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:
- Chiari II malformation
- spina bifida
- Dandy Walker malformation
- thanatophoric dysplasia
- cystic hygroma
- diaphragmatic hernia
- corpus callosal agenesis
- fetal hydronephrosis
- umbilical vein varix
It is also associated with the banana sign.
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:
- significant anterior angulation for obtaining images of the calvaria should be avoided as fetal orbits could simulate a lemon sign
- this sign usually disappears >24 weeks, which may be due to the reduced pliability of the fetal calvaria with advancing gestational age or an increase in the 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
For an abnormal head shape resembling the lemon sign, one should also consider craniosynostosis, most notably of the bicoronal (or bilateral coronal) type.