Acute non-traumatic abdominal pain in pregnancy requires a considered imaging approach due to the increased risks of fetal demise associated with undiagnosed diseases such as perforated acute appendicitis. Ultrasound is the first-line modality due to its wide availability and ability to diagnose a range of abdominal diseases. However, due to changes in the location of organs caused by displacement by the gravid uterus, identification of structures such as the appendix may not be possible, and as such MRI has gained a growing role in this settting.
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Clinical presentation
The presentation of acute diseases may be delayed due to a number of factors. The symptoms are the same as those frequently found as part of pregnancy, namely nausea and vomiting, and laboratory tests such as leukocytosis are also found as a normal feature in pregnancy. Furthermore, symptoms may not localize or may present in an atypical manner due to the anatomic alterations of pregnancy.
Acute appendicitis in pregnancy occurs at a similar frequency as outside of pregnancy, and is the most common reason for emergency non-obstetric surgery in pregnancy. There is an increased risk of fetal demise with complications of acute appendicitis such as perforation, and there are also risks associated with anesthesia and increased rates of fetal loss with negative appendectomies, underlining the importance of prompt and accurate imaging 9.
Radiographic features
Ultrasound findings of acute abdominal diseases are identical to those outside of pregnancy, albeit more difficult to establish due to the aforementioned anatomic changes. MRI should be considered as the second-line test in this setting, with no role for plain radiographs. The decision to perform a CT should be made after thorough considerations of the risks and benefits of imparting a high radiation dose to the mother and fetus.
MRI protocol
An MRI protocol for assessing non-traumatic abdominal pain in pregnancy encompasses sequences that allow for assessment of bowel, solid organs, ovaries and uterus, although not for detailed examination of the fetus.
Bowel diseases
Acute appendicitis
With increasing gestation, the cecal pole and appendix move out of the pelvis into the right side of the abdomen up towards the right upper quadrant, and it may be difficult to positively identify the appendix. The principal features of acute appendicitis include enlargement of the diameter to >7 mm, thickening of the wall to >2 mm, luminal fluid and adjacent free fluid or gas. Finding linear diffusion restriction is also helpful. Low signal appendicoliths may be seen in the lumen. In the absence of identifying the appendix, with no edema or fluid or restricted diffusion, it is unlikely that there is acute appendicitis 7,8.
Crohn disease
The appearances of bowel in Crohn disease in pregnancy are identical to those outside of pregnancy.
Bowel obstruction
Ultrasound may identify dilated fluid-filled loops of small bowel, but MRI additionally enables more confident identification of the cause of obstruction and the transition point.
Hepatobiliary diseases
Cholelithiasis, choledocholithiasis, cholecystitis and pancreatitis
There is an increased rate of gallstone formation in pregnancy, and therefore a significant proportion of women will present with various facets of gallstone disease. The imaging features on ultrasound and MRI are identical to those outside of pregnancy, and due to the epigastric location, there is little change to the expected findings by the gravid uterus.
Urogenital diseases
Pyelonephritis
Usually diagnosed clinically, signs of pyelonephritis may be encountered on MRI, seen as foci of signal alteration including high signal edema on T2 weighted images and restricted diffusion. Complicating features such as intrarenal or perinephric abscesses can also be identified.
Ureteric stones
Physiologic hydronephrosis is commonly seen in pregnancy, caused by a combination of hormone-mediated smooth muscle relaxation and physical compression of the ureters between the uterus and psoas muscles. The ureter will be seen to taper smoothly in these cases. If a standing column of urine ends above or below the level of the sacral promontory, a ureteric stone should be looked for carefully; a low-signal intraluminal structure may be identified.
Adrenal infarction
There is an increased risk of venous thromboembolism in pregnancy, and the adrenal glands can undergo acute non-hemorrhagic infarction due to venous thrombosis. On MRI, there is edema, thickening and lengthening of the adrenal gland limbs, with edema of the surrounding fat 6.
Gynecologic diseases
Degenerating fibroids
Fibroids usually shrink during pregnancy, but may also undergo acute degeneration, with carneous degeneration being the most common type. On ultrasound, the fibroid will have a heterogeneous echotexture, with tenderness on probe pressure, and on MRI there may be heterogeneous signal particularly on T2 weighted images with internal high signal indicating necrosis, and high signal on T1 weighted images indicating hemorrhage.
Ectopic or heterotopic pregnancy
These are usually diagnosed confidently on ultrasound, but there may be doubts and in these instances MRI is helpful. A heterogeneous mass may be seen adjacent to one of the ovaries, with variable signal which may include foci of high signal on T1 weighted images indicating hemorrhage, and there may also be signs of a hemoperitoneum, indicating ruptured ectopic pregnancy. The presence of an intrauterine gestation sac in addition to an adnexal gestation sac indicates a heterotopic pregnancy.
Ovarian torsion
Ovarian torsion is seen most frequently in the first trimester, thought to be due to rapid enlargement of the uterus leading to increased mobility of the broad ligament and vascular pedicles of the ovaries. Torsion may occur secondary to an ovarian mass, which may be a benign follicular cyst or a malignant lesion such as a cystadenoma or dermoid cyst. Features on imaging include ovarian enlargement, edema and surrounding fluid. The uterus may be displaced away.
Ovarian hyperstimulation syndrome
Ovarian hyperstimulation syndrome may occur as a result of fertility therapy, but can also occur spontaneously. The ovaries are found to be massively enlarged, with peripheral cysts. A large quantity of fluid, including blood, may be detected if there has been cyst rupture.
Obstetric diseases
Findings related to placental abnormalities such as placenta previa, placenta accreta and placental abruption are described in detail in the linked articles.
Vascular diseases
There is an increased rate of thrombosis in pregnancy, and most thromboses are readily detected using ultrasound. MRI may be helpful in areas not accessible by ultrasound, such as the ovarian veins. A lack of a flow void on T2 weighted images (i.e. the normal low signal void is not present and instead there is high signal) or low signal on gradient-echo images are key signs.
Differential diagnosis
A dilated ureter caused by physiologic hydronephrosis, and a dilated ovarian vein, may both mimic a dilated appendix, but the former two can be followed for a longer distance within the retroperitoneum, and the signal changes on SSFSE and gradient-echo sequences will enable these structures to be differentiated. On gradient-echo images, the ureter and ovarian vein will have a high signal and lie close to each other anterior to the psoas, whereas on SSFSE images, the ureter maintains a high signal but the ovarian vein will have flow voids within.
Practical points
the use of MRI in all trimesters of pregnancy is considered to be safe by the American College of Radiology's Manual of MRI Safety, but patients should be consented about the as-yet-unknown long term effects of MRI on the fetus
the use of gadolinium contrast is relatively contraindicated in pregnancy due to gadolinium crossing the placental barrier and the unknown effects on the fetus
the appendix may be difficult to identify on MRI, particularly in the latter stages of pregnancy, due to movement outside of the usual position in the right iliac fossa and compression of bowel loops
the presence of restricted diffusion in the region of the cecal pole helps to diagnose acute appendicitis
physiologic hydronephrosis is commonly seen in pregnancy, more frequently on the right side, with gradual tapering of the ureter as it is compressed between the uterus and psoas muscle
a dilated ovarian vein is another tubular retroperitoneal structure that may be mistaken for an appendix but can be followed caudally from the ovary to its cranial drainage to the IVC on the right and left renal vein on the left
if fetal assessment is not carried out, a statement explicitly stating this should be added to the report
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