Placental abruption

Placental abruption (or abruptio placentae) refers to a premature separation of the normally implanted placenta after the 20^th week of gestation and before the 3^rd stage of labour. It is a potentially fatal complication of pregnancy and is a significant cause of third-trimester bleeding/antepartum haemorrhage.

Epidemiology

The estimated incidence is ~1% of all pregnancies. The rate of placental abruption is thought to have dramatically increased in the past few years.

Risk factors

A number of risk factors have been associated with placental abruption, including:

• pre-eclampsia and maternal hypertension: up to 50% of cases
• previous placental abruption (recurrence rate 19-25%) ¹¹
• prolonged rupture of membranes
• maternal age: pregnant women who are younger than 20 years or older than 35 years are at greater risk
• maternal trauma
• cigarette smoking
• cocaine or other amphetamine use
• thrombophilia
• chorioamnionitis
• short umbilical cord
• multiparity
• multifetal pregnancies

Clinical presentation

Patients typically present with painful vaginal bleeding with "board-like" abdominal tone. Bleeding can occasionally be 'concealed' as in a retroplacental haemorrhage. 

Other features include:

• uterine contractions and irritability
• lumbar pain
• maternal/ fetal compromise secondary to exsanguination

Pathology

The exact aetiology is unknown, but the final pathophysiology is likely to rupture of a spiral artery with haemorrhage into the decidua basalis leading to separation of the placenta. The small vessel disease seen in abruptio placentae may also result in placental infarction.

According to the position of the abruption within the placenta it can be classified as:

• marginal placental abruption: most common by far
• retroplacental abruption 
• preplacental abruption 

Radiographic features

Ultrasound

Ultrasound is almost always the first (and usually the only) imaging modality used to evaluate placental abruption, but an index of suspicion should be maintained for the diagnosis since ultrasound is relatively insensitive for the diagnosis ⁹. This is partly because a retroplacental haematoma may be identified only in 2-25% of all abruptions.

The sonographic signs of placental abruption include:

• retroplacental haematoma (often poorly echogenic)
• intraplacental anechoic areas
• separation and rounding of the placental edge
• thickening of the placenta: often to over 5.5 cm
• thickening of the retroplacental myometrium: usually should be 1-2 cm unless there is a focal myometrial contraction
• disruption in retroplacental circulation
• intra-amniotic echoes due to intra-amniotic haemorrhage
• blood in the fetal stomach
• intermembranous clot in twins

The echogenicity of haematomas depends upon their age. Acute haematomas imaged at the time of symptoms tend to be hyperechoic or isoechoic compared to the adjacent placenta. As the haematoma is commonly isoechoic to the placenta, it may be mistaken for focal thickening of the placenta. A 'normal' ultrasound does not exclude a placental abruption-particularly as the blood may have escaped through the vagina in the case of external haemorrhage

In other cases, the retroplacental haematoma may be hypoechoic or of heterogeneous echogenicity.

CT

Since placental abruption is a concern in a pregnant patient who has undergone traumatic injury, CT is occasionally the first imaging modality used to evaluate the placenta.

The appearance of the placenta in the trauma patient is reviewed at "traumatic abruption placenta scale (TAPS)".

MRI

MR imaging can accurately detect placental abruption and should be considered after negative US findings in the presence of late pregnancy bleeding if the diagnosis of abruption would change management.

Haemorrhage due to abruption appears as an area of medium to high signal intensity on T1 and high signal intensity on a T2 weighted image, located between the placenta and uterine wall.

Treatment and prognosis

Given the low sensitivity for detecting placental abruption on ultrasound, if there is a high clinical suspicion, then it is likely prudent to treat based on the clinical suspicion ⁹.

If an abruption is detected, then the larger the size of the abruption, the greater the fetal morbidity. The presence of associated concurrent fetal bradycardia carries a poorer prognosis. Management for small abruptions is usually conservative - serial sonographic examinations with measurement of the retroplacental clot volume, antepartum heart rate and maternal symptoms has been suggested.

The recurrence rate of placental abruption is thought to vary between 19-25% ¹¹

Complications

• intrauterine growth restriction (IUGR): particularly when the abruption exceeds 30-40% of the placental area
• fetal demise: with a large unattended abruption
• maternal exsanguination

Differential diagnosis

A number of conditions can simulate the appearance of placental abruption.

For an isoechoic haematoma in an acute to subacute abruption on ultrasound consider:

• causes of placentomegaly
• focal myometrial contraction: transient

For a hypoechoic haematoma on ultrasound consider:

• uterine leiomyoma
• poorly echogenic subplacental space:
  • may also simulate a retroplacental haematoma
  • this appearance is often due to prominent veins in the decidua basalis
  • often colour Doppler may help define the anatomy of this space