Abnormal peripheral myelination Onion bulb formation Repeated episodes of segmental demyelination and remyelination lead to the accumulation of supernumerary Schwann cells around axons, which is referred to as onion bulb formation. This finding affects peripheral nerves. This feature is characteristic of chronic demyelinating neuropathies as well as certain hereditary neuropathies. Onion-bulb formation may be observed in electron microscopic examinations of peripheral nerve biopsy material. Often, this examination is performed on the sural nerve. Onion bulb formations HP:0007084 UMLS:C1847906 HP:0007013 HP:0007022 HP:0008276 HP:0003389 HP:0003383