AIN

Acute Immune Neuropathies

AIDP
AMN
AMSAN
Demyelination
Active
Macrophage-mediated

Acute Immune Demyelinating Polyneuropathy (AIDP)

H&E stain

Endoneurium: Granular myelin remnants
Perineurium, Epineurium & Vessels: Normal

H&E stain

Gomori trichrome stain

Myelin remnants: More staining with VvG than Gomori trichrome

VvG stain

Large & Small Axons
Relatively preserved compared to myelin loss
Focal loss in some endoneurial areas

Neurofilament stain

Acid phosphatase stain

Histiocytic cells: Associated with myelin remnants

Acid phosphatase stain

CD163 stain

Stages of Macrophage (Histiocyte)-mediated Myelin Stripping (Active Demyelination) ^1,²

Histiocytes
- Attracted to myelinated axon
  - Initially associated with: Outside of Schwann cell basal lamina
  - May be preferentially located at
    - Internode
      - Common in AIDP or CIDP Subacute
    - Node of Ranvier
      - Not common inNeurofascin-155 or Contactin-1 antibody disorders
    - Axon (AMAN)
- Penetration through Schwann cell basal lamina
- Locate between basal lamina & myelin
- Histiocytes extend processes
  - Grow between layers of myelin (Between adjacent major dense lines)
- Phagocytosis & degradation of myelin
  - Generates myelin debris & lipid droplets within histiocytes
- Migration of histiocytes out of Schwann cell basal lamina
Demyelinated Axons
- No associated myelin
- Surrounded by Schwann cell basal lamina
- May be associated with
  - Non-myelinating Schwann cell processes
  - Histiocytes
- May become chronic

Macrophage
Adherent to Schwann cell basal lamina
Outside myelin sheath

Macrophage mediated demyelination: Macrophage process
Macrophage is inside Schwann cell basal lamina
Macrophage process extends under outer myelin layer (Arrows)
Also see; Active demyelination

Macrophage mediated demyelination: Macrophage process
Histiocyte processes: Extend between layers of myelin (Arrows)

Electron micrograph: From Robert Schmidt MD

Demyelination: Schwann cells containing Myelin & Lipid debris

Myelin debris: Multiple partially degraded myelin fragments in phagocytic cells

Toluidine blue stain

Lipid debris & droplets: Multiple rounded regions in phagocytic cells

Demyelinated Axons: Surrounded by cell processes

Electron micrographs: From Robert Schmidt MD

Demyelinated axons (Arrows)
Axons, without myelin, surrounded by phagocytic cells (Histiocytes)
Axon cytoplasm is dark, containing closely spaced neurofilaments.
Phagocytic cells contain myelin debris, lipid droplets & inclusions.

Demyelinated axons
Patially surrounded by Schwann cell basal lamina (Above, Arrow)
May be near cells containing myelin debris

Demyelinated axons

Demyelinated axon
Surrounded, in part, by Schwann cell basal lamina and some Schwann cell processes
A macrophage (Top; Right) is located outside the basal lamina