Fetal spina bifida: intracranial 'lemon' and 'banana' signs (ISUOG)
Intracranial markers of open spina bifida
Direct visualization of the spinal defect in spina bifida may be difficult; therefore, indirect intracranial signs — 'lemon' and 'banana' — have become particularly important in clinical practice. Their assessment in 130 fetuses with open spina bifida was performed in the study by Van den Hof MC, Nicolaides KH, Campbell J, Campbell S (Am J Obstet Gynecol 1990; 162: 322-327), which is referenced in the ISUOG Practice Guidelines for routine ultrasound examination in the second trimester (2022).
Lemon sign
The lemon sign describes the characteristic fetal skull shape on transverse scanning of the head. Precise measurement criteria and the gestational age at which the sign retains diagnostic significance are not detailed in the provided fragments [clarification needed].
Banana sign
The banana sign reflects changes in the structures of the posterior cranial fossa. The specific morphological description is not provided in the available fragments [clarification needed].
Early markers in the first trimester (11–13 weeks)
In addition to the classic second trimester signs, several additional markers of open spina bifida have been proposed for screening at 11–13 weeks:
| Marker | Source |
|---|---|
| Assessment of intracranial translucency and posterior brain | Chaoui R, Nicolaides KH. Ultrasound Obstet Gynecol 2011; 38: 609-612 |
| 'Crash sign' — first trimester sonographic marker | Ushakov F et al. Ultrasound Obstet Gynecol 2019; 54: 740-745 |
| Choroid plexus to head size ratio | Chaoui R et al. Ultrasound Obstet Gynecol 2020; 55: 81-86 |
| Biparietal diameter to transverse abdominal diameter ratio < 1 | Simon EG et al. |
| Small biparietal diameter | Karl K et al. Ultrasound Obstet Gynecol 2012; 40: 140-144 |
The choice of scanning plane (midsagittal or axial) for assessment of intracranial translucency and posterior brain in the first trimester remains controversial (Chaoui R, Nicolaides KH, 2011).
Differentiation of open and closed forms
Prenatal diagnosis of open and closed spina bifida was addressed by Ghi T, Pilu G, Falco P et al. (Ultrasound Obstet Gynecol 2006; 28: 899-903). Details of differentiation between the forms are not provided in the available fragments [clarification needed].
Frequently asked questions
On what sample were the 'lemon' and 'banana' signs validated?
On 130 fetuses with open spina bifida in the study by Van den Hof MC, Nicolaides KH, Campbell J, Campbell S (Am J Obstet Gynecol 1990; 162: 322-327).
What early markers of spina bifida are available at 11–13 weeks?
Assessment of intracranial translucency and posterior brain (Chaoui, Nicolaides 2011), 'crash sign' (Ushakov et al. 2019), choroid plexus to head size ratio (Chaoui et al. 2020), small biparietal diameter (Karl et al. 2012), and biparietal diameter to abdominal diameter ratio < 1 (Simon et al.).
Which plane is preferable for assessment of the posterior brain in the first trimester?
The choice between midsagittal or axial plane is discussed in the study by Chaoui R, Nicolaides KH (Ultrasound Obstet Gynecol 2011; 38: 609-612); a definitive conclusion is not provided in the available fragments [clarification needed].
What does a biparietal diameter to transverse abdominal diameter ratio < 1 signify?
This is a potential marker of open spina bifida during screening at 11–13 weeks (Simon EG et al.).
Where is the differentiation of open and closed spina bifida addressed?
In the study by Ghi T, Pilu G, Falco P et al. (Ultrasound Obstet Gynecol 2006; 28: 899-903); specific criteria are not detailed in the provided fragments [clarification needed].