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I was disappointed to see a recent editorial in one of our sister sport medicine journals. The journal issue concerned was a supplement devoted to sport related concussive injuries. While such noble sentiments are to be supported nevertheless the terminology of concussion once again is becoming confused.1 The authors use the term “mild traumatic brain injury” to describe the clinical entity of concussion. A seemingly small change yet one that has important implications for the understanding of the clinical problem, and more importantly serves to confuse clinicians reading published articles on the topic. It may be useful for clinicians to understand the background of this issue.
One of the major limitations in this field is that there is no universal agreement on the standard definition or nature of concussion.2–4 The historical context of this injury refers to a transient disturbance of neurological function caused by “shaking” of the brain that accompanies low velocity brain injuries.5–7 The clinical manifestations of concussion as a transient neurological syndrome without structural brain injury have been known since the 10th century AD.8
Following pioneering experimental work demonstrating the transient and functional nature of concussion by Denny-Brown et al, the term “acceleration concussion” was proposed as the generic descriptor that should be applied to all forms of traumatic brain injury.9 Implicit in this concept is that the term concussion should be synonymous with traumatic brain injury. A variation on this view holds that concussion refers to the mechanism of injury and motion of the brain within the skull rather than any clinical symptoms or pathology.10, 11 In some quarters, this view has been modified to incorporate a threshold effect beyond which permanent or structural brain damage may occur, and that the degree of pathological damage is dependent upon the direction and magnitude of rotational forces on the brain following impact.12, 13
In an attempt to resolve this confusion, the Committee on Head Injury Nomenclature of the Congress of Neurological Surgeons proposed a “consensus” definition of concussion, which was subsequently endorsed by the American Medical Association and the International Neurotraumatology Association.14, 15 This definition has now become the accepted definition by most researchers in this field. The Congress of Neurological Surgeons definition states that concussion is: “ … a clinical syndrome characterised by the immediate and transient post-traumatic impairment of neural function such as alteration of consciousness, disturbance of vision or equilibrium due to mechanical forces”
Developing in parallel with the term concussion has been the term “mild brain injury”. Jennet et al proposed the Glasgow Coma Scale (GCS) as a prospectively validated prognostic scale for the assessment of traumatic brain injury.16 This scale distinguished mild, moderate, and severe brain injury on the basis of a standardised score at six hours following injury. The time frame was used in order to exclude “trivial” injuries from the analysis and to allow resuscitation to occur with stabilisation of the underlying brain injury prior to assessment.
Because the GCS was designed to be applied six hours after brain injury, the full spectrum of brain injury must also encompass a “minimal” injury subset that falls below the threshold for a GCS mild injury as measured at six hours. In clinical practice, the majority of sporting concussions falls into this group. In lay parlance, the typical descriptors of these minimally significant injuries include being “dinged” or “having one's bell rung”. Concussion is a subset of GCS mild brain injury, however, the converse is not true and the terms cannot be used interchangeably.
How then can we resolve the difficulties in concussion definition? Considering that the principal limitation of the Congress definition is the duration of symptoms in some cases—that is, not transient—the alternative does not adequately reflect the common clinical condition of concussion seen on sporting fields throughout the world each week. Several common features that incorporate clinical, pathological and biomechanical injury constructs that may be utilised in defining the nature of a concussive head injury include2:
• Concussion may be caused either by a direct blow to the head or elsewhere on the body with an “impulsive” force transmitted to the head
• Concussion results in an immediate and short-lived impairment of neurological function
• Concussion may result in neuropathological changes however the acute clinical symptoms largely reflect a functional disturbance rather than structural injury
• Concussion may result in a graded set of clinical syndromes that may or may not involve loss of consciousness. Resolution of the clinical and cognitive symptoms typically follows a sequential stereotyped course
• Normal conventional (computer tomography and magnetic resonance scanning) neuroimaging studies.
In attempting to resolve this problem, an “all encompassing” definition along the lines of the neurosurgical approach or even simply defining concussion as “a (transient) post-traumatic impairment in neurological function” remains inadequate. It may be that with further research a more specific time limited categorisation may be incorporated, however, at the present time, evidence is lacking as to a precise separation from more severe categories of brain injury.
The use of the term “mild traumatic brain injury”, however, is inadequate to define the problem and more importantly is inappropriate to understand the conceptual relationship between mild brain injury as defined by the GCS and the historical understanding of concussion. To use the terms interchangeably is incorrect conceptually and adds to, rather than detracts from, the existing confusion in understanding the problem. We should all speak with one voice on this issue—concussion!
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