Can retinal nerve fiber layer (RNFL) thickness be a marker for distinguishing bipolar depression from unipolar depression?

Objective: We aimed to compare retinal nerve fiber layer (RNFL) thickness and ganglion cell complex (GCC) thickness in bipolar disorder (BD) and major depressive disorder (MDD).

Method: The study included thirty MDD, thirty-two BD participants in depressive episode, and thirty-seven controls matched according to age, gender, body mass index (BMI), and smoking status. Optic coherence tomography (OCT) measurements were performed for both participants and controls. The RNFL and GCC thickness were measured and recorded automatically by a spectral OCT device. Participants were also subjected to Hamilton Depression Rating Scale (HAM-D).

Results: RNFL superior thickness was significantly lower in BD participants, compared to the MDD participants and controls (p = 0.001). GCC inferior (p = 0.022) and inferonasal (p = 0.005) thickness were detected lower in BD group, compared to the control groups. In the BD group, HAM-D scores were negatively correlated with RNFL-temporal (p = 0.049, r= -0.357), GCC inferotemporal (p = 0.02, r= -0.416) and superotemporal thickness (p = 0.002, r= -0.546).

Conclusions: RNFL thickness were lower in BD participants compared to the MDD and controls and, GCC thickness were lower in BD participants compared to the controls. Our findings support the hypothesis that neurodegeneration is part of the pathogenesis of BD. Future research are needed to confirm the lack of RNFL thickness in MDD, which could have immediate therapeutic consequences as well as implications for distinguishing BD from MDD.