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Lemon sign

The lemon sign, noted on antenatal imaging, is one of the many notable fruit inspired signs. It is a feature when there appears to be an indentation of the frontal bone (depicting that of a lemon). It is classically seen as a sign of a Chiari II malformation and also seen in the majority (90-98%) of fetuses with spina bifida.



The following conditions are associated with the lemon sign: 

It is also associated with the banana sign

Radiographic assessment

The lemon sign is seen on axial imaging (usually antenatal ultrasound, although antenatal MRI will also demonstrate this sign) through the head and relates to concavity (not just flattening) of the frontal bones. 

Several diagnostic points should be remembered about this sign: 

  • when obtaining images of the calvarium, the transducer should not be angled downward anteriorly, as the fetal orbits may simulate the lemon sign
  • this sign is seen more often in fetuses less than 24 weeks and may not be present in older fetuses (usually disappears after 24 weeks 4)
  • this may be due to the decreased pliability of the fetal calvarium with advancing gestational age or the increased intracranial pressure with associated hydrocephalus 
  • this sign may be rarely seen in normal patients (~1% of cases) and in those with other non-neural axis abnormalities.

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Inspired signs

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