Disclosure
Embolization is useful in a broad spectrum of clinical situations. Embolization can be particularly effective in hemorrhage, regardless of whether the etiology is trauma, tumor, epistaxis, postoperative hemorrhage, or GI hemorrhage. It can be performed anywhere in the body that a catheter can be placed, including the intracranial vasculature, head and neck, thorax, abdomen, pelvis, and extremities. With the availability of coaxial microcatheters, superselective embolizations can be performed. In most patients, embolization for hemorrhage is preferable to surgical alternatives.
Identifying hemorrhage Embolization procedures begin with diagnostic angiography to identify the source of bleeding. For example, in epistaxis, angiography of the external carotid artery with attention to the internal maxillary artery can be helpful. In pelvic fractures, the internal iliac arteries are examined angiographically. Selective and superselective angiography is more sensitive in finding the source of bleeding than are nonselective studies. Consequently, clinical suspicion and the results of other imaging studies such as contrast-enhanced CT and radionuclide scans with technetium Tc 99m–labeled RBCs are important in guiding angiographic examination. In intra-abdominal bleeding, such as after complex trauma, CT scan may identify the site of acute bleeding, because acute bleeding often demonstrates higher density (Hounsfield units) than older blood; this is termed the "sentinel clot sign." Hemorrhage is identified by active extravasation of contrast outside of the confines of the vessel lumen. The angiographic appearance depends on the rate and location of bleeding. The extravasating contrast medium may flow towards the dependent part of the viscus; in the bowel, the extravasated contrast may outline the mucosa. When the bleeding site and artery have been identified on the initial angiogram, a catheter, often a 3F microcatheter, is placed as selectively as possible into the bleeding artery to confirm the bleeding and to stop it with embolization. Angiography in the setting of lower GI hemorrhage generally does not demonstrate unique diagnostic findings that explain the cause of bleeding, only the site of active bleeding. Thus, angiographic examinations during the episode of acute, brisk bleeding are required. In cases of recurrent occult GI bleeding, angiography can be performed electively in hopes of identifying a distinct finding such as angiodysplasia, arteriovenous malformations, or intestinal varices. Occasionally, provocative protocols using angiography and infusions of lytic agents or heparin have been used, though these techniques have provided mixed results. Carbon dioxide angiography may increase the yield of angiography in the acute GI bleeding. Agents and coils When a vessel requires embolization, coils are typically the agent of choice (see Image 1). Coils are available in a variety of shapes and sizes; the largest coils measure 15 mm in diameter when deployed. Such a coil would be large enough to fill the common iliac artery, for instance. Once microcatheter technology (3F or smaller) became available, microcoils were developed to embolize increasingly smaller vessels. Microcoils assume a deployment diameter as small as 1 mm. In addition, some coils are straight when deployed; thus, the coil has the same diameter as the wire from which it is made (the term "straight coil" is a misnomer). The advantages of coils include their high radiopacity and that they can be deployed with high accuracy Coils are made from stainless steel, platinum, and titanium wire. Some are coated with Dacron fibers to elicit greater thrombogenic reactions. A coil is generally packaged in a deployment needle, which is used to load the coil into the catheter. Once inside the catheter, the coil is advanced with a standard angiographic wire. Typically, the stiff end of the guidewire is used to introduce the coil into the catheter and the floppy end to deploy the coil. The tip of the catheter is positioned at the desired point of embolization and the coil is pushed out the end. As it is deployed, the coil assumes its shape and final diameter. Most often, a "nest" of coils is deployed to occlude the artery fully. Care must be taken when deploying microcoils, since they assume the shape of the catheter through which they are deployed. This formation can cause occlusion of the delivery catheter, and the embolization may fail. Particulate embolic agents useful in the setting of acute hemorrhage include polyvinyl alcohol (PVA) and an absorbable gelatin sponge (Gelfoam). Recently, Embosphere particles (starch microspheres) have become available in a variety of sizes and may be useful in some settings. These agents are mixed with an iodine-contrast agent for fluoroscopic visualization and injected through a catheter or microcatheter. PVA is available in particle sizes ranging from 50-2000 micrometers (see Image 2). An appropriate range of particle size must be chosen based on the size of the vessels to be occluded. The smaller the particles, the more distal the embolization, and the greater the likelihood of tissue necrosis. Extreme care should be taken not to reflux the particle/contrast mixture outside the intended area of embolization. PVA and Embosphere particles are permanent occlusive agents. Gelfoam is generally a temporary occlusive agent though it can incite an inflammatory response, contributing to permanent thrombosis. Once injected, Gelfoam induces a thrombogenic reaction, occluding the vessel. However, once occluded, thrombolytic enzymes degrade the clot and Gelfoam, recanalizing the occluded vessel over a period of days to weeks. Gelfoam can be useful in trauma where a temporary occlusion is desired while either surgical repair of the injury is undertaken or the body's natural healing processes repair the damage. Gelfoam is available as either a sponge, which can be cut into pledgets or from which a slurry can be made, or as powder particles that average approximately 50 micrometers in diameter. Until recently, acrylic glue has not routinely been used in the setting of acute hemorrhage. A recent report, however, included a series of 16 patients in whom cyanoacrylate glue was used for embolization in a variety of anatomic sites, including the GI tract, kidney, and liver, among others. Embolization was successful in most patients, even in the setting of failed embolization using coils and particles. Other embolic agents that are not useful in the setting of hemorrhage should be mentioned for comparison. Absolute alcohol and surgical gelatin powder (Gelfoam) cause occlusion at the capillary level, leading to tissue necrosis. Because of tissue necrosis, these agents typically are reserved for tumor embolization where cell death is the intended result. They should not be used for hemorrhage. Another embolization agent, a detachable balloon, is used in specific circumstances, such as intracranial arteriovenous malformations. Balloons are not routinely used in patients with hemorrhage.
In the head and neck, embolotherapy most often is performed for epistaxis and traumatic hemorrhage. Epistaxis A common indication for embolization in the head and neck is epistaxis. Otorhinolaryngologists differentiate anterior and posterior epistaxis on anatomic and clinical bases; however, this discussion considers anterior and posterior epistaxis the same entity. Epistaxis results from a number of causes, including environmental factors such as temperature and humidity, infection, allergies, trauma, tumors, and chemical irritants (see Image 3-5). Treatment of epistaxis is placed in 3 categories as follows:
An advantage of embolization over surgical ligation is the more selective blockade of smaller branches. By embolizing just the bleeding branch, normal blood flow to the remainder of the internal maxillary distribution is retained. Complications of embolization may include the reflux of embolization material outside the intended area of embolization, which, in the worst case, may result in stroke or blindness. Embolization has been proven more effective than arterial ligation. Although embolization has a higher rate of minor complications, no difference in the rate of major complications was found. Trauma For traumatic hemorrhage, the technique of embolization is the same as for epistaxis. Because of the size of the arteries in the head and neck, microcatheters are often required. Endovascular treatment of ruptured cerebral aneurysms is controversial and is mentioned for the sake of completeness. Interested readers are encouraged to refer to the Bibliography. |
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In the thorax, the 2 indications for embolization in relation to hemorrhage are (1) pulmonary arteriovenous malformations (PAVM), and (2) hemoptysis. Pulmonary arteriovenous malformations PAVMs usually are congenital lesions, although they may occur after surgery or trauma. The congenital form is highly associated with hereditary hemorrhagic telangiectasia, also termed Rendu-Osler-Weber syndrome. There is a genetic predisposition to this condition. It is also associated with liver AVMs; however, preemptive treatment of the latter is presently unwarranted. Screening head CT scans or MRIs are important to exclude intracranial AVMs. PAVMs can be single or multiple, and if large enough, can result in a physiologic right-to-left cardiac shunt. Clinical manifestations of the shunt include cyanosis and polycythemia. Stroke and brain abscesses can result from paradoxical embolism. PAVMs also may hemorrhage, which results in hemoptysis. Treatment options for PAVMs include surgery and transcatheter therapy. The treatment objective is to relieve the symptoms of dyspnea and fatigue associated with the right-to-left shunt. In addition, if the patient suffers from paradoxical embolism, treatment prevents further episodes. As a result of the less invasive nature of the procedure and excellent technical success rate, embolization currently is considered the treatment of choice for PAVM, whether single or multiple. Transcatheter embolization of PAVMs is performed with coils of appropriate size or detachable balloons. Potential complications of the procedure include pulmonary infarction, pleurisy, and loss of coils into the systemic circulation. Embolotherapy is the clear treatment of choice for PAVMs. Patients (and asymptomatic siblings) should be periodically screened for new PAVMs using chest radiography or perhaps thin-section CT. Generally PAVMs larger than 3 mm in diameter are embolized. Hemoptysis Bronchial artery embolization is performed in patients with massive hemoptysis, defined as 500 mL of hemoptysis within a 24-hour period. Etiologies vary and include bronchiectasis, cystic fibrosis, neoplasm, sarcoidosis, tuberculosis, and other infections. Rarely, Rasmussen aneurysms (pulmonary artery aneurysms from tuberculosis) can cause massive hemoptysis and should be considered when bronchial artery angiography proves negative. Untreated, massive hemoptysis carries a high mortality rate. Death most often results from asphyxiation rather than exsanguination. Bronchoscopy is performed first, both to treat any intrabronchial source of bleeding with cauterization and to localize the source of bleeding. Radiographic studies, including chest radiographs and CT, localize the responsible lesion. Medical and surgical treatments for massive hemoptysis usually are ineffective, with mortality rates ranging from 35-100%. Embolization has an initial success rate of 95%, with less morbidity and mortality than surgical resection. Consequently, transcatheter embolization has become the therapy of choice for massive hemoptysis, with surgical resection currently reserved for failed embolization or for recurrent massive hemoptysis following multiple prior embolizations. The embolization technique involves catheterization and angiography of the descending thoracic aorta. A 5F reverse curve catheter such as a Mikaelsson catheter or Shetty catheter or double curve catheter such as a cobra-shaped catheter or Headhunter is used to cannulate the origins of the bronchial arteries, which typically arise at the level of the T3-T7 thoracic vertebrae. A Mikaelsson catheter has a characteristic hook on the end, which is particularly useful for cannulating branch arteries coming off at right angles to the aorta, such as the bronchial arteries (see Image 6). Often, conventional 4F hydrophilic cobra catheters are useful, particularly in concert with coaxial microcatheters. The number and distribution of the bronchial arteries vary widely. In the most common situation, one artery supplies the right lung and two arteries supply the left; however, as many as four arteries may supply each side. In as many as 40% of patients, the arteries to both the right and left arise as a common trunk with one of the supreme intercostal arteries. Care must be taken to avoid embolization of common-origin intercostal arteries, which may result in infarction of the musculature supplied by the intercostal branches. Most importantly, spinal arteries must be identified during angiography. Spinal arteries can have common origins with intercostal branches and are identified by a characteristic "hairpin" loop overlying the vertebral column. The presence of spinal arteries may be a contraindication to embolization, although using a microcatheter or infusion wire to deliver the embolic agent beyond the origin of the spinal artery typically renders this issue moot. Permanent embolic agents (eg, coils, PVA) are used. One strong disadvantage of coils is a more proximal occlusion, which prevents or hinders repeat embolization in the same arterial distribution (see Images 7-9).
Many indications for embolization in the abdomen and pelvis exist. For embolization of hemorrhage, the most common indication is acute GI hemorrhage. GI hemorrhage usually is categorized as upper GI (UGI) or lower GI (LGI) hemorrhage, since the etiologies and treatments differ considerably. UGI bleeding is defined as bleeding from the esophagus to the ligament of Treitz, while LGI bleeding is defined as occurring distal to the ligament of Treitz, usually in the colon or rectum. Bleeding in the small bowel is rare; the most notable exception is a Meckel diverticulum in the pediatric population. Solid organ injury, usually to the liver and spleen, can readily be treated with embolization. Other indications exist, such as gynecologic/obstetric-related hemorrhage and pelvic ring fractures. Etiologies of GI hemorrhage UGI bleeding has many etiologies, including peptic ulcer, hemorrhagic gastritis, esophagitis, gastroesophageal varices, neoplasm, and Mallory-Weiss tear. UGI endoscopy initially is performed to localize the source of bleeding and possibly to treat the cause of bleeding. However, endoscopy fails to find the source of bleeding in 10% of patients. Endoscopic treatment failures also can occur. With endoscopic diagnostic or treatment failure, angiographic evaluation and embolization are the next step. The results of endoscopy are important to the radiologist performing angiography, since the angiographic evaluation can be guided to a particular arterial distribution. The two most common causes of LGI bleeding are diverticular disease and angiodysplasia. Less common causes include inflammatory bowel disease, neoplasia, and bowel ischemia. Endoscopy in the LGI tract is more problematic than in the UGI tract. When the colon is filled with blood and stool, the source of bleeding can be difficult to identify. As much as 40% of the colonoscopy is nondiagnostic regarding the source of bleeding. When endoscopy fails to identify the source of bleeding, a nuclear medicine study can help. Frequently, a nuclear medicine study can localize the source of bleeding to a particular arterial distribution, thus guiding the angiographic evaluation. The radionuclide scan can also help in determining the rate of bleeding, as arteriography is not helpful if bleeding has stopped. Treatment of GI hemorrhage Once the source of bleeding is identified, an appropriate embolization procedure can be planned. The technique for embolization is different for UGI bleeding and LGI bleeding. The vascular supply in the UGI tract is so richly collateralized that relatively nonselective embolizations can be performed without risk of infarcting the underlying organs. Conversely, the LGI tract has less collateral supply, which necessitates more selective embolizations, often with microcatheters and microcoils. Before microcatheter technology was available, vasopressin infusions were the initial treatment of choice in LGI hemorrhage. Vasopressin is a locally active vasoconstrictor that effectively controls LGI bleeding in most patients when delivered via a catheter. However, vasopressin is relatively contraindicated in patients with coronary insufficiency. For these patients, concomitant administration of nitroglycerine by infusion or patch is required. Unfortunately, bleeding treated with this procedure tends to recur, necessitating either an embolization procedure or colonic resection. Prior to microcatheter technology, embolizations for LGI bleeding could not be performed as distally in the artery as is possible with a microcatheter. Consequently, proximal embolizations may result in intestinal ischemia or infarction, which may mandate subsequent colonic resection. Microcatheter technology has led to substantial improvement in the effectiveness of embolization. Several series have reported using both coils and particulate embolization with good results. Because of high success and low complication rates, superselective embolization currently is considered the treatment of choice for LGI bleeding. Solid organ hemorrhage Outside the GI tract, there are organ specific considerations when performing embolizations in the abdomen. For instance, the liver has a dual blood supply, with 75% of the total supply from the portal vein and 25% from the hepatic artery. The hepatic artery invariably is responsible for hemorrhage resulting from trauma due to its higher blood pressure compared to the portal vein. Therefore, all embolizations in the liver are performed in the hepatic artery and not in the portal vein. Because of the dual blood supply, occlusion of large branches of the hepatic artery can be performed without risk of necrosis. In patients with normal livers (ie, no hepatic dysfunction), occlusion of the entire proper hepatic artery results in hepatic necrosis in 10% of patients. The remainder experiences a transient rise in their liver enzymes, which normalize after a few days. However, embolizations should be performed as distally as possible to minimize the risk of necrosis. An angiographic study of a patient in whom embolization of the liver was performed for trauma-induced hemorrhage is seen in Images 10-12. In contrast, embolizations of the spleen always should be performed as distally as possible. Occlusion of the splenic artery can result in splenic necrosis and the possibility of a splenic abscess postembolization. If occlusion of the entire splenic artery is contemplated for traumatic hemorrhage, total splenectomy instead of embolization or total splenectomy postembolization should be performed. In patients with traumatic splenic hemorrhage in whom embolization is contemplated, a 3F or smaller microcatheter should be used. PVA or platinum coils are the embolic agents of choice. Antibiotic prophylaxis is required both before and after the embolization (see Images 13-15). Other types of hemorrhage Further indications for hemorrhage embolization in the abdomen and pelvis include postpartum, postcesarean, and postoperative bleeding. Differential diagnoses for postpartum bleeding include laceration of the vaginal wall, abnormal placentation, retained products of conception, and uterine rupture. Conservative measures for treating postpartum bleeding include vaginal packing, dilatation and curettage to remove retained products, IV and intramuscular medications (eg, oxytocin, prostaglandins), and uterine massage. When conservative methods fail, surgical methods to control hemorrhage are used, including ligation of the internal iliac arteries and hysterectomy. With the availability of transcatheter embolization, surgical risks are avoided. Several series in the literature compare iliac-artery ligation to transcatheter embolization. The studies conclude that embolization is a safe and effective procedure for controlling pelvic hemorrhage, avoids surgical risks, preserves fertility, and shortens hospital stays. In addition, embolization is advocated when conservative methods fail. Surgery typically is reserved for patients in whom embolization fails. Finally, embolization of the internal iliac arteries is valuable in patients with hemodynamically unstable pelvic fractures. Protocols for trauma include treatment of associated soft-tissue injury first, followed by stabilization of the pelvic ring. Patients with persistent hemodynamic instability are candidates for embolization. As in other clinical settings, angiography is used to identify the source of hemorrhage, and a selective embolization is performed.
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