"...is an essential reference for investigators, graduate and medical students in the mechanisms mediating and detection of trauma related neuronal injury. It is an inspiration for clinicians to see how the groundwork is being laid toward the development of biomarkers, which could revolutionize the world of TBI."
Biomarkers for Traumatic Brain Injuryby Svetlana Dambinova
Due to injuries sustained in sports and in combat, interest in TBI has never been greater. Biomarkers for Traumatic Brain Injury will fulfil a gap in our understanding of what is occurring in the brain following injury that can subsequently be detected in biological fluids and imaging. This knowledge will be useful for all researchers and clinicians interested in the biochemical and structural sequelae underpinning clinical manifestations of TBI and help guide appropriate patient management. Current and prospective biomarkers for the assessment of traumatic brain injury (TBI), particularly mild TBI, are examined using a multidisciplinary approach involving biochemistry, molecular biology, and clinical chemistry. The book incorporates presentations from outstanding researchers and clinicians in the area and describes advanced proteomic and degradomic technologies in the development of novel biomarker assays. For practical purposes, the focus of this volume is on detection of blood-based biomarkers to improve diagnostic certainty of mild TBI in conjunction with radiological and clinical findings. It represents contributions from internationally-recognized researchers at the forefront of traumatic brain injury many of whom are recipients of grants and contracts from the United States Department of Defense for research specifically on developing diagnostic tests for TBI. The book will be essential reading for scientists, pharmacologists, chemists, medical and graduate students.
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Biomarkers for Traumatic Brain Injury
By Svetlana A. Dambinova, Ronald L. Hayes, Kevin K. W. Wang
The Royal Society of ChemistryCopyright © 2012 Royal Society of Chemistry
All rights reserved.
Clinical Relevance of Biomarkers for Traumatic Brain Injury
KERSTIN BETTERMANN AND JULIA E. SLOCOMB
Traumatic brain injury (TBI) is defined as damage to the brain due to sudden trauma, either by penetrating or, more commonly, by closed head injury. TBI can be either focal or diffuse and is classified as mild, moderate, or severe, as defined in Table 1.1 by the U.S. Department of Defense and Veterans Administration Traumatic Brain Injury Task Force.
Moderate and severe TBI have long been known to lead to impairments in motor and vision function, cognition, attention, memory, and executive functions. However, in recent years it has been discovered that even mild TBI (mTBI) can also have lasting and commonly disabling effects including headache, concentration difficulties, insomnia, mood disturbances, and dizziness. Regardless of severity, TBI involves two phases – the primary injury, which is acquired during impact, and the secondary injury, which is due to brain damage that evolves throughout the subsequent hours and days. Although some of the contributing factors of this secondary damage are understood, such as cerebral hyperemia or dysfunction of cerebrovascular autoregulation, diagnosis and treatment of secondary brain injury remains rudimentary despite years of research. As mild TBI typically is not associated with any changes on brain MRI and is difficult to assess by standard diagnostic workup, its pathophysiology is not well known. There is no objective test for mild TBI, other than history and neurological examination, and there remains great disparity in our ability to identify mild TBI and predict clinical outcome. Therefore, there is an unmet need to improve the diagnosis of patients with mild TBI and to identify individuals who are susceptible to secondary brain injury. Recent advances in biomarker research may help to fill these gaps by improving diagnostic certainty. In the future, biomarkers of mild TBI may help to guide therapeutic decisions and could be useful in predicting clinical outcome.
1.2 Epidemiology and Health Care Economics
Globally, TBI is the leading cause of mortality and morbidity in individuals under the age of 45 years. Approximately 1.7 million people in the United States sustain a TBI each year, resulting in more than 235 000 hospitalizations and 50 000 deaths. The rate of death as a result of TBI is three times higher in males than in females. However, estimated morbidity and mortality rates for TBI are probably imprecise due to inaccurate recording of cause of death, cause of injury, circumstances of injury, and inconsistencies in the diagnosis of TBI. A significant number of mild TBI cases probably go unrecorded, as many victims do not seek medical attention. The National Institute of Neurological Disorders and Stroke estimates that between 2.5 and 6.5 million Americans alive today have had one or multiple TBIs. Mild TBI, which accounts for 80–90% of all cases, is the most prevalent form of brain injury. Among civilians, TBI is most common in children ages 0–4 years, adolescents age 15–19 years, and adults age 65 or older.
U.S. military combat missions, which have increased in recent years, have created an additional large population of young TBI survivors, mostly due to blast injuries. As of 2006, roughly 30% of injured U.S. soldiers returning from Iraq to Walter Reed Army Medical Center were diagnosed with TBI. While the precise number of TBIs among U.S. troops remains unknown, it has been estimated by the U.S. Department of Defense that 212 742 service men and women have been diagnosed with TBI between January 2000 and May 2011.9 TBI is commonly referred to as the "signature injury" of the wars in Iraq and Afghanistan, as TBI is thought to be more prevalent now than in any past wars in U.S. history.
Given that TBI is most common among young adults, costs to society can be significant. Depending on the severity of their injuries and disabilities, these survivors often require specialized care for the rest of their lives. An estimated 5.3 million Americans currently have long-term disabilities following TBI, resulting in an estimated $60 billion in health care expenditures annually. For individuals, the significant financial burden that results from TBI depends on factors including duration of acute care, duration of rehabilitation and long-term care, therapies, prescription costs, loss of employment, and need for medical equipment. The U.S. National Institutes of Health estimates that lifetime care for a survivor with severe TBI can cost between U.S.$ 600 000 and U.S.$ 1 875 000.
In light of the growing TBI epidemic, use of biomarkers for the diagnosis of TBI and its potential sequelae is essential to optimize patient care and help improve clinical outcome. Additionally, biomarkers could be potentially useful to identify individuals at risk for secondary injury following the initial impact. Such individuals are more prone to subsequent brain damage and long-term disability if they are released prematurely to return to high risk behaviors and activities. Biomarkers could thus play an important role in the care of patients with TBI.
Currently, a diagnosis of TBI is based on medical history, findings on neurological examination, clinical assessment scales, and neuroimaging, such as computed tomography (CT) of the head and magnetic resonance imaging (MRI), single-photon emission computed tomography (SPECT), or positron emission tomography (PET) of the brain. While a diagnosis of moderate to severe TBI is often self evident from patient history and associated signs of injury or abnormalities on neurological examination and neuroimaging, a diagnosis of mild TBI frequently remains difficult. Multiple classification schemes for mild TBI exist; although there is some overlap, there is also controversy regarding the qualifying diagnostic criteria, such as presence of loss of consciousness and duration of changes in sensorium. It has been generally accepted that mild TBI is defined by an injury that is significantly severe enough to cause transient confusion, impairment of memory function, and disorientation with or without accompanying loss of consciousness. Following these transient initial findings, the patient may first seem to have returned to his/her normal neurological baseline and has a normal neurological examination. However, frequently patients with mild TBI then develop non-specific symptoms such as fatigue, headaches, visual disturbances, memory difficulties, inability to concentrate, inattentiveness, sleep disturbances, dizziness and imbalance, emotional disturbances, depression and/or seizures, which can occur immediately following the injury or after a delay of days to several weeks. These symptoms may be subtle and can go unrecognized until the patient or others notice work performance difficulties, personality changes, or inability to perform complex tasks. Family or patients may notice increased sensitivity to light, irritability, mood changes, confusion, and mental slowing. However, as the neurological examination and neuroimaging studies are frequently normal, the diagnosis of mild TBI remains challenging and there are no objective prognostic predictors following mild TBI.
Multiple clinical assessment scales to grade the severity of TBI have been utilized, including the Cantu or the American Academy of Neurology grading systems. The most widely known and replicated scale is the Glasgow Coma Scale (GCS), which assesses best motor, eye opening, and verbal response that can be observed spontaneously and following verbal or painful stimulation. While the GCS has proven its utility in the clinical management and prognosis of severe TBI patients, it cannot provide information about the pathophysiological mechanisms responsible for a patient's neurological deficits. In addition, specific patient populations are difficult to assess with the GCS, particularly those who suffer from mild or moderate TBI. Many mild head trauma patients with a GCS between 13 and 15 are also intoxicated with drugs and alcohol that frequently confound clinical neuropsychological examinations. Head injuries may also be overlooked in multi-trauma patients.
Neuroimaging techniques can provide additional clinical information, but the diagnostic capabilities of these techniques are limited by their sensitivities, accessibility, high capital costs, and service requirements. CT is the most rapid and widely available neuroimaging technique, yet its usefulness in detecting diffuse brain damage from axonal shear injury is limited. In critically ill patients and in those with contraindications to undergo MRI, brain MRI frequently cannot be obtained. Furthermore, limited availability, high cost, and the time to acquire and to analyze images limit the use of functional MRI, PET, and SPECT scans. SPECT and PET are capable of detecting regional changes in blood flow but cannot necessarily detect structural damage. Furthermore, MRI and CT often do not provide information that can predict outcome of mild TBI and concussion (for details on neuroimaging in TBI, see also Chapter 2). Typical findings on brain MRI following moderate TBI are shown in Figure 1.1.
A rare, but potentially life-threatening, complication following TBI that is primarily observed in athletes is the second impact syndrome. Individuals who have sustained an acute TBI frequently develop a delayed disruption of the cerebrovascular autoregulation leading to diffuse cerebral hyperemia, a hypermetabolic state, and dysfunction of the blood-brain barrier. During this phase, the brain is highly susceptible to severe tissue damage from recurrent head injury. A second impact during this critical phase can cause malignant cerebral edema, which can be refractory to any treatment and may cause brain herniation, devastating secondary cerebral injury, and death. It would be of great clinical importance to identify individuals at risk for second impact syndrome. Biomarkers could potentially be useful to identify these individuals and help guide the decision of when it is safe to return the athlete to play or for a soldier to return to active duty.
Long-term sequelae following TBI include vision changes, speech and cognitive impairment, personality and mood changes, abnormal behavior, paralysis, sensory deficits, gait dysfunction, seizures, chronic pain, and various degrees of disability. Biomarkers may be useful in predicting these sequelae and to help optimize treatment and preventive strategies following TBI.
1.4 Potential Clinical Applications of TBI Biomarkers
Detection of markers of brain tissue injury is complicated by the presence of the blood-brain barrier (BBB). Any potentially useful biomarker has to penetrate this barrier to become released into the bloodstream, cerebrospinal fluid, or other body fluid in sufficient quantities to be reliably identified. From a practical standpoint, a simple point of care blood test would be ideal. Although the concept is intuitively simple, markers of brain injury are difficult to detect because of the complexity of the pathophysiological changes that take place in the brain following TBI, including the activation of multiple molecular pathways of neurodegeneration and neuroregeneration, metabolic compromise, oxidative stress, inflammation, vascular dysfunction, and secondary tissue injury, making it difficult to isolate brain-specific biomarkers.
An ideal TBI biomarker must be brain-tissue specific, sensitive, and should be detectable in blood within minutes of the onset of symptoms; ideally, it should also be inexpensive and easy to measure. Moreover, the concentration of the biomarker should correspond to lesion size, location, and functional outcome. Its predictive value needs to be compared to the sensitivity and specificity of brain MRI, clinical assessment scales, and the neurocognitive function tests that serve as reference standards for the diagnosis and prognosis of TBI. Currently, there is no sufficiently specific and sensitive single biomarker or panel of biomarkers validated by large clinical trials. Thus, many questions regarding the clinical utility of biomarkers for TBI remain.
Reliable and cost-effective biomarkers could help establish a diagnosis of TBI, especially in those with mild cases, where TBI which often goes undetected. In patients who have no immediate access to brain MRI or have contraindications to receiving an MRI or advanced neuroimaging, biomarkers could improve diagnostic certainty, allowing timely identification and triaging for further management at specialized medical centers. Furthermore, in the acute setting and during early clinical follow-up, they could be useful predictors of clinical outcome and potential complications, such as the development of malignant cerebral edema or ischemia that can follow TBI. They could also serve useful in identifying individuals who are at risk for second impact syndrome, guiding the decision of when someone who has suffered a TBI can safely assume his/her activity.
If biomarkers are proven to be sensitive and specific for diagnosis of mild TBI, diagnostic errors could be reduced, adverse outcomes could be minimized, and patients requiring hospital admission could be clearly identified and treated. All of these measures could theoretically help reduce health care expenditures and improve the quality of patient management. However, it remains to be shown whether TBI biomarkers have sufficient predictive value and/or produce likelihood ratios that are acceptable for standard use in clinical practice.
1.5 Human Biomarkers for TBI
Types of biomarkers for TBI currently include – but are not limited to – proteins, enzymes, protein degradation products, cytokines and, more recently, micro-RNAs. For an overview of human studies of CSF and blood biomarkers for TBI, see Table 1.2. This table serves as an overview of the most commonly studied biomarkers in humans and is not meant to be complete; the interested reader is encouraged to refer to the existing literature for more in-depth information.
Great variability in methodologies and study populations exist in the current TBI biomarker literature. Different outcome measures, sampling times, inclusion/exclusion criteria, types of "healthy controls," definitions of severity of TBI, diagnostic biomarker cut-off levels, and measurement methods for biomarkers have been used, making it difficult to compare the efficacy and reliability of multiple biomarkers described in the different human studies. In addition, there is very limited research on mild TBI to date. Thus far, the large majority of research in TBI biomarkers has focused on S100beta, neuron-specific enolase (NSE), myelin basic protein (MBP), and glial fibrillary acidic protein (GFAP).
One of the most widely researched biomarkers for TBI is S100beta, a calcium-binding protein that can be detected in serum and cerebrospinal fluid (CSF) following TBI. Within the human body, S100beta has been found in the cytoplasm of astroglia; in Schwann cells, adipocytes, and chondrocytes; and in melanoma cells; therefore, it cannot be considered a brain-specific biomarker. It is believed that S100beta is released into the central nervous system (CNS) following glial cell damage. If the brain injury is accompanied by breakdown of the BBB, sufficient quantities of S100beta will be released into the bloodstream, allowing detection. Following severe TBI, S100beta serum levels are significantly elevated and correlate well with poor clinical outcome and abnormal head CT findings. In most studies, the biomarker has generally been found to peak within the first 12 hours following injury, although this time course has been variable between studies. Overall, S100beta serum levels have high sensitivity but only poor specificity for diagnosis of moderate to severe TBI. In contrast to many other potential biomarkers of TBI, studies have been conducted to assess its diagnostic use for mild TBI. Some studies have shown a correlation between elevated S100beta in serum following mild TBI and clinical outcome, but other studies could not confirm these findings and revealed conflicting results. Cut-off levels for diagnosis of mild TBI also varied widely between studies. The most significant drawback to the diagnostic use of S100beta is that it is not specific for TBI and cannot distinguish between brain injury and peripheral multi-trauma. Elevated serum S100beta levels are not only observed after TBI, but may also reflect release due to neurodegenerative processes associated with aging or breakdown of the BBB from other etiologies. Ultimately, S100beta lacks the specificity for TBI to make it a reliable diagnostic tool for clinical use.
Excerpted from Biomarkers for Traumatic Brain Injury by Svetlana A. Dambinova, Ronald L. Hayes, Kevin K. W. Wang. Copyright © 2012 Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Meet the Author
Svetlana A Dambinova DSc, PhD is a Distinguished Professor at the WellStar College of Health and Human Services, Kennesaw State University, USA. She has more than 35 years of experience in the fields of molecular neurobiology, neurochemistry, and laboratory medicine. She spent most of her academic life at the Institute of Experimental Medicine, the Institute of Human Brain and Pavlov's Medical University in St Petersburg, Russia, during her career from a predoctoral student to a Professor. Ronald L Hayes PhD is Founder & President and a Distinguished Principal Investigator at the Center of Innovative Research, Banyan Biomarkers, Inc, USA. His research involves studying the pathobiology of traumatic brain injury and he employs contemporary biochemical approaches to examine proteolytic mechanisms of cell injury and death. Kevin KW Wang PhD is Founder, Chief Operations & Scientific Officer and Executive Director at the Center of Innovative Research, Banyan Biomarkers, Inc, USA.
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