Birefringence and depolarization characteristics of myelinated retinal nerve fibers assessed with polarization-sensitive optical coherence tomography

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PURPOSE: To quantify birefringence and depolarization features of myelinated retinal nerve fibers using polarization-sensitive optical coherence tomography and to evaluate their utility for noninvasive assessment. STUDY DESIGN: Retrospective observational case series. METHODS: Seven eyes from 7 patients with clinically diagnosed myelinated retinal nerve fibers underwent polarization-sensitive optical coherence tomography. We quantified phase retardation (birefringence), polarimetric entropy (depolarization), and intensity within myelinated retinal nerve fibers and adjacent retinal nerve fiber layer. Optic-axis orientation maps were generated to infer myelin lamellar orientation relative to axons and to delineate depth-resolved lesion extent. RESULTS: Myelinated retinal nerve fibers showed significantly higher phase retardation and polarimetric entropy than adjacent retinal nerve fiber layer (p = 0.02 and 0.008, respectively). Retardation and entropy exhibited greater variability than intensity across lesions (p < 0.05). Optic-axis orientation within lesions was consistently near-perpendicular to the local nerve fiber direction (all seven lesions; overall mean angle, 81°). Within lesions, greater depolarization and birefringence were associated with stronger posterior shadowing independent of lesion thickness. Optic-axis images also revealed segments in which nonmyelinated retinal nerve fiber layer overlaid portions of the lesion. CONCLUSION: Elevated birefringence, increased depolarization, and optic-axis orientation perpendicular to axons constitute distinctive optical signatures of retinal myelination. These contrasts may allow polarization-sensitive optical coherence tomography to delineate lesion boundaries and internal heterogeneity in three dimensions more precisely than conventional optical coherence tomography, supporting differential diagnosis, phenotyping, and longitudinal monitoring.

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