LEMON SIGN

Frontal bone convacity due to inward scalloping of the frontal bones of the calvarium such that the head appears shaped like a lemon. It may be visualized when the spinal defect is small and difficult to see.

• Scalloping of frontal bones seen on axial view at the level of BPD.
• Seen in 98% of fetuses prior to 24 weeks and 13% after 24 weeks.
• Positive predictive value 81-84%.
• False positive rate 1%.
• Lack of the lemon sign in the third trimester is thought to be due to strengthening of the fetal skull rather than increasing hydrocephalus.
• May be present in 1% of fetuses with an open spinal defect.
• Link to open spinal defects.

Concave Contour	Flattened Contour
Temporal area widest part of cranium. Almost always pathological. Seen earlier in the second trimester.	Cranium sides more flattened. Seen at a later stage. May be seen in pathological or normal pregnancy.

True lemon sign
Lemon Skull- Normal variant Normal cisterna magna. Normal cerebellum. No ventriculomegaly.
	False Positive Open Spinal Defect: Lemon sign (normal variant). Appearance of hanging choroid plexus due to non-visualization of the medial wall of the lateral ventricle. Normal cerebellum and cisterna

PATHOGENESIS

1. ? arise by same mechanism that produces the Arnold Chiari II (ACII) malformation.

Spinal cord tethered by meningomyelocele
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Growth and elongation of fetal spine
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Downward displacement of caudal intracranial structure
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Decreased pressure in subarachnoid space
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Decreased pressure in calvarium
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Indrawing of pliable frontal bones Þ LEMON SIGN

As the gestation progresses: Progressive hydrocephalus and increased intracranial pressure combined with thickening of calvarium results in resolution of the sign in late 2nd or 3rd trimester.

2. Due to a mesenchymal dysplasia of cranium (pressure of the classical craniolacunia) i.e. primary skeletal developmental dysplasia.

Initially thought to be highly specific for Arnold Chiari II malformation, as it is not seen in any of the form of hydrocephalus, however:

1. Seen in the 1-2% N fetuses.
2. Seen in cephalocele (33%).
3. Seen in diffuse anomalies.