What causes the banana sign on prenatal ultrasound?
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Answer:
The banana sign is a cerebellum that is flattened, curved and wrapped tightly around the brainstem, with effacement or complete obliteration of the cisterna magna, seen on the axial transcerebellar view at prenatal ultrasound. It is a hallmark of the Chiari II malformation and therefore of open spinal dysraphism, and it is one of the two cranial markers that make the diagnosis reliable even when the spinal defect itself is not directly visualised. It can be present from 12-14 weeks and persists through the second trimester. Its companion, the lemon sign, is bifrontal scalloping of the skull, and the two behave differently: the lemon sign usually resolves after about 24 weeks as the calvarium ossifies, whereas the banana sign reflects a structural posterior fossa deformity and persists. After successful in utero repair of a myelomeningocele, hindbrain herniation reverses in the majority of fetuses, so the banana sign may normalise on follow-up imaging, in contrast to postnatal repair.
Why is it called so?
The flattened cerebellum curving around the brainstem takes the shape of a banana.
Pathophysiology
In open spinal dysraphism, cerebrospinal fluid leaks continuously from the exposed neural placode, so the fetal ventricular system never becomes adequately distended. The developing posterior fossa fails to expand to its normal volume and remains small. The cerebellum, which continues to grow, is therefore compressed within an undersized posterior fossa: it is displaced caudally through the foramen magnum and is flattened and moulded around the brainstem, obliterating the cisterna magna. Reduced intracranial pressure similarly allows the soft frontal bones to indent, producing the lemon sign. Closing the spinal defect in utero stops the CSF leak and allows the posterior fossa to develop, which is why the hindbrain findings can reverse.
Alternative names: Banana cerebellum
Other associated named signs: Lemon sign, obliterated cisterna magna, ventriculomegaly, and the intracranial translucency sign at 11-13 weeks
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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