Chapter 209 Intracranial Pressure Monitoring
Acquired brain injury is a common neurologic emergency typically caused by head trauma, brain disease (tumors, meningoencephalitis, hypoxic injury), metabolic derangements, prolonged seizures, or surgical trauma. Increased intracranial pressure (ICP) often is associated with these processes and may affect outcome seriously. Because the intracranial contents (blood, cerebrospinal fluid [CSF], and brain parenchyma) are encased in a rigid container, there is limited space available for expansion of the contents. As volume increases in the cranial vault from any cause (edema, hemorrhage, mass), there must be a reciprocal decrease in the other volumes for ICP not to increase beyond limits compatible with life.1,2
When compensatory mechanisms in the brain are exhausted, ICP increases and cerebral blood flow is compromised, resulting in secondary injury. Secondary injury is a complex sequence of events that leads to further elevations in ICP, reduced cerebral blood flow, tissue hypoxia, and ischemia. This ultimately perpetuates neuronal death and may result in brain herniation.1,2 Thus, secondary injury is a major contributor to the mortality of animals with acquired brain injury. The primary goal in the treatment of these animals is to minimize the impact of the secondary injury by appropriate and timely treatment to maintain adequate cerebral blood flow. In the clinical setting, cerebral blood flow is reflected most accurately by cerebral perfusion pressure (CPP). CPP is dependent on the mean arterial pressure (MAP) and the ICP, and this relationship is expressed by the formula: CPP = MAP − ICP.1,2 By measuring the ICP, the clinician is able to assess whether CPP is maintained adequately in a patient with severe brain disease or injury.3,4
Although a growing number of studies in humans have suggested decreased mortality rates and improved long-term outcome with ICP-guided therapy, a randomized clinical trial showing that ICP monitoring improves outcome has not been done. The “Guidelines for the Management of Severe Traumatic Brain Injury” (published in 1995 and revised in 2007) outline the evidence-based recommendations for using ICP monitoring to improve the treatment and outcome from severe brain injury.4 Similar guidelines and recommendations were published in 2004 for the management of severe brain injury in infants and children. As yet, no specific guidelines have been established in veterinary medicine for treating severe brain injury. The standard of care has been primarily that of repeated and careful assessments of an animal’s neurologic status in an attempt to detect increases in ICP. Unfortunately, most clinical signs indicating life-threatening intracranial hypertension (ICH) occur as a result of damage to brain tissue, and therapies administered at this point often are ineffective. There are potential benefits gained by monitoring ICP, especially when one expects prolonged and/or life-threatening ICH (Box 209-1).4,5
ICH, Intracranial hypertension; ICP, intracranial pressure.
ICP monitoring commonly is done through a burr hole in the skull or a craniectomy site. It can be measured directly or reflected through measurement of CSF pressure or brain tissue pressure.2,3 CSF pressure measurements can be taken from the lateral ventricles or the cerebral subarachnoid space; brain tissue pressure measurements are taken intraparenchymally from within a cerebral hemisphere. Measurement of ICP from the brain’s surface may be taken epidurally or subdurally over a cerebral convexity1,2,4 (Figure 209-1).
Although there are very few data in veterinary medicine with respect to the role of ICP monitoring in patients with brain disease, several studies in animals have shown that ICP can be monitored accurately. Historically, CSF pressure was measured using a manometer and needle puncture of the cisterna magna. This method requires that the patient undergo general anesthesia and does not allow for ongoing ICP measurements needed to guide the clinician in treatment decisions. In addition, CSF pressures measured at the cisterna magna may not accurately reflect more compartmentalized elevations in ICP. In animals with global ICH, there is the added risk of brain herniation through the foramen magnum with this method.
Pressure transducers convert ICP into a graded electrical signal that is recorded and displayed. They can be situated either intracranially or extracranially depending on the system used. Extracranial strain gauge type transducers communicate with the intracranial compartment via fluid-filled tubing and require that ICP measurements be taken at fixed reference points. Pressure transducers situated intracranially are incorporated into the tip of a catheter and implanted into one of several compartments of the brain. Some of the important considerations in choosing a transducer are listed in Box 209-2.2-4