PERIPHERAL NERVE: Chronic Demyelination

Active demyelination
Onion bulbs
Segmental demyelination
Subperineurial edema
Teased fibers


Myelin Loss

Neurofilament stain
Large & Small axons: Present; Reduced numbers

VvG stain
Myelin: Mostly absent

Toluidine blue stain
Few well myelinated axons

Toluidine blue stain
Demyelinated large axons, Scattered (Arrow)

Toluidine blue stain
    Onion bulb, Early (Arrow): Reduplicated basal lamina

Segmental demyelination

Teased fibers
  Myelination is absent or thin in
  some internodes but not others

P0-Silver stain

Myelin pathology: Multifocal motor neuropathy
  Top: Normally myelinated motor axon in muscle
  Bottom: Segmental demyelination of a motor axon in Multifocal motor neuropathy

Hypomyelinated Axons

From child with Dejerine-Sottas


Early onion bulb formations
Note the few extra layers of basal
lamina around several thinly
myelinated larger axons.

Electron micrograph: From Robert Schmidt MD

Toluidine blue

Toluidine blue stains of plastic nerve sections

Late, large onion bulbs
Abundant connective tissue
around thinly myelinated axons.
Onion bulbs may contain 0, 1,
or several axons.
Large onion bulbs
Several layers of basal lamina,
connective tissue & Schwann cells
around thinly myelinated axons.

Onion Bulbs: Obsolete
Onion bulbs with multiple layers of Schwann cells & basal lamina: Associated axon is lost

Electron micrographs: From Robert Schmidt MD

Chronic Immune Demyelinating Polyneuropathy (CIDP)

Remaining axons
  Thinly myelinated
  Reduced numbers
Onion bulbs: Small
Subperineurial edema

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