Normal uterus.
Uterus
Several uterine conditions can be diagnosed. These include congenital anomalies and submuscosal fibroids.
Congenital anomalies
With proper configuration, the internal contours of the uterus can be visualized. Anomalies include uterus arcuate, septate, bicornuate, or uterus didelphys. Uterine anomalies can result in impaired vascularization of the uterus and the space for an expanding pregnancy; this can increase the rate of miscarriage. Approximately 12%–15% of women with recurrent miscarriage have uterine malformation. Of all of the possible uterine malformations, complete or partial septation of the uterus is the commonest. It occurs about 33% of the time amongst major anomalies, but is the most receptive to treatment. Successful surgery leads to a term delivery rate of about 75% and a live birth rate of 85%.[5] Although these anomalies can be suspected on 2D ultrasounds, 3D ultrasound gives coronal views of the uterus and the depth of the septum can be measured.
Uterine pathology
Polyps, fibroids, and adenomyosis can often be distinguished on ultrasound. Conventional ultrasound will help localize the location of the fibroid and categorized it a pedunculated, subserosal, intramural or submuscosal lesion. It is further possible to distinguish between type 0, type 1 and type 2 submucosal fibroids, although 3D ultrasound is more useful in this regard. Figure 5-2 shows an intramural myoma.
Uterine myoma.
Submuscosal fibroids
Submuscosal fibroids are typically hypoechoic and displace the basalis layer of the endometrium. Evaluation of the amount of extension into the myometrial layer is important in determining the type of surgical procedure to plan.
Polyps have a more homogeneous echogenicity and have a pedunculated attachment to the uterine wall without interruption of the endometrial lining. Color or power Doppler modes are useful to distinguish myomas from polyps or uterine malignancies. Myomas have a typical vascularization pattern of the capsule that forms a circle, whereas endometrial polyps have a vascular pedicle. Malignancies have an abundant irregular vessel distribution.[6] Saline infusion sonohysteroscopy (SIS) increases the specificity of the exam; polyps often show motion with injection of sterile saline. They also have a less round contour and are often elongated in appearance. See Figure 5-3.
Postsurgical evaluation of the endometrium can also be performed with ultrasound. Sagittal and transverse views can be utilized to evaluate the endometrium after removal of polyps or fibroids. SIS can also be used to evaluate an endometrium on a patient receiving tamoxifen therapy as a more detailed view of the endometrial cavity is obtained.
An isthmocoele is a fluid filled area in the region of a previous cesarean section. This may be a cause of fluid accumulation in the uterine cavity or be associated with abnormal uterine bleeding and infertility. The persistence of menstrual blood in the cervix might negatively influence cervical mucus quality, obstruct the passage of sperm through the cervical canal, affect sperm quality, or affect implantation of an embryo in the endometrial cavity if fluid is present. Surgical repair of isthmocele may correct bleeding, fluid accumulation and restore fertility. Repair is usually done with the hysteroscope and the area is cauterized.
Nabothian follicles can be visualized within the cervix. These are small sonolucent cysts seen near the region of the cervical canal.
Pregnancy analysis
Ultrasound is a critical modality in the management of intrauterine pregnancy.[8] Before the gestational sac appears, the endometrium is markedly echogenic and the arcuate vessels are somewhat prominent. This appearance, however, is nondiagnostic of pregnancy. Although the blastocyst begins to implant in the endometrium at one-week post conception (or three postmenstrual weeks) the first definitive ultrasound sign of pregnancy is the gestational or chorionic sac. In ultrasound images, the chorionic sac appears as a thick, echogenic rind surrounding a sonolucent center. It appears embedded deep in the thick endometrium or decidua on one side of the cavity line, not in the middle of it. The sonolucent center is actually fluid within the chorionic sac. The sac already contains the amnion, a bilaminar embryonic disc, and the yolk sac; these structures are too small to be imaged even with the high magnification of current scanners. The echogenic rind is the result of a trophoblastic decidual reaction. Early in gestation, the entire chorionic sac is surrounded by chorionic villi. These villi are symmetrically located. Some villi will bud and branch into secondary tertiary villi and become chorion frondosum (forerunner of the placenta). Other primary villi will regress and become chorion leave. The appearance of the gestational sac predates the fusion of the decidua parietalis to decidua capsular. One can occasionally image a sonolucent area caused by implantation bleeding that separates the sac from the contralateral decidua. The sac can first be seen at approximately 34.8 days plus or minus two days. The discriminatory level of human chorionic gonadotropin (hCG) has been reported to range from 935 mIU/mL to 2,388 mIU/mL.[7] The presence of fibroids, coexisting intrauterine devices or multiple pregnancies can create exceptions. The chorionic sac grows approximately 1 mm/day in mean diameter in early pregnancy. With further growth, first the yolk sac and later the embryo become visible sonographically inside the chorionic cavity. The yolk sac has a very bright echogenic rim around a sonolucent center. When it first appears at approximately five weeks it may be only 1 mm–2 mm in diameter. Discriminatory size of the yolk sac is reported to range from 8 mm to 13 mm in mean diameter. In abnormal pregnancies, the yolk sac may be enlarged or irregular, or sometimes is described as “floating.”
The embryonic period begins at three weeks post conception or five weeks from the last normal menstrual period (LNMP). Before the embryo is 18 mm in length a true “crown” and “rump” do not exist. Therefore, early embryonic size (EES) is measured as the greatest length along the long axis of the embryo. Initially at the somite stage, the embryo is a linear structure 2 mm–3 mm in length. The rostral neuropore closes and develops into the forebrain prominence then the head. The caudal neuropore elongates into a tail. As it grows the embryo is a C-shaped tadpole-like structure. The primitive heart has great prominence, often allowing the sonographer to provide early detection of the cardiac activity pulsations before visualizing an embryo distinct from the adjacent yolk sac. With further development, the tail regresses, the head unfolds from its flexed position, and limb buds develop and are replaced by hands and feet. By 18 mm–22 mm, a recognized crown rump length (CRL) can be measured. By 10 postmenstrual weeks the previous tadpole-like form is now recognizable as a human fetus. The fetal periods begins at 70 days post-LNMP.
In early pregnancy the demonstration of viability may be necessary to exclude ectopic pregnancy, to rule out embryonic demise in cases of vaginal bleeding or abdominal pain, or to clarify uncertain dates. Cardiac activity starts at 21–22 days of embryonic or conceptual age, or approximately 36 days post LNMP. At this stage the embryonic pole is approximately 1 mm–3 mm. Transabdominal sonography can detect cardiac activity by 41–43 days gestation.
A very important clinical question is, “What is the embryonic size without cardiac activity that is diagnostic for pregnancy failure?” Consensus is that the absence of cardiac activity on real-time transvaginal sonography in embryos measuring more than 4 mm is always associated with embryonic demise.[10] Slow embryonic heartbeat of 80–85 or less for a sustained period of time is considered ominous.[13] Rapid embryonic heart rate and arrhythmias in early pregnancy have also been studied.[14] When bradycardia, tachycardia, or arrhythmias are noted the patient should be scheduled for follow-up ultrasounds. The chance of poor outcome is higher if other pathologic ultrasound findings are noted such as growth discrepancy, subchorionic hemorrhages, and abnormalities of the yolk sac or fetal heart rate changes noted earlier.[16]
Assessment of the amnionicity and chronionicity of a multifetal pregnancy is important as these pregnancies have an increase in complications; the number of chorionic sacs can be determined by five postmenstrual weeks. The determination of the number of fetuses occurs at the sixth postmenstrual week, after the onset of cardiac activity when the number of heartbeats can be determined. The number of yolk sacs is not a reliable marker for twin gestation. The detection of the individual amniotic sacs in the case of monochorionic-diamniotic twins becomes possible at 7.15 postmenstrual weeks or immediately thereafter because the amnion separates from the embryo and is easily imaged.
Gestational age assessment is an important role of transvaginal ultrasound. In the past, embryonic crown rump length (CRL) was considered the most accurate method of sonographically dating a pregnancy. With current methods, chorionic sac size expressed as a mean sac diameter can be correlated by the postmenstrual age before visualization of an embryo. Measurements of the sac are taken at the interface of trophoblastic tissue and the fluid filling the sac. Such measurements ideally come from two planes at right angles to each other. One or more tables are usually included in the software of commercially available ultrasound machines.
In dating an early pregnancy, the following are important landmarks. The gestational sac is always present by the fourth week of pregnancy, the yolk sac by the sixth week of pregnancy. The fetal pole with heartbeats by the seventh week of pregnancy, the single brain ventricle in 82% of cases by the seventh week of pregnancy, the falx by the tenth week of pregnancy and midgut herniation by the eighth week of pregnancy.[16]
Assessment of abnormal intrauterine pregnancy
Twenty-five percent of pregnant patients bleed in the embryonic period. Of these, up to 50% will abort. In the author’s experience, many “subchorionic bleeds” resolve spontaneously and the pregnancy carries on significantly. In many cases of threatened abortion, no abnormal ultrasound findings are noted.
On ultrasound, a hydatidiform mole appears as multiple small sonolucent areas which correspond to the grapelike vesicles seen on pathologic analysis. Sonographically the changes are rarely seen prior to 10 weeks because the trophoblastic proliferation and hydropic changes are not yet present.
When assessing a patient with suspected blighted ovum, it should be realized that in a normal pregnancy, once the mean diameter of the gestational sac on transvaginal ultrasound exceeds 8 mm–13 mm, a yolk sac should become visible.
Embryonic resorption is present more frequently in fertility patients who have undergone superovulation or assisted reproductive technology.
Subchorionic hemorrhage appears as a crescent shaped sonolucent structure outside of the chorionic sac in a patient with threatened abortion. The presence of a fetal heart leads to a better prognosis. Approximately 20% of cases of threatened abortion exhibit this finding.[7]
Ectopic pregnancy
If a patient has a positive pregnancy test and bleeding, she must be evaluated for ectopic pregnancy with or without the presence of pain. The ultrasonographer must be aware that the corpus luteum can mimic an ectopic pregnancy.
Heterotopic pregnancy is the concomitant presence of both an intrauterine and an ectopic pregnancy. The incidence is 1 in 30,000 spontaneous pregnancies and in 1 in 6,000 pregnancies with assisted reproductive technologies (ART).[17] Patients with prior tubal damage seem more susceptible in the latter category. In a normal pregnancy the quantitative HCG will raise a minimum of 66% every 48 hours. Once HCG levels surpass a discriminatory level, it should be possible to image an intrauterine gestation. Absence of a gestational sac at this level either indicates a failing intrauterine or an ectopic pregnancy. If uncertain, the physician may consider performing a dilation and curettage (D&C) for these patients to assess if trophoblastic tissue is present. It is very important from both a clinical and medicolegal perspective to avoid giving methotrexate to an early ongoing intrauterine gestation. The decision to give methotrexate should never be made on evaluation of a single HCG value. A pseudo sac (seen in 20% of women with ectopic pregnancy) can mimic an intrauterine gestation.[16] A true gestational sac is intradecidual whereas a pseudo sac develops within the uterine cavity. Color Doppler with an early intrauterine pregnancy (IUP) can often identify a high-velocity, low resistance trophoblastic arterial flow pattern associated with developing chorionic villi, but this is not a strict diagnostic sign.
A finding of a well-developed sac with decidual casting outside of the uterine cavity is strongly suggestive of an ectopic pregnancy; 15%–28% of ectopic pregnancies develop a yolk sac or cardiac activity. The strictest criterion, with the highest specificity (100%) but the lowest sensitivity (15%–20%) is identifying an extra uterine gestational sac that contains a yolk sac or an embryo. Many of these are in the ampullary portion of the tube where the HCG can continue to double. Free fluid in the pelvis is a suspicious but not diagnostic sign.
Interstitial pregnancy accounts for 2%–4% of ectopic pregnancies. These pregnancies are associated with a higher degree of morbidity and mortality. Diagnosis is suggested when ultrasound images show eccentric implantation of the gestational sac at the superior fundal portion of the uterus. In almost all cases, there is myometrial thinning or absent endometrium around portions of the sac. Rarely visible myometrium has been reported to surround completely a small interstitial ectopic pregnancy. To increase the predicative value of sonography to make this diagnosis, Ackerman and associates described the interstitial line sign.[17] This consists of visualizing a thin, echogenic line that extends from the central uterine cavity echo to about the periphery of the interstitial gestational sac. When used in combination with the ultrasound finding of an incomplete mantle and an eccentric sac, this line may be useful for diagnosing an interstitial pregnancy.
Cervical pregnancy occurs in 0.15% of normal pregnancies, but increases by a factor of 10 with ART. The diagnosis is based on detection of a gestational sac that contains a yolk sac, embryo, or cardiac activity within the cervix.
Diagnosis of abdominal pregnancy is made upon the finding of an empty uterus that is separate from the fetus. It can be confused by a congenital uterine duplication anomaly that consists of one empty uterus and one containing a fetus. Other ultrasound findings include a lack of visualization of surrounding myometrium, extra uterine placental tissue, oligohydramnios and an abnormal fetal lie.
Chapter 11, “Abnormal First Trimester Pregnancy,” includes ultrasound images associated with pregnancy assessment.
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