ABSTRACT
The aim of this study was to specify the sonographic, anatomical and morphological aspects of the fetal anal sphincter and to compare them with pathological and physiological findings. The sphincter was examined by serial sectioning and staining of embryo and fetal tissue and by real-time ultrasound. Its function was analysed using amniotic fluid digestive enzyme assays in cases of anorectal atresia and cystic fibrosis. Morphological findings indicate that the functional components of the anal sphincter do not differentiate before 30 weeks and therefore do not account for the observed anal continence at 22 weeks. Ultrasound measurements of the sphincter indicate three developmental phases: 1) slow growth from 14 to 19 weeks; 2) rapid growth from 19 to 30 weeks; 3) subsequently, no further increase, but contractions indicative of peristaltism. Amniotic fluid digestive enzyme assays indicate that anal sphincter maturation begins with perforation of the anal membrane at 12 weeks. Comparison of pathological cases (anorectal atresia and cystic fibrosis) suggests two possible explanations of fetal anal obstruction: increasing viscosity of digestive secretion or the presence of the three anal sphincter muscles, even if still immature. Our results clarify the evacuation and retention of meconium during fetal life and the role of the terminal part of the digestive tract, notably the anal sphincter. Prenatal diagnosis of anorectal atresia is therefore possible before 20 weeks of gestation by measurement of amniotic fluid digestive enzymes and ultrasonography, thus enabling better neonatal management.
