William T. Hosek, M.D., FACEP
I. Introduction and Indications
One in every thirteen women presenting to the emergency department (ED) in their first trimester of pregnancy with abdominal or pelvic pain or vaginal bleeding will eventually be diagnosed with an ectopic pregnancy.(1-4) Because the history and physical exam are unreliable for detecting or excluding the presence of an ectopic pregnancy(2,,4), ultrasound has become more than just an adjunctive diagnostic tool. Traditionally, the serum quantitative B-hCG has been the initial test ordered in the evaluation for possible ectopic pregnancy. Pelvice ultrsound would be performed only if the quantitative B-hCG was above a certain level.(4) Unfortunately, limiting the use of ultrasound in this way resulted in delayed diagnosis of ectopic pregnancy, a great number of undiagnosed ectopic pregnancies being released from the ED, and a significant number of ectopic pregnancies rupturing prior to definitive treatment. Ultrasound as the initial step in the evaluation of all women presenting to the ED in their first trimester with abdominal pain or vaginal bleeding provides the best chance of early diagnosis of ectopic pregnancy.(5,6,7) Given the financial and logistical challenges of providing resources for around-the-clock immediate ultrasound, clinician-performed pelvic sonography has been incorporated into the practice of emergency medicine.(8) Pelvic ultrasound performed by emergency physicians significantly shortens emergency department length of stay for women with an intrauterine pregnancy and reduces the incidence of ectopic pregnancy rupture in discharged patients.(5,9,10)
The uterus is a pear-shaped organ that lies posterior to the bladder and anterior to the sigmoid colon. The uterus consists of the body, an upper broad fundus and the lower neck or cervix. The uterus in the adult (post-pubertal) female is approximately 8 cm long, 5 cm wide and 3 cm deep, and is comprised of three layers: the outer serosa, the muscular middle layer and the inner endometrium. The uterine fundus may point towards the anterior abdominal wall (anteverted) or back towards the spine (retroverted). When a sharp angle exists between the cervix and fundus, the uterus is said to be flexed.
Fallopian Tubes and Ovaries
Each fallopian tube is approximately 10 cm in length and consists of four segments (from proximal to distal: interstitial cornu, isthmus, ampulla and infundibulum). Ovaries are elliptical and approximately 4 cm in length, 3 cm in width and 2 cm in height. Although the location of the ovaries can vary, they are usually anterior to the internal iliac artery.(11)
III. Scanning Technique and Normal Findings
A thorough pelvic ultrasound will scan completely through the uterus and adnexal region in both the longitudinal and transverse planes. The transabdominal ultrasound employs ideally a 3.5 MHz curvilinear probe and begins just above the pubic symphysis in a longitudinal axis. It is best performed when the patient has a full bladder, which not only provides a good window for viewing underlying structures, but also displaces overlying bowel (Figure 1, illustration 1).
Figure 1: Transabdominal
ultrasound in longitudinal plane showing the uterus (fundus) and a distended
Illustration 1: Corresponding medical illustration.
The endometrial lining appears as an echogenic line on ultrasound (the endometrial stripe) and identifies the center of the uterus in the longitudinal plane. The ultrasound probe is moved from side to side through the entire width of the uterus in the longitudinal plane looking for evidence of an intrauterine pregnancy. The probe is next placed in the transverse plane just above the pubic symphysis directed down towards the uterus (Figure 2 and illustration 2).
Figure 2: Transabdominal ultrasound
in a transverse plane showing bladder, uterus and right ovary.
Illustration 2: Drawing of corresponding anatomy.
Once the endometrial stripe is identified, the uterus is scanned from cervix to fundus in the transverse plane. Finally, although not often visualized in the transabdominal ultrasound, the ovaries are located by sweeping laterally from the uterus. Before moving on to the transvaginal ultrasound, the hepatorenal space should be scanned for the presence of free fluid, which appears anechoic when present. It is important to remember that the ultrasound is providing the clinician with only one thin slice through a given region. Therefore, free fluid may be missed if a region is not thoroughly scanned.
TThe transvaginal ultrasound is best performed on patients with an empty bladder because of reduced patient discomfort and less uterine distortion from a distended bladder. The transvaginal probe is typically a 5 MHz probe and is covered with a probe condom. Bacteriostatic Surgi-lube is placed inside and outside of the condom to ensure smooth transmission of ultrasound waves. Patients may experience less discomfort with self-insertion of the probe into the vaginal canal. The probe is then positioned adjacent to the cervix (Illustration 3). A transvaginal ultrasound in the longitudinal axis will appear as shown in Figure 3. In this standardized format, a uterine fundus pointing towards the anterior abdominal wall is anteverted. A uterine fundus pointing in the direction of the posterior wall is retroverted (Figure 4, video clip 1). The longitudinal scan begins midline at the endometrial stripe and the uterus is scanned from right to left through its entire length (Video clip 2). The probe is then moved laterally to the adnexal area to view the ovaries.
Figure 3: Transvaginal ultrasound
in a longitudinal plane showing an anteverted uterus.
Illustration 3: Longitudinal view of uterus with the transvaginal probe in the vaginal canal.
Figure 4: Transvaginal ultrasound in a longitudinal plane showing a retroverted uterus.
Video clip 1: Transvaginal ultrasound showing a retroverted uterus in the longitudinal plane.
Video clip 2: Transvaginal ultrasound showing an antiverted uterus in the longitudinal plane.
Once scanning is complete in the longitudinal axis, the probe is rotated 90 degrees counterclockwise so that the probe is now in the transverse plane (Illustration 4). In this orientation, the left side of the screen corresponds to the patient’s right side (similar to what is seen with a CT scan). The transverse scan begins with identification of the endometrial stripe and then moves throughout the entire length of the uterus. In order to help locate the ovaries, the cornual flare, that portion of the uterus at its junction with the fallopian tubes, is identified (Figure 5). Once the cornual flare is located, the probe is moved laterally along the fallopian tube to locate the ovary (Figure 6 and video clips 3A & B). Ovaries are often identified by the presence of follicles, which appear hypoechoic or anechoic (Figures 7 and 8). During the transvaginal ultrasound, adnexal structures can be brought into view by using one hand to compress the lower abdominal wall while the other hand manipulates the transducer.
Illustration 4: Drawing of the transverse view of the uterus with probe in vaginal canal.
Video clip 3a
Video clip 3b
Video clip 3c
Video clips 3: Transvaginal ultrasound beginning with a short axis view of the uterus and moving toward the right ovary (clips 3a and 3b) and the left ovary (clip 3c).
Figure 5: Transvaginal ultrasound demonstrating the endometrial stripe and the cornual flare (the uterine fundus at its junction with the fallopian tube).
Figure 6: Once the cornual flare is located, the probe is tilted in the direction of the fallopian tube.
Figure 7 & Figure 8: Transvaginal ultrasound of the ovary showing multiple
follicles (Figure 7) and a single large follicle (Figure 8).
The first question to ask when evaluating a patient with pain or bleeding in their first trimester of pregnancy is “Is there evidence of a definitive intrauterine pregnancy on ultrasound?” The earliest sign of pregnancy seen with transvaginal ultrasound is the intradecidual sign, an anechoic sac without a distinct chorionic ring. This may be seen as early as four weeks of gestation (Figures 9 and 10). However, a thin walled sac within the uterus is not definitive evidence of an intrauterine pregnancy and could also represent a decidual cyst or endometrial breakdown during an ectopic pregnancy (the pseudogestational sac). Definitive sonographic evidence of an intrauterine pregnancy is established when a gestational sac containing a yolk sac is identified in two planes within the endometrium. This occurs around the fifth week of gestation (Figures 11 and 12).
Figure 9: Transvaginal ultrasound in longitudinal plane showing an anechoic sac within the endometrium
Figure 10: Transvaginal ultrasound in transverse plane (magnified) showing an anechoic sac
with a thin ring (decidual sac)
Figure 11: Transvaginal ultrasound in a longitudinal plane showing a gestational sac containing a yolk sac within the endometrium (a definitive IUP).
Figure 12: Transvaginal ultrasound in short axis showing a gestational sac with yolk sac within the right horn of a bicornuate uterus ( a definitive IUP).
At six weeks gestation, the heart tubes of the embryo fuse and both a fetal heart beat and a fetal pole can be identified (Figures 13, 14 and15 and video 4). At seven weeks gestation, the fetal head and extremities can be identified. The amniotic sac is often seen as it begins to expand into the chorionic sac (Figure 16). Ten weeks gestation marks the end of the embryonic period, and on ultrasound, the amniotic sac will obliterate the chorionic sac (not quite complete in Figure 17).
Figure 13 and 14: Transvaginal ultrasounds showing a gestational sac containing both a yolk sac and fetal pole within the endometrium.
Figure 15: Transvaginal ultrasound with Doppler over the fetal pole showing the presence of a fetal heart beat.
Video clip 4: Transvaginal ultrasound showing an intrauterine gestational sac with yolk sac, fetal pole and heart beat.
Figure 16: Transvaginal
ultrasound at 7 weeks gestation.
Figure 17: Transvaginal ultrasound at 10 weeks gestation (end of the embryonic period).
The introduction of fertility medications has greatly increased the incidence of multiple gestations. Figure 18 is a transvaginal ultrasound in the transverse plane showing dichorionic, diamniotic twin gestations. The ultrasound shows two separate gestational sacs (dichorionic), each with its own yolk sac (diamniotic) and fetal pole. Because multiple gestations are inherently high risk and because fertility medicines have dramatically increased the incidence of heterotopic pregnancy (simultaneous intrauterine and ectopic pregnancy), early obstetrical consultation should be considered in any patient with multiple gestations or a history of fertility treatments.
Figure 18: Transvaginal ultrasound showing twin gestations with two separate gestational sacs (dichorionic) and two yolk sacs (diamniotic).
IV. Pathologic Findings
Signs suggestive of abnormal embryonic development include a gestational sac greater than 10 mm in diameter without a visible yolk sac (Figure 19), a gestational sac greater than 18 mm in diameter without a fetal pole (Figure 20), or a collapsed gestational sac (Figure 21). Additionally, when the difference between the mean sac diameter and crown rump length (CRL) is less than 5mm, there is a significant risk of spontaneous abortion. (12) Other signs associated with a poor prognosis include the absence of a fetal heart beat in an embryo with a CRL of at least 5 mm (Figure 22) and a fetal heart beat less than 90 beats per minute. Gestational trophoblastic disease (molar pregnancy) may present with multiple, small, irregular cystic lesions within the endometrium (Figures 23 and 24).
Figure 19: Transvaginal
ultrasound in a longitudinal plane showing a gestational sac greater than
8 mm in diameter without a yolk sac (fetal demise). Figure 20: A gestational sac greater
than 18 mm without a fetal pole (fetal demise).
Figure 21: Transvaginal ultrasound in a longitudinal plane showing a collapsed gestational sac (fetal demise).
Figure 22: Transabdominal ultrasound with Doppler showing the absence of fetal heart tones in an eight-week gestational pregnancy (fetal demise).
Figure 23 and 24: Pelvic ultrasounds of two patients with confirmed molar pregnancy.
Ectopic pregnancy is the implantation of the blastocyst outside the endometrial lining of the uterus. This may occur within the peritoneal cavity, fallopian tubes, ovaries, cervix or within a scar of prior uterine surgery. 95% of all ectopic pregnancies occur within the fallopian tubes. (13) The most definitive sonographic sign of ectopic pregnancy is the visualization of an extrauterine gestational sac containing a yolk sac, embryo or fetal heart beat (Figure 25 and video clip 5). This occurs in approximately 14% of ectopic pregnancies. (14) Figure 25 shows a gestational sac with a fetal pole. A fetal heart beat was detected on ultrasound exam. On closer inspection, the gestational sac is clearly outside the endometrial cavity indicating an ectopic pregnancy. Figure 26 is a transabdominal ultrasound showing an empty uterine cavity in long axis. A suspicious mass or sac is seen just outside the uterine cavity. Closer examination of this adnexal area (Figure 27) reveals the presence of twin ectopic pregnancies.
Figure 25: Transvaginal ultrasound in a longitudinal plane showing an ectopic gestational sac.
Video Clip 5: An ectopic pregnancy confirmed in the operating suite. This video clip begins with a short axis view of the uterus on transvaginal ultrasound. The sonographer then moves the probe to the left adnexa where a gestational sac with yolk sac and fetal pole is seen.
Figures 26 and 27: Transabdominal ultrasounds showing twin ectopic pregnancies.
The pseudogestational sac is an intrauterine anechoic sac-like structure that may be mistaken for an early viable pregnancy. It actually represents endometrial breakdown in the presence of an ectopic pregnancy. A pseudogestational sac may be differentiated from a true gestational sac by its central location, oval shape, and lack of a thick chorionic ring. (12, 15) Figure 28 shows a longitudinal view of the uterus with a pseudogestational sac. The endometrial sac does not have a surrounding chorionic ring and free fluid is visible in the posterior cul-de-sac. This patient was found to have a ruptured ectopic pregnancy in the operating room.
Figure 28: Transvaginal ultrasound in a longitudinal plane showing pseudo-gestational sac.
A tubal ring is an anechoic sac surrounded by a thick, echogenic wall clearly separate from the ovary. It is highly suggestive of a tubal ectopic pregnancy. A corpus luteal cyst may appear strikingly similar to a tubal ring on ultrasound, so the structure should be carefully examined in relation to the ovary. The sonographer can apply pressure to the fallopian tube with the transducer to look for separation of the ovary and tubal ring. An echogenic ring that separates from the ovary suggests a tubal ectopic pregnancy. (16, 17) Conversely, because intraovarian ectopic pregnancies are so rare, a thick-walled structure within the ovary is almost always a corpus luteal cyst. (18)
Some studies suggest that the walls of the tubal ring of ectopic pregnancy are often more echogenic than the ovarian parenchyma (19) or endometrium. (20) In contrast, the walls of the corpus luteum are often less echogenic than the endometrium. (20) Color Doppler is not helpful in distinguishing between the two structures because of similar flow patterns.
Figures 29 – 32 are ultrasounds of unruptured ectopic pregnancies. The ultrasounds each show an anechoic sac surrounded by a thick chorionic ring outside the uterus.
Figures 29-32 are transvaginal ultrasounds demonstrating ring-like structures situated between the uterus and ovary. All of these patients were found to have tubal ectopic pregnancies in the operating suite.
Complex Adnexal Mass
When an ectopic pregnancy can be visualized by TVS, it will most commonly appear as a complex, or inhomogenous, adnexal mass. (14) The mass might represent an early ectopic pregnancy before the appearance of the gestational sac, a failing ectopic pregnancy, a ruptured ectopic pregnancy surrounded by coagulated blood, or a corpus luteal cyst. Unfortunately, the variable appearance of these masses can make definitive identification difficult. (13,14)
Color Doppler may be useful in confirming the presence of a suspicious adnexal mass. Since the developing gestational sac has increased blood supply, a “ring of fire” appearance with color Doppler may be seen. When this color pattern is identified, it is important to check for separation of the mass from the ovary given that the corpus luteum can have a similar color flow pattern. An ectopic pregnancy will separate from the ovary when pressure is applied with the transducer. Figure 33 is a short axis view of the fallopian tube during TVS showing a suspicious adnexal mass between the uterus and left ovary. When color Doppler was applied over the area (Figure 34), a “ring of fire” flow pattern was observed. This was confirmed to be an ectopic pregnancy in the operating room.
Figures 33 and 34: Transvaginal ultrasound in a transverse plane showing a suspicious left adnexal mass (Figure 33). A “ring of fire” flow pattern is observed when color Doppler is applied (Figure 34).
Figure 35 is a transvaginal ultrasound of a left ovary. There is a suspicious mass immediately adjacent to the ovary. The “ring of fire” pattern observed with the application of color Doppler (Figure 36) does not distinguish between an ectopic pregnancy and an exophytic corpus luteal cyst. Separation of the mass and ovary occurred when transducer pressure was applied to the ovary, suggesting ectopic pregnancy. This was confirmed in the operating suite.
Figures 35 and 36: ectopic pregnancy.
Even in the absence of an adnexal mass on gray scale, an asymmetrical color pattern when right and left adnexa are compared might suggest ectopic pregnancy. (21) Figure 37 is a transvaginal ultrasound in a short axis plane through a normal right fallopian tube. Here, color Doppler has identified the fallopian tube just anterior to the iliac vessels. Blood supply above the iliac vessels in the area of the fallopian tube is normal. Marked increase in color flow (blood flow) of the contralateral adnexal region might suggest the presence of an embedded ectopic pregnancy.
Figure 37 is a transvaginal ultrasound with color Doppler showing normal color flow of an adnexal area as. Figure 38 is a transvaginal ultrasound with color Doppler showing increased color flow of an adnexal region in a patient with an ectopic pregnancy.
Figure 37: A normal right fallopian tube on TVS in short axis with color Doppler.
Figure 38: Using color Doppler, an increased flow pattern is observed over the adnexal area of a patient with an ectopic pregnancy.
Free fluid in the pelvic cul-de-sac is a frequent normal physiologic finding in women. Figure 39 is a transabdominal ultrasound in the transverse plane across the uterus which shows a small amount of physiologic free fluid in the pelvic cul-de-sac.
Figure 39: Transabdominal ultrasound in the transverse plane showing free fluid in the pelvic cul-de-sac.
However, free fluid is also associated with both ruptured (more commonly) and unruptured ectopic pregnancies. Additionally, the likelihood of rupture increases along with the increase in quantity of free fluid. (22) Since clotted blood in the pelvic cul-de-sac after tubal rupture can obscure active hemorrhage, a brief scan through the hepatorenal space can often lead to quick identification of free fluid. In the first trimester, the presence of free fluid in the hepatorenal space of a symptomatic patient without an intrauterine pregnancy is virtually diagnostic of a ruptured ectopic pregnancy. Immediate obstetrical consultation is advised in this scenario.
Figure 40 is a transabdominal ultrasound showing a large amount of free fluid in and around the hepatorenal space in a patient with a ruptured ectopic pregnancy. Despite the amount of hemorrhage, free fluid was not readily seen on transvaginal ultrasound because of distorted landmarks from clotted blood (Figure 41).
Figure 40: Ultrasound of the hepatorenal space (ruptured ectopic) demonstrating anechoic (black)
free fluid between the liver and right kidney.
Figure 41: Transvaginal ultrasound in long axis in a patient with a ruptured ectopic pregnancy.
Interstitial pregnancy is the implantation of the gestational sac into the proximal portion of the fallopian tube within the muscular wall of the uterus. (12, 23) Ultrasound findings suggestive of interstitial pregnancy include an eccentrically located gestational sac surrounded by a thin (less than 5 mm) or incomplete myometrial mantle, an empty uterine cavity, and the “interstitial line” sign: an echogenic line extending from the endometrium into the cornual region and abutting the midportion of the interstitial mass or gestational sac. (24)
Figure 42 is a transvaginal ultrasound in the transverse plane showing a large
eccentrically located mass in the uterus. This was found to be a left interstitial pregnancy in the operating room.
Figure 42: Transvaginal ultrasound in a transverse plane showing an interstitial pregnancy.
Cervical pregnancy is defined as implantation of the gestational sac within the cervix below the level of the internal os. Although relatively uncommon (1% of all ectopic pregnancies), cervical pregnancy may result in massive hemorrhage. Therefore, when a gestational sac is identified within the cervix, it is imperative to distinguish ectopic pregnancy from the cervical phase of an abortion.
Sonographic findings suggestive of cervical implantation include:
- An empty uterine cavity
- An hour-glass shaped uterus
- A gestational sac containing a yolk sac, fetal pole or heart beat within the cervix (25)
- A gestational sac within the cervix with a closed internal os
- Peritrophoblastic blood flow on color Doppler (26)
Sonographic findings suggestive of spontaneous abortion include:
- A crenated gestational sac
- Sliding of the gestational sac within the cervix when the transducer is gently pressed against the cervix (the “sliding sign”) (26)
- A dilated internal os
- Movement or regression of the gestational sac on repeat ultrasound
Figures 43-45 are images from a transvaginal ultrasound in a patient with a suspected cervical pregnancy. This patient was ultimately treated with embolization and subsequent D&C. Pathology specimens showed trophoblastic tissue invading the cervical tissue consistent with cervical pregnancy.
Figure 43: Transvaginal ultrasound in long axis showing a gestational sac with yolk sac within the cervix.
Figure 44: Transvaginal ultrasound in a short axis view of the cervix showing a fetal pole within the gestational sac.
Figure 45: Transvaginal ultrasound showing peritrophoblastic blood flow on color Doppler suggesting implantation of the gestational sac into the cervix.
Heterotopic pregnancy is the simultaneous presence of an intrauterine and extrauterine pregnancy. While an unlikely event (1 in 7,000 spontaneous pregnancies), its incidence jumps to approximately 1% of all assisted reproductive pregnancies. (27) Due to the high-risk nature of these pregnancies, evaluation and management of this subset of patients should be performed in consultation with the obstetrical department.
Pregnancy of Unknown Location & Serum Human Chorionic Gonadotropin
Pregnancy of unknown location (PUL) is a term used to describe cases where patients with a positive urinary pregnancy test have no sonographic signs of an intra- or extrauterine pregnancy. (28) This can be seen with an early viable IUP, ectopic pregnancy, or a failing intra- or extrauterine pregnancy. Serum human chorionic gonadotrophin (B-hCG) is often used as a guide in management of women with a PUL. The “discriminatory level” is the level of hCG at which a viable intrauterine pregnancy should be visualized using transvaginal ultrasound. Today, the discriminatory level is institution-dependent and usually between 1000 mIU/L and 2000 mIU/L. Patients presenting to the ED with a PUL and an hCG above the discriminatory level should be considered to have ectopic pregnancy. (29) Patients with a PUL and an hCG below the discriminatory level can usually be managed by serial hCGs as an outpatient. (28) The primary goal in these cases is to identify a viable IUP, where the hCG would be expected to rise at least 35-53% over a 48 hour period. (30, 31) This percentage has been lowered from the traditional 66% (32) to reduce the chances of terminating a viable intrauterine pregnancy. However, given that women with either ectopic pregnancy or miscarriage can have a 48 hour rise in hCG similar to that seen in a viable pregnancy, close patient follow-up is advised.
V. Pearls and Pitfalls
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