To summarize ophthalmological signs of monoclonal gammopathy of undetermined significance (MGUS) and to present a case report. Summary of the literature data and presentation of the history of a 46-year-old female patient. In MGUS, pathological, but non-malignant plasma cells produce abnormal monoclonal immunoglobulin. Its prevalence is 0.15%, but it increases with age. As yearly 1–2% of MGUS patients develop multiple myeloma, frequent hematological follow-up is necessary. Corneal opacifications in MGUS have been described in a few dozens of patients in the literature. These may be nummular or crystal-like, or even present with white or grey line-forming depositions in the stroma. They may be centrally or peripherally localized. In our patient, bilateral, branching, geographical corneal opacifications were detected predescemetally, that were progressing and reaching the optical centre during follow-up. With 0.15 best corrected visual acuity, penetrating keratoplasty was performed (postoperative best spectacle-corrected visual acuity 0.6). Masson trichrom staining of the explanted cornea verified protein deposition, immunhistochemistry identified kappa light chain immunglobulin deposition in the posterior stroma, surrounded with inflammatory cells. Serum electrophoresis and bone marrow biopsy of our patient proved MGUS, therefore, hematological follow-up is going on. In the case of progressive, atypical corneal opacification, the hematological diagnosis of monoclonal gammopathy must be excluded – monoclonal gammopathy of ocular significance –, as delay in proper diagnosis and treatment of the systemic disease may have devastating consequences. Orv Hetil. 2018; 159(39): 1575–1583.

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