Intrathecal Plasma Cell Activity as Measured by the Kappa Free Light Chain Index Is Associated With Retinal Layer Atrophy in Early Multiple Sclerosis.

BACKGROUND: The relationship between intrathecal kappa free light chain (κ-FLC) synthesis and retinal layer atrophy in multiple sclerosis (MS) is unknown. OBJECTIVE: To investigate whether the κ-FLC index is associated with peripapillary retinal nerve fiber layer (pRNFL) and ganglion cell plus inner plexiform layer (GCIPL) thickness. METHODS: Patients with newly diagnosed clinically isolated syndrome or relapsing MS and available cerebrospinal fluid (CSF) analysis and optical coherence tomography (OCT) were included. Clinical and magnetic resonance imaging (MRI) data were also assessed. κ-FLC concentrations were measured by nephelometry, and the κ-FLC index was calculated as (CSF κ-FLC/serum κ-FLC)/albumin quotient. RESULTS: A total of 100 patients at a median age of 33 (25th-75th percentile: 25-39) years and a female predominance (58%) were included. In multivariable linear regression analysis adjusted for age, sex, disease duration, number of T2-hyperintense and number of contrast-enhancing MRI lesions, the κ-FLC index was associated with pRNFL (log-transformed κ-FLC index: β: -2.51; 95% confidence interval [CI]: -4.01, -1.01; p = 0.001) and GCIPL thinning (log-transformed κ-FLC index: β: -1.24, 95% CI: -2.32, -0.17; p = 0.024). CONCLUSIONS: Intrathecal plasma cell activity, as measured by the κ-FLC index, is associated with neuroaxonal damage, as reflected by pRNFL and GCIPL atrophy.