Differential diagnosis of the 'Lemon Sign' in the fetal Skull
·Meningomyelocele- The ‘lemon sign’ describes seal- loping of the fetal frontal bones in axial view at the level of BPD. When seen with meningomyelocele it is usually associated with Arnold—Chiari type II or III malformation with an abnormal cerebellar position.
·Normal fetus- The incidence quoted varies from 0.66 to 1.3%. No associated intracranial abnormality is apparent, i.e. normal sized ventricle and normal cerebellum. The spine is normal.
·Encephalocele- This is a part of the spectrum of neural tube defects and is often associated with type II or III Arnold—Chiari malformation. The majority of these are occipital in location. Rarely they may occur in the frontal region where they themselves may mimic a ‘lemon sign’.