Chapter 41: Traumatic Brain Injury

MECHANISMS OF INJURY AND PATHOPHYSIOLOGY

INJURY PATTERNS

Mechanism of Injury: It is important to know the cause of injury to determine severity of injury, triage the patient to the appropriate level of care, and anticipate possible complications.
Primary Injury: Occurs on initial impact, either diffuse or focal. Often results in skull fractures. Focal injuries include contusions, lacerations, and hemorrhages. Diffuse brain injuries include concussions and diffuse axonal injury.
- Concussion: Diffuse brain injury without associated head computerized tomography (CT) findings. Symptoms may include headache, disorientation, emesis, or brief loss of consciousness.¹
- Diffuse axonal injury: Shearing injury caused by acceleration and deceleration resulting in trauma between the gray and white matter.¹
- Epidural hematoma: Rapidly progressing arterial bleed occurring between the skull and dura.² Often secondary to middle meningeal artery and occasionally caused by a venous bleed.¹ Patients may have loss of consciousness followed by a lucid period prior to neurologic deterioration. Elliptical shape on head CT due to limitations of fluid spread by dural attachment at suture lines. See Figure 41-1.
- Subdural hematoma: Venous bleed occurring between the dura and arachnoid. Caused by rupture of the bridging veins during rapid movement of the brain within the skull. Acute subdural hematomas progress rapidly and are associated with high morbidity and mortality.² Concave shape on neuroimaging, extending throughout the subdural space. See Figure 41-2.
- Intracerebral hematomas: Tearing of small vessels in any area of the brain. Symptoms vary depending on location of bleed, but often occur in the frontal and temporal lobes.²
Secondary Injury: Occurs due to decreased perfusion of surviving tissue that causes reduction in oxygen delivery, metabolite delivery, and metabolic waste and toxin clearance. Additional injury results from cerebral herniation syndromes, which can cause focal ischemic injury and brainstem compression.³
- Vasogenic edema: Swelling of interstitial space. Capillary leak from cerebral vasculature into interstitial space due to disruption of blood–brain barrier, occurring primarily in the white matter. Secondary to ischemia, hemorrhage, brain injury, infection, and tumors.²
- Cytotoxic edema: Swelling of brain cells. Secondary to altered osmotic state caused by ischemia with associated extracellular acidosis secondary to anaerobic metabolism and cell membrane rupture.²