From nuchal translucency to intracranial translucency: towards the early detection of spina bifida.

It is now clear that the vast majority of major fetal abnormalities can be diagnosed prenatally by ultrasound, that most of these abnormalities can be detected in the first trimester of pregnancy and that women want firsttrimester rather than later diagnosis. It is also clear that effective diagnosis of fetal abnormalities often necessitates the identification of easily recognizable markers which direct the attention of the sonographer to the specific abnormality. Good examples of such markers are the scalloping of the frontal bones (the ‘lemon’ sign) and caudal displacement of the cerebellum (the ‘banana’ sign), observed in the second trimester in most fetuses with open spina bifida, and increased nuchal translucency thickness (NT) which identifies in the first trimester the majority of fetuses with major aneuploidies, lethal skeletal dysplasias and a high proportion of major cardiac defects. It is now widely accepted that increased NT at 11–13 weeks is the single most effective marker of trisomy 21 and all other major aneuploidies. First-trimester screening by a combination of maternal age, fetal NT, nasal bone, Doppler assessment of blood flow in the ductus venosus and across the tricuspid valve together with maternal serum free β-hCG and PAPP-A can identify more than 95% of all major aneuploidies for a screen-positive rate of less than 3%. A major remaining challenge in first-trimester ultrasonography has been the diagnosis of open spina bifida. This challenge, however, may now have been resolved by the realization that open spina bifida can be suspected by an easily detectable marker within the brain in the same mid-sagittal plane of the fetal face as for measurement of NT and assessment of the nasal bone. In normal fetuses the fourth cerebral ventricle presents as an intracranial translucency (IT) parallel to the NT, while in fetuses with open spina bifida there may be absence of the IT1. In almost all cases of open spina bifida there is an associated Arnold–Chiari malformation, which is thought to be the consequence of leakage of cerebrospinal fluid into the amniotic cavity and hypotension in the subarachnoid spaces leading to caudal displacement of the brain and obstructive hydrocephalus. In the second trimester of pregnancy the manifestations of Arnold–Chiari malformation are the lemon and banana signs and in the first trimester caudal displacement of the brain results in compression of the fourth ventricle and loss of the normal IT1–4.