ANESTHESIA FOR INTERVENTIONAL NEURORADIOLOGY

Advances in interventional neuroradiology over the past 20 years have been remarkable.^[58] Procedures that were once highly risky experimental procedures are now routinely performed at many centers. Relatively commonly performed interventional neuroradiologic procedures include embolization of cerebral and dural AVMs, coiling of cerebral aneurysms, angioplasty of atherosclerotic lesions, and thrombolysis of acute thromboembolic stroke.^{[59] [60]} Interventional neuroradiologic procedures are sophisticated diagnostic and therapeutic endeavors involving advanced manipulation of patient conditions to provide optimal operative outcomes. These procedures may involve deliberate hypotension, deliberate hypercapnia, or deliberate cerebral ischemia as part of the procedure; a requirement for rapid transition between deep sedation/analgesia and the awake, responsive state; and severe potential procedural complications.^[61] As such, involvement of experienced anesthesiologists is valuable to ensure an optimal outcome.

Interventional neuroradiology is the radiologically guided endovascular approach to lesions of the central nervous system or its related circulatory structures to deliver therapeutic agents.^[59] The development of digital subtraction angiography was the basis for growth of the field.^[58] Within a few years of the development of digital

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High-resolution fluoroscopy and digital subtraction angiography are combined to obtain real-time images of the patient's vascular anatomy, through which catheters can be manipulated.^[61] Vascular access for the neuroradiologist is usually obtained through the femoral artery, but the carotid or brachial arteries may also be used. The site of sheath placement is typically infiltrated with local anesthetic to minimize patient discomfort. Radiologic contrast media are used. It must be noted that digital subtraction angiography delivers greater radiation doses than even fluoroscopy does; attention to radiation safety policies in the neuroradiology suite is therefore essential.^[59] Blood pressure may need to be increased or decreased at various points during the procedure to facilitate the intended therapy. Once the catheter or catheters have been placed in the vicinity of the lesion, the definitive procedure commences.

Many cerebral aneurysms are amenable to endovascular treatment. Current practice is to embolize the aneurysm with Guglielmi detachable metallic coils.^[63] Middle cerebral artery aneurysms are difficult to treat endovascularly. Conversely, aneurysms of the posterior circulation or the cavernous segment of the internal carotid artery are more amenable to embolization. Aneurysms with large necks or large neck-to-aneurysm size ratios are more favorably treated surgically. Aneurysms associated with large intraparenchymal hematomas may be approached surgically to allow concomitant decompression.^[63] Patients may present electively with unruptured aneurysms or may present emergently for repair after rupture with subarachnoid hemorrhage. Patients with subarachnoid hemorrhage may suffer further rupture, hydrocephalus, or cerebral vasospasm.^{[63] [64] [65]} Anesthesiologists managing patients during these procedures must be prepared for aneurysm rupture at any time, and tight postprocedural blood pressure control will be essential to reduce the potential for rupture or rerupture after the procedure.^{[59] [60] [63]} Aneurysm rupture may be treatable with further embolization.

Brain AVMs are another lesion amenable to endovascular therapy. ^[66] Techniques used to obliterate AVMs include coil embolization, detachable balloons, particulate material, and glue.^[60] Frequently, the endovascular approach to intracranial AVMs is combined with the surgical approach, with initial embolization followed by subsequent surgical resection. ^{[67] [68]} Multiple episodes of embolization are often necessary to limit the occurrence of hemorrhagic complications. Complications of AVM embolization include embolization of foreign material to undesired locations, cerebral edema, and intracranial bleeding.^{[59] [60]} As in the case of cerebral aneurysm rupture, hemorrhage may be treatable by further embolization.

Both intracranial and extracranial vessels with stenotic atherosclerotic lesions have been treated with balloon angioplasty.^{[59] [60] [69]} Increasingly, patients with extracranial carotid artery stenoses are undergoing carotid angioplasty/stenting rather than conventional carotid endarterectomy. However, at present, indications for selection of carotid angioplasty/stenting over carotid endarterectomy are not clear, and the results of comparative outcome trials are some years away. Current indications for carotid angioplasty/stenting include medical contraindications to carotid endarterectomy, postirradiation carotid stenosis, restenosis of previous endarterectomy, high cervical lesions, and contralateral carotid occlusion.^[69] A notable feature of carotid angioplasty/stenting is that profound bradydysrhythmias often occur at the time of balloon inflation. Placement of transvenous pacing wires or a pulmonary artery catheter with the ability to accept pacing wires before the procedure may be reasonable in some patients.^[59] Movement attributable to external temporary cardiac pacing is undesirable in the face of the ongoing vascular procedure. Intracranial arterial angioplasty/stenting has been performed, but at this time, the procedure remains risky, with complication rates reported to be 12% to 28%.^{[70] [71]}

Thrombolysis of acute occlusive stroke, performed in a timely fashion, may result in reversal of neurologic deficits. A single, controlled trial has demonstrated improved neurologic outcome after thrombolytic therapy for acute stroke (thrombolysis within 4 to 6 hours of stroke onset).^[72] Meta-analysis of other studies does demonstrate a trend toward improved neurologic outcome after thrombolytic treatment of acute stroke.^[73] Thrombolytic therapy is, however, associated with an increased early mortality and increased rate of intracranial hemorrhage when compared with conventional therapy. Risk factors for complications after thrombolysis of acute stroke include severe hemispheric stroke, extended early infarction signs on preprocedural CT, delay of more than 3 hours in treatment, uncontrolled hypertension, aspirin use, diabetes mellitus, and old age.^[74]