Author(s):
Rajat Mohanka
NYIT College of Medicine ’24

Harris Liou
Mayo Clinic Alix School of Medicine ’23

Hamad Khalil Hamad
Geisinger Commonwealth School of Medicine ’23

The use of bleeding control procedures goes back throughout history, but the modern era of medicine has ushered in new minimally invasive ways to stop bleeding. Common techniques in World War 2 were vessel ligation or ultimately amputation. Since WW2, there has been an increase in hemorrhaging control technique advancements, especially in emergent and traumatic situations. Embolic agents were created to increase the effectiveness of controlling bleeds without invasive procedures. In the early 1970s, the embolizing vascular injuries from trauma was first described by Rosch and Dotter, with a follow up from Bookstein and Goldstein a year later.

An increasing number of  interventionists are being integrated into emergency procedures to prevent hemorrhaging in a safe and efficient manner, given the high success rate of using embolization techniques. Embolization techniques have expanded quickly from hemorrhage control to treating endovascular disorders such as aneurysms.

One form of embolization is balloon embolization, which was once used in controlling aneurysms (first described in 1974 by Serbinenko). But as years went on, the downside of using balloon embolization was seen as balloons forcing the aneurysm to adapt to the shape of the balloon, which had a high incidence of aneurysm rupture. This changed in 1988, when Hilal first used coils as an embolization technique to treat aneurysms. Coil embolization is a technique of passing metal coils within the vessel of abnormal blood flow. Once detached and inserted in the region, the coil will adapt around the aberrant vessel to occlude blood flow to that vessel. This became a prominent technique used in treating aneurysms due to the reduced pressure on the blood vessel walls relative to the balloons     .

Building on his prior advances, in 1989 Guglielmi first developed a GDC (Guglielmi-detachable coil) embolization system. The embolic material is a soft platinum coil that is passed through the vessel via a catheter or microcatheter. The coil is then positioned (can be readjusted if not in the correct position). Then a current is stimulated which results in the release of the coil into the aneurysm. The first GDC coil was used in a clinical setting around 1990 to treat a carotid artery aneurysm.

The coils used in current coil embolization procedures are made of soft platinum or steel and range in length from 1 to 300 mm and in diameter from 1 to 27 mm.  The coil shapes include straight, helical, tornado, conical, J-shaped, C-shaped, and complex three-dimensional shapes.  The coil can be bare or embedded with fibers, such as nylon, wool, silk, or Dacron to increase coagulation.  Generally, the coil is placed into a catheter and the catheter is inserted into an artery, usually the femoral artery.  The catheter is then advanced to an aneurysm or other affected blood vessel using X-ray guidance.  When the catheter reaches the appropriate area, the coil is released from the catheter into the aneurysm or vessel.  A blood clot will form around the coil, resulting in filling of the aneurysm or total obstruction of the vessel.

This procedure has a growing number of important applications in various fields of medicine.  Transcather arterial embolization is now one of the firstline interventions for massive arterial bleeding from the upper GI, which can be resistant to endoscopic therapy and has a high mortality rate when treated with surgery.  Arresting the bleeding can be achieved either by coiling proximal vessels to the bleed or the preferred method of superselective catheterization where distal vessels immediately supplying the bleed are identified and coiled.  Arterial embolization is considered safe above the ligament of Treitz due to the high collateral supply to the stomach and duodenum.  Transcather arterial embolization is also one of the first line treatments for closing the patent ductus arteriosus, whereby a catheter is used to deliver a Gianturco coil to the aortic side of the ductus arteriosus.  Additionally, the procedure is used to treat intractable severe hematuria due to hemorrhagic cystitis and/or prostate cancer.  The success rate of this operation depends on being able to locate the visceral or prostatic artery responsible for the bleed.  After catheterization of the femoral artery with a 5Fr or 6Fr sheath, selective angiography of the internal iliac arteries is done using a 5Fr Cobra or the Simmons-type 2 catheter.  Based on the angiography findings, a superselective 3Fr coaxial microcatheter is used to deliver platinum microcials to the culprit vessel. In patients receiving hemodialysis via an arteriovenous fistula endovascular coil embolization is used to occlude competing collateral veins, which can lower flow rates and thereby hamper effective hemodialysis. Coil embolization also found high success in treating chylous leaks that occur above the diaphragm.  In this procedure, a microcatheter delivers coils and glue matrix to the chyle leakage site, thereby occluding it.  In patients preparing to undergo liver resection, portal vein embolization is used to redirect blood flow to the future liver remnant, causing it to hypertrophy and improve its functional reserve.  Lastly coil embolization can be used to block or reduce blood flow to tumors, reducing their access to oxygen and nutrients and thereby causing them to regress and/or reducing their growth potential.

Building on its origin as an effective method to stop hemorrhage, coil embolization continues to improve medicine, having significantly expanded its indications and techniques. Small microcatheters and innovative coil designs have allowed practitioners to provide increasingly safe and effective treatment to high-risk patients. Research is active in many facets of coil embolization, as the field demonstrates its potential to add value to and minimize complications in patient care.

Balloon-assisted coiling is an evolving technology that allows for the temporary inflation of a balloon catheter over the neck of an aneurysm during coil placement. Ideal for intracranial aneurysms, this technique increases coil packing density and reduces procedure time. Recent advancements include increased compliance (conformity to the vessel anatomy), higher inflation and deflation speeds, and increased visibility. New products with novel features continue to be developed and evaluated.

A recent innovation similar to balloon-assisted coiling is the use of non-occlusive remodeling nets. Its indications and functionality are similar to those of balloons, but this patent stent-like device allows for continued blood flow through the vessel during coil placement. This significantly reduces the risk of ischemia to downstream tissues, which has been reported as a complication of balloons. Several other stent-like technologies have been introduced and are under further development.

Design and diversity of coils have had a long history of innovation since the conception of coil embolization. Recently, hydrogel (a material that expands in liquid) was applied to platinum coils to increase packing density and occlusion upon placement. Although these coated coils were effective at lowering hemorrhage recurrence, they were limited by stiffness and restricted placement time. Nonetheless, new designs have incorporated softer coils and delayed expansion times to address these issues.

There are other materials which seek to revolutionize embolization techniques beyond the use of coils.  Researchers have developed a bioactive decellularized cardiac extracellular matrix‐based hydrogel to address the shortcomings of coil embolization. Designed to take on a semi-solid state under pressure, the hydrogel can be injected through catheters and instantly solidified upon ejection into the target. The solid gel can then reliably stop blood flow without depending on the patient’s coagulability or causing imaging artifacts. Trials on pigs have demonstrated the material’s ability to induce a fibroinflammatory response at the embolization site, after which it biodegrades. Importantly, it is producible at a lower price than traditional coils and can thus add value to patient care. Representing a recent rise in research on liquid embolic agents, this innovation holds great potential to improve the practice of embolization.

There are also emerging techniques to aid in embolization planning via e simulation with accurate 3D printed vascular models based on patient imaging. For example, interventional radiologists at UCLA recently reported 3D printing a silicone model of a superior cerebellar artery aneurysm, on which they tested different stents before selecting one to assist in the actual coil embolization. Although such simulations are limited in several regards, such as inaccurate replication of vessel elasticity, improvements in 3D printing technology may allow for better interventional planning in the near future.

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