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Onion bulb formation

Last revised by Dr Francis Deng on 05 May 2020

Onion bulb formation is a histopathologic finding in hypertrophic neuropathies such as Charcot-Marie-Tooth type 1A (CMT1A) and chronic inflammatory demyelinating polyneuropathy (CIDP).


Onion bulb formations are concentric layers of Schwann cell processes and collagen surrounding axons 1,3. They are the result of repeated cycles of demyelination and remyelination 3. These formations in microscopic cross-section resemble the bulb of an onion.

Radiographic features


Onion bulb formation in the hypertrophic neuropathies correlates with the imaging findings of nerve hypertrophy (bulbous thickening) with the following signal characteristics 2,4:

  • T1 C+ (Gd): usually demonstrate enhancement
  • T2: usually hyperintense

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