Jennifer Gonzalez, Au.D., Ph.D., CCC-A
Speech and Hearing Sciences, College of Health Solutions, Arizona State University
Concussion, also known as mild traumatic brain injury or mTBI, encompasses injuries caused by bumps, blows, or jolts to the head or body resulting in rapid movement of the head and brain(Centers for Disease Control and Prevention, 2018). Concussion injuries induce chemical changes in the brain as well as stretching and damaging of neurons and supporting cells in the central nervous system that can significantly and deleteriously impact brain function, giving rise to alterations in cognitive, vestibular, and ocular function as well as onsets of fatigue, posttraumatic headache and migraine, and anxiety, emotional, and mood disorders (Centers for Disease Control and Prevention, 2016, 2018; Charek, Collins, & Kontos, 2018).
Globally, an estimated 42 million people experience mTBI per year, of which 3.8 million occur in the United States during sporting and recreational events (Harmon et al., 2013; Hernandez et al., 2015; Marar et al., 2012; World Health Organization, 2018; Gardner & Yaffe, 2015). The Centers for Disease Control and Prevention (2018) reported that there were about 640,000 emergency department visits related to all severities of TBI in 2013, 70 to 90% of which were categorized as mTBI. Additionally, in 2012, 325,000 of TBI-related emergency department visits concerned children and teens, with 283,000 related to sports and recreation. Most instances of concussion reportedly occur in football, girls basketball, girls soccer, and boys wrestling; however, the highest rates of concussion have been reported in football, boys lacrosse, and boys ice hockey, the last of which represents the highest proportion of total head injuries over any other sport studied (Harmon et al., 2013; Hernandez et al., 2015; Marar et al., 2012). An additional concern, especially when considering the dangers of second impact syndrome (e.g., swelling and herniation of the brain and death), is that an estimated 50% of mTBI incidents go unreported (Harmon et al., 2013; Hernandez et al., 2015).
The functional effects of concussion on the individual often can be observed firsthand and behaviorally measured; however, it is difficult to objectively document the physical changes to the brain resulting from the injury via imaging, as standard imaging techniques, including magnetic resonance imaging (MRI) and computerized tomography (CT), rarely identify evidence of structural change associated with sport-related concussion (Useche & Bermudez, 2018). While MRI and CT tend to fall short in revealing structural abnormalities after mTBI, multiple studies have documented the effectiveness of diffusion tensor imaging (DTI), at uncovering abnormalities following blast-related and sport-related concussion, specifically in white matter areas of the brain, including the corpus callosum, cerebellum, fornix, hippocampus, thalamus, cingulum, and fusiform gyri (Alhilali et al., 2014; Borich et al., 2013; Cubon et al., 2011; Hayes et al., 2015; Hernandez, 2015; Hulkower et al., 2013; Khong et al., 2016; Kleiven, 2006; Laksari et al., 2018; McAllister et al., 2011; Virji-Babul et al., 2013).
Cortical locations of injury, such as coup – where the maximal injury or contusion to the brain is beneath the location of the initial hit to the head, and contrecoup – where the maximal injury or contusion is sustained at a point opposite to the location of the hit, are commonly known and typically include surface-focused descriptions of the cortical lobes involved (i.e., frontal, temporal, parietal, or occipital). However, another effect of TBI that may be less well known – strain in the corpus callosum – occurs as the result of a torqueing around the midline of the two cerebral hemispheres when the brain is set intomotion. Comprised of approximately 200 million heavily myelinated nerve fibers, the corpus callosum is the largest white matter structure in the brain and connects the right and left cerebral hemispheres at midline (Benavidez et al., 1999; Goldstein et al., 2019). Spanning about 10 centimeters in length along the midsagittal plane, it has four main divisions – the rostrum, genu, isthmus/body, and splenium and is consideredone of the most vulnerable cortical white matter structures to damage in the form of stretching, twisting, and shearing of axons sustained from a blow to the head (Hernandez, 2015; Khong et al., 2016). This damage also results in the loss of myelin surrounding those axons, leading to less effective interhemispheric neural transmission of homotopic and heterotopic transcallosal projection neurons.
While corpus callosum strain may be a less well-known form of injury, it has been documented in the literature to be the most reliable indicator for concussion cases in football (Laksari et al., 2018; Kleiven, 2006; Hernandez et al., 2015). Such damage to the corpus callosum significantly impacts central auditory and vestibular function, especially for tasks that require interhemispheric transfer of information, including binaural integration, binaural separation, temporal ordering/sequencing, vision, eye movement control, tolerance of visual motion, and dynamic balance. Due to the high likelihood of central auditory and vestibular dysfunction following concussion, including measures of both central auditory and vestibular function in the post-concussion workup is strongly recommended.
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