What are the diagnostic criteria for closed neural tube defects in fetal ultrasound?

Diagnostic Criteria for Closed Neural Tube Defects in Fetal Ultrasound

The primary diagnostic criteria for closed neural tube defects (NTDs) in fetal ultrasound include assessment of the conus medullaris position, as this is the most specific marker for closed spinal dysraphism and significantly improves detection rates. 1

Understanding Neural Tube Defects

• Neural tube defects result from failure of the embryonic neural tube to close properly by approximately 4 weeks' gestation, with approximately 20% of these defects being closed (having skin covering) 2
• Clinical consequences depend on the site and severity of the defect, with closed NTDs generally having better outcomes than open NTDs 2
• Most cases are sporadic with multifactorial inheritance, though some may be associated with specific genetic disorders or environmental factors such as anticonvulsant medications (e.g., valproate) and inadequate maternal folate intake 2

Diagnostic Approach for Closed Neural Tube Defects

Primary Ultrasound Markers

• Conus medullaris position assessment: Abnormal low-lying position of the conus medullaris is the most specific marker for closed spinal dysraphism 1
• Intact skin covering: Unlike open NTDs, closed defects maintain skin coverage over the affected area 2
• Subtle spinal anomalies: May include lipomas, dermal sinuses, or tethered cord that require careful examination 3

Advanced Imaging Techniques

• Three-dimensional ultrasound: Using multiplanar views can achieve diagnostic accuracy within one vertebral body in approximately 80% of patients with spinal defects 3
• Fetal MRI: Should be considered as an adjunct to ultrasound when closed NTD is suspected, as it can:
  • Provide detailed topography and contents of NTD sacs 4
  • Detect additional CNS abnormalities not apparent on ultrasound 4
  • Change or modify management decisions in approximately 21% of cases 4

Differential Diagnostic Features

• Closed vs. Open NTDs: Closed NTDs typically lack the classical cranial signs associated with open spinal defects (no "lemon" shaped head or "banana" cerebellum) 3
• Gestational age at detection: Closed spinal dysraphism is typically detected later (median 22.6 weeks) compared to cranial dysraphism (13.3 weeks) or open spinal dysraphism (22.0 weeks) 1

Prognostic Implications

• Closed spinal dysraphism without other anomalies has a significantly higher live-birth rate (100%) compared to those with associated anomalies (33.3%) 1
• Careful assessment of lesion level is important for prognosis, with a "watershed" between L3 and L4 marking a high probability of being wheelchair-bound with higher lesions 3

Screening Algorithm

1. First trimester: Basic anatomical screening to rule out major cranial defects 1
2. Second trimester: Detailed assessment including:
   • Systematic evaluation of the entire spine in multiple planes 3
   • Assessment of conus medullaris position 1
   • Evaluation for associated anomalies 1
3. Follow-up imaging: Consider MRI when ultrasound findings are suspicious or inconclusive 4

Common Pitfalls and Caveats

• Closed NTDs are more difficult to detect than open defects and may be missed on routine screening 1
• Isolated assessment of maternal serum AFP is insufficient for diagnosis of closed NTDs as levels may not be elevated 2
• The absence of classical cranial signs (lemon and banana signs) does not rule out a closed neural tube defect 3
• Comprehensive assessment should include evaluation for other associated anomalies that may affect prognosis and management decisions 1