What causes Lemon Sign in fetal skull on ultrasound?
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Answer:
The Lemon Sign is caused by abnormal inward scalloping or flattening of the frontal bones in the fetal skull, most commonly associated with open spina bifida (myelomeningocele) and Chiari II malformation, though it can occur with other non-neural tube defects; it is seen in 64-89% of cases, particularly before 24 weeks gestation, and often accompanies ventriculomegaly (82% of cases).
Why is it called so?:
The sign is named for the lemon-like contour of the fetal skull on transverse axial ultrasound views, resulting from the flattened or concave appearance of the frontal bones that mimics the shape of a lemon.
Pathophysiology:
In spina bifida with Chiari II malformation, traction on the brainstem and cerebellum by the tethered spinal cord causes posterior fossa overcrowding and reduced pressure in the supratentorial subarachnoid spaces; this leads to collapse of these spaces, eversion of the frontal bones, and their characteristic inward bowing or scalloping.
Alternative names: None
Other associated named signs: Banana sign (cerebellar vermis dysmorphology)
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