The term amyloidosis refers to a group of diseases that lead to extracellular deposition of amyloid. Amyloid is a group of proteins that have in common a characteristic ultrastructural appearance of nonbranching fibrils with variable length and a diameter of 7–10 nm. The proteins forming amyloid can form a tertiary structure known as a β-pleated sheet, which enables the proteins to bind Congo red stain and show birefringence under polarized light (Fig 10-12).
The type of amyloid protein that is deposited and the location of the deposition depend on the etiology of the underlying condition. Amyloid deposition can occur in the vitreous when the protein forming the amyloid is transthyretin. Multiple genetic mutations can result in various amino acid substitutions in the transthyretin protein and allow for a conformational change to a β-pleated sheet. The most common mutations were originally described in familial amyloid polyneuropathy (FAP). Systemic manifestations in patients with FAP include vitreous opacities and perivascular infiltrates (Fig 10-13), peripheral neuropathy, cardiomyopathy, and carpal tunnel syndrome. If amyloid protein is identified in the vitreous, referral to evaluate for systemic disease should be considered.
The mechanism by which the vitreous becomes involved is not known with certainty. Because amyloid deposits are found within the walls of retinal vessels and in the RPE and ciliary body, amyloid may gain access to the vitreous through these tissues. In addition, because transthyretin is a blood protein, it may gain access to the vitreous by crossing the blood–aqueous or blood–retina barrier.
Figure 10-12 Amyloid of the vitreous. A, Congo red–stained material, aspirated from the vitreous, shows red-orange staining. B, Photomicrograph of the material shown in part A under polarized light; the material exhibits apple-green birefringence (dichroism).
(Courtesy of Nasreen A. Syed, MD.)
Figure 10-13 Perivascular sheathing of retinal vessels (arrow) associated with vitreous amyloidosis.
(Courtesy of Hans E. Grossniklaus, MD.)
Excerpted from BCSC 2020-2021 series: Section 4 - Ophthalmic Pathology and Intraocular Tumors. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.