On obstetric ultrasound at 18 weeks, the 'lemon sign' and 'banana sign' are associated with which neural tube defect?
- A Open spina bifida (myelomeningocele) with Arnold-Chiari type II malformation ✓
- B Anencephaly
- C Encephalocele
- D Agenesis of corpus callosum
Explanation
In open spina bifida (myelomeningocele) with associated Arnold-Chiari type II malformation, the loss of CSF through the spinal defect causes downward herniation of the hindbrain, producing two cranial ultrasound signs: (1) Lemon sign — frontal bones appear indented or scalloped, giving the head a lemon shape on axial view (due to reduced intracranial pressure); (2) Banana sign — the cerebellum is pulled downward, wraps around the brainstem, and appears banana-shaped rather than crescent-shaped. These signs are detected on routine anomaly scan and prompt detailed assessment of the spine. Both signs normalise after in-utero repair.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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