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

Prenatal diagnosis of open spina bifida is carried out by ultrasound examination in the second trimester of pregnancy. The diagnosis is suspected by the presence of a ‘lemon-shaped’ head and a ‘bananashaped’ cerebellum, thought to be consequences of caudal displacement of the hindbrain. The aim of the study was to determine whether in fetuses with spina bifida this displacement of the brain is evident from the first trimester of pregnancy.  In women undergoing routine ultrasound examination at 11–13 weeks’ gestation as part of screening for chromosomal abnormalities, a mid-sagittal view of the fetal face was obtained to measure nuchal translucency thickness and assess the nasal bone. In this view the fourth ventricle, which presents as an intracranial translucency (IT) between the brain stem and choroid plexus, is easily visible. We measured the anteroposterior diameter of the fourth ventricle in 200 normal fetuses and in four fetuses with spina bifida.  In the normal fetuses the fourth ventricle was always visible and the median anteroposterior diameter increased from 1.5 mm at a crown–rump length (CRL) of 45 mm to 2.5 mm at a CRL of 84 mm. In the four fetuses with spina bifida the ventricle was compressed by the caudally displaced hindbrain and no IT could be seen.  The mid-sagittal view of the face as routinely used in screening for chromosomal defects can also be used for early detection of open spina bifida.  In the 1980s the main method of screening for open spina bifida was by maternal serum α-fetoprotein at around 16 weeks of gestation, and the method of diagnosis was amniocentesis and measurement of amniotic fluid α-fetoprotein and acetyl cholinesterase. Although it was possible to diagnose the condition by ultrasonographic examination of the spine, the sensitivity of this test was low1,2. However, the observation that spina bifida was associated with scalloping of the frontal bones (the ‘lemon sign’) and caudal displacement of the cerebellum (the ‘banana sign’), has led to the replacement of biochemical assessment with ultrasonography, both for screening and for diagnosis of this abnormality3. In the last 10 years there has been widespread uptake of routine ultrasound examination in the first trimester of pregnancy, and in the UK the National Screening Committee has recommended that this scan should be offered to all pregnant women. The 11–13-week scan is used for measurement of the fetal crown–rump length (CRL) to determine gestational age, for diagnosis of major abnormalities such as anencephaly and to screen for trisomy 21 and other aneuploidies. The latter relies on accurate measurement of the fetal nuchal translucency (NT) thickness and assessment of the nasal bone, which necessitates examination of the mid-sagittal view of the fetal face4,5. Extensive studies have reported that in addition to aneuploidies the 11–13-week scan can identify the majority of all major fetal abnormalities6. However, in the case of spina bifida the diagnosis is usually missed at this scan. A screening study at 11–13 weeks in 61 972 pregnancies undergoing measurement of fetal NT reported that none of the 29 fetuses with spina bifida was detected7. Correspondence to: Prof. K. H. Nicolaides, Harris Birthright Research Centre for Fetal Medicine, King’s College Hospital, Denmark Hill, London SE5 9RS, UK (e-mail: fmf@fetalmedicine.com) Accepted: 20 July 2009