Placenta accreta spectrum (PAS)
Placenta accreta spectrum (placenta accreta spectrum, PAS) encompasses a range of pathologies classified according to the depth of chorionic villi invasion. There are three degrees of severity:
- Placenta accreta — placental tissue directly contacts the myometrium without the decidua basalis.
- Placenta increta — villi invade into the myometrium.
- Placenta percreta — the placenta invades through the serosa and can affect adjacent organs, for instance, causing tenting of the bladder wall.
Ultrasonography is the primary diagnostic method. Mild cases are typically characterized by the disappearance of the retroplacental clear zone and the appearance of vascular bridges (so-called "tornado vessels") with multidirectional blood flow. Deeper invasion reveals placental lacunae with turbulent blood flow and marked subplacental hypervascularity. Magnetic resonance imaging (MRI) serves as a problem-solving modality. On T2-weighted images, the pathology manifests as irregularity of the uteroplacental interface and dark intraplacental bands. Diffusion-weighted imaging (DWI) allows reliable tissue differentiation due to the naturally high signal intensity of the placenta.
Placental abruption
Placental abruption is accompanied by hematoma formation and carries a high risk for the fetus. The condition typically manifests between 24 and 26 weeks of gestation. Risk factors include maternal age, arterial hypertension, and trauma. Hemorrhages are classified by location: preplacental, retroplacental, and intraplacental.
Ultrasound diagnosis of abruption is challenging, with a sensitivity not exceeding 25%. The appearance of a hematoma heavily depends on the age of the extravasated blood, and a fresh retroplacental hematoma can be difficult to differentiate from a normal hypoechoic zone (thickening of the zone over 2 cm should raise suspicion). The presence of ultrasound findings correlates with an adverse prognosis.
Computed tomography (CT) is frequently performed in trauma cases to rule out internal organ injuries. Reliable CT signs of abruption include areas of decreased attenuation spanning the entire thickness of the placenta and forming an acute angle with the myometrium (unlike physiological uterine contractions, which form obtuse angles). An additional marker of hemorrhage is hyperdense amniotic fluid.
Retained products of conception (RPOC)
Retained products of conception or placental tissue (retained products of conception, RPOC) frequently occur after first- and second-trimester miscarriages or abortions. Less commonly, placental fragments persist postpartum, especially in cases of undiagnosed PAS. The pathology carries the risk of profuse bleeding and infection.
On ultrasound, retained products appear as a heterogeneous mass within the uterine cavity. Color Doppler imaging (CDI) reveals active vascularization, distinguishing true tissue remnants from avascular blood clots. On contrast-enhanced CT or MRI, these masses appear as polypoid structures arising from the endometrium and showing avid contrast enhancement. The MR signal from such structures is heterogeneous due to blood breakdown products of varying ages.
Uterine arteriovenous malformations (AVM)
An arteriovenous malformation (uterine arteriovenous malformation, AVM) is an acquired vascular anomaly that often presents with heavy uterine bleeding following intrauterine interventions. The imaging appearance of AVMs and vascularized placental remnants can be similar.
On transvaginal B-mode ultrasound, an AVM appears as a focal hypoechoic area within the myometrium. Color Doppler imaging demonstrates intense color signals with aliasing artifacts and a feeding vessel. MRI and CT visualize tangles of tortuous vessels with characteristic signal voids extending into the endometrium. Selective angiography confirms the diagnosis by demonstrating early venous drainage. The condition is effectively treated with endovascular embolization.
Briefly: answers to topical questions
What laboratory changes are observed in massive uterine bleeding due to placental pathology?
Massive uterine bleeding in placental abruption can lead to coagulopathy and disseminated intravascular coagulation (DIC). The mechanism of DIC is related to the entry of large amounts of active thromboplastins from the placental abruption site into the maternal circulation. Recurrent bleeding can lead to anemia.
To differentiate placental abruption on CT, remember the rule: the hematoma forms an ACUTE angle with the myometrium, whereas a normal uterine contraction forms an OBTUSE angle.
Class and exam questions
How to differentiate an arteriovenous malformation from vascularized retained products of conception?
The main diagnostic criterion is that an AVM does not regress after beta-hCG levels return to normal. In addition, AVMs are characterized by aliasing artifacts on ultrasound and signal voids on MRI.
Why is ultrasound not always reliable in diagnosing placental abruption?
Ultrasound sensitivity is only about 25%. Sonographically, an acute hematoma is extremely difficult to distinguish from a normal retroplacental hypoechoic zone, and the diagnosis is usually established based on clinical presentation.
What MRI findings indicate placenta accreta spectrum?
Key MRI findings include loss of the uteroplacental zone, thinning of the myometrium, low-intensity T2 bands within the placenta, and invasion into adjacent structures.
What else to study on this topic
- MRI diagnosis of adenomyosis and uterine endometriosis
- Evaluation of tubal infertility: hysterosalpingography and sonohysterography
- Ultrasound signs of ectopic pregnancy ("ring of fire" sign)
- CT and MRI in gynecological emergencies
- Diagnostic imaging of complications following transvaginal sling placement
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