Spina bifida (neural tube defect)

Definition

Pathophysiology and typical signs

• The pathomechanism of open spina bifida is a loss of cerebrospinal fluid through the defect, which leads to a drop in intracranial pressure.
• As a result, the cerebellum is pulled caudally, which is referred to as Arnold-Chiari malformation type II.
• The lemon sign, a lemon-shaped skull contour, is typically best seen between 16 and 24 weeks and often disappears again in the third trimester.
• In the banana sign, the cerebellum is deformed into a banana shape and the cisterna magna is obliterated.

Diagnosis

Further diagnostics and early markers

• Further sonographic indicators of open spina bifida include ventriculomegaly and a small biparietal diameter.
• The most important early marker in the first trimester is the absent or narrowed intracranial translucency.
• Direct detection is achieved by visualising the defect in the longitudinal and transverse sections of the back.

Typical imaging findings

• Ultrasound: The cerebellum is not dumbbell-shaped but curved forwards and banana-like in shape, wrapping around the brainstem. The cisterna magna is obliterated and no longer definable.
• Ultrasound: The skull contour is not oval but indented bilaterally at the frontal bones, so that in cross-section the head takes on the shape of a lemon. The arrow marks the indentation; the remaining head circumference is appropriate for gestational age.

Keep learning in the app

Further reading (selection)

1. StatPearls: Spina Bifida (NCBI Bookshelf)
2. DocCheck Flexikon, Spina bifida

Cross-references