Approach to Central Nervous System Injury Initial Considerations

Important considerations in an acute setting of central nervous system (CNS) trauma are:

1. Consciousness

2. Intracranial pressure

3. Cerebral perfusion

4. Cranial structures volume changes (the Kellie-Monroe principle) (1,2)

A dramatic change in any of these usually signifies an impending or already developing trend and directly affects the patient's survival and prognosis.

Consciousness is defined as the ability to be aware of oneself and one's surroundings and is loosely attributed to the activity of the reticular formation, an extensive and fragile neuronal network (2).

Intracranial pressure (ICP) is the normally positive pressure of the cerebrospinal fluid (CSF) present in the cranial cavity. It ranges from 5 mm Hg in an infant to 15 mm Hg in an adult (2).

Cerebral perfusion pressure (CPP) equals mean blood pressure minus ICP and physiologically should be higher than 70 mm Hg in adults and 60 mm Hg in children. Another measure is the cerebral perfusion rate: it is well known from emergency practice that if cerebral perfusion falls below 12 mL of blood per mg of neural tissue per minute, irreversible brain damage occurs (3).

Since the cranial space is closed, the Kellie-Monro principle asserts that changes in one of the intracranial components (e.g., CSF or blood) will result in compensatory alteration in the others (e.g., brain volume.) In practice, increases of ICP cause herniation of the brain matter through natural openings such as the tentorium hiatus (uncal herniation, commonly associated with the ipsilateral compression of cranial nerve III and dilatation of the ipsi-lateral pupil) or the foramen magnum (hindbrain herniation). Both can result in brainstem compression and death (conning) (4).

Both direct impact and contrecoup injuries, in which the moving brain careens onto the skull opposite the point of impact, can result in focal bleeding beneath the calvarium. Such bleeding can result in an intracerebral focal contusion or hemorrhage as well as an extracerebral hemorrhage. Axonal injury increasingly has been recognized as a structural sequela of brain injury. Interestingly, a prominent locus of axonal damage has been the for-nices, which are important for memory and cognition. More severe and diffuse axonal injury has been found to correlate with vegetative states and the acute onset of coma following injury (2,4).

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