Assessing Pre-test probability

What is pre-test probability?

Defining the pre–test probability is a standard procedure in medical practice which refers to the likelihood of a condition being present before a diagnostic test is performed. The higher greater the underlying risk of the condition, the higher the diagnostic accuracy most medical tests will have.

Why is pre-test probability important?

The assessment of pre–test probability is essential in reducing false positive diagnoses, especially as a recent study by Philips et al demonstrated at least 2 ultrasound signs of PAS were found in 98% of low risk pregnancies, all of which had spontaneous separation at delivery. However, it is interesting to observe which signs were found frequently and which were not. This appears to differentiate between the commonly described signs in terms of specificity as it is the highly subjective signs which were most commonly seen.

Commonly seen PAS signs: Infrequently seen PAS signs:

*Lacunae * Placental bulge

*Sub-placental hyper-vascularity * Bridging vessels

*Irregular bladder wall * Myometrium <1mm

* Loss of clear zone

Risk Factors for PAS

Damage to the uterus predisposes to developing PAS in subsequent pregnancies. However, the depth of the original myometrial injury determines the likely severity of PAS as well as the morbidity associated with it.

Superficial uterine damage

* Curettage, * Endometrial ablation

* Infection (severe endometritis) * Intra-uterine septal resection/resection for Asherman's syndrome

* Previous radiotherapy * Techniques for assisted reproduction (IVF)

As only the surface of the uterine cavity is damaged, it is highly unlikely that the placenta will be significantly invasive so this tends to give rise to abnormally adherent placenta (Accreta - FIGO grade 1).

This is not usually associated with neovascularity on the superficial surface of the uterus or re-modelling of the deep myometrial arteries so often has very few imaging signs. The myometrium can also reasonably thick (>5mm) so an extirpative approach may be suitable. If it is not, or there is significant bleeding, the lack of massive neovascularity means the hysterectomy is more akin to one performed for uterine atony than for abnormally invasive placenta.

Full-thickness uterine damage

This arises when entire uterine wall has been scarred from endometrium to serosal surface and is usually a result of a surgical incision but may be from a perforation or previous uterine rupture/trauma.

The most common cause is caesarean delivery but it can also be a result of myomectomy where the cavity has been breached.

Caesarean scar and placenta previa: a toxic combination

Abnormal placentation can occur anywhere in the uterus where there is a full thickness scar. However, the combination of an anterior low lying placenta (2cm from the internal cervical os) or placenta previa with a scar from previous caesarean remains the most dangerous due to several factors;

1) A placenta previa increases the risk of vaginal bleeding and the potential for emergency delivery

2) Poor contractility of the lower segment means a previa is more likely to bleed more heavily even with normal placentation

3) Lower segment incision runs the risk of transecting the placental bed which will result in significant hemorrhage.

4) The placental bed is in close proximity to other structures including the bladder and ureters making collateral damage at hysterectomy a greater risk, and the blood supply to the placental bed may come from not only the uterine arteries but other arteries lower in the pelvis making vascular control much harder in the event of hemorrhage.

5) In the lower uterine segment the ureters cross in close proximity to where the uterine arteries enter the uterus and lie within the narrowest portion of the bony pelvis. Hence, lateral placental bulging in this region makes surgical dissection even more technically difficult and risks ureteric injury.