Prenatal screening is a routine aspect of modern obstetric care, designed to identify potential developmental issues in a fetus. Neural tube defects (NTDs) are serious congenital anomalies affecting the brain and spinal cord. Early and accurate detection is paramount for informing parents and planning specialized obstetrical management and postnatal care. Ultrasound is the primary non-invasive tool for this assessment, providing visual evidence of these structural changes.
Understanding Neural Tube Defects
Neural tube defects result from the incomplete development of the embryonic structure that forms the central nervous system. This structure, known as the neural tube, normally closes between the third and fourth weeks of pregnancy. When this process fails at any point along the embryo’s length, an NTD occurs.
The two most frequently encountered types of NTDs are anencephaly and spina bifida. Anencephaly involves a failure of the upper part of the neural tube to close, which results in the absence of a large part of the brain, skull, and scalp. Spina bifida involves incomplete closure along the spine, leading to a defect in the backbone that protects the spinal cord and nerves.
Spina bifida is further categorized into open and closed defects, a distinction that has significant implications for detection. Open spina bifida, such as myelomeningocele, means the neural tissue is exposed through the defect in the back. Closed defects, such as spina bifida occulta, are covered by skin and are structurally less severe, making them much more challenging to identify prenatally.
The Role of Ultrasound in Detection
Ultrasound is the primary non-invasive method for screening and diagnosing neural tube defects during pregnancy. This imaging technology allows for the visualization of the fetal anatomy, identifying structural abnormalities in the spine and brain. The standard time for a detailed fetal anatomical survey is typically between 18 and 22 weeks of gestation.
During this mid-trimester scan, the sonographer examines the entire fetal spine to assess the alignment of the vertebral bodies and the intactness of the skin covering. Visualization of the fetal head is equally important, as many spinal defects create secondary changes in the brain structure. Detection is significantly higher for open defects, particularly anencephaly, which can often be identified as early as the first trimester due to the profound absence of the cranial vault.
Open spina bifida is associated with a leakage of cerebrospinal fluid, which causes structural changes in the fetal head, making the defect easier to spot. Closed spina bifida defects, which retain an intact skin covering, often lack these cranial signs and require meticulous, direct visualization of the spine. The accuracy of ultrasound in detecting open NTDs approaches 98% sensitivity.
Specific Ultrasound Markers
The diagnosis of open spina bifida often relies on identifying specific indirect signs visible in the fetal skull, which result directly from the cerebrospinal fluid leak. One recognized sign is the “Lemon Sign,” which is a scalloping or flattening of the frontal bones. This compression gives the fetal head an appearance similar to a lemon when viewed in a transverse plane.
Another specific finding is the “Banana Sign,” which describes an abnormal shape of the cerebellum. Normally dumbbell-shaped, the cerebellum is displaced downward due to the fluid leakage caused by the open spinal defect. This displacement, known as an Arnold-Chiari II malformation, pulls the cerebellum into a curved, crescent shape resembling a banana. The Banana Sign is often accompanied by the obliteration of the cisterna magna, the fluid-filled space behind the cerebellum.
While these cranial findings indicate an underlying open spinal defect, direct visualization of the spine remains necessary. The sonographer looks for a disruption of the three ossification centers of the vertebra, which normally form a closed ring. A visible gap in this ring, often accompanied by a fluid-filled sac protruding from the back, directly confirms the open spinal lesion. The presence of cranial signs guides the sonographer to perform a targeted examination to pinpoint the defect’s location and extent.
Limitations and Confirmatory Testing
Despite its high accuracy, ultrasound detection of neural tube defects has limitations that affect its diagnostic reliability. Factors such as the sonographer’s skill, the fetus’s position, and a high maternal body mass index can hinder image quality. Closed NTDs, which do not cause characteristic cranial changes, are also significantly more difficult to detect via ultrasound alone.
When an NTD is suspected or ultrasound images are suboptimal, additional testing is often required to confirm the diagnosis. One initial screening step involves measuring Maternal Serum Alpha-Fetoprotein (MSAFP) levels, a protein produced by the fetus. Elevated levels of MSAFP in the mother’s blood suggest the presence of an open NTD, where the protein leaks into the amniotic fluid and then into the maternal bloodstream.
A more definitive diagnostic procedure is amniocentesis, typically performed between 15 and 20 weeks of gestation. This test involves extracting a small sample of amniotic fluid to directly measure the levels of alpha-fetoprotein and acetylcholinesterase. Elevated levels of both substances in the amniotic fluid confirm an open neural tube defect, providing the necessary information for patient counseling and management planning.