What is an onion bulb formation?
Onion bulb formation is the pathologic hallmark of the hypertrophic neuropathies, in which repeated segmental demyelination and remyelination have occurred.
When viewed in transverse sections, onion-bulb formations are multiple concentric layers of intertwined, attenuated Schwann cell processes surrounding the remaining nerve fibers.
The Schwann cell processes are separated from each other by layers of collagen fibers.
The onion bulb formation may be seen in any condition with chronic segmental demyelination and remyelination but are frequently seen in
- Charcot–Marie–Tooth (CMT) disease,
- Dejerine–Sottas syndrome,
- Refsum’s disease, and
- chronic relapsing idiopathic (inflammatory) demyelinating neuropathy