45: Increased Intracranial Pressure and Traumatic Brain Injury

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CHAPTER 45 Increased Intracranial Pressure and Traumatic Brain Injury

Matthew Hall, MD

1 Define elevated intracranial pressure

Elevated intracranial pressure (ICP) is usually defined as a sustained pressure of greater than 20 mm Hg within the subarachnoid space. The normal ICP is approximately 10 to 18 mm Hg.

2 What are the determinants of intracranial pressure?

The space-occupying contents of the skull, the brain (85%), the cerebrospinal fluid (CSF) (10%), and cerebral blood volume (15%), are all contained in the virtually fixed volume of the cranium. Once compensatory mechanisms are exhausted (mostly translocation of CSF), any increase in volume will cause a rise in ICP.

3 How is intracranial pressure measured?

Various techniques are available, including ventricular catheters, subdural-subarachnoid bolts, epidural transducers, and intraparenchymal fiber-optic devices. The standard method is a ventriculostomy, in which a plastic catheter is introduced into the anterior aspect of the lateral ventricle via a burr hole in the cranium. The catheter is attached to saline-filled tubing, and the pressure is transduced. In addition, this catheter can be used to drain CSF via a pop-off valve set at 20 cm H2O. Another common method is the subarachnoid bolt, which is also placed through a burr hole but does not require insertion through brain tissue or identification of the position of the ventricle. A third technique involves the insertion of a fiber-optic bundle through a small burr hole. The fiber-optic bundle senses changes in the amount of light reflected off a pressure-sensitive diaphragm at its tip. However, the fiber-optic bundle is subject to drift in the measurements over time and cannot be recalibrated in situ. In addition, the intraparenchymal and subdural methods are limited by the inability to withdraw CSF and manage ICP. Newer generations of fiber-optic ICP monitors allow simultaneous measurement of ICP, local cerebral blood flow (CBF) with Doppler, pH, PO2, and PCO2 and avoid the potential infectious problems of a fluid-filled transducing system.

4 Summarize the conditions that commonly cause elevated intracranial pressure

See Table 45-1.

TABLE 45-1 Common Causes of Elevated Intracranial Pressure

Increased CSF Volume Increased Blood Volume Increased Brain Tissue Volume
Communicating hydrocephalus Intracerebral hemorrhage (aneurysm or AVM) Neoplasm
Obstructing hydrocephalus Epidural or subdural hematoma Cerebral edema (CVA, encephalopathic, metabolic, traumatic)
  Malignant hypertension Cysts

AVM, Arteriovenous malformation; CSF, cerebrospinal fluid; CVA, cerebrovascular accident; ICP, intracranial pressure.

5 Describe the symptoms of increased intracranial pressure

Symptoms associated with increased ICP alone include headache, vomiting, papilledema, drowsiness, loss of consciousness, and behavioral changes. Several other symptoms such as pathologic (decerebrate) posturing, oculomotor nerve palsy, abnormalities of brainstem reflexes, and abnormal respiratory patterns (including apnea), are probably caused by brainstem distortion or ischemia secondary to impending herniation. The Cushing reflex, consisting of hypertension and reflex bradycardia, is likely caused by medullary ischemia and occurs when ICP approaches systemic arterial pressure.

6 Discuss the possible consequences of increased intracranial pressure

In addition to producing the previously mentioned symptoms, increased ICP can lead to a decrease in cerebral perfusion pressure, which may result in regional or global cerebral ischemia. Further elevations in ICP may result in herniation of brain contents (across the falx cerebri, tentorium, or foramen magnum).

7 What are the determinants of cerebral perfusion pressure?

Cerebral perfusion pressure (CPP) is usually defined as the difference between mean arterial pressure (MAP) and ICP or central venous pressure, whichever is higher.

8 What is intracranial elastance? Why is it clinically significant?

Intracranial elastance, commonly misnamed intracranial compliance, refers to the variation in ICP in accordance with intracranial volume. Because intracranial components can shift their volumes to an extent (e.g., CSF movement from the intracranial compartment to the spinal compartment), ICP remains somewhat constant over a certain range of volume. However, when compensatory mechanisms are exhausted, ICP rises rapidly with further increases in volume (Figure 45-1).

image

Figure 45-1 Intracranial elastance. ICP, Intracranial pressure.

9 How is cerebral blood flow regulated?

CBF is coupled to cerebral metabolic rate by an uncharacterized mechanism but may involve potassium, hydrogen, calcium ions, adenosine, nitric oxide, and prostaglandins. In general, increases in the cerebral metabolic rate for oxygen (CMRO2) lead to increases in CBF, although the increase in flow is delayed by 1 to 2 minutes. Several other parameters influence flow. Specifically an increase in the partial pressure of carbon dioxide in arterial blood (PaCO2) is a powerful cerebral vasodilator, with CBF increasing/decreasing 1 to 2 ml/100g/min with a 1-mm Hg change in PaCO2. Similarly, a decrease in the partial pressure of oxygen (PaO2

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