Crystalloidal Paraprotein Deposits in the Cornea: An Ultrastructural Study of Two New Cases with Tubular Crystalloids that Contain IgGk Light Chains and IgGγ Heavy Chains

The fine structure and immunoprotein content of the crystalloids are described in two cases of paraproteinemic crystalloidal keratopathy, both of which had clinical features thought by the referring ophthalmologists to be those of atypical lattice-type corneal dystrophy (presumably because of lattice-like lines). Most keratocytes in one case were surrounded by a mantle of densely packed tubular crystalloids. Individual tubules were annular in cross section with mean dimensions as follows: overall diameter, 29.32 nm (SD 1.26); internal diameter (core), 8.53 nm (SD 1.12); wall thickness, 10.39 nm (SD 0.85) (n = 10). Crystalloids were extracellular and found only in the corneal stroma, with none in Bowman's layer or Desce-met's membrane. In the second case, the tubules had a similar distribution but formed geometric arrays with no clear relationship to, or envelopment of, the keratocytes. The tubules were thin-walled, with mean dimensions as follows: overall diameter, 26.12 nm (SD 1.12); internal diameter (core), 15.46 nm (SD 1.12); wall thickness, 5.33 nm (SD 0) (n = 10). In both cases the tubules were K-light chain- and γ-chain-positive. Laboratory investigations revealed the presence of two IgM-k paraproteins and an IgG-K paraprotein in the serum of the first patient. The second patient had an IgG-K paraproteinemia and bone marrow changes consistent with low-grade non-Hodg-kin's lymphoma. These cases emphasize and extend the morphological range of corneal IgG crystalloids; the second case also demonstrates that corneal IgG crystalloids may be an early indicator of an underlying immunoproliferative disease.