The role of light and sound technologies in the detection of vasa praevia.

Velamentous vessels across the internal os are rare (1 in 5000) but the high perinatal mortality (50-70%) has not improved with traditional methods of detection. In late pregnancy, Low Intrauterine Transcervical Endoscopy (LITE) can aid detection. Using LITE, such vessels have been 'seen' in 3 of 5000 patients examined; one case was detected fortuitously in 2500 patients examined during 1965-70, and two were detected in a special subgroup from 2500 patients examined during 1971-89 in which LITE was performed prior to labour with the express aim of excluding vasa praevia. Detection occurred before labour and in the absence of antepartum haemorrhage. In all three cases, the cervix was not dilated beyond 1 cm and a single velamentous vessel was found. The absence of labour and haemorrhage probably contributed to a zero perinatal mortality. Recently, ultrasound has been used in the detection of vasa praevia. Combination of abdominal real-time ultrasonography with LITE has permitted comparison of the efficacy of these two techniques; two of the patients with vas praevium were ultrasound negative but LITE positive. The risk of vasa praevia should be considered (a) when ultrasonography has shown a bilobed or succenturiate placenta, a low-lying placenta where the cord has a battledore insertion, a migrating placenta, or a suspicion of velamentous vessel traversing the internal os, (b) when there is a multiple or IVF pregnancy, or (c) when there is intrapartum bleeding or an irregularity in fetal heart rate. Inexperienced practitioners using LITE risk the rupture of a vas praevium. However, efficiently performed for those patients at increased risk of fetal hypoxia, or to detect meconium pollution of the amniotic sac or prior to amniotomy, LITE may fortuitously reveal a vas praevium and thereby enable action that prevents iatrogenic or spontaneous rupture.