What transverse width defines a widened cavum septum pellucidum (CSP) on fetal ultrasound, and what is its clinical significance?

Widened Cavum Septum Pellucidum: Definition and Clinical Significance

A widened cavum septum pellucidum (CSP) is generally defined as a width >95th percentile for gestational age, which ranges from approximately 6-7 mm in the second trimester, though this varies by gestational week. 1, 2

Defining Abnormal CSP Width

The CSP width increases linearly throughout pregnancy and must be interpreted using gestational age-specific percentile values rather than absolute measurements:

• Between 15-28 weeks: The 95th percentile ranges from 3.7-7 mm depending on gestational week 1
• Mean CSP width in normal fetuses: Approximately 4.5 mm (range 1.8-7.4 mm) in the second trimester 2
• CSP width correlates with both gestational age and biparietal diameter (BPD): CSP width = 0.658 + (0.064 × BPD in mm) 2

The CSP should always be visualized between 18-37 weeks of gestation (or with BPD 44-88 mm) in normal fetuses 3. Absence before 18 weeks or after 37 weeks is a normal finding.

Clinical Significance

Association with Aneuploidy

A widened CSP has significant associations with chromosomal abnormalities, particularly trisomy 18:

• Trisomy 18: CSP width is above the 95th percentile in 92% of cases, with mean width of 7.9 mm (range 3.5-12.8 mm) 2
• Trisomy 21: CSP width is above the 95th percentile in 42% of cases, with mean width of 5.7 mm (range 2.8-10.5 mm) 2
• Trisomy 13: CSP width is above the 95th percentile in 37.5% of cases 2

Key Management Principles

When a widened CSP is identified, the critical distinction is whether it is isolated or associated with other anomalies:

1. With other structural anomalies: The risk of aneuploidy is substantially elevated, warranting genetic counseling and consideration of diagnostic testing 2, 4

2. Isolated widened CSP (no other structural or chromosomal abnormalities):

   • Appears to have normal neurodevelopmental outcome 5, 6
   • Should prompt detailed anatomic ultrasound examination to exclude associated anomalies 2
   • Consider aneuploidy screening if not previously performed
   • Postnatal developmental follow-up is recommended 6

Important Caveats

• CSP width/BPD ratio may be a more useful marker than absolute width, with a cutoff of 0.081 showing 83% sensitivity and 61% specificity for aneuploidy detection 4
• MRI should be offered when absence of CSP is suspected on ultrasound, as many cases represent narrow CSP rather than true absence—a distinction with prognostic implications 5
• The finding of a widened CSP should trigger a comprehensive search for associated brain anomalies and other structural defects, as isolated findings carry a much better prognosis

In summary: Use gestational age-specific percentiles (>95th percentile = widened), distinguish isolated from non-isolated cases, and recognize that isolated widened CSP without aneuploidy appears benign with normal neurodevelopmental outcomes.