Pathomechanics Of Traumatic Brain Injury

A forceful blow to the resting movable head usually produces maximal brain injury beneath the point of cranial impact (coup injury). A moving head hitting against an unyielding object usually produces maximal brain injury opposite the site of cranial impact (contrecoup injury) as the brain bounces within the cranium. When the head is accelerated prior to impact, the brain lags toward the trailing surface, thus squeezing away the

CSF and allowing for the shearing forces to be maximal at this site. This brain lag actually thickens the layer of CSF under the point of impact, which explains the lack of coup injury in the moving head injury. On the other hand, when the head is stationary prior to impact, there is neither brain lag nor disproportionate distribution of CSF, accounting for the absence of contrecoup injury and the presence of coup injury. Many sport-related concussions involve a combined coup-contrecoup mechanism but are not considered to be necessarily more serious than an isolated coup or contrecoup injury.28 If a skull fracture is

Table 14-3

Traumatic Intracranial Lesions

Type

Mechanism

Injured Structures

Signs and Symptoms

Cerebral contusion

Object hits skull Skull hits object

Injured vessels bleed internally Progressive swelling may injure brain tissue not originally harmed

Loss of consciousness, partial paralysis, hemiplegia, unilateral pupil dilation, altered vital signs

Cerebral hematoma Epidural

Subdural

Intracerebral

Severe blow to head; skull fracture

Force of blow thrusts brain against point of impact Depressed skull fracture, penetrating wound, acceleration-deceleraton injury

Middle meningeal artery

Subdural vessels tear and result in venous bleeding Torn artery bleeds within brain substance

Neurologic status deteriorates in

10min-2hr Neurologic status deteriorates in hours, days, or weeks Rapid deterioration of neurologic status

Second impact syndrome

Sustains second injury before symptoms from first injury resolve

Brain loses autoregulation of blood supply; rapidly swells and herniates

Typically occurs within 1wk of first injury; pupils rapidly dilate, loss of eye movement, respiratory failure, eventual coma

Table 14-4 Types of Skull Fractures

Type

Description

Depressed

Portion of the skull is indented toward the brain

Linear

Minimal indentation of skull toward the brain

Nondepressec

Minimal indentation of skull toward the brain

Comminuted

Multiple fracture fragments

Basal/basilar

Involves base of skull

present, the first two scenarios do not pertain because the bone itself, either transiently (linear skull fracture) or permanently (depressed skull fracture) displaced at the moment of impact, may absorb much of the trauma energy or may directly injure the brain tissue (Table 14-4). Focal lesions are most common at the anterior tips and the inferior surfaces of the frontal and temporal lobes because the associated cranial bones have irregular surfaces.19,20

There are three types of stresses that can be generated by an applied force when considering injury to the brain: compressive, tensile, and shearing. Compression involves a crushing force whereby the tissue cannot absorb any additional force or load. Tension involves pulling or stretching of tissue, and shearing involves a force that moves across the parallel organization of the tissue. Uniform compressive stresses are fairly well tolerated by neural tissue, but shearing stresses are very poorly tolerated.17,19,20

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