Traumatic Brain Injury

Central nervous system (CNS) injury is the most common cause of death from injury. Two million people per year in the United States suffer traumatic brain injuries (TBIs), many as the result of motor vehicle crashes and falls. Approximately 50,000 deaths per year and 500,000 hospital admission are attributable to head injury. Most of these victims are between the ages of 16 and 30 years. The increasing use of seat belts and airbags has resulted in an estimated 20% to 25% reduction in these traffic fatalities. However, the incidence of penetrating injury to the brain and spinal cord is increasing. As awareness of the correct methods for brain injury management grows, guidelines for TBI have developed and been shown to improve outcome.

I. ANATOMY AND PHYSIOLOGY

A.

The skull is particularly thin in the temporal region and thick in the occiput. The floor of the cranial cavity is divided into three regions: anterior (frontal lobes), middle (temporal lobes), and posterior (lower brainstem and cerebellum).

B.

The meninges cover the brain in three layers: dura mater (fibrous membrane that adheres to the internal surface of the skull), arachnoid membrane, and pia mater (attached to the surface of the brain). Cerebrospinal fluid (CSF) circulates between the arachnoid and pia mater in the subarachnoid space.

C.

The brain is composed of the cerebrum, cerebellum, and the brainstem. The brain-stem consists of the midbrain, pons, and medulla. The reticular activating system (responsible for state of alertness) is within the midbrain and upper pons. The car-diorespiratory centers reside in the medulla. Small lesions in the brainstem can cause profound neurologic deficit.

D.

The Monro-Kellie doctrine states that the total volume of intracranial contents must remain constant because of the rigid bony cranium. With an expanding mass lesion, the intracranial pressure (ICP) is generally within normal limits. As cerebral edema worsens and brain swelling increases, CSF and blood volume within the skull decrease to compensate until the point of decompensation on the pressure-volume curve is reached; ICP then dramatically increases.

E. Cerebral perfusion pressure (CPP) = Mean arterial pressure - ICP.

Maintenance of cerebral perfusion is essential in the management of patients with severe closed head injury. Normal cerebral blood flow (CBF) is approximately 50 mL/100 g brain/minute. CBF <20 mL/100 g brain/minute represents cerebral ischemia, and cell death occurs at approximately 5 mL/100 g brain/minute. In addition to cerebral ischemia in response to injury, the injured brain loses its ability to autoregulate blood flow, increasing susceptibility of the injured brain to further ischemia. The generally acceptable range during active therapy in traumatic brain injury is CPP >60 to 70 mmHg.

II.

TBIs are categorized as mild (80%), moderate (10%), or severe (10%), depending on the level of neurologic dysfunction at the time of initial evaluation. Determination of the Glasgow Coma Scale (GCS) score as early as possible and then serially

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is essential. Loss of consciousness (LOC) is an important indicator of TBI. Classification of TBI is based on the GCS.

A. Mild head injury

• GCS score of 13 to 15
• Brief period of LOC
• Prognosis is excellent
• Mortality rate <1%

B. Moderate head injury

• GCS score of 9 to 12
• Typically, confused and may have focal neurologic deficits; able to follow simple commands
• Prognosis is good
• Mortality rate <5%

C. Severe head injury

• GCS of ≤8—generally, the accepted definition of coma
• Unable to follow commands
• Until recently, mortality >40%
• Most survivors have significant disabilities
• Airway control is essential
• Elevated ICP is a common cause of death and neurologic disability