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.
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
- 1. Oh SJ. Color Atlas of Nerve Biopsy Pathology. CRC Press. (2001) ISBN:1420039806. Read it at Google Books - Find it at Amazon
- 2. Duggins AJ, McLeod JG, Pollard JD et-al. Spinal root and plexus hypertrophy in chronic inflammatory demyelinating polyneuropathy. Brain. 1999;122 ( Pt 7) (7): 1383-90. doi:10.1093/brain/122.7.1383 - Pubmed citation
- 3. Tracy JA, Dyck PJ, Klein CJ, Engelstad JK, Meyer JE, Dyck PJB. Onion-bulb patterns predict acquired or inherited demyelinating polyneuropathy. (2019) Muscle & nerve. 59 (6): 665-670. doi:10.1002/mus.26452 - Pubmed
- 4. Cellerini M, Salti S, Desideri V, Marconi G. MR imaging of the cauda equina in hereditary motor sensory neuropathies: correlations with sural nerve biopsy. (2000) AJNR. American journal of neuroradiology. 21 (10): 1793-8. Pubmed