyHb.

These findings indicate that vasodilatory propagation plays an important role in the regulation of brain microcirculation and that its impairment by OxyHb could, in part, explain the cerebral hypoperfusion that is observed after SAH.