Lecture Notes

Importance of Cavum Septi Pellucidi in Fetal Imaging

• The cavum septi pellucidi (CSP) is a key imaging landmark:
  • Indicates normal midline prosencephalic (forebrain) development.
  • Reassuring if clearly visualized on fetal ultrasound.
  • Helps exclude many complex cerebral malformations.

Clinical Significance of Absent CSP

• Absence of CSP is a significant marker suggesting potential cerebral anomalies:
  • Often prompts fetal MRI for further evaluation.
• Caution on ultrasound:
  • Paired fornix columns may mimic CSP, causing false reassurance.

Cerebral Anomalies Commonly Associated with Absent CSP

• Holoprosencephaly spectrum
  • Alobar, semi-lobar, lobar forms.
• Septo-optic dysplasia (De Morsier syndrome)
  • Absent CSP, optic nerve/chiasm hypoplasia, hypothalamic-pituitary dysfunction.
• Corpus callosum agenesis/dysgenesis
  • Commonly associated with anterior callosal agenesis.
• Severe hydrocephalus (aqueductal stenosis)
  • Compression or absence of CSP due to enlarged ventricles.
• Schizencephaly
  • Typically involves frontoparietal cortical clefts.

Embryological and Anatomical Considerations

• Septum pellucidum forms with the anterior corpus callosum.
  • Absence of CSP suggests anterior callosal malformation.
  • Hippocampal commissure may mimic corpus callosum when anterior segment is absent.

Septo-optic Dysplasia (SOD) – De Morsier Syndrome

• Triad:
  1. Absent CSP
  2. Optic nerve/chiasm hypoplasia or dysplasia
  3. Hypothalamic-pituitary dysfunction
• Prenatal diagnosis is difficult:
  • Spectrum from mild vision loss to severe endocrine issues.
  • Severity not always predictable antenatally.
• Postnatal assessment includes:
  • Endocrine function testing.
  • Ophthalmological evaluation.
• Differentiate from mild holoprosencephaly:
  • SOD: Thalami normally separated.
  • HPE: Partial or full thalamic fusion.
• Low maternal estriol may support prenatal suspicion of SOD.

Truly Isolated Absence of CSP

• Rare and difficult to confirm antenatally.
• Postnatal MRI required for accurate diagnosis.
• Neurodevelopmental prognosis is mixed:
  • Some cases have normal outcomes.
  • Others show significant developmental challenges.

Imaging Pitfalls & Differential Diagnosis

• Ultrasound:
  • Fornix columns can mimic CSP.
• MRI:
  • More reliable to detect associated structural abnormalities.
  • Detects anomalies missed on ultrasound.

Summary of Disorders & Prognosis Associated with CSP Absence

• Holoprosencephaly: Fused thalami, callosal anomalies – severity determines outcome.
• Septo-optic dysplasia: Optic nerve hypoplasia, normal thalami – variable visual/endocrine effects.
• Corpus callosum agenesis: Colpocephaly, high third ventricle – variable prognosis.
• Aqueductal stenosis: Ventriculomegaly, absent CSP – depends on hydrocephalus.
• Schizencephaly: Cortical cleft to ventricle – outcome depends on cleft extent.
• Isolated CSP absence: No other anomalies – often favorable outcome.

Clinical Management Recommendations

• If CSP is absent on prenatal scan:
  • Fetal MRI is recommended.
  • Assess endocrine and visual function early postnatally if SOD suspected.
• Provide clear parental counseling on outcome variability.