DMS333WI2021: Lemon Sign

Week 9: Forum/Discussion Board

Hello everyone,

As we all know, it is very important for us to not only get great pictures during OB scans but also look out for any abnormalities. One such abnormality that can get unnoticed if a sonographer is not paying attention is the lemon sign of the fetal skull. To recap, the fetal skull or calvarium should be smooth, bilaterally symmetrical, and usually of ovoid shape. The bone should be fairly echogenic and be continuous all around the circumference. We can see a normal skull in the picture below:

On the other hand, the lemon sign occurs when the frontal bones of the skull start to appear flattened or scalloped, giving the skull the shape of a lemon. The lemon sign has been linked to spina bifida for it is believed that a decrease in intraspinal pressure in fetuses with spina bifida causes the brain to shift downward. It is also a classic sign of Chiari II malformation.

The Lemon Sign | Radiology

https://radiopaedia.org/articles/lemon-sign-1?lang=us

https://pubs.rsna.org/doi/abs/10.1148/radiol.2281020448?journalCode=radiology

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Sometimes I've encountered fetal heads that I feel like I can't really tell if the shape is looking lemonish....are there any techniques out there that I can utilize if I'm pondering whether to call a lemon head sign or not? Great example btw.

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Hi Dr. Wilson,

Based on my research, I would say that the best way is to make sure that you are in a true axial plane, at the level of BPD. So for us new students it is very important to keep the anatomical landmarks in mind and try to find different windows so that we can see the calvarium all across and are not creating a false sign of pathology. We need to be mindful of shadowing and how different angles of ultrasound can change the appearance of structures. For lemon sign, the frontal indentation is bilateral and a lot of the times the baby will also have issues with the cerebellum where is it shaped like a banana and ventriculomegaly might be seen as well. There was an article I was reading that mentioned if baby is in the cephalic position then do coronal TV scan to check for cranial abnormalities (https://www.karger.com/Article/Fulltext/364806).Also, the lemon sign disappears in about the late second and third trimester, so if I had any doubt I would just document a possible lemon sign in my report and the patient can get rescanned just to make sure. I rather be safe than sorry. :)

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