Could structural changes in the retinal layers be a new biomarker of mental disorders? A systematic review and thematic synthesis.

It has recently been suggested that alterations of the layers of the retina could be a biomarker of specific mental disorders since they originate in the same embryonic layer as the brain and both are interconnected through the optic nerve. The purpose of this article is to offer a systematic review of the literature and a thematic synthesis on the current state of the alterations of the retina layers identified by optical coherence tomography in patients with schizophrenia, bipolar disorder and major depression. For this purpose, we performed a bibliographic search, a systematic review of the studies and a thematic synthesis of the reported findings. Patients with schizophrenia have more abnormal findings followed by patients with bipolar disorder, with very few findings in depression. The nerve fiber layer is the retinal layer with more abnormal findings both in schizophrenia and in bipolar disorder, while no study in major depression found alterations in it. Of the clinical parameters, the duration of the illness correlates significantly and inversely with the thickness of the different layers in all disorders. When interpreting these data, it is necessary to take into account the limitations and differences of the studies, especially the mean length of the disorders. Given that this was very different among the 3 disorders (more than doubled in the case of schizophrenia respect to major depression), the differences in the results found could be due more to the effect of the length of illness than to the disorder itself. In summary, optical coherence tomography findings are promising, since they could provide biomarkers of neurodegeneration and/or neuroprogression of both schizophrenia and bipolar disorder.