Lacunae. The finding of multiple hypoechoic vascular spaces within the placental parenchyma has been associated with PAS (FIGURES 3 and 4). The pathogenesis of this finding is probably related to alterations in placental tissue resulting from long-term exposure to pulsatile blood flow.11
Finberg and colleagues introduced a grading system for placental lacunae in 1992 that is still used:
Grade 0: no lacunae seen
Grade 1: 1 to 3 lacunae seen
Grade 2: 4 to 6 lacunae seen
Grade 3: multiple lacunae seen throughout the placenta.12
The sensitivity and specificity of lacunae as an independent marker for PAS have been reported to be 77% and 95%, respectively.13 Despite these findings, several studies report a range of sensitivity (73% to 100%) and negative predictive value (88% to 100%).14 Even in Finberg’s original work, 27% of cases of confirmed PAS had Grade 0 or Grade 1 placental lacunae and 11% of cases of placenta previa, without PAS, demonstrated Grade 2 lacunae.12 There is agreement, however, that, the more lacunae, the higher the risk of PAS.
AT OUR INSTITUTION…
…we define placental lacunae as anechoic spaces within the placenta, surrounded by placental tissue on all sides and measuring ≥5 mm at their greatest diameter. We utilize color Doppler US to evaluate the presence or absence of blood flow within the lacunae. To optimize visualization of low-velocity blood flow within lacunae, we use bidirectional (high-definition) color Doppler US at ≤5–10 cm/sec, with color filters set at the lowest level and color gain maximized.