Researchers recently discovered that fractional anisotropy could aid in distinguishing the different patterns of optic nerve damage between neuromyelitis optica spectrum disorder (NMOSD) and multiple sclerosis (MS), as published in European Radiology.

Xie and colleagues found that fractional anisotropy could differentiate between NMOSD and MS by quantifying the characteristics of NMOSD- and MS-related optic nerve impairment in a retrospective study that included 31 patients with NMOSD and 25 patients with MS.

The lesions of NMOSD optic neuritis (NMOSD-ON) were longitudinally extensive. They tended to involve the posterior segment of the optic nerve, whereas the patients with MS optic neuritis (MS-ON) had short-segment involvement, consistent with the results of magnetic resonance imaging studies.


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“We also found that NMOSD caused more severe optic nerve damage than MS, with more pronounced changes in the optic nerve fiber structure and more aggressive demyelination, leading to a widespread decrease in [fractional anisotropy],” the authors wrote.

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Furthermore, fractional anisotropy values may also have the ability to reveal potential damage in the optic nerves of patients with NMOSD who have not yet presented with vision impairment. When inflammation involves the nerve, the absence of myelin and damage to the cell cause changes in the diffusion direction of water molecules in the nervous fibers, leading to changes in fractional anisotropy values.

Previous studies showed that fractional anisotropy values were significantly decreased in patients with optic neuritis, suggesting that the optic nerve’s cellular structural arrangement and microstructural integrity have been destroyed. Thus, fractional anisotropy is thought to help quantify visual impairment in NMOSD-ON and differentiate it from MS-ON.

Both NMOSD and MS are 2 major inflammatory demyelinating diseases of the central nervous system. Antibody-mediated humoral immunity with complement involvement dominates NMOSD-ON, specifically attacking astrocytes, resulting in its damage and death. The aquaporin-4 is another major antigenic target in the pathogenesis of NMOSD. This disease usually presents bilateral, longitudinally extensive, and optic nerve damage involving the chiasm.

“We speculate that the greater tendency of NMOSD-ON to involve the posterior segment of the optic nerve may result from a greater abundance of [aquaporin-4] protein in this region,” the authors concluded.

Reference

Xie Y, Zhang Y, Yao Y, Liu D, Chen B, Zhu W. Fractional anisotropy helps to differentiate the optic nerve impairment between neuromyelitis optica spectrum disorders and multiple sclerosis. Eur Radiol. Published online April 14, 2022. doi:10.1007/s00330-022-08779-3