The Lemon Sign

The lemon sign refers to the shape of the fetal skull at ultrasonography (US) when the frontal bones lose their normal convex contour and appear flattened or inwardly scalloped. This gives the skull a shape similar to that of a lemon. The sign is seen on transverse sonograms of the fetal cranium obtained at the level of the ventricles.

Transverse cranial sonogram of a 20-week-old fetus with spina bifida. Image obtained at the level of the ventricles demonstrates the lemonlike configuration of the fetal skull due to biconcavity (arrows) of the frontal bones.

            Transverse cranial sonogram of an 18-week-old fetus demonstrates the normal contour of a fetal skull.                       

 The lemon sign has a strong association with spina bifida. Although the exact pathogenesis is unknown, it has been postulated that the decrease in the intraspinal pressure in neonates with spina bifida causes the brain to shift downward. This shift decreases the intracranial pressure, which is reflected onto the fetal cranium. The frontal bones are the most vulnerable to the decreased intracranial pressure and respond by flattening or scalloping inward.

As the fetus matures, the lemon sign disappears because the frontal bones become stronger and are able to withstand the decreased pressure. In addition, the majority of neonates with spina bifida develop hydrocephalus as they mature. This increase in intracranial pressure can lead to reversal of the flattening. However, this theory does not explain why the lemon sign is present in fetuses with a normal posterior fossa.

Therefore, an alternative theory has been proposed that the lemon sign might be due to a primary skeletal developmental disorder and that the contour of the skull is a result of mesenchymal dysplasia of the cranium.      

The lemon sign is very useful in the detection of spina bifida in a high-risk population before 24 weeks of gestation. The lemon sign is not exclusive to spina bifida. It has been seen in a variety of conditions such as encephalocele, Dandy-Walker malformation with encephaloceles, thanatophoric dysplasia, cystic hygroma, diaphragmatic hernia, agenesis of the corpus callosum, fetal hydronephrosis, and umbilical vein varix and two-vessel cord.

Detection of the spinal defect by using prenatal US can be very difficult and depends on the experience and skill of the sonographer. The spinal defect may be detected approximately 80% of the time when the examination is performed by a highly qualified sonographer who is carefully evaluating the spine. In contrast, the sensitivity for detection of a spinal lesion is lower than 50% when US is performed in a low-risk population, by an inexperienced sonographer, or by using less-advanced equipment.

            Longitudinal sonogram of a fetal spine demonstrates a spinal defect (arrow) covered by membrane. 


                    

The lemon sign is a useful tool to aid in the detection of spina bifida. Detection of the lemon sign does not require the high level of skill that is needed for US evaluation of the spine. If the lemon sign is present, this should signal the possibility of spina bifida and should prompt the sonographer to look for other cranial markers of spina bifida and to perform a more detailed evaluation of the spine.