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:


  • absent bony calvarium above the orbits
  • orbits well visualized
  • absence of supratentorial brain
  • residual brain
  • possibly spina bifida
  • polyhydramnios


  • 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


  • 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


  • calvarium present
  • decreased brain tissue
  • head circumference 2-3 standard deviations below that expected for the menstrual age


  • 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.