Sonolucent Skulls
Diffuse demineralization of the skull almost always occurs with fetal skeletal dysplasia syndromes. In the case of severe demineralization of the bony calvarium, the cranium is thin without an acoustic shadow and so poorly ossified that the intracranial structure can easily be seen. This increased visualization of the intracranial structures may be confused with such abnormalities as exencephaly due to acrania or acalvaria. Unlike exencephaly, however, there is an intact but poorly ossified skull. Careful scanning reveals concomitant limb anomalies.
Fig 1, Fig 2
Differential Diagnosis
The main differential diagnoses of the sonolucent skull are as follows:
- osteogenesis imperfecta (most common)
- hypophosphatasia (rare)
- achondrogenesis type I (rare)
- acalvaria
Fig 1: Hypophosphatasia Cross-sectional scan of skull: thin and sonolucent calvarium (arrow = falx cerebri)
Fig 2: Compressible skull Poorly ossified skull associated with osteogenesis imperfecta is easily compressed (arrow) and the cerebral gyri is clearly visualized
Video clips of sonolucent skulls
Sonolucent skull: The thin skull is so poorly ossified that cerebral gyri and sulci could be seen easily
Acalvaria: The head with intact skin but no skull, the gyri and sulci of brain can easily be seen
Differential diagnosis
The differential diagnoses and some sonographic features of skull defects are as follows:
Anencephaly:
- absent bony calvarium above the orbits
- orbits well visualized
- absence of supratentorial brain
- residual brain
- possibly spina bifida
- polyhydramnios
Excencephaly
- calvarium absent
- disorganized supratentorial brain tissue
Amniotic band syndrome (ABS)
- asymmetric or bizarre cephalocele
- other deformities, including limb amputation
Limb-body wall complex
- asymmetric or bizarre encephaloceles
- similar to ABS except fetal body adherent to placenta
- usually more severe than ABS
- associated bizarre defects of fetal body
Cephalocele
- midline defect
- extracranial cyst or brain tissue
- possibly ventriculomegaly
- lemon sign may be present
Open neural tube defects
- lemon sign
- banana sign
- mild ventriculomegaly
- spinal defect
Fetal demise
- overlapping sutures (Spalding’s sign)
- poor visualization of intracranial structures
- associated findings of fetal demise
Microcephaly
- calvarium present
- decreased brain tissue
- head circumference 2-3 standard deviations below that expected for the menstrual age
Craniosynostosis
- complete or partial
- deformed skull
- possibly microcephaly
- abnormal cephalic index.
In addition, considerable overlap may be found in these features among abnormalities. For instance, the lemon sign was originally described with opened NTD, however, it may not be present in an opened NTD identified in the third trimester of pregnancy, and a mild lemon sign may be identified in normal fetuses.