In 1932, Joseph Baer performed serial barium examinations in pregnant women that documented the progressive upward displacement of the appendix.1 It since has become an accepted obstetric truism that the appendix is located higher in the gravid abdomen and that the pain associated with acute appendicitis migrates upward as gestation advances.2 Interestingly, contemporary studies now suggest there is little change in the anatomic location of the appendix with advancing pregnancy and that the pain associated with acute appendicitis is located mainly in the right lower quadrant regardless of gestational age.3 In term gestations, rapidly diagnosing appendicitis is important because the rates of appendiceal rupture, septicemia, and fetal mortality all are increased.4,5 Therefore, verifying the location of the appendix in term pregnant women would be useful because misconceptions regarding appendiceal location and its associated pain when inflamed may lead to diagnostic delay.
Magnetic resonance imaging (MRI) technique provides maximal tissue characterization, multiplanar capability, and minimal fetal risk.6 In addition, recent studies have demonstrated that MRI permits good visualization of the appendix in the term gravid abdomen without exposing the woman or fetus to ionizing radiation.6 Because of these advantages, MRI technique is an optimal modality to use when performing imaging in these women. The primary objective of this study was to use MRI technique to locate and verify the position of the appendix in relation to the iliac crest in a cohort of pregnant women at term. We also sought to document appendiceal rotation because early studies describe the progressive inward and upward rotation of the long axis of the appendix to near-vertical orientation by the third trimester. Finally, we sought to investigate any correlation between neonatal birth weight and appendiceal displacement; we hypothesized that fetal size may affect displacement even among a cohort with a narrow gestational age range.
MATERIALS AND METHODS
This study was approved by the institutional review board of the University of Texas Southwestern Medical Center at Dallas. This is a secondary analysis of participants enrolled from January 2001 to January 2002 in a study comparing the accuracy of estimated fetal weight using MRI or ultrasonography. Women were eligible for enrollment if they had reached 39 to 40 completed weeks of gestation, required a scheduled cesarean delivery, and had an uncomplicated prenatal course. Eligible participants were recruited at the time the cesarean delivery was scheduled or in the preoperative holding area the day of surgery. Pregnancies complicated by diabetes, hypertension, or chronic medical conditions were excluded. Informed consent was obtained the morning of the participant's elective surgery and immediately before the MRI acquisition.
A single-shot fast spin-echo sequence with the following parameters was used to obtain images: echo time (effective) 50 to 100 ms, field of view 12 to 36 cm, 256×128 matrix, band width 31.2 or 62.5 Hz, 0.5 average (number of excitations 0.5), and slice thickness 8 mm. A 1.5 T GE Signa magnet was used (GE Healthcare, Waukesha, WI). The average acquisition time was 90 seconds. Approximately 40 images (without gaps) of 8-mm slice thickness covering 320 mm of the gravid uterus were obtained uniformly in every participant. No gadolinium contrast was used during the imaging sequence.
The images were reviewed by a radiologist who was board certified in body imaging (T.A.), using a GE Workstation 4.1 (GE Healthcare, Waukesha, WI). The T2-weighted images were viewed in both the coronal and axial planes to determine the location of the base of the appendix. If the appendiceal base was visualized, the distance from the base of the appendix to the iliac crest was measured in millimeters using the coronal image. In cases in which either an axial or coronal sequence was not able to verify the location of the appendix, the image was reformatted using software to create a corresponding image for confirmation. In instances in which the appendix was not visualized, the distal cecum was identified in both the coronal and axial planes and used as a surrogate for appendiceal location. All distances were measured along a line drawn perpendicularly from the base of the appendix or distal cecum to a line connecting the iliac crests (Fig. 1). The long axis of the appendix also was recorded using clock-position notation.
To test the hypothesis that the appendix is upwardly displaced at term, we compared our results with Baer's originally reported data. That report specifically noted that the appendix was a mean of “two fingerbreadths” above the iliac crest, with a range of one fingerbreadth below to four fingerbreadths above the crest in late pregnancy. We converted these parameters to facilitate statistical comparison, estimating 30 mm to be roughly equivalent to two fingerbreadths (range -15 mm to +80 mm, 0 mm being at the iliac crest). To evaluate the rotational axis of the appendix, we compared our data with Baer's assertion that the long axis was horizontal in 20% of women (3 o'clock by our scheme) and vertically oriented (12 o'clock) in 60% of women in the late pregnancy.
Statistical analyses were performed using SAS 9.1 (SAS Institute, Inc., Cary, NC). The one-sample Student t-test was used to examine the primary hypothesis, and the Shapiro-Wilk test was used to test the normality of our data. Pearson's correlation was used to evaluate the relationship between birth weight and appendiceal location and axis. Results were considered significant at P<.05.
All participants were multiparous and at at least 39 weeks of gestation. More than 80% of the women were of Hispanic ethnicity. A total of 125 women were eligible for enrollment, of whom 80 gave consent. Of these 80 women, 72 had imaging studies that included the region of the appendix and were available for analysis. The entire appendix needed to be visualized to determine axial rotation (clock position); 52 women met this criteria and were analyzed.
The mean distance of the appendix to the iliac crest was 45±28 mm (95% confidence interval 38–52). Because the 95% confidence interval for distance in this study did not include the hypothesized value (from Baer's original work) of 30 mm, a test was conducted on our data set that confirmed normality (result 0.14). Therefore, the statistics could be compared reliably, and the distance from the iliac crest was significantly greater than previously published at P<.001. The median axial rotation using clock position was 5 o'clock (first and third quartiles 3 and 6, respectively) with no significant difference when compared with the landmark study. No significant correlation existed between neonatal birth weight and appendiceal displacement or axis with r=.09 and .04, respectively.
In this study of 72 women with optimal MRI studies, the appendix is confirmed to be located well above the iliac crest in the late third trimester, and indeed may be further displaced than Dr. Baer originally believed. This information is in contrast to recent publications that suggest the appendiceal location essentially is unchanged in late pregnancy.7,8 Interestingly, even in Baer's own day, his assertions also were challenged because other investigators could not corroborate his findings using similar radiographic techniques.
In the early 1920s, Fink found no upward displacement of the appendix using fluoroscopy, and Hoffman cited the appendix to be below the iliac crest in 90% of pregnant women.9,10 More recently, Popkin and colleagues published a series in which the McBurney incision was deemed most optimal to remove the appendix in pregnancy and suggested that a study confirming its location in pregnancy be performed.8 Similarly, Hodjati et al conducted an intraoperative analysis and found no significant upward displacement among 191 pregnant women.7 The findings of Mourad and associates seemed to corroborate these contemporary observations by reporting right lower quadrant pain as the most common symptom of appendicitis among 67 gravid women.3 Previously used techniques such as fluoroscopy are not recommended in pregnancy; MRI is an effective and safe modality to confirm the position of the appendix.
Oto and colleagues used MRI to evaluate the position of the appendix in pregnancy across gestation, and their findings suggest an upward migration with advancing gestation.11 Their study, however, contained few term gestations, which are an important subset to study because the morbidity of ruptured appendicitis peaks at this time. Although our study results were not revealing with respect to appendiceal rotation or any birth weight correlation, our findings were consistent with Oto et al's initial MRI study of appendiceal location.
A legitimate criticism of the current study is that is not known whether any participants previously had undergone appendectomy or prior abdominal surgery besides cesarean delivery. Theoretically, a significant surgical history could affect the position of the appendix (or the cecum in cases in which the appendix was not visualized) and give misleading results. A study by Lyell et al, however, seems to refute the majority of this concern by reporting the extremely low incidence of bowel adhesions and adhesions to other pelvic structures after previous cesarean delivery.12 Additionally, we hypothesize that the incidence of other major abdominal surgery or prior ruptured appendicitis in our young, uncomplicated patient cohort was probably minimal.
The upward displacement of the appendix in the term pregnant woman is confirmed in this study. Although the diagnosis of appendicitis in pregnancy may require a high-degree of suspicion owing to nonclassical symptoms, the anatomic position of the appendix in pregnancy can be assumed reliably. More importantly, this knowledge may lead to a more rapid diagnosis and decreased fetal and maternal morbidity.
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