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Retinothalamic White Matter Abnormalities in Amblyopia.

Purpose
Amblyopia is associated with a broad array of perceptual and neural abnormalities in the visual system, particularly in untreated or unsuccessfully treated populations. Traditionally, it has been believed that the neural abnormalities are confined to the visual cortex and subcortex (e.g., lateral geniculate nucleus). Here, we investigate the presence of neuroanatomical abnormalities earlier in the visual stream, in the optic nerves and tracts, of participants with two predominant forms of amblyopia.


Methods
We used diffusion magnetic resonance imaging and probabilistic tractography to compare the microstructural properties of five white matter visual pathways between 15 participants with amblyopia (eight anisometropic, five strabismic, and two exhibiting both etiologies), and 13 age-matched controls.


Results
Participants with amblyopia exhibited significantly smaller mean fractional anisotropy in the optic nerve and optic tract (0.26 and 0.31 vs. 0.31 and 0.36 in controls, respectively). We also found greater mean diffusivity in the optic radiation compared to controls (0.72 μm2/s vs. 0.68 μm2/s, respectively). Comparing etiologies, the abnormalities in the precortical pathways tended to be more severe in participants with anisometropic compared to strabismic amblyopia, and anisometropic participants' optic nerves, optic tracts, and optic radiations significantly differed from control participants' (all, P < 0.05).


Conclusions
The results indicate that amblyopia may be associated with microstructural abnormalities in neural networks as early as the retina, and these abnormalities may differ between amblyopic etiologies.