MRI next step when ultrasound is inconclusive for suspected appendicitis in pregnancy

By MD James E. Barone

Magnetic resonance imaging is the most cost-effective next step when ultrasound is inconclusive for suspected appendicitis in pregnant women, a new study suggests.

"One of the most interesting and important findings is that in almost all situations diagnostic imaging is warranted for a pregnant woman with an equivocal ultrasound and suspected appendicitis prior to surgery," lead author Dr. Zachary J. Kastenberg, from Stanford University in California, told Reuters Health by email.

The study, in the October issue of Obstetrics and Gynecology, used computer-based decision analysis modeling to compare costs and outcomes for MRI, CT scanning and diagnostic laparoscopy. The first trimester was not included due to the theoretical risk of miscarriage or harm to fetal development by MRI.

Accuracy rates of MRI, CT and laparoscopy were derived from previous published studies. Other factors included in the model were maternal and fetal outcomes, probability of developing acute lymphocytic leukemia after radiation exposure, costs and incremental cost-effectiveness ratios based on quality-adjusted life-years (QALYs).

The model used the assumption that 30% of pregnant women suspected of having appendicitis will actually have it. Reported series cite negative appendectomy rates of 30% to 50% during pregnancy when no CT or MRI is obtained.

The most-effective choice for possible appendicitis was MRI with a negative appendectomy rate of 2.8% and a delayed diagnosis rate of 8%. CT scanning yielded rates of 7.5% and 5.8%, respectively, the researchers found.

While laparoscopy eliminates false negatives, the false positive rate -- finding a normal appendix -- was estimated at 70%. Dr. Kastenberg called this a "moot point," however, because for laparoscopy to be the preferred diagnostic option the negative appendectomy rate would have to be less than 1%.

"It's a timely paper addressing a very challenging clinical scenario that is anxiety provoking for patients, families and clinicians," said Dr. Ali Tavakkoli, a general surgeon at Brigham and Women's Hospital in Boston who was not involved in the research.

"The paper's message clinically makes sense," he told Reuters Health.

MRI and CT had similar rates of premature births of about 13,500 per 100,000 cases of appendicitis and fetal loss rates of just over 4,000 per 100,000. Laparoscopy was associated with 16,700 premature births and 11,000 occurrences of fetal loss. One additional cancer-related death would result for every 13,699 CT scans performed.

"The results of our study can be used to inform both clinicians and patients that, in a setting where MRI is not feasible, the risks of obtaining a preoperative CT are less than the risks of proceeding directly to the operating room in the vast majority of situations," said Dr. Kastenberg. "CT is not a benign technology and the risks of fetal radiation are real. However, they are overshadowed by the risks of misdiagnosis."

The authors concluded that MRI was "the most cost-effective strategy with a cost of $6,767 per QALY gained relative to CT." But since MRI may not be available at all hours in every hospital, CT scanning is the better second choice, at $560 per QALY gained relative to laparoscopy.

From a strict cost-effectiveness standpoint, the risk of inducing a childhood cancer did not outweigh the excess risk of fetal loss from diagnostic laparoscopy.

The study is limited by being model-based. Also, it didn't include observation and re-examination as a management option, and prospective data on the true incidence of radiation-induced childhood cancers are lacking.

Since that paper only dealt with possible appendicitis in the second and third trimesters of pregnancy, the question of what to do after an equivocal ultrasound in the first trimester remains open.

"The risk of abdominal MRI in the first trimester of pregnancy is not well established," Dr. Kastenberg said, "and there is not currently enough existing data to effectively address this issue with a decision analytic model. This is an important area for further experimental and population-based research."

Dr. Tavakkoli argued that the case for fetal harm is weak.

"I am not convinced by the single reference to possible fetal harm from a first-trimester MRI," he said. "I was on a committee to establish a protocol for working up abdominal pain in pregnancy at our hospital, and we decided on MRI as the next choice after ultrasound in all trimesters."

SOURCE: http://bit.ly/1eFdpHH

Obstet Gynecol 2013; 122:821-829.