Fetal Chiari Malformation Surveillance
Immediate Diagnostic Confirmation
When Chiari malformation is suspected on prenatal ultrasound, perform fetal MRI at or after 22 weeks gestation to confirm the diagnosis, characterize the specific type and severity, and identify associated anomalies that will guide counseling and delivery planning. 1
Detailed multiplanar ultrasound examination should be performed immediately to evaluate for characteristic findings including the "banana sign" (cerebellar compression), "lemon sign" (frontal bone scalloping), obliteration of cisterna magna, and ventriculomegaly, though these classic signs often become obscured after 25 weeks gestation 2
After 24 weeks gestation, use coronal views of the posterior horn of the lateral ventricle to identify the "triangle sign" (bilateral downward-triangle shape) or "square sign" (quadrilateral angular shape), which correlate with lower and upper spinal lesions respectively and maintain diagnostic accuracy when traditional signs fade 2
Evaluate tectal morphology on sagittal views for elongation or "beaking" of the tectum, which occurs in 66% of Chiari II cases and increases in frequency with severity of posterior fossa abnormalities 3
Perform comprehensive spinal survey to identify the level and extent of associated myelomeningocele, as this is present in virtually all Chiari II cases and determines prognosis 2, 4
Advanced Imaging Protocol
Fetal MRI is essential for complete characterization and should include T2-weighted ultrafast sequences in axial, coronal, and sagittal planes to evaluate posterior fossa contents, brainstem position, and associated supratentorial abnormalities. 1
MRI provides superior soft tissue contrast compared to ultrasound and can identify associated anomalies including corpus callosum abnormalities, cortical malformations, and syringomyelia that may not be apparent on ultrasound 5, 1, 4
Timing of fetal MRI is critical: perform at or after 22 weeks gestation when brain structures are sufficiently developed for detailed evaluation, though earlier imaging (18-22 weeks) may be valuable in specific clinical scenarios 6
Consider complementary sequences including T1-weighted and T2*-weighted gradient sequences based on specific findings, though these should be used judiciously with careful risk-benefit analysis 1
Genetic Evaluation
Offer amniocentesis or chorionic villus sampling for karyotype analysis and chromosomal microarray, as Chiari malformations can be associated with chromosomal abnormalities and genetic syndromes. 7, 6
Genetic testing is particularly important when additional structural anomalies are identified, as genetically transmitted disorders account for approximately one-third of cases with multiple anomalies 7, 6
Obtain complete family history to identify potential hereditary patterns or recurrence risks 6
Ongoing Surveillance Schedule
Perform serial ultrasound examinations every 2-4 weeks for growth monitoring, assessment of ventriculomegaly progression, and evaluation for development of additional complications. 8, 6
Monitor for progressive ventriculomegaly, which may require postnatal ventriculoperitoneal shunt placement and influences delivery planning 2
Assess for development of hydrops or other signs of deterioration that would necessitate earlier delivery 6
Evaluate amniotic fluid volume at each visit, as polyhydramnios can develop secondary to impaired fetal swallowing from brainstem dysfunction 8
Follow-up ultrasound at 32 weeks gestation is critical for reassessing findings, finalizing delivery planning, and coordinating multidisciplinary postnatal care 7, 6
Delivery Planning
Coordinate multidisciplinary consultation with pediatric neurosurgery, neonatology, and maternal-fetal medicine prior to delivery to establish a comprehensive postnatal management plan. 6
Cesarean delivery is not routinely indicated for Chiari malformation alone, but may be necessary based on obstetric indications or severe associated anomalies 6
Plan delivery at a tertiary care center with immediate access to pediatric neurosurgery and neonatal intensive care 6
Postnatal Confirmation
Obtain postnatal brain MRI with diffusion-weighted imaging within the first weeks of life to precisely characterize the malformation pattern and identify associated abnormalities that may not have been apparent on prenatal imaging. 7
Postnatal MRI provides definitive anatomic detail and serves as baseline for surgical planning and long-term developmental surveillance 7, 5
Implement developmental surveillance using standardized tools at regular intervals, as children with Chiari malformations and associated myelomeningocele are at increased risk for developmental disabilities 7
Critical Caveats
Do not rely solely on classic "lemon" and "banana" signs after 25 weeks gestation, as these findings frequently resolve or become less apparent in the third trimester despite persistent underlying pathology 2
Chiari I malformation with syringomyelia can rarely be detected prenatally as early as 18 weeks, presenting with abnormal fetal positioning and limited joint movement, though this is exceptional 4
The severity of posterior fossa findings on prenatal imaging correlates with the frequency of supratentorial abnormalities like tectal beaking, which can serve as an indicator of overall malformation severity 3