The overall goal of this procedure is to simply and accurately measure intracranial pressure in an animal model. In this case, the rat. This is accomplished by first burring, two small holes in the skull.
Next, an ICP monitoring screw and an anchoring screw are inserted into the skull and are cemented into place After screw insertion, a fiber optic pressure transducer is inserted into the monitoring screw for ICP measurement in the epidural space. In the final step, an airtight seal is formed around the pressure transducer in monitoring screw to seal the cranial vault. Ultimately, results can be obtained that show an intracranial pressure physiological trace through the use of an epidural fiber optic pressure transducer.
Visual demonstration of this method is critical as the key steps are difficult to learn because the brain could easily be damaged during the procedure and confound experimental results. The main advantage of this technique over existing methods like invasive interventricular or intraparenchymal ICP measurements is that our epidural ICP measurement technique prevents damage to the brain by keeping the meninges intact. After confirming total anesthesia using a foot pinch, place the rat prone on a warming plate and position the rat's nose in an anesthetic nose cone, shave the scalp, and then wash and disinfect with chlorhexidine, infiltrate the skin at the site of incision with long acting local anesthetic while maintaining anesthesia.
Secure the head in a stereotaxic frame and insert the ear bars until head is stabilized. Ensure that breathing is not impaired. Once the animal is in position, make a 1.5 centimeter skin midline head incision.
Next blunt, dissect the soft tissue and surrounding muscles to clearly locate lambda and bgma. Retract the skin and connective tissue stem any bleeding by applying pressure to the exposed skull using sterile gauze swabs. Using a dental drill with a one millimeter tip burr bur a hole two millimeters wide into the right parietal bone for stroke studies.
Birth a hole, two millimeters lateral and two millimeters posterior from Breg MA to avoid the superior sagittal sinus and to ensure the placement of the ICP sensor is over the ischemic territory, bur the hole to a depth where the skull above the dura becomes translucent. Then use a 0.5 millimeter tip burr to remove the skull at the base of the hole when the skull begins to crack. Use 45 degree forceps to remove all remaining skull, ensuring the base of the hole is cleared of debris.
To make the monitoring screw drill a 0.7 millimeter hole through a hexagonal headed screw using a lathe and a 0.7 millimeter drill bit, insert the monitoring screw into the hole by turning it approximately 1.5 turns. Being careful not to damage any underlying tissue. Use a one millimeter drill bit to burr a second hole for an anchoring screw in the left parietal bone, two millimeters lateral and two millimeters posterior from breg ma, but do not completely penetrate the skull.
Insert a two by four millimeter hexagonal headed screw into the second hole. This screw helps to anchor the dental cement and the monitoring screw to the skull. Use a transfer pipette to mix and then apply dental cement, monomer and polymer to the base of the head of the screws, allowing the cement to dry for at least 10 minutes using white correction fluid, mark the fiber optic sensor four millimeters from the tip, fill the hole of the monitoring screw with sterile saline and ensure no air bubbles are present within the screw.
Insert the intracranial pressure or ICP probe four millimeters into the screw so that the tip of the probe is level with the end of the screw. Ensure the tip does not pierce the dura. Adjust the tip of the probe within the screw until an ICP trace reflecting ventilation and blood pressure pulse waves can be observed.
An airtight seal is imperative to an accurate ICP reading to create one first mix of viscous biocompatible caulking material monomer and polymer in a one-to-one ratio, apply a thin layer of caulk around the probe and the head of the monitoring screw. Being careful not to displace the ICP probe. After allowing the first layer to set for five minutes, apply a second layer of caulking material around the entire monitoring screw and probe.
Ensure that no liquid leaking from any crevices in the caulking material. Remove the ear bars and allow the rat to remain in the prone position. During ICP monitoring at the completion of ICP monitoring, the ICP sensor may be removed by gently pulling the catheter from the screw and caulking material, immediately place the fiber optic sensor into 1%zyme solution to prevent tip corrosion.
The hole remaining in the caulking material should be covered with an additional layer of caulking material shown. Here is a representation of ICP readings over 10 seconds at baseline. The average ICP in a wistar rat is six millimeters of mercury.
The events of shorter amplitude reflect blood pressure pulse waves, while the events of longer periodicity show ventilation events to validate the position of the fiber optic sensor in each experiment, ICP traces should be tested for responsiveness to abdominal compressions. Here the abdomen was temporarily compressed, which resulted in reduced cerebral venous return in increasing intracranial volume, and thus increasing ICP arterial pressure dropped only after the initial ICP rise. Periods of apnea are observed in most experiments involving spontaneously breathing animals.
These events are identified on physiological records shown here by an absence of respiratory deflections on respiratory and arterial pressure traces. An equivalent alteration in the ICP trace validates the ICP probe positioning here. A typical ICP trace after the removal of the ear bars is shown, the initial insertion of the ear bars results in a slight compression of the skull and consequent disruption, intracranial volume, and thus an increased ICP.
If the sensor is positioned correctly, ICP will drop at least four to five millimeters of mercury with the removal of the ear Bars once mastered. This technique can be done in approximately 30 minutes if performed correctly. After watching this video, you should have a good understanding of how to correctly and accurately measure intracranial pressure using a minimally invasive technique.
The most difficult part of this technique involves screw insertion here. Care must be taken when bearing through the skull to prevent damage to the underlying meninges and brain.
