Countercoup, or contrecoup, brain injury occurs at sites remote to the actual impact site on the head. The etiology and significance of this type of injury have been debated intermittently for centuries. The classic countercoup injury involves a contusion remote from, and classically opposite to, the actual site of impact to the head. This model requires refinement to appreciate the variation in the actual sites of injury remote from the impact site. Countercoup injury is a focal phenomenon and is unlike diffuse axonal injury or brain edema which may also result from trauma. Countercoup injury has also been considered to play a role in visual abnormalities following minor head injuries.
Countercoup injuries classically occur when the moving head strikes a stationary object; whereas, a coup injury is associated with a moving object impacting on a stationary head. Classic evidence of both coup and countercoup injury is intracerebral hemorrhage or contusion in a focal area noted on CT scan or MRI scan. Countercoup lesions arise from forces within the intracranial cavity which are not directly related to the site of the focal blow, but instead, are related to stress on the brain and its structure caused by the force of the blow on an already moving head.
In the positive pressure theory, these stresses have been caused by the brain lagging the initial movement of the skull followed by compensatory movements at the time of impact. The negative pressure or cavitation theory is that the movement of the brain in one direction generates stress to the opposite area of the brain and causes damage to the cerebral tissue at that point. There is also a rotational shear stress theory which incorporates rotation of the brain in addition to the displacement along the axis of the trauma.
Finally, there is the angular acceleration theory which postulates that the brain is tethered in some areas such, for example, the brainstem, causing specific areas potential to greater acceleration and slowing within the brain. Also the difference in brain and CSF density has been implicated in countercoup injury. This idea hypothesizes that the brain initially moves away from the impact site to be injured on the opposite area of the skull from the impact. Various experimental models have been used for these investigations. The fact that no one explanation accounts for all aspects of countercoup injury may show that a variety of mechanisms contribute to the final result of countercoup injury. The actual injury to the brain often occurs in the irregular surface of the temporal fossa and at the area of the frontal poles. Sometimes the severity of the injury at the countercoup site may be greater than that occurring at the coup site. Indeed, there are several reports of patients with countercoup injuries in the absence of coup injuries.
Countercoup brain injury is one type of traumatic brain injury of which several may coexist in a particular patient. Vehicular accidents and falls are the most common etiologies. These injuries predominate men at younger ages with more even distribution in older adults. Intracerebral hemorrhage occurs in 13% to 48% of adults with traumatic brain injury. Of these patients, 13% to 77% have countercoup hemorrhages which most often occur with occipital and temporal impact.
Location of countercoup brain injuries has been the subject of several studies sometimes with conflicting results. Temporal bone injuries in one study had the best correlation with countercoups injuries also in the temporal lobe area on the opposite side. These patients were younger and often involved in vehicle accidents. Another study found that posterior impact injuries in the occipital area were most likely to cause countercoup injuries in either the frontal or temporal lobe areas. Many of these patients simply fell backward; also some of them were older.
A large series of head injury patients with evidence of traumatic intracerebral hemorrhage on CT was recently studied. The authors attempted to establish criteria for countercoup brain injury by using a location greater than 90 degrees from the site of the coup impact. This means that many countercoup lesions are not exactly opposite the side of the coup impact; the purpose is to establish objective criteria for both coup and countercoups injuries. There was an association between posterior impact sites and temporal intracerebral hemorrhage from countercoup lesions. Frontal impact generated primarily hemorrhages in the area of the initial impact. Countercoup intracerebral hemorrhage was more likely to show hemorrhagic progression possibly requiring surgery with lateral hemorrhages being the most likely to progress. Frontal impacts were least likely to cause countercoup hemorrhages whereas posterior impacts sometimes did cause temporal or frontal countercoup injuries. The total incidence of countercoup hemorrhages in this study was 32.9% of cases although the range of comparison and other studies range from 20% to 77%. The lesion most likely to demonstrate hemorrhagic progression was a countercoup hemorrhage in the temporal region.
The countercoup brain injury has also been implicated in extra-axial brain lesions from trauma. One case report indicates a case of mild head injury in which a skull fracture was noted on the opposite side of the head from the impact of the fall. This patient clinically had only a mild injury to his head and had no loss of consciousness. The patient had a contralateral fracture with no impact to that side; there was no brain or other type of lesion on either CT or MRI. The authors hypothesized that the force of the impact caused local bending of the skull with no fracture at the site of the impact; this force was then the transmitted through the skull to the opposite side to cause the fracture.
There are also reports of epidural hematomas being located in a contralateral location from the side of the trauma and head injury. These countercoup epidural hematomas were sometimes accompanied by acute epidural hematoma at the site of impact but not invariably. The countercoup epidural sometimes was larger than the epidural associated with the coup impact. There are at least five reported cases of this nature of which four patients were taken to surgery. It is hypothesized that the dura may have detached from the inner table of the skull and the small arteries may have caused the accumulation of blood. This would occur without a laceration to a larger artery such as the middle meningeal artery and would not be caused by a fracture of the skull.
The main aspect of patient history includes obtaining a careful evaluation of the circumstances involved in the trauma and any other significant aspects of the history.
The physical findings depend on the exact location of the countercoup injury.
Traumatic brain injury patients require a thorough general trauma evaluation. These patients need a detailed neurological examination and require a head CT for initial assessment of brain injury.
Treatment of countercoup brain injury comprises basic traumatic brain injury care such as close clinical monitoring, repeat CT or MRI as indicated, possible monitoring of intracranial pressure, and close monitoring of other forms of traumatic brain injury.
Countercoup brain injury is one manifestation of traumatic brain injury and must be integrated with multiple other types of brain injury in trauma. The deficits caused by countercoup injury depend on the location affected.
The prognosis of countercoup injury is variable based on the extent of the affected areas and the presence of other types of head injury such as subarachnoid hemorrhage, diffuse axonal injury, and the other multiple types of traumatic brain injury. In one study the outcome at 6 months was favorable in 48% of cases and unfavorable in 52%.
Traumatic brain injury may cause multiple neurological deficits some of which might be permanent. The exact type of deficit is dependent on the area of the brain affected by the countercoup injury.
Patient education as to the multiple aspects of traumatic brain injury and its manifestations such as countercoup injury is necessary to make people aware of the possible life-altering nature of these injuries and to deter dangerous behavior.
Countercoup brain injury has multiple manifestations in addition to the primary one of intracerebral hemorrhage which is used to define countercoup injury in most cases. The essential factors in countercoup injury are an impact and some form of acceleration, deceleration, rotational stress, or angular stress. Both these are necessary as simply a direct blow to the stationary head would be less likely to cause countercoup injury. Also acceleration, deceleration, and rotation injury such as might occur in shaken baby syndrome in the absence of impact directly to the head is not noted to cause countercoup injury.
Since countercoup injury is only one type of traumatic brain injury, it is difficult to make specific recommendations for treatment. The patient must have a treatment plan to include multiple types of traumatic brain injury such as subdural hematoma, epidural hematoma, subarachnoid hemorrhage, cerebral edema, diffuse axonal injury, and coup injuries as well as the subject of this paper, countercoup brain injury.