Canadian researchers found significantly reduced cerebral blood flow and steadily increasing mean diffusivity 12 months after the athletes returned to play. These abnormalities persisted even though their symptoms, such as headache and dizziness, had dissipated.
"Despite concussion being conventionally described as a transient disturbance in brain function, there is growing evidence that neurobiological recovery may be incomplete at return to play," wrote the authors, led by Nathan Churchill, PhD, from St. Michael's Hospital in Toronto. "It is important to determine whether disturbances in brain physiology seen at this time are permanent or resolve over a longer time interval so as to better understand the biological mechanisms of neurologic sequelae associated with a history of concussion."
The current findings add to previous research by Churchill and colleagues that also found longer-lasting and deeper adverse effects from concussions than previously thought. Their 2017 study discovered subtle changes in brain structure and activity after concussed athletes returned to action. In 2016, MR images showed changes in brain size, cerebral blood flow, and structural white matter several months and, in some cases, years after the injury.
In this new study, Churchill's team consecutively recruited 24 college athletes (mean age 20.0 ± 1.9 years) from eight different sports who experienced a concussion, along with 122 healthy controls (mean age 20.3 ± 2.0 years) with no concussions. Individuals in both groups underwent 3-tesla MRI scans (Magnetom Skyra, Siemens Healthineers) with a protocol that included 3D T1-weighted magnetization prepared rapid acquisition gradient echo (MPRAGE) imaging, 2D pulsed arterial spin labeling (ASL), and diffusion-weighted imaging (DWI). Resting-state functional MRI (fMRI) was also performed with multislice T2-weighted echo-planar imaging (EPI).
Initial MR images were acquired an average of four days after the injury to assess the early symptomatic phase of the concussion. Follow-up scans were performed when the athletes returned to action and again one year after that return. Each scan gathered information on global functional connectivity, cerebral blood flow, white-matter fractional anisotropy, and mean diffusivity.
In reviewing the results, the researchers found no significant aftereffects in global functional connectivity or fractional anisotropy one year after the return to action. Resting-state fMRI measurements of brain connectivity were normal at that time, while fractional anisotropy showed normal water molecule flow in the white matter of the athlete's brains.
However, the researchers found significantly reduced cerebral blood flow one year later, with an average decrease of 10 mL/100 g of blood per minute compared with healthy controls. Interestingly, cerebral blood flow was normal when the athletes returned to play. MR images also indicated increasing significant elevations in mean diffusivity. The elevations were primarily in the posterior and superior corona radiata from the early symptomatic phase of the concussion through the return to play and one year later, which could indicate edema in that brain region.
The MRI measures also showed an association between the clinical severity of the concussion and the recovery timeline, "reinforcing that neurobiological recovery may be highly variable across individuals and partly dependent on initial clinical presentation," the researchers added. "These findings provide novel insights into the response of the brain to concussion and help to better understand how interindividual heterogeneity in long-term brain recovery is related to clinical measures."
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