Every year, approximately 7 in 10,000 babies in the U.S. are born with spina bifida. Even though this neural tube defect forms during the first few weeks of pregnancy, the vast majority of pregnant patients do not learn of the diagnosis until standard screenings, such as blood alpha-fetoprotein or triple testing, are able to detect it in the second trimester.
However, new insights are emerging to support spina bifida detection via ultrasound as early as the first trimester. 3D ultrasound could have particular utility in informing accurate diagnostic decisions as quickly as possible.
Currently, the literature has identified a small number of screening methods that are most promising for first-trimester spina bifida detection, including sonographic markers of the "crash sign," brainstem/brainstem occipital bone distance ratio and the maxillo-occipital line. Other methods, such as biparietal diameter and intracranial translucency, may be additional options worth exploring.
The Presence of the "Crash Sign"
In one retrospective review published in Obstetrics and Gynaecology that examined 53 confirmed cases of spina bifida, a little more than 90 percent of fetuses had what's known as the "crash sign" within the axial ultrasound view of the fetal head. This marker involves the visibly misshapen and displaced mesencephalon (the midbrain) against the occipital bone (at the lower back of the skull).
That sign — which was found between 11 and 14 weeks gestation in the studied patients — occurs when the cerebrospinal fluid (CSF) spills out from the defect itself. As pressure at the cranial end exceeds that of the lower spinal end, the mesencephalon moves caudally before "crashing" into the occipital bone.
Importantly, the presence of the crash sign yielded zero false positives in the study, which included ultrasound images from 40 healthy controls.
Brainstem/Brainstem Occipital Bone Distance Ratio
This metric compares the brainstem's (BS) diameter to its distance to the occipital bone (BSOB). A BS/BSOB ratio that exceeds the 95th percentile is thought to be a highly reliable marker for open spina bifida between 11 and 14 weeks of pregnancy.
When viewing sagittal scans for a retrospective comparative study, also published in Obstetrics and Gynaecology, assessment of the BS/BSOB successfully detected 26 out of 27 cases of confirmed spina bifida. Among 1,003 healthy controls, there were no false positives.
Maxillo-Occipital Line
This metric involves the straight line from the maxilla's superior border to the inner border of the occipital bone. In a healthy fetus, the mesencephalon meets the brainstem above the maxillo-occipital (MO) line. With open spina bifida, it can be seen below where the mesencephalon meets the thalamus in the brainstem.
Similar to the BS/BSOB ratio, abnormalities in the MO line measurement helped detect 26 out of 27 open spina bifida cases in first-trimester sagittal scans within the comparative Obstetrics and Gynaecology study. However, it did yield one false positive among the 1,003 healthy controls.
Other First-Trimester Markers
Additional markers to detect spina bifida — which have either been found to be less promising than crash sign, BS/BSOB and MO line, or require more investigation — include measurements of the biparietal diameter, occipital bone-Sylvius aqueduct distance and intracranial translucency:
- Biparietal Diameter (BPD): Clinicians are familiar with taking BPD to assess fetal size in the second trimester, but the metric also has some utility in early pregnancy to detect possible neural tube defects. When assessing BPD under the fifth percentile, spina bifida detection rates for this method have been seen to range anywhere from 50 to 66.7 percent accuracy.
- Ratio and Aqueduct of Sylvius (AoS): This indicator measures the distance between the occiput to the AoS (which connects the third and fourth ventricles in the brain). Lower values are thought to be indicators for neural defects because of the mesencephalon's displacement. AoS values under the fifth percentile were linked to a 77.8 percent detection rate in the Obstetrics and Gynaecology comparative study.
- Intracranial Translucency (IT): This method involves examining the fourth ventricle as a potential indicator for spina bifida. In fetuses with the defect, the IT values have been lower — below the fifth percentile — due to collapsing from the fourth ventricle. Using sagittal views, the comparative Obstetrics and Gynaecology study found IT values to be effective 52.3 percent of the time when diagnosing spina bifida.
Emerging Utility of 3D Visualization
Advanced imaging modalities such as 3D visualization showing the coronal plane can help providers more clearly assess these diagnostic markers during the first trimester compared to static scans — especially in cases of adverse fetal lie or where visualization is otherwise difficult with standard ultrasound. As researchers note in Sonography, 3D platforms can be particularly helpful to determine IT values and may give lackluster spina bifida detection rates increased utility due to enhanced picture quality.
Treatment Options and Patient Education
Spina bifida is a complex diagnosis, which means there is a good deal of patient education needed throughout the pregnancy. (The Spina Bifida Association's Expectant Parent's Guide is an excellent resource for physicians to share with patients.)
Start by counseling patients on what to expect with a spina bifida diagnosis. Emphasize that although there is no cure, surgical treatments to seal the gap can be successful. Interventions may be performed prenatally or postnatally:
- Prenatal interventions involve the in-utero repair for the neural tube defect, conducted via uterine incision or endoscopically before 25 weeks gestation. Compared to postnatal surgery, it can be advantageous in a few areas, such as the lower risks of hydrocephalus and postnatal neurologic sequelae. However, it also involves risks to mother and fetus (including premature birth, uterine dehiscence and fetal demise). Given the complexity of this surgery and the fact that not all medical centers can perform it, this intervention necessitates an in-depth discussion of the pros and cons.
- Postnatal interventions involve early surgical repair, typically within three days after delivery. This is currently considered the gold standard for spina bifida. However, it is commonly associated with hydrocephalus, though that can be treated by draining CSF into the abdominal area with a ventriculoperitoneal shunt. Pregnant patients will need to deliver at a hospital with the proper neurosurgery team to provide immediate care and should expect a NICU stay of about two weeks or more for their child.
Even with successful interventions, patients need to understand that spina bifida commonly requires an expanded care team over the long-term.
Often, physical and occupational therapy may be needed to support movement, as well as insight from orthopedists, urologists and other specialists. This depends on the extent of adverse outcomes, which may include bowel and bladder problems or issues with mobility where children may need assistive devices or a wheelchair.
Keep in mind that this information may be difficult for patients to hear, so take time to listen and answer questions patients may have throughout their pregnancy. Look for opportunities to remain optimistic about outcomes and possibilities to move forward, and always work to make diagnostic decisions as expediently as possible for long-term care planning. First-trimester biomarkers should be just one of many tools to accomplish these goals.
