A question of angle of view

We read with interest the papers published in the March 2009 issue of Ultrasound in Obstetrics and Gynecology on the diagnostic and prognostic value of the angle of progression of the fetal head during labor. Barbera et al.1 calculated the angle of progression at fetal head engagement (station 0) from computed tomographic (CT) scans of non-pregnant women and found a mean value of 99°. This angle was defined as that between the long axis of the pubic symphysis and the midpoint of the line connecting the two ischial spines. They calculated that an angle of head descent of 120° corresponds to station +3 and reported that once the angle was ≥ 120°, all women delivered vaginally2. Kalache et al.3, using the same method of angle measurement during labor in a selected population with a prolonged second stage, reported that there was a calculated 10% probability of Cesarean section when the angle was 120°. In their model, all women would deliver vaginally when the angle reached values close to 140°. If an angle of 120° really does correlate to station +3, then clinical evaluation is sufficient because the fetal head is then very low and there should be no Cesarean section at this level. The two papers therefore give opposing information regarding the 120° angle threshold. To try to understand these differences, we measured the progression angle using different positions of the ultrasound probe in the same women, at the same point of time in labor, during constant uterine contraction. Figure 1 shows two images from the same woman, taken at almost the same time (64 s apart), with angles of progression of 148° (1a) and then 120° (1b). In the second case, the probe was held 6 cm higher than the level of the bed, by rotating the probe around the pubic symphysis. Modifying the position of the ultrasound probe gave an angle of progression of nearly 30° less, even though the reading was obtained later in time. All other parameters were constant. Figure 2 shows the superimposition of the first and the second images. Two angle of progression measurements in the same woman, taken 1 min apart. The second image (b) shows an angle that is 30° smaller, i.e. with less descent of the fetal head, although the uterus tone had increased slightly. Superimposition of Figures 1a (yellow, the initial image) and 1b (red, the second image) with artificial rotation so that the two images of the pubic symphysis correspond. The head seems to have progressed in the latter image only because of the change in the position of the ultrasound probe. The initial calculations were based on CT bony landmarks, but ultrasound measurements use the non-bony pubic symphysis landmarks. The angle between the pubic symphysis and the pelvic inlet plane might not be constant between women. The exact position of the ultrasound plane (strictly sagittal or not) and the position of the probe according to the pubic symphysis (transpubic or infrapubic positioning) could change the image of the pubic symphysis, and subsequently the angle measurement. We must be careful using these techniques because there is still a need for studies to help us to understand the meaning of the angle measurements. Further research is required and an exciting period of development of ultrasound during labor lies ahead. Y. Paltieli*, J. Nizard , * Department of Obstetrics and Gynecology, Bnai Zion Medical Center, Haifa, Israel, Department of Obstetrics and Gynecology, CHI-Poissy-Saint-Germain-en-Laye, Université de Versailles-Saint-Quentin-en-Yvelines, 78300 Poissy, France