Interventional Neuroradiology of the Skull Base, Head, and Neck

Angiographic Equipment

During endovascular interventional procedures, it is essential to define the blood supply of a lesion with the highest possible resolution in all projections. Arteries and their collateral channels act as conduits for embolic material to the lesion. Of equal importance is visualization of the many dangerous collateral vessels that lead to the intracranial circulation or to the blood supply of the cranial nerves. Superb visualization and an excellent knowledge of the vascular anatomy are of paramount importance, as is an understanding of the potential clinical symptoms of embolization into an unwanted vascular territory and occlusion of the blood supply to normal tissues.

The anatomic complexity of the neck, skull base, and face makes it essential that sophisticated C-arm positioners be used to allow variable projections of the area of interest. Biplanar visualization is helpful for superselective catheterization of the small, tortuous channels that feed the lesion. High-resolution techniques, such as electronic “road mapping,” are essential. For example, injection of embolic agents suspended or diluted in contrast material during real-time high-resolution subtracted fluoroscopy (road mapping) permits visualization of the progressive occlusion of the vessels that feed the lesion. Monitoring the procedure with high-resolution fluoroscopy helps to avoid the complications of reflux of embolic material or flow into collateral branches and, thus, preserves the blood supply to normal tissues.

Angiographic Catheters

The catheters most commonly used for diagnostic angiography are 4- and 5-Fr polyethylene catheters. Superselective catheterization of tiny arteries that feed tumors, fistulas, and other lesions facilitates the most effective embolization—frequently beyond sites of anastomoses to dangerous collaterals—and is achieved by the use of microcatheters. Wire-directed microcatheters approximately 2 to 3 Fr in size have extremely flexible distal portions that are variable in length. They are used with micro guidewires of 0.008 to 0.018 in in diameter. Flow-directed catheters also are used for tortuous vessels that feed high-flow lesions.

If it is impossible to catheterize feeders to a tumor or vascular malformation selectively, percutaneous puncture of the hypervascular lesion can be performed under image guidance to gain access to the intralesional blood supply. Injections of contrast material during digital subtraction angiography (DSA) are performed to ensure that no filling of the arterial supply to normal tissues occurs. Injections of the embolic agent are made under low pressure during fluoroscopy with road-mapping or DSA.

Embolic Agents

Several embolic agents are available for the various lesions found at the skull base and in the face and neck. The choice of embolic material for a given patient depends on the goal of the procedure; the selectivity of the accomplished catheterization; the angioarchitecture and flow dynamics of the lesion; and the proximity of the catheter tip to the blood supply to vital structures such as the brain, cranial nerves, eyes, and skin or to potential collateral channels to these organs. These embolic agents include particulate materials, metallic coils, liquid sclerotic and embolic agents, and detachable balloons; each has its own place in the interventionalist's armamentarium.

Commonly used particulate materials are Gelfoam (Upjohn Pharmaceuticals, Kalamazoo, MI), polyvinyl alcohol (PVA) foam products, and spherical microparticles. Gelfoam breaks down 72 hours after embolization, and this lack of permanence detracts from its efficacy if surgery is not performed within a few days after the procedure. Gelfoam has been used as a preoperative embolization material for neoplasms to be operated in 48 hours, as well as for patients with epistaxis, to slow the bleeding sufficiently so that the body's normal hemostatic mechanisms stop the hemorrhage. Gelfoam powder should always be used with care, because its particles (approximately 50 µm in size) in solution act as a liquid that easily passes through tiny arteries—which may result in skin necrosis or damage to cranial nerves—or through collateral channels that communicate with the intracranial circulation.

PVA is more permanent than Gelfoam, but much of the efficacy of the vascular occlusion is a result of a combination of PVA plus intravascular thrombus. The stellate-shaped particles slow the intravascular flow and thrombus forms. However, this thrombus may be metabolized before fibrosis occurs, which results in partial or complete recanalization over weeks to months. PVA is easy to use and is supplied as uniform particles within a narrow size range (150 to 1250 µm). In most patients with neoplasms, the smallest size (150 µm) is used because the particles can easily be injected through a small microcatheter placed selectively into tiny feeding arteries to penetrate the vascular bed of the tumor.

Because of the stellate shape of PVA particles, they do not form a tightly packed embolic mass, which allows recanalization as the interspersed thrombus undergoes lysis. Various types of beads and spheres have been produced to overcome this limitation, such as Embosphere microspheres (Biosphere Medical, Rockland, MA) or Embozene Microspheres (Boston Scientific, Fremont, CA). Their more uniform size and spherical geometry produce more complete and more permanent vascular occlusion. They are available in six sizes that range from 40 to 1200 µm. They are very easy to use and come in a set size range preloaded in a syringe, to which contrast material is added. A major advantage of microspherical particles is that occlusion of the microcatheter used to embolize them is significantly less common than with PVA. Thus we currently almost always use microspherical particles, rather than PVA, for particulate material embolization procedures.

Similar spheres might also be filled with a chemotherapeutic or other agent for more prolonged treatment of a nonsurgical neoplasm. Microfibrillary collagen (Avitene; Avicon, Fort Worth, TX) is a hemostatic agent that is mixed with contrast material for embolization, or it can be mixed with other embolic agents such as PVA and ethanol.

The two broad categories of metallic coils are those that are pushed from a catheter with a metal coil pusher or guidewire and those released by breaking a bond between the coil and the pushing wire by either mechanical or electrolytic means; the latter is the technique used for treating intracranial aneurysms. Detachable coils are available in various lengths, shapes, and sizes to suit the size and volume of the intended target of embolization. In head and neck lesions, metallic coils are used for the occlusion of vessels that measure a few millimeters to 1 cm or more in diameter; the coil size prohibits their moving more distally into the embolized lesion. For tumors, they are best used for occluding the feeding artery after particulate material embolization; the particles are embolized deeply into the lesion, and the coils produce the final occlusion of the feeding artery. Coils are used to occlude bleeding vessels in an emergency, as in epistaxis or after trauma. Their use for primary treatment of an arteriovenous malformation (AVM) or arteriovenous fistula (AVF) is discouraged because the proximal occlusion of any feeding artery simply leads to the establishment of collateral channels that may be difficult to access via endovascular techniques. However, if the vascular lesion is to be surgically removed soon after embolization, coil occlusion of the feeding arteries may be performed as a low-risk method of decreasing surgical blood loss.

Detachable balloons with a valve mechanism to keep the balloon inflated are available in Europe and intermittently in the United States. They are used primarily for fistulas with a single artery-to-vein connection. Most experience has been with intracranial posttraumatic carotid–cavernous sinus fistulas, but this technique has also been used for vertebral artery–vertebral venous fistulas (usually posttraumatic) or for any other type of fistula in the face or neck. Detachable balloons with a valve mechanism have also been used for occlusion of a parent artery that leads to an unclippable aneurysm, a dissected carotid or vertebral artery that produces embolization into intracranial vessels, and a carotid artery to be sacrificed at tumor surgery. Detachable balloons have largely been replaced by mechanical and electrolytically detachable coils, given the inconsistent availability of balloons and marked proliferation of detachable coils of various shapes, lengths, and sizes. For medium- to large-vessel occlusion and cavernous sinus fistula embolization, the large number of detachable coils to achieve a similar effect to balloon embolization can greatly increase the overall procedural cost.

Numerous liquid embolic agents are available, the most commonly used being absolute alcohol (100% ethanol) and various tissue adhesives, which include cyanoacrylates and Onyx (ev3 Endovascular, Plymouth, MN, and ev3 Neurovascular, Irvine, CA). Absolute ethanol is extremely toxic to the endothelium and is highly effective at producing sclerosis of vascular lesions, such as venous and lymphatic malformations. Although it has also been used to treat AVMs and dural AVFs, the problem with these fast-flowing lesions is the need to increase the “dwell time” of the ethanol to interact with the intima, which often requires temporary balloon occlusion more proximally. Alcohol has also been used for tumors via endovascular and percutaneous access, particularly surgically inaccessible recurrent tumors.

The injection of ethanol is extremely painful; it requires deep sedation or, more commonly, general anesthesia. It is necessary to see where the embolic agent is going, thereby making opacification necessary; however, opacification of the ethanol with a liquid contrast agent produces dilution of the ethanol and, thus, decreases its effectiveness, which is maximal if used undiluted. Metrizamide powder was used previously for opacification, because a powder does not dilute the ethanol but provides acceptable opacification. Unfortunately, metrizamide powder is no longer available in the United States.

Sodium tetradecyl sulfate 3% (Sotradecol) is also an excellent sclerosing agent, is less painful on injection than ethanol, and may be opacified, although it seems to be less effective than ethanol for venous malformations and is not considered an acceptable alternative when used alone. Doxycycline, a tetracycline-like antibiotic, is effective at producing sclerosis of venous, lymphatic, and other slow-flow vascular malformations in which the dwell time is adequate. It can also be used for fast-flowing AVMs and AVFs if temporary proximal occlusion of the embolized feeder is possible. Tetracycline has been associated with the darkening of the tooth enamel in young children; hence this drug is usually reserved for patients older than 11 years, whose permanent teeth have been formed. Bleomycin, a glycopeptide antibiotic and antineoplastic agent, can also be injected into venous and lymphatic malformations with less pain than with alcohol and with good success. Bleomycin in combination with sodium tetradecyl sulfate has also been used with good results for the treatment of low-flow venous malformations of the head and neck.

Cyanoacrylates, such as isobutyl-2-cyanoacrylate (IBCA) or N -butyl-2-cyanoacrylate (NBCA), produce polymerization of rapidly flowing blood within seconds. They not only produce immediate thrombosis and have tissue adhesive properties but also initiate a giant cell inflammatory reaction of the vessel wall. These embolic liquids are excellent for a lesion with rapidly flowing blood, such as an AVM or AVF. Neoplasms have slow flow, so there is no need to use these agents, which are associated with more risk than particulate materials injected intra-arterially or other agents injected percutaneously.

The newest “liquid” agent is Onyx, an ethylene vinyl alcohol copolymer that contains dimethyl sulfoxide as the agent to facilitate absorption through endothelial barriers. This agent is not truly a tissue adhesive but rather coats the intima and gradually fills the lumen of the vessel. It is an important addition to the armamentarium of the neurointerventionalist because it slowly permeates the tiny vessels that feed a vascular lesion, creeping into these feeders during fluoroscopic visualization and control, without the rapid setup time required by the cyanoacrylates. Although Onyx has become the most commonly used agent for the embolization of AVMs and AVFs, it can be difficult to use. Forward propagation of the agent requires a “seal” to be made around the embolizing microcatheter; an improper seal does not allow for propagation and attempts to produce the seal may result in reflux of the agent and the potential for trapping the catheter in the artery. Given the potential for catheter tip entrapment, a detachable tip dimethylsulfoxide (DMSO)-compliant microcatheter, the Apollo (ev3 Endovascular, Plymouth, MN, and ev3 Neurovascular, Irvine, CA) provides a solution for prolonged injection times and the forming of a vascular plug to promote antegrade penetration into the embolization target. Percutaneous injection of Onyx under combined ultrasound and fluoroscopic guidance can be performed for superficial lesions and may be particularly helpful for lesions inaccessible by traditional endovascular approaches.

Any embolization procedure with a liquid agent should be approached with trepidation, because occlusion of the end arteries to the face, tongue, and cranial nerves may lead to necrosis, and intracranial embolization may occur if the liquid passes through tiny collateral channels to vessels that feed normal structures. Liquid embolization should be performed only when superselective catheterization can be done directly into the feeders of the lesion—or with percutaneous puncture of the lesion and direct visualization of the flow of the agent —to prevent unwanted extension to the blood supply of vital structures.

Provocative Testing to Ensure the Safety of Embolization

Two important techniques help ensure the safety of vascular occlusion at the skull base. The first involves the injection of lidocaine 1% without preservatives into an arterial feeder considered a candidate for embolization; this is done to predict the potential of permanent cranial nerve palsy from the embolization procedure. The cranial nerve is temporarily anesthetized if there is blood flow leading to it from the vessel catheterized for embolization. Critics suggest that a false-positive test may occur because a liquid anesthetic can be injected into the capillary bed, whereas particles used for embolization stop short of terminal arterioles, so devascularization is rare. It is likely that a negative test result is truly negative, which is reassuring.

The second provocative test is the temporary balloon occlusion test (BOT) of the internal carotid or vertebral artery, with the addition of blood flow measurement for precise quantification of the potential effects on cerebral blood flow during temporary or permanent occlusion of the artery during surgery or endovascular therapy. Carotid or vertebral artery occlusion might be necessary during the surgical removal of a skull base tumor; during the temporary occlusion of a major intracranial artery, such as during surgical clipping of an aneurysm; for the thrombosis of an unclippable aneurysm via occlusion of the parent artery; or if carotid occlusion has to be performed to close a carotid–cavernous sinus fistula or any other type of traumatic vascular lesion.

First, a complete angiographic evaluation of all cerebral vessels is performed to evaluate their contribution to the particular lesion and the adequacy of collateral flow via the anterior and posterior communicating arteries. If communicating arteries are present, the traditional BOT is performed, which consists of occlusion of the internal carotid artery (ICA) with a nondetachable balloon catheter placed at the level of the future permanent occlusion. After systemic heparinization, the balloon is inflated and the patient is evaluated for 30 minutes. Careful neurologic examinations are performed throughout the occlusion period with special emphasis on the neurologic functions subserved by the vessel being tested. If the patient develops a neurologic deficit, the balloon is immediately deflated. If the patient has no neurologic deficit from the carotid occlusion, it is assumed that blood flow is adequate for permanent occlusion to occur. However, such a qualitative test does not provide precise quantification of the blood flow to the hemisphere at risk. Blood flow values greater than 20 to 25 mL/100 g of tissue per minute allow normal neuronal function, and values less than that precipitate neuronal dysfunction. It is conceivable that blood flow values slightly greater than 20 to 25 mL/100 g/min would leave the patient without symptoms while on the angiography table, but superimposed intraoperative or postoperative hypotension, decreased cardiac output, or decreased oxygenation might precipitate cerebral infarction.

Different methods have been used during temporary occlusion to evaluate the physiologic effects of the BOT on cerebral blood flow to predict the risk of cerebral infarction after definitive occlusion. These methods include electric studies such as evoked potentials or electroencephalography, measurements of arterial stump pressures distal to the site of the temporary occlusion, induced hypotensive challenges, and transcranial Doppler studies. Also, several cerebral blood flow imaging methods—including xenon-enhanced CT, single photon emission computed tomography (SPECT; Fig. 136.1 ), positron emission tomography (PET), and MRI and CT perfusion studies—have been used to evaluate cerebral blood flow during BOT to determine the potential risk of ischemia after permanent occlusion. We prefer to use SPECT because it adds little to the basic BOT and easily provides the needed information. After the balloon has been inflated for a short time, the radionuclide is injected intravenously and the patient is scanned a few hours after the completion of the angiographic study (the radiopharmaceutical “sticks” to the brain tissue during its initial circulation in proportion to the blood flow to that tissue) (see Fig. 136.1 ).

Unfortunately, these techniques have turned out to be imperfect predictors of the ischemic risk associated with permanent carotid occlusion. Clinical deficits have been reported after permanent occlusion, even when there has been a negative BOT with accompanying normal physiologic or cerebral blood flow studies. Many, if not most, of these postocclusion deficits are probably a result of marginal cerebral perfusion accompanied by hypotension or decreased cardiac output, clot propagation from the residual stump, or de novo emboli at the time of vascular occlusion.

Paragangliomas

Paragangliomas, also known as chemodectomas or glomus tumors, are neoplasms related to chemoreceptor tissue. They usually are benign but are locally invasive and highly vascularized. Most glomus tumors originate within the temporal bone (48%), which includes lesions along the promontory of the middle ear (glomus tympanicum tumors) and lesions related to chemoreceptor tissue in the jugular bulb (glomus jugulare tumors). Tumors related to the vagus body (11%) in the high cervical region are called glomus vagale tumors, and tumors related to the carotid body (35%) at the common carotid artery bifurcation in the neck are called carotid body tumors (see Fig. 136.1 ). Multiple tumors are found in approximately 10% of patients ( Fig. 136.2 ), and a familial form exists.