Absent Cavum Septi Pellucidi

Anne Kennedy, MD

DIFFERENTIAL DIAGNOSIS

Common

Less Common

Holoprosencephaly Spectrum
- Alobar Holoprosencephaly
- Semilobar Holoprosencephaly
- Lobar Holoprosencephaly

Rare but Important

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Scan technique is very important
- Cavum septi pellucidi (CSP) should be the landmark for measurement of biparietal diameter and head circumference
- Coronal images also helpful if fetal head position makes correct axial plane difficult
  - CSP seen between frontal horns inferior to corpus callosum
Cavum septi et vergae is an anatomic variant that may cause confusion
- Cavum vergae is posterior continuation of CSP, usually obliterates posterior to anterior so not seen in standard 18-20 week scan
- If present → elongated, larger, fluid-filled space which may be confused with interhemispheric cyst
- No other abnormalities present
Normal CSP signifies normal midline brain development
- If absent, significant malformation may be present though not immediately apparent

Helpful Clues for Common Diagnoses

Incorrect Scan Plane
- If scan plane too high or too low CSP not seen
- Fornices are normal structures which create parallel echoes inferior to normal location of CSP
- CSP appears as a fluid-filled “box”: White line/black space/white line appearance
- Fornices: Series of parallel black and white lines without intervening fluid-filled space
Agenesis of the Corpus Callosum
- Mild ventriculomegaly or colpocephaly
- Lateral ventricles are parallel instead of divergent
- Coronal images show “steer horn” appearance of frontal horns
- Lack of normal anterior cerebral artery branch pattern into callosomarginal and pericallosal arteries on midline sagittal view
- Stenogyria: Radiating “sunburst” pattern of medial cerebral gyri
- MR very helpful to look for additional brain malformations
  - Heterotopia, lissencephaly, gyral abnormalities
Chiari II Malformation
- Ventriculomegaly with “boxy” appearance of frontal horns
- “Banana” sign: Cerebellar prolapse into foramen magnum causes cerebellum to curve around brainstem
- Obliteration of cisterna magna
- “Lemon” sign: Bifrontal concavity
Severe Hydrocephalus
- Severe hydrocephalus leads to “blown out cavum”
  - Fenestrations appear in leaves of septum secondary to elevated cerebrospinal fluid pressure
  - Eventually leaves of cavum may be so thinned as to be invisible or torn
- Most likely to occur with aqueductal stenosis

Helpful Clues for Less Common Diagnoses

Alobar Holoprosencephaly
- Supratentorial brain without division into cerebral hemispheres
- “Rind”, “cup”, or “pancake” of cerebral tissue associated with monoventricle ± dorsal cyst
- Head may be large if large dorsal cyst, or small and rounded in shape
- Associated with severe facial malformation
- Look for stigmata of trisomy 13
Semilobar Holoprosencephaly
- Some division into cerebral hemispheres posteriorly but fusion anteriorly
- Head shape often round
- Look for stigmata of trisomy 13 or 18
Lobar Holoprosencephaly
- Two cerebral hemispheres are formed
- May be single gyrus in continuity across midline
- Midline fusion anomalies seen
  - Fused fornices create a round “mass” in the third ventricle

Helpful Clues for Rare Diagnoses

Septo-Optic Dysplasia
- Absent CSP associated with variable hypothalamic pituitary dysfunction and visual impairment
- Mild dilatation of frontal horns
- “Flat top” appearance to frontal horns
- MR used to exclude additional malformations
  - Optic nerves and chiasm beyond resolution of fetal MR at present
- Diagnosis is confirmed by clinical and ophthalmological evaluation of infant
Schizencephaly
- Cortical defect extending from ventricular surface to pia
- May be “closed” or “open”, unilateral or bilateral
- Size of defect varies from very small to giant, involving most of frontoparietal cortex
- Lateral ventricle on side of defect is distorted → tenting toward area of parenchymal loss
- Bilateral giant open lip schizencephaly looks very similar to hydranencephaly
  - Key observation is that walls of a schizencephalic cleft are lined by grey matter
  - Important distinction as hydranencephaly is lethal
  - Giant open lip schizencephaly is associated with neurological impairment but is not necessarily lethal
Syntelencephaly
- Considered variant of holoprosencephaly spectrum by some
- Two hemispheres are present but with some fusion of the dorsal aspect of cerebral hemispheres
  - Most common site of fusion is posterior frontal lobe

Other Essential Information

Absent CSP is often the tip-off for underlying brain abnormality
Agenesis of corpus callosum is associated with many other brain malformation as well as innumerable syndromes
- Counsel parents that outcome highly variable
Septooptic dysplasia associated with visual impairment/blindness as well as potential hypothalamic pituitary dysfunction