The role of ultrasound in the management of ectopic pregnancy.

The case fatality rate for ectopic pregnancy has fallen dramatically in the last 30 years and currently stands at about 1.7 per 10 000 cases.¹ Much of this can be attributed to earlier diagnosis through ultrasound and human chorionic gonadotrophin (hCG) measurement. However, serious morbidity and occasionally mortality does still occur.

Studies evaluating the sensitivity of transvaginal ultrasound in the diagnosis of ectopic pregnancy typically do so within an early pregnancy or acute gynaecology clinic setting and either report the sensitivity of a single scan at first presentation or as part of a diagnostic algorithm where women with an inconclusive scan are reviewed 48–72 hours later if their symptoms and hCG levels permit them to be managed as outpatients. In such a setting about 75 per cent of ectopic pregnancies will be detected on an initial transvaginal ultrasound and 85–99 per cent of ectopic pregnancies will be diagnosed on a subsequent scan before treatment.^{2 3 4 5} Women without any evidence of an intrauterine or ectopic pregnancy on an initial scan are classified as having a pregnancy of unknown location (a PUL).⁶ In most series, only seven to 20 per cent of these women will subsequently be found to have an ectopic pregnancy.⁷ The remainder will eventually be found to have a viable or non-viable intrauterine pregnancy. In a small number of women presenting to an early pregnancy clinic, pregnancy location cannot be confirmed on subsequent scans and the management of these women with an ongoing PUL will be based on the pattern of their hCG levels and symptoms.

There are a number of reasons why an ectopic pregnancy may not be identified at an initial transvaginal ultrasound. Poor equipment, poor ultrasound technique, an increased body mass index, fibroids and other ovarian pathology can make clear visualisation of the pelvic adnexae difficult. It might also simply be that the ectopic pregnancy is too small or it is too early in the disease process for it to be visualised. Ectopic pregnancies that are not identified at an initial scan have a significantly lower hCG concentration and gestational age at the time of initial scanning than those that are seen on an initial scan.⁸

It should be emphasised that unless an intrauterine pregnancy has been clearly identified on initial abdominal scanning (and no risk factors for a heterotopic pregnancy are present) an ultrasound examination for a suspected ectopic pregnancy should always include a transvaginal scan. An inconclusive pelvic ultrasound examination that does not include transvaginal ultrasound is an incomplete investigation. The higher frequencies used in a transvaginal scan improve resolution and allow early pregnancy features to be detected up to a week earlier than an abdominal ultrasound examination.

The role of hCG in diagnosing ectopic pregnancy

The resolution of transvaginal ultrasound means, in the great majority of cases, the diagnosis of an ectopic pregnancy should be based on a positive visualisation rather than the inability to visualise an intrauterine pregnancy.⁹ However, hCG levels and the so called ‘discriminatory threshold’ or ‘discriminatory zone’ (the hCG level at which a viable intrauterine pregnancy should always be seen) still have an important role in diagnosis and management. This level is typically 1500 to 2000IU/l for transvaginal ultrasound and 5000-6000IU/l for transabdominal ultrasound.^{10 11} A viable intrauterine pregnancy should nearly always be seen on transvaginal ultrasound at an hCG level above the discriminatory zone with studies reporting a sensitivity of over 95 per cent for an intrauterine pregnancy at these hCG levels. It should be remembered that at an hCG level of 2000IU/l a gestation sac may only be 3–4mm in size and a non-viable intrauterine pregnancy or recent miscarriage may appear as an empty uterus at hCG levels above the discriminatory zone. The discriminatory zone will also be affected by a sonographer’s experience, the quality of equipment used and patient body habitus. It should be used with caution if treatment is going to be initiated purely on the basis of the finding of an empty uterus and an hCG level above the discriminatory zone.

Endometrial appearances in ectopic pregnancy

There is no specific appearance or thickness that suggests the presence of an ectopic pregnancy. The uterine cavity will usually be empty, although in up to 20 per cent of ectopic pregnancies a ‘pseudosac’ will be present. This can be distinguished from an early intrauterine pregnancy because a pseudosac, which consists of fluid within the endometrial cavity, will usually develop symmetrically within the uterine cavity and lack a well-defined rim of surrounding echoes. An early intrauterine pregnancy will be intra-decidual and appear as an eccentrically placed hyperechoic ring within the endometrial cavity. Differentiating a failed intrauterine pregnancy from a pseudosac can be much more difficult, with their appearances sometimes being similar.

What does pelvic free fluid mean?

A small amount of free fluid in the pelvis is a common finding, but echogenic free fluid in the Pouch of Douglas or Morrison’s pouch increases the likelihood of ectopic pregnancy significantly. Blood and clot from a ruptured ectopic or tubal miscarriage typically has a ‘ground glass’ appearance. However, significant quantities of free fluid may also be present following a ruptured haemorrhagic cyst. The amount and extent of free fluid seen on ultrasound scan can provide an indication of blood loss – blood in Morrison’s pouch suggests that at least 500ml of blood is in the pelvis.¹² If free fluid is seen in the pelvis then it is important that an abdominal scan is also undertaken to check for possible blood in Morrison’s pouch, the paracolic gutters and sub-diaphragmatic space.

Appearance of specific types of ectopic pregnancy

• Tubal pregnancy – 95 per cent of extrauterine pregnancies are in the Fallopian tube, usually in the ampullary region. An adnexal mass that moves separately to the ovary will usually be seen. It may range in appearance from an inhomogeneous mass through to a distinct gestational sac with or without a fetal pole, yolk sac or fetal heart. The latter finding will usually be in the presence of an empty uterus and hCG level well above the discriminatory threshold. The corpus luteum can be seen as a ‘ring of fire’ on colour Doppler and will be on the ipsilateral side in 70–85 per cent of tubal pregnancies.⁹ About 60 per cent of tubal pregnancies will appear as an inhomogeneous mass, 20 per cent will appear as a hyperechoic ring (sometimes called a ‘bagel sign’) and only 13–15 per cent will have an obvious gestation sac and fetal pole, with or without fetal cardiac activity.^{4 13}
• Interstitial pregnancy – one to five per cent of ectopic pregnancies lie in the interstitial part of the Fallopian tube(the part of the Fallopian tube that traverses the myometrium). The gestation sac will lie outside the cavity in the interstitial area surrounded by a thin continuous rim of myometrium. An ‘interstitial line’ is often seen consisting of a thin echogenic line that extends from the central uterine cavity to the periphery of the interstitial sac and presumably represents the endometrial canal of the interstitial part of the Fallopian tube.¹⁴
• Cervical pregnancy – these occur in less than one per centof ectopic pregnancies. The uterine cavity will appear empty and cervix may appear barrel shaped with the gestation sac or trophoblastic mass lying below the level of the internal cervical os. There may be a negative ‘sliding organ sign’ – in a miscarriage a gestation sac lying within the cervical canal will slide against the endocervical canal when transducer probe pressure is applied to the cervix, but an implanted cervical pregnancy will remain fixed.
• Caesarean section scar pregnancy – in women with a previous caesarean section these may make up to six per cent of all ectopic pregnancies. Appearances can be similar to a cervical pregnancy, but gestation sac or trophoblastic mass is located anteriorly at the level of the internal os covering the visible or likely site of the caesarean section scar.
• Ovarian pregnancy – this is a much rarer form of ectopic pregnancy, but will appear as a cystic structure or gestation sac within or on the ovary. The gestation sac cannot be separated from the ovary on gentle palpation.
• Abdominal pregnancy – most of these pregnancies will be the result of tubal abortion with re-implantation in the abdominal cavity, usually on the broad ligament. Rarely, a primary implantation in the abdominal cavity will occur. It is often difficult to distinguish from a tubal pregnancy on ultrasound.
• Heterotopic pregnancy – risk factors include in vitro fertilisation and super-ovulation-based fertility treatments. In women with no history of fertility treatment, the risk is less than one in 10 000 pregnancies. They may occur in between one and three per cent of women undergoing fertility treatment and should be looked for in such women presenting with early pregnancy bleeding or pain.

A final word

Always take a careful history, pinpointing if possible the exact dates of the last menstrual period, the dates of a positive pregnancy test, and dates of early pregnancy bleeding and pain. Early pregnancy transvaginal ultrasound requires a high level of operator skill and experience. Ultrasound is a dynamic investigation and, as a gynaecologist, relying on the written interpretation of ultrasound images alone will expose you to a greater risk of delayed or misdiagnosis. Where possible, review the images with your radiologist or, if your work setting permits, be present when the scan is performed so ultrasound images can be correlated with patient history, symptoms, hCG levels and examination findings.

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