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Author: Jeffrey J. Bazarian
The current method of diagnosing a concussion on the athletic field, battlefield, or even in the emergency department is unreliable and likely inaccurate: It relies on self-report of symptoms from the person who is injured or from a witness, if there is one. The symptoms that indicate concussion are a brief loss of consciousness, period of amnesia, or confusion. There is no x-ray, blood test, or scan to help make the diagnosis.
Concussion Diagnosis: Shortcomings
The current method of concussion diagnosis has several obvious drawbacks. Someone hit on the head may not be able to recall the details of the injury precisely because the part of the brain that controls short-term memory is affected. Yet the patient’s recollection of events is usually required to make the diagnosis. The conversation in an emergency department goes something like this:
Doctor: “Tell me what happened, Mr. Smith.”
Mr. Smith: “I dunno, Doc. I have no idea.”
Doctor: “All right. You clearly have a concussion. You can go home now.”
It’s hard for anyone to have confidence in a diagnosis of brain injury when this is how it is established.
Recollection of the events associated with an injury may also be altered by drug or alcohol use, or preexisting dementia, both common among patients presenting to emergency departments. And how does one make this diagnosis in preverbal children? Finally, two patient groups, warfighters and athletes, may well remember injury events but not want to tell a healthcare provider because they want to be back with their unit or team.
Not surprisingly, given the current state of diagnosis, concussions are often overlooked. A recent study showed that a third of athletes did not realize they had a concussion (Meehan et al. 2013). This might be understandable given that most athletes don’t have a medical degree. However, hospital-based healthcare providers don’t fare much better: Three studies of head-injured patients presenting to emergency departments found that concussions were missed in 56–89 percent of cases (Delaney et al. 2005; De Maio et al. 2014; Powell et al. 2008)!
A Diagnostic Blood Test
Fortunately, help is on the way. In the near future it may be possible to diagnose concussion, also known as mild traumatic brain injury (mTBI), with a simple blood test. Although still in the research stage, such a blood test could remove much of the doubt associated with trying to determine if a concussion has occurred.
After a concussion, proteins are released from the breakdown of the brain’s primary cell type, neurons, as well as from supporting cells such as astrocytes and oligodendrocytes (figure 1). This process of axonal injury provides an opportunity to detect these proteins in the blood (Zetterberg et al. 2013).
The traditional thinking has been that proteins released during axonal injury diffuse into the space between brain cells (interstitial space), then into the fluid surrounding the brain (cerebrospinal fluid), and finally across the normally closed blood-brain barrier to reach the peripheral circulation, where they can be detected in a blood sample. Traditional thinking also held that a head blow hard enough to cause concussion also transiently opened the blood-brain barrier, allowing brain proteins to pass.
As it turns out, how proteins get from the brain into the blood may be a bit more complicated, which has implications for interpreting the results of a blood test in the context of a head injury.
It now appears that there is an alternative route for brain proteins to gain access to the blood, and it may sometimes be blocked during a concussion, preventing markers of brain damage from reaching the peripheral circulation. In this route, called the glymphatic pathway, brain proteins diffuse into the interstitial space and then into the brain’s lymphatic system, which empties into the blood (Brinker et al. 2014; Plog et al. 2015).
Brain researcher Maiken Nedergaard of the University of Rochester determined that flow along the glymphatic pathway was reduced in mice that were sleep deprived and in mice subjected to repeated subconcussive head hits (Plog et al. 2015). These findings have important implications if they are confirmed in humans: Serum brain protein levels could be falsely negative among patients chronically sleep deprived or subject to repeated head hits—such as student athletes and warfighters.
Detecting Brain Proteins in the Blood
It has taken many years for scientists to develop techniques to detect the very low concentrations of brain proteins in the blood. Table 1 depicts the status of research and development on the key brain proteins currently under investigation. Several proteins, such as amyloid-beta protein 42 (Ab42) and neuron-specific enolase (NSE), were found to be poor diagnostics of concussion and have not been studied further (Zetterberg et al. 2013).
The table shows that serum levels of most brain proteins are elevated after mTBI, and several of them—S100B, UCH-L1, GFAP,1 alpha-II spectrin, and tau—provide reasonable diagnostic accuracy for distinguishing concussion from nonconcussion (Zetterberg et al. 2013). Only S100B is in use clinically, but not in North America. It is used clinically in several European and Asian countries, not for diagnosis but to determine who should undergo a computed tomography (CT) scan of the head.
Levels of S100B, UCH-L1, and GFAP can also predict which concussed patients will have intracranial hemorrhage on a head CT scan (Papa et al. 2012; Welch et al. 2016). Intracranial hemorrhage occurs in about 5–10 percent of mTBI patients, and early detection and neurosurgery can be lifesaving. So it is important for a putative marker of brain injury to be able to detect not only concussion but also the subset of concussed patients with intracranial hemorrhages.
Levels of alpha-II spectrin and tau appear to predict postconcussion outcomes such as symptom reduction and timing of return to sports.
Limitations of Brain Injury Markers
There are several limitations to the use of these proteins as brain injury markers. First, they are much more sensitive than they are specific, and their negative predictive value is higher than their positive predictive value. This may be because these proteins exist in small amounts in tissues outside the brain; for example, S100B is found in cartilage and fat cells, and tau in peripheral nerves. Thus, these markers are better at ruling out concussion (or intracranial hemorrhage) than ruling it in.
Second, these markers have not been tested in the first hour after injury, which would be important for applying them in nonhospital settings such as the athletic field, battlefield, or scene of a mass casualty. Data on marker accuracy come from studies of marker levels in the 3- to 12-hour postinjury window, with a minority examining levels at 1 hour (Rothoerl et al. 2000; Shahim et al. 2014; Townend et al. 2006).
Third, the serum levels of brain markers tend to go up with physical exertion. The reasons for this are not clear, but this effect complicates interpretation of an increased level after an injury incurred during a sporting event or combat operation (Shahim et al. 2015; Stålnacke et al. 2003, 2006).
Finally, there is large interindividual variation in the serum levels of these proteins in normal, uninjured persons (Kiechle et al. 2014; Shahim et al. 2014). This fact complicates efforts to find a single cutoff value separating concussed from nonconcussed individuals.
These limitations need to be addressed if brain markers are to provide clinical value to the management of patients with head injuries.
The low specificity problem could be addressed by combining results from two or more proteins rather than a single marker.
The lack of information on marker accuracy within the first hour of injury requires research on blood samples obtained immediately after injury, which may be most practical in athletic settings.
Determining the effects of physical exertion on marker levels would be a simple matter of serum sampling at various time points after aerobic activity.
Finally, the difficulty in defining a single postinjury cutoff might be addressed by obtaining preinjury marker levels and then examining changes from baseline, rather than a single postinjury value. Of course, this solution will help address the problem only in patient cohorts where a baseline is obtainable, such as athletes and perhaps warfighters.
In terms of actual implementation, moving brain markers into clinical use in the United States would require approval by the Food and Drug Administration. And the development of point-of-care devices would be necessary for these tests to become part of return to play decision making on athletic fields across the country.
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1 S100B is S100 calcium-binding protein B, UCH-L1 is ubiquitin carboxyl-terminal hydrolase L1, and GFAP is glial fibrillary acidic protein.