Fran Lowry

December 08, 2009

December 8, 2009 (Chicago, Illinois) — Magnetic resonance imaging (MRI) is a useful and accurate adjunct to ultrasound for the diagnosis of placenta accreta, one of the top causes of maternal and fetal deaths in the United States, according to the results of a study presented here at the Radiological Society of North America 95th Scientific Assembly and Annual Meeting.

"Our recommendations are that women who are at high risk for placenta accreta, for example women who have had multiple [caesarean deliveries], should get ultrasound with particular attention to the placenta and, if ultrasound is inconclusive, MRI should be considered to make the diagnosis for this condition," said Michele A. Brown, MD, from the University of California, San Diego.

Placenta accreta is an abnormal attachment of the placenta with some degree of invasion into the wall of the uterus. If it is not diagnosed during pregnancy, it can lead to severe complications at the time of delivery.

These include massive blood loss in the mother requiring multiple transfusions of blood products and hysterectomy. Maternal death occurs in 7% of cases.

The frequency of placenta accreta has increased enormously in recent years. In 1950, this condition affected 1 in 30,000 deliveries; currently, estimates range from 1 in 2500 to 1 in 533 deliveries. The severity of placenta accreta is also increasing.

The reason is largely because of the increasing numbers of cesarean deliveries that are being done in the United States. The more cesarean deliveries a woman has had, the higher her risk, Dr. Brown said.

"Currently in the United States, about 29% of births are by [caesarean delivery]," she told Medscape Radiology. "This creates a scar in the uterus that disrupts a certain layer of the endometrium, which is very important for keeping the placenta in place in the uterus."

Other risk factors for placenta accreta include previous uterine surgery other than caesarean delivery, such as dilation and curettage and placenta previa, where the placenta covers the cervical opening. A combination of placenta previa, which is readily seen on ultrasound, and placenta accreta is a particularly ominous one, Dr. Brown said.

Also, being older than 35 years of age is an independent risk factor.

Diagnosis of placenta accreta is difficult. Ultrasound is usually used, but its detection rate ranges from 33% to 100%, Dr. Brown said.

This study tested the accuracy of MRI in detecting placenta accreta in 71 women who were at high risk, based on previous caesarean delivery or suspicious ultrasound examination.

The investigators found that MRI identified 88% of cases positive for placenta accreta and 96% of cases that were negative, for an overall accuracy rate of 90%.

"MRI is good at both detecting placenta accreta and ruling it out," Dr. Brown told Medscape Radiology. "We found that it had a higher accuracy than ultrasound, so we believe that it is complementary to ultrasound. Any time there is an inconclusive ultrasound, MRI could give the answer as to whether it is positive or negative."

Detection of placenta accreta prior to delivery is absolutely crucial, she added.

"You really have to call up the troops to get everything in place to deal with this emergency. If you are not prepared beforehand, the outcome is usually pretty dire," she said.

MRI plus ultrasound should be done in all high-risk patients. "This is an important, deadly disease of pregnancy that people should be more aware of," Dr. Brown warned.

"If you connect your risk factors with clear ultrasound examinations and potentially MRI, then your outcome is going to be much better, because an emergency hysterectomy in the peripartum setting has a much higher mortality than a planned one. You really need to be on top of it during pregnancy to make sure you get the best treatment."

Mary Mahoney, MD, from Cincinnati Children's Medical Center in Ohio, who was invited to comment on this study by Medscape Radiology, said she found the study fascinating because it highlights a huge problem.

"Older mothers and [caesarean deliveries] are things that we are seeing more of, so the incidence of placenta accreta has increased dramatically. It is astonishing how much it is increasing. So this is another use of a technology that can really make a huge difference."

Dr. Mahoney added: "I thought it was interesting that the study showed more benefit with MRI in terms of ruling out placenta accreta. Not having to get 14 units of blood on board, [not having to get] anesthesia and surgery in place, and [not having to assemble] an emergency team, to be able to say that everything is actually going to be OK, is very important. I thought that was a big part of the study."

Dr. Brown and Dr. Mahoney have reported no relevant financial relationships.

Radiological Society of North America (RSNA) 95th Scientific Assembly and Annual Meeting: Abstract SSJ10-01. Presented December 1, 2009.

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