apple-green birefringence

Amyloidosis is a group of diseases characterized by extracellular deposition of misfolded proteins that aggregate into insoluble beta-pleated sheet fibrils. These deposits can be composed of various proteins (AL - immunoglobulin light chains, AA - serum amyloid A, ATTR - transthyretin, Abeta - amyloid precursor protein, among others) depending on the underlying disease process. Regardless of the specific protein type, the beta-pleated sheet structure is the unifying pathologic feature. To diagnose amyloidosis histologically, tissue biopsy is stained with Congo red dye, which binds to the beta-pleated sheet structure. Under regular light microscopy, amyloid deposits appear as amorphous, eosinophilic, extracellular material. However, when viewed under polarized light, Congo red-stained amyloid exhibits a distinctive apple-green birefringence - this optical property occurs due to the regular, repeating beta-sheet structure interacting with polarized light. Clinically, amyloidosis can affect virtually any organ system, with common presentations including nephrotic syndrome (kidney involvement), restrictive cardiomyopathy (cardiac involvement), hepatosplenomegaly, peripheral neuropathy, macroglossia (classically seen in AL amyloidosis), and carpal tunnel syndrome. Systemic amyloidosis types include AL (associated with plasma cell dyscrasias like multiple myeloma), AA (secondary to chronic inflammatory conditions like rheumatoid arthritis, IBD, or chronic infections), and ATTR (either hereditary or wild-type/senile systemic amyloidosis). Localized forms include Abeta amyloid in Alzheimer disease and islet amyloid polypeptide (IAPP) in type 2 diabetes. Diagnosis requires tissue biopsy (commonly abdominal fat pad, rectal, or affected organ biopsy) with Congo red staining and polarized light microscopy showing the pathognomonic apple-green birefringence. Additional workup includes immunohistochemistry or mass spectrometry to identify the specific amyloid protein type, which guides treatment approach.