Placenta accreta spectrum disorders are abnormal placentations often demonstrated as irregular or absent retroplacental clear space and dark intraplacental bands on ultrasound.
Description
Placenta accreta spectrum disorder, also known as abnormal placentation, refers to a range of conditions where the placenta abnormally adheres to the uterine wall, due to a defect in the decidua basalis layer. The spectrum encompasses three conditions – placenta accreta, increta, and percreta – distinguished by the depth of placental invasion into the uterine wall.
Pathogenesis
Placenta accreta spectrum disorders result from defective decidualisation of the endometrium. This usually occurs in the presence of scarring or damage to the endometrial layer, most commonly from a prior caesarean delivery. The placental chorionic villi directly attach to the myometrium instead of the decidua, and in severe cases, can invade into or through the myometrium.
Subtypes
- Placenta accreta: Characterised by superficial invasion of the placental villi into the myometrium without penetration of the uterine serosa.
- Placenta increta: Involves deeper invasion into the myometrium.
- Placenta percreta: The most severe form, where the placental villi penetrate the entire myometrial layer, potentially invading adjacent organs such as bladder and rectum.
Epidemiology, Risk Factors & Associations
- Prior caesarean section (30-40% risk)
- Placenta praevia, particularly in women with a previous caesarean delivery (5-10% risk)
- Increasing maternal age and parity
- Prior uterine surgery or curettage
Clinical Features
Typically asymptomatic, but may present with painless vaginal bleeding in the third trimester.
Complications
- Massive haemorrhage at delivery
- Need for blood transfusion
- Hysterectomy
- Premature birth
- Maternal mortality
- Foetal morbidity and mortality
Pathological Features
Histopathology
- Macroscopic: Placenta adhered to the uterine wall, with potential invasion through the myometrium.
- Microscopic: Absence of decidua basalis, with chorionic villi attached to the myometrium.
Radiological Features
General Features
- Loss or thinning of the normal hypoechoic retroplacental zone
- Presence of numerous vascular lacunae within the placenta
- Dark intraplacental bands
- Increased vascularity of the uterine serosa-bladder interface
US
- B-mode: Loss of retroplacental clear space, multiple vascular lacunae.
- Placenta accreta (placenta attaches deeply into the myometrium): thinning or absence of the normal hypoechoic subplacental myometrium.
- Placenta increta (placenta invades into the myometrium to the serosa)
- Placenta percreta (placenta penetrates through the serosa to invade other structures): A
focal bulge in the uterine wall is seen.
- Colour Doppler: Increased vascularity of the uterine serosa-bladder interface.
MRI
- T2WI: Low-signal-intensity bands within the placenta, focal interruptions in the myometrial wall.
- T1WI: Heterogeneous signal intensity of the placenta.
- T1 C+: Irregular or absent enhancement of the uterine-bladder interface.
Diagnosis
Made on imaging, usually ultrasound, but MRI may be used for detailed evaluation when ultrasound findings are inconclusive or to determine extent of invasion in placenta percreta.
Differential Diagnosis
- Normal placental adherence: Differentiated by presence of a clear, well-defined retroplacental space and lack of placental lacunae.
- Placenta praevia: Differentiated by presence of placenta covering the internal cervical os.
Management
If diagnosed before delivery, delivery should be planned at a tertiary care centre capable of managing massive haemorrhage and prepared for possible hysterectomy. Delivery is usually by caesarean section, often followed by hysterectomy if placenta cannot be safely removed. Conservative management with preservation of the uterus can be attempted under certain conditions.