Concussion and Mild Traumatic Brain Injury: Definitions, Distinctions, and Diagnostic Criteria

Noah D. Silverberg, Rael T. Lange, and Grant L. Iverson

INTRODUCTION

A traumatic brain injury (TBI) is a disturbance in brain physiology and function caused by an external mechanical force. Giza and Hovda [1–4] described the complex interwoven cellular and vascular changes that occur following mild TBI (MTBI) as a multilayered neurometabolic cascade. The primary mechanisms include ionic shifts, abnormal energy metabolism, diminished cerebral blood flow, and impaired neurotransmission. MTBI may or may not result in macrostructural brain damage visible on computed tomography (CT) or magnetic resonance imaging (MRI).

TERMINOLOGY

A concussion, by definition, is an MTBI. By convention, MTBI is typically used in civilian trauma and military settings. Concussion is the preferred term in sport. Much of the sports medicine literature uses the more precise phrase “sport-related concussion.” Because concussions in sport typically fall on the milder end of the MTBI spectrum, some suggest that concussions represent a subgroup of MTBI and should not be used synonymously with MTBI [5,6].

MTBI can also be subclassified into uncomplicated and complicated on the basis of neuroradiologic findings. As originally conceptualized, a complicated MTBI is diagnosed if the person meets operational criteria for MTBI (see the following) and has some trauma-related macroscopic intracranial abnormality (e.g., subarachnoid hemorrhage, intraparenchymal hemorrhage, subdural hematoma, epidural hematoma, or contusion) on acute CT [7]. MRI can detect abnormalities missed by CT [8–11]. More sophisticated experimental quantitative neuroimaging methods may identify neurometabolic, functional, and microstructural changes associated with MTBI [12–14], but these changes have not been considered part of the definition of complicated MTBI. Moreover, the reliability, validity, accuracy, and usefulness of these experimental techniques in clinical practice have not been established.

MTBI is distinguished from moderate-to-severe TBI on the basis of the acute clinical presentation, in particular, the duration of loss of consciousness (LOC), the level of consciousness (as measured by the Glasgow Coma Scale), and the duration of posttraumatic amnesia (PTA) (see definitions in the following).

DEFINITIONS

There is no universally agreed-upon definition of MTBI. Commonly cited definitions have been proposed by the (a) MTBI Committee of the Head Injury Interdisciplinary Special Interest Group of the American Congress of Rehabilitation Medicine (ACRM) MTBI Committee [15], (b) Centers for Disease (CDC) Control and Prevention working group [16], (c) World Health Organization (WHO) Collaborating Centre Task Force on MTBI [17], (d) Department of Veterans Affairs and The Department of Defense (VA/DoD) [18], and (e) Demographics and Clinical Assessment Working Group of the International and Interagency Initiative toward Common Data Elements (CDE) for Research on TBI and Psychological Health [19].

The ACRM, CDC, WHO, VA/DoD, and CDE definitions of MTBI have areas of agreement and discrepancy. All specify that MTBI is caused by a transfer of mechanical energy from an external force. All definitions require clinical evidence of brain injury but differ in how the lower threshold of this criterion is operationalized. Any one of the following constitutes sufficient evidence of brain injury, according to all five definitions: LOC (of any duration), PTA, or a focal neurological sign (e.g., seizures). All five definitions also consider an immediate alteration in mental status to be sufficient but characterize this construct differently. The WHO definition requires that altered mental status is evidenced by transient confusion or disorientation. Other definitions have a less stringent threshold, allowing for altered mental status to be demonstrated by signs and symptoms such as feeling “dazed” or slowed thinking. Retrograde amnesia is sufficient clinical evidence of brain injury for all definitions except the WHO. Note that a frank LOC is not required by any definition.

Some definitions (e.g., WHO and CDE) require that potential confounds of altered mental status such as psychological trauma and alcohol intoxication be considered. For example, MTBI may be ruled out for a patient who cannot recall aspects of the event but has no other evidence of brain injury, responded to the event with intense fear, and experienced acute traumatic stress symptoms (e.g., detachment, derealization, and/or depersonalization).

The CDE definition is for TBI of all severities and so does not have a ceiling threshold. The ACRM and WHO definitions concur that injuries involving a reduced level of consciousness (i.e., Glasgow Coma Scale score of less than 13) or LOC greater than 30 minutes, or a period of PTA lasting more than 24 hours, should not be classified as “mild.” The CDC definition is similar, but recognizes that case ascertainment typically occurs by interview or survey where Glasgow Coma Scale scores are not available. Table 6.1 summarizes the clinical features that differentiate mild from moderate-to-severe TBI. In addition to these clinical features, the VA/DoD uses neuroimaging to stratify TBI severity. Specifically, the guidelines specify that “abnormal structural imaging (e.g., MRI or CT scanning) attributed to the injury will result in the individual being considered clinically to have greater than mild injury” (18; p. 19). The other definitions of MTBI do not exclude patients with trauma-related intracranial abnormalities on structural imaging.

TABLE 6.1 Clinical Features That Differentiate Mild From Moderate-to-Severe TBI

Integrating the ACRM, CDC, WHO, VA/DoD, and CDE definitions, a diagnosis of MTBI should be considered when a person has (a) an LOC immediately following a head trauma event, and/or (b) amnesia for a period that includes or abuts the moment of head impact, and/or (c) an alteration in mental status immediately following a head trauma event; where (a) to (c) cannot be fully explained by factors other than brain injury (e.g., psychological trauma, drug or alcohol intoxication, sedation for pain or intubation, or massive blood loss). In a patient who remembers the moment of head impact and surrounding events, altered mental status is most compellingly demonstrated by confusion (e.g., inability to follow commands or answer orientation questions). Clinical judgment is necessary to differentiate confusion induced by brain injury from being startled by an unexpected event. If the alteration in mental status is equivocal and not accompanied by an LOC or amnesia, contextual factors such as head impact velocity and acute signs (e.g., vomiting) could raise suspicion of an MTBI in a probabilistic manner. An LOC greater than 30 minutes or a PTA period of more than 24 hours would indicate at least a moderate TBI. Obviously, the MTBI classification range includes an extraordinarily broad spectrum of injury severity. Injuries characterized by seconds of confusion to injuries involving several minutes of traumatic coma, several hours of PTA, and a focal contusion visible on day-of-injury CT qualify as MTBIs.

APPLYING THE DEFINITION IN CLINICAL PRACTICE

Patients with MTBI may present for medical attention shortly after (e.g., in the emergency department) or long after (e.g., in a clinic) their altered mental status has resolved. Establishing the diagnosis then rests on a clinical interview, and when possible, an interview of people who observed the head trauma event and a review of medical records from the first medical contacts (e.g., ambulance crew report). Some considerations for clinicians are provided in the following.

A. A careful and deliberate approach should be used that retrospectively assesses for altered consciousness or gaps in memory (retrograde and posttraumatic). One cannot assume that a very careful and thorough approach was taken by health care providers at the scene or in the emergency department. A diagnosis of MTBI can be missed initially, especially in the presence of traumatic bodily injuries that require urgent care. Alternatively, MTBI may be misdiagnosed based on inaccurate history and an inadequate differential diagnosis.

B. Clinicians should be careful not to misinterpret PTA for LOC (e.g., a patient who is experiencing PTA and who was walking and talking following injury often incorrectly states that “I woke up in the emergency room”). Patients cannot give a first-person account of the presence and duration of LOC because, by definition, they will be amnestic for that period. It is best to clarify when a patient is (a) making an inference rather than providing an experiential account, and (b) relaying information that they were told.

C. As many as 50% of patients are intoxicated with alcohol when they sustain an MTBI. Although the best available evidence suggests that alcohol intoxication has a modest impact on Glasgow Coma Scale scores [20,21], severe intoxication must be considered as a potential explanation for confusion and amnesia. Traumatic bodily injuries disrupting hemodynamic or respiratory function can also complicate the identification of mental status changes associated with TBI. Finally, acute medical interventions (e.g., intubation or administration of sedating medications) can make it difficult to estimate the duration of TBI-related mental status changes.

D. Reduced awareness of one’s surroundings at the scene of the event and an inability to later remember parts of the traumatic event are diagnostic criteria for acute stress disorder [22]. In other words, these symptoms are characteristic of psychological “shock” and not necessarily caused by brain injury. Amnesia for the impact and following moments is more likely attributable to brain injury when it is (a) preceded by a temporally graded retrograde amnesia, (b) associated with confusion and disorientation, (c) initially dense but then resolves gradually (i.e., becomes less and less patchy), (d) not preceded by intense fear and related symptoms (e.g., rapid heart rate, muscle tension, depersonalization).

E. Neuroimaging is adjunctive. An urgent CT scan may be indicated to rule out the need for neurosurgical intervention. When identifying MTBI is difficult due to the presence of confounds (e.g., multitrauma and alcohol intoxication), imaging or other neurodiagnostic techniques may be helpful [19].

F. Standardized symptom scales can help track recovery from MTBI, but do not establish the diagnosis of MTBI. Symptoms such as headache, dizziness, fatigue, difficulty concentrating, irritability, and light and noise intolerance are common following MTBI, but they are not specific to MTBI. Orthopedically injured patients report some of these symptoms in the first week following injury. Such symptoms are sometimes endorsed by healthy people and often by people with depression, pain, and other health conditions. These factors are important to consider when a person reports symptoms months following an injury.

G. Neuropsychological testing can be used to examine the consequences of an MTBI, but cannot be used as the basis for the initial diagnosis. Neuropsychological test results can be influenced by numerous demographic, situational, pre-existing, co-occurring, and injury-related factors.

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