- Cutaneous amyloidosis may be either secondary to systemic disease or localized to the skin.
- Various types of primary cutaneous amyloidosis exist, the most common being macular amyloidosis, lichenoid amyloidosis and biphasic (macular and lichenoid amyloidosis).
- The amyloid in all forms of primary cutaneous amyloidosis is formed from keratin from damaged keratinocytes, with the exception of nodular amyloidosis, in which it is derived from immunoglobulin light chains.
- In systemic amyloidosis, the amyloid localizes around blood vessels, skin adnexae and surrounding adipocytes in the earlier stages, eventually spreading to diffusely involve the dermis with progression of the depositions.
- Lichen and macular amyloidosis show globular deposits of amyloid, primarily within the dermal papillae.
- Nodular amyloidosis displays massive deposition of amyloid throughout the dermis and often extending into the subcutis.
- Special stains for amyloid, such as Congo red with apple-green birefringence, are positive in all forms of amyloidosis.