Placenta Accreta Spectrum Notes

Placenta Accreta Spectrum

Introduction

Placenta accreta spectrum (PAS) has emerged as a critical concern in modern obstetrics due to its strong association with intractable postpartum hemorrhage (PPH), a leading cause of maternal morbidity and mortality. The condition demands careful consideration, especially in patients diagnosed with placenta previa, as the latter is a significant risk factor for PAS. Early recognition and management are paramount to improving maternal outcomes.

Definition

Placenta accreta spectrum encompasses a range of conditions previously grouped under the term "morbidly adherent placenta." This condition is characterized by the abnormal attachment of placental villi to the uterine wall. In a normal pregnancy, placental villi attach to the endometrium or decidua basalis. In PAS, these villi attach directly to the myometrium or invade it, leading to complications during delivery.

Types of Placenta Accreta Spectrum

The classification of PAS is based on the depth of placental villi attachment and invasion into the uterine wall:

  1. Placenta Accreta:

    • In this most common type, the villi adhere superficially to the myometrium without significant invasion.

  2. Placenta Increta:

    • Here, the placental villi invade into the myometrium, extending beyond the normal boundary but not penetrating the entire thickness of the uterine wall.

  3. Placenta Percreta:

    • The most severe form, where placental villi penetrate through the entire myometrium and attach to or invade the serosa. In extreme cases, they may extend to adjacent pelvic structures such as the urinary bladder or bowel.

FIGO Classification

The International Federation of Gynecology and Obstetrics (FIGO) provides a standardized classification for PAS:

  • Grade 1: Placenta Accreta

  • Grade 2: Placenta Increta

  • Grade 3: Placenta Percreta

    • Grade 3A: Villi attached to the serosa.

    • Grade 3B: Villi attached to the bladder.

    • Grade 3C: Villi attached to other pelvic structures.

Etiopathogenesis

Multiple pathological factors contribute to the development of PAS:

  1. Absent Decidua Basalis or Defective Decidualization:

    • Normally, the decidua basalis forms at the site of placental attachment. It provides a boundary that limits the depth of placental invasion. When absent or defective, it facilitates abnormal placental adherence.

  2. Absence of Nitabuch's Layer:

    • Nitabuch's layer, a fibrinoid layer located between the trophoblast and the decidua basalis, plays a crucial role in regulating blastocyst penetration.

    • It guides the penetration depth of the blastocyst, ensuring it does not invade too deeply. Its absence leads to uncontrolled invasion, with chorionic villi attaching to the myometrium or beyond.

  3. Hyperinvasiveness of the Cytotrophoblast:

    • Increased invasiveness of the cytotrophoblast may contribute to PAS. The exact mechanisms are still under investigation, but it is believed that some cytotrophoblasts exhibit an increased capacity to invade the uterine wall.

Risk Factors

Key risk factors for PAS include:

  1. Placenta Previa in the Present Pregnancy:

    • The presence of placenta previa significantly elevates the risk of PAS.

  2. Previous History of Cesarean Section:

    • Prior cesarean section is a major risk factor, with the risk escalating with each subsequent cesarean delivery. The scarred uterine segment may promote abnormal placental implantation.

Other notable risk factors:

  • History of uterine surgeries such as myomectomy, curettage, or endometrial ablation.

  • Previous history of PAS.

  • Risk factors associated with placenta previa, such as multiparity and advanced maternal age.

Markers

Several markers have been studied to assess the risk of PAS:

  1. Maternal Serum Alpha-Fetoprotein (MSAFP) Levels:

    • Elevated MSAFP levels ( ≥2.5\geq 2.5 Multiples of Median (MOM)) are associated with a higher risk of PAS.

  2. Beta-hCG Levels:

    • Similarly, elevated beta-hCG levels ( ≥2.5\geq 2.5 MOM) can suggest an increased risk.

It is important to note that these markers are not definitive diagnostic tools but rather indicators that warrant further investigation.

Diagnosis
Ultrasound with Color Doppler

  • Ultrasound with color Doppler is the primary imaging modality for diagnosing PAS. Characteristic findings can be observed from the second trimester, ideally leading to a definitive diagnosis by 28 weeks.

Key ultrasound findings, as described in Williams Obstetrics 26th Edition, page 762, include:

  1. Large Placental Lacunae (Placental Lakes):

    • Numerous, large, irregular sonolucent areas within the placenta give a moth-eaten appearance. Color Doppler reveals increased vascularity within these lacunae.

  2. Thinning of the Retroplacental Myometrium:

    • The distance between the serosa and myometrium is reduced to less than 1 mm, indicating loss of the normal myometrial thickness.

  3. Bridging Vessels:

    • Blood vessels extend from the placenta to the serosal interface, a prominent feature in placenta percreta cases.

  4. Disruption of the Continuous White Line Reflecting Bladder-Uterine Serosa Interface:

    • Blood vessels growing from the placenta disrupt the normal continuous white line at the bladder-uterine serosa interface. Doppler studies reveal hypervascularity in this region.

  5. Loss of Normal Hypoechoic Area Behind the Placenta Representing Decidua Basalis:

    • The normal hypoechoic area, which represents the decidua basalis, is absent, indicating abnormal placental adherence.

MRI

  • Magnetic Resonance Imaging (MRI) is reserved for cases where ultrasound findings are inconclusive. MRI can provide additional details regarding the extent of placental invasion.

Management
Antenatal Diagnosis

  • When PAS is diagnosed antenatally:

    • Plan a cesarean section followed by hysterectomy between 34 weeks and 35 weeks and six days to optimize maternal and neonatal outcomes.

    • Perform hysterectomy with the placenta in situ to minimize blood loss and complications.

    • General anesthesia is generally preferred.

    • Employ a classical cesarean section, carefully avoiding the placental site.

    • Close the uterine incision after fetal delivery.

    • Do not attempt to remove or deliver the placenta.

Preoperative Measures

  • Ureteric catheterization aids in identifying the ureters, particularly when the placenta invades the bladder (Grade 3B). However, studies suggest that it does not significantly reduce the risk of urinary tract injury.

  • Arterial catheterization of the internal iliac artery with a balloon catheter is generally discouraged due to the risk of embolization and other complications.

Intrapartum Management

  • If PAS is diagnosed unexpectedly after delivery, presenting as intractable PPH:

    • Emergency hysterectomy is often necessary to control bleeding and save the patient's life.

Uterus-Preserving Management

  • In situations where the patient strongly desires to preserve the uterus:

    • Leave the placenta in situ.

    • Ligate the umbilical cord as close to the placental insertion as possible.

    • Close the hysterotomy incision.

    • Control bleeding using compression sutures or packing.

    • Perform serial ultrasounds and MRI to monitor placental involution.

    • Serial beta-hCG levels are not particularly useful in this context.

    • Approximately twenty percent of these patients will eventually require hysterectomy despite conservative management.

    • Methotrexate is no longer routinely recommended due to limited evidence of its effectiveness and potential side effects.

Conclusion

In patients with placenta previa or a history of cesarean section, a high index of suspicion for PAS is crucial. Early diagnosis, meticulous planning, and appropriate management are essential to reduce morbidity and mortality associated with this challenging condition.