Traditional MRI only provides purely anatomical data and does not offer any information about neural fiber conduction functionality. DTI, a specialized form of diffusion-weighted MRI, has made notable strides in neuroscience. Previous studies have found that after tumor removal and the reconstruction of white matter fibers, the FA value increases, and the MD value decreases [18]. Recent research indicates that an MD value under 0.0021 and an FA value over 0.1689 can forecast favorable postoperative visual recovery outcomes [10]. In the research conducted, it was found that the FA values of the optic pathway in the group of patients were inferior when contrasted with the control group, whereas the ADC values were superior. This further confirms the conclusion that PA patients have abnormal neural fiber conduction.
Given the shared embryological origin of the eyes and the central nervous system, they are anatomically interconnected. Lima Rebouças and colleagues hypothesize that since the GCL-IPL and GCC layers are mainly composed of the bodies of ganglion cells, they reflect the thickness of the cerebral cortex and gray matter [19]. In contrast, CP-RNFL primarily consists of the axons of retinal ganglion cells, mainly reflecting the volume and microscopic changes in white matter. Whole-brain analysis has shown that a thicker CP-RNFL, GC-IPL, and GCC correspond with higher white matter microstructural integrity. Ito, Y identified a correlation between the thickness of the macular GCC layer, GCL, and dementia, implying that retinal structure can serve as an indicator of brain health [20]. There are many studies currently using OCT to measure the thickness of the optic disc and macular layers. Our results are consistent with previous findings, showing quadrant-specific thinning of CP-RNFL and the macular region GCL, IPL, and GCC. Combined with DTI results, this indicates that PA patients may have both gray and white matter structural and functional impairments [1–3].
Another anatomical retinal marker is retinal blood flow density. Our findings have similarities and differences from previous research. Previous studies have found reduced RPCP and superficial macular capillary plexus density in PA patients [21–22]. This reduction is speculated to result from tumor growth compressing the optic chiasm, causing axoplasmic flow obstruction and reduced blood supply, leading to neural atrophy and ganglion cell death, which ultimately manifests as retinal vasoconstriction [23]. Our study identified reduced blood flow density in both the optic disc RPCP and all layers of the macular region, which aligns with the results of Gilda Cennamo and others [22]. We understand that the visual conduction pathway includes four tiers of neurons. Bipolar cells in the nuclear layer represent the second tier, and ganglion cells represent the third. The retina within the nuclear layer is supplied by the central artery, wherein the RPCP originates from the small arteries around the optic disc and extends radially from the optic disc. This supplies blood and nutrients to the ganglion cells. After the atrophy of ganglion cells, the corresponding RPCP density reduces.
For the first time, our research delves into the association between retinal blood flow density and the functionality of neural fibers within the visual pathway. We found that the blood flow density of all macular layers positively correlates with radiative FA values and negatively with ADC values, indicating that the impairment of radiative neural fiber conduction function is related to reduced macular blood flow density. The lateral geniculate body is the fourth tier of the visual conduction pathway, with radiations made up of its axons responsible for transmitting visual information to the primary visual cortex. Previous research and our preliminary studies confirm that in addition to retrograde degeneration after chiasmal compression, anterograde degeneration also occurs [25]. This forward degeneration is evident in DTI as a reduction in radial FA values and an increase in ADC values. Meanwhile, we observed the strongest correlation between macular region SVP density and optic nerve ADC value, optic tract FA value, and radiative ADC value, suggesting that the macular region SVP could serve as an indicator for evaluating the function of the visual pathway, particularly reflecting the radiative neural conduction function. However, further studies with larger samples are required to confirm this finding.
Lastly, our study found that the density of macular SVP and peripapillary RPCP negatively correlates with tumor height. We speculate that tumors with a larger vertical diameter tend to compress the optic chiasm, causing retrograde degeneration, which then reduces retinal blood flow density. Nonetheless, the correlation between the density of blood flow in the retina and the diameter of the tumor is still a subject of debate. For example, a study by Guangxin Wang et al. did not observe any association between the density of blood flow around the optic disc and the diameter of the tumor. Conversely, Xuqian Wang et al. discovered a notable association, which aligns with the results of our research [28].
Limitations of our research include the limited number of patients studied, preventing subgroup analysis, including studies on tumors compressing the optic chiasm to varying degrees. Our study is cross-sectional and does not compare the pre- and post-treatment visual pathway DTI and retinal OCTA characteristics in PA patients. To gain a more comprehensive understanding of visual pathway damage in PA patients, additional long-term studies are warranted.