[Paraneoplastic optic neuritis in an unknown small cell lung carcinoma defined by CRMP-5-IgG].

CASE REPORT: The case of a 64-year-old patient with bilateral, progressive and painless diminution of visual acuity is presented. Ophthalmologic evaluation revealed optic neuritis and vitreous cells in both eyes, at different stages. Suspecting a paraneoplastic optic neuritis, the study of antibodies was requested. This showed positivity to the marker CRMP-5-IgG. After mediastinoscopy, a small cell lung carcinoma was diagnosed.

DISCUSSION: Autoantibody CRMP-5-IgG defines a paraneoplastic entity of combined optic neuritis and vitreous inflammatory cells. The serological positivity avoids the vitreous biopsy and expedites the search for cancer. In our case, it allowed the diagnosis a previously unidentified tumor.