The lemon sign

For my blog this week I wanted to learn more about spina bifida. Occasionally in the clinic we will see a case of spina bifida occulta when scanning a lumbar spine. However, I wanted to learn more about how more severe cases are diagnosed in utero. I found an interesting article which talked about the “lemon sign” on ultrasound. This sign is strongly associated with spina bifida. The image on the left shows the normal contour of a skull in an 18 wk fetus in utero. The image on the right is of a 20 week old fetus with spina bifida (note the lemon shape). The scans are transverse at the level of the ventricles. The lemon shape is caused by a loss of the normal shape of the frontal bones and it is suspected that a decrease in intracranial pressure causes the brain to shift downward, thus resulting in the depressed skull. For some reason the frontal bones are most sensitive to the pressure change. Interestingly this sign will disappear as the fetus matures so this sign is most useful in ultrasound before 24 weeks, especially in cases of open spina bifida. The lemon sign can also indicate other conditions but seeing this sign in an ultrasound prior to 24 weeks means the ultrasound tech should evaluate the spine more closely looking for other signs of spina bifida. In addition, the cranium needs to be evaluated for the 4 signs of spina bifida – ventriculomegaly, microcephaly, obliteration of the cisterna magna (w/absent cerebellum), and abnormal anterior curvature of the cerebellum.

Spina bifida occurs about seven times in every 10,000 live births. Folic Acid supplementation before and during pregnancy has been shown to decrease the risk of spina bifida in the fetus. Ultrasound or amniocentesis tests for spina bifida are usually ordered if alphafetaprotein (AFP) results are elevated in the mother’s second semester quadruple screen blood test. Effects of spina bifida on the fetus can be mild to severe and can include spinal abnormalities, hydrocephalus, paralysis, and urinary and/or fecal incontinence. Prenatal diagnosis is important, allowing the parents the option of in-utero surgery, surgery after birth, or pregnancy termination.

Images and References:

Spina Bifida Association. Accessed December 8, 2009 at http://www.spinabifidaassociation.org/site/c.liKWL7PLLrF/b.2642323/k.8E10/Spina_Bifida.htm

Thomas, M. (2003). The Lemon Sign. Radiology. RSNA Journals Online. Accessed December 8, 2009 at http://radiology.rsna.org/content/228/1/206.full