Anatomic Pathology: Muscle and Peripheral Nerve Pathology

• The lesion called “onion-bulb” is produced through repetitive cycles that build up the concentric array of flattened Schwann cell processes and their basement membranes.

• Animal models have demonstrated that a single demyelinating/remyelinating event is insufficient to create onion bulbs, whereas recurrent and continuing demyelinating insults can produce onion bulbs in great numbers.

• Human diseases in which demyelination is occurring constantly with remyelination for prolonged periods often show onion bulbs on biopsy. These diseases include inherited hypertrophic neuropathies (Charcot-Marie-Tooth disease type I) and chronic inflammatory demyelinating polyradiculoneuropathy.

• By observations in human disease and experimental models, the presence of onion bulbs suggests that a demyelinating process has been active for at least several months. The longer the duration of the demyelinating process, the more onion bulbs are seen.

• Although onion bulbs are far more likely seen in diseases of primary demyelination, demyelination secondary to axonal disease may also cause onion bulb formation. This occurrence depends on the chronicity of the process and on how many times the axon sheds its myelin sheath before dying.