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Embolic Agents:

Choice of particles for uterine artery
embolization for leiomyomata

With the emergence of uterine embolization for fibroids, there has been renewed interest in the properties of embolic material (particles) used to block the arteries. This page will review the theory behind the use of embolic particles in UAE and describe the materials currently in use.

FDA Approval Status
When reviewing this material, it is important to realize that while both PVA and Emboshperes® are approved for general embolization by the FDA, neither is specifically approved by the FDA for the treatment of uterine fibroids. To obtain this approval, multi-center studies are underway with both products. The standard for this type of approval is high. Not only must the material be shown to be safe and effective for treating fibroids, but uterine embolization must be shown to be safe and effective when compared to surgical treatment. So each study must compare the results of UAE with surgery. The PVA study is using myomectomy as the comparison; the Embospheres® study is comparing UAE to hysterectomy.

Embolization: how much blockage of the artery is enough?

When UAE was first performed, it was assumed that the artery had to be completely blocked to successfully treat the fibroids. This has become the common practice during uterine embolization.

Some experts in this field have theorized that the artery may not need to blocked entirely, but only the vessels going to the fibroids. This theory has been supported by research into a new particle, called Embospheres® Microspheres. This particle is a clear acrylic flexible sphere. Studies have shown that it is more likely than other types of particles to block the fibroid vessels without closing the entire uterine arteries. The particles appear to be equally effective to others in use. It is not yet clear whether Emboshperes® are safer than other agents, but their introduction has caused many interventionalists to reassess the extent of embolization and many now attempt to leave the main uterine artery open and try only to occlude the fibroid feeding vessels.

Both Emboshperes® and PVA can be used in a similar way, leaving at least partial flow in the uterine arteries. It is not yet known if one particle type is superior to the other for UAE. At Georgetown, we are just beginning a study to compare these 2 products, asking patients to allow us to randomly assign to one or the other particle and then measuring the outcome. This will allow us to determine is there is an advantage of one product over the other.

Polyvinyl Alcohol Particles (PVA)

PVA is ground from blocks of foam and then separated into different size groupings. The commonest sizes used for uterine embolization are 355 to 500 micron and 500 to 710 micron. A micron is one thousandth of a meter and so these particles are about the size of grains of coarse sand.

PVA has a number of desirable characteristics. It is a particulate material capable of penetrating the fibroid blood supply and blocking it. PVA is relatively inexpensive and easy to deliver. Most interventional radiologists have extensive experience in its use and both animal studies and the published experience in patients suggest that it is safe without any known long-term side effects.

There are some potential downsides. The particles swell after they mix with saline or contrast. Once wet, they tend to aggregate and may clump within vessels after injection. As a result, PVA may completely block the uterine artery. As discussed above, there is not yet agreement among experts in this area as to the extent to which the artery ought to be blocked. It is fair to say that the majority of interventionalists completely block the artery and that does effectively treat the fibroids. However, there is an ongoing debate as to whether complete blockage of the artery is needed and whether it might be safer to just block the blood supply to the fibroids. At this time, PVA is in common use for embolization.

Gelfoam (gelatin sponge)

Gellfoam or gelatin sponge is a dissolvable sponge-like material that has been used for many years in surgery. It comes in small flat rectangular blocks about the size of a matchbook. In this form, it can be applied to a minor area of bleeding during an operation and helps a clot form. It has also been used for embolization of fiboids as well, although the reported experience is very limited. For embolization the sponge is compressed flat and then cut into tiny squares (called pledgets) with a sterile scissors. Prior to injection, many operators will load these pledgets into syringes and mix them until they break down into a slurry. Whether used as a slurry or as cut pledgets, gelatin sponge usually results in complete occlusion of the uterine artery at the end of the embolization. Because it generally behaves as a temporary agent in other vascular beds, it has been assumed that the gelfoam will dissolve and the artery will reopen in several days to a few weeks. This reopening of the uterine artery has not been documented in any study to date and therefore it is unclear whether this actually occurs. Its use has been suggested for patients that may want to preserve their fertility because of the perception that the artery will reopen, but further study is needed to determine the long-term rate of reopening (called recanalization) of the uterine arteries. To provide some perspective, reopening of the main uterine arteries has been reported in over 80% of patients after PVA embolization. This research by Dr. Razavi at Stanford suggests that permanent occlusion of the uterine arteries is uncommon after a PVA embolization.

Embosphere® Microspheres

Embosphere® Microspheres are clear acrylic microspheres that were previously used as a micro-carrier for cell culture, which helped confirm their biocompatibility. They received FDA approval in May of 2000 for general use for embolization. There are some attractive characteristics of this material. The spheres are compressible and this allows easy passage through a micro-catheter with a lumenal diameter smaller than that of the spheres. Studies by Derdelyn and Pelage have shown that these spheres are more uniform in size and the particle sizes do not changed in liquids. They have little tendency to clump after injection and animal studies indicate that they have less tissue reaction than is typically seen with PVA.

In September of 2001, we reported the initial US results with Embospheres from a three center study. The initial clinical experience with Embospheres® is very promising. The symptom change following UAE was substantial, resulting in a dramatic decrease in the impact of symptoms both during menstrual periods and between periods. MRI 3 months after treatment showed no evidence of injury to the normal uterine tissue and fibroid reductions similar to those seen after PVA embolization. This study confirmed that blockage of the main uterine artery is not necessary to achieve successful treatment of the fibroids.

Summary

At this time there is not clear evidence if one of the embolic agents in use is superior to others. Currently, most physicians choose the material to fit the individual patient circumstances and their preference. It is hoped that with additional study, the use of products will be able to be tailored to the anatomy of a patient to optimize the results.


UAE HOME | WHAT ARE FIBROIDS? | TREATMENT OPTIONS | UAE PROCEDURE
PATIENT'S GUIDE | FIBROID REGISTRY | FINDING A PHYSICIAN
PHYSICIAN'S RESOURCE | ADENOMYOSIS | MEET OUR STAFF