Abstract
Injuries and illnesses can alter the normal bilateral symmetry of the brain, and determining the extent of this disruption may be useful in characterizing the pathology. One way of quantifying brain symmetry is in terms of bilateral correlation of diffusion tensor metrics between homologous white matter tracts. With this approach, we hypothesized that the brains of patients with a concussion are more asymmetrical than those of healthy individuals without a history of a concussion. We scanned the brains of 35 normal individuals and 15 emergency department patients with a recent concussion. Fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD), and radial diffusivity (RD) were determined for regions of interest (ROI) defined by a standard white-matter atlas that included 21 bilateral ROIs. For each ROI pair, bilateral correlation coefficients were calculated and compared between the two subject groups. A symmetry index, defined as the ratio between the difference and the sum of bilateral measures, was also calculated for each ROI pair and compared between the groups. We found that in normal subjects, the extent of symmetry varied among regions and individuals, and at least subtle forms of structural lateralization were common across regions. In patients, higher asymmetry was found overall as well as in the corticospinal tract specifically. Results indicate that a concussion can manifest in brain asymmetry that deviates from a normal state. The clinical utility of characterizing post-concussion pathology as abnormal brain asymmetry merits further exploration.
Highlights
The functional and anatomical lateralization of the brain is well recognized, what is more obvious, and can be taken for granted, is its normal structural symmetry
The extent of symmetry varied among regions and individuals, and at least subtle forms of structural lateralization were common across regions
Values of Fractional anisotropy (FA) tended to be higher, and mean diffusivity (MD), radial diffusivity (RD), and axial diffusivity (AD) lower in our patient sample compared to controls, with a notable inconsistency in the middle cerebellar peduncle
Summary
The functional and anatomical lateralization of the brain is well recognized, what is more obvious, and can be taken for granted, is its normal structural symmetry. Concussion Brain Asymmetry homologous pairs of tracts are generally high in healthy individuals (1–3). This verification supports an assertion that bilateral asymmetry reflects disruption of a normal state when found in patients with injuries or illnesses and that determining the extent of this disruption is useful in characterizing the pathology (2, 4, 5). To examine abnormal brain asymmetry, at least three considerations are needed: (1) the extent and similarity of the impact of an injury or illness on bilateral regions vary; (2) the degrees of normal symmetry vary among regions (1); and (3) no two injuries or illness progressions are alike (2)
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