Techniques for catheter placement

The precise placement of intraventricular catheters is of paramount importance when considering the surgical nuances of implanting Ommaya reservoirs. Proper catheter placement is important to avoid damaging eloquent cortex or subcortical structures along the catheter's trajectory. In addition, proper targeting of the catheter so that its tip is within the ventricle is crucial to avoid administering toxic chemotherapy directly into brain parenchyma. Inadvertent administration of chemotherapy into brain parenchyma can cause mild to severe leukoencephalopathy, the destruction of the myelin sheaths which cover nerve fibers. Ideally, the ventricular catheter is placed so that all of the catheter outlet holes, typically extending two centimeters from the catheter tip, are within the ventricular compartment. This goal is readily achieved by placing the catheter tip at the ipsilateral foramen of Monro. In addition to ensuring that all of the outlet holes are within the CSF space, catheter tip placement at the foramen of Monro also lessens the likelihood of the catheter tip becoming entangled in the choroid plexus or septal veins causing hemorrhage at initial surgery or revision. Intraoperative confirmation of the catheter tip position can be achieved using a variety of techniques, including pneumoencephalography, frame-based or frameless stereotaxy7, and/or endoscopy.6'8 Postoperative CT confirmation is routinely obtained prior to chemotherapy delivery. If safe delivery of chemotherapy remains a concern due to equivocal CT confirmation, radioactive tracer studies can be performed, followed by whole brain scintography at various time-points (up to 24 hours) to document safe tracer distribution throughout the CSF, but excluding brain parenchyma.

The surgical technique for Ommaya reservoir placement utilizing either pneumoencephalography or frameless stereotaxy will be briefly described. For either method, a horseshoe shaped incision is centered 10 cms from the nasion and 3 cms from the midline (mid-pupillary line) on the right (nondominant) side. A burr hole is created using a craniotome. A subgaleal pocket is created using blunt dissection posterior to the incision and the Ommaya reservoir is inserted. The dura is coagulated using bipolar cautery and opened with an 11 blade.

When using pneumoencephalography, the ventricular catheter is initially placed using the Ghajar guide (Neurodynamics, New York, NY) or free-hand based on anatomic landmarks. The catheter tip is typically placed 5.5 cms from the inner table of the skull. Approximately 10 mis of CSF are drained and sent for cytology. Air is then injected into the catheter under continuous fluoroscopy until both the ipsilateral lateral ventricle and third ventricle are air-filled (Fig. 3A, 3B).

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Figure 3. Fluoroscopic Ommaya placement: Intraoperative A/P (A: top panel), and lateral (B: bottom panel), fluoroscopic images demonstrating air-filled right lateral ventricle (a). The catheter tip (b) can be seen approaching the Foramen of Monroe (c).

A vascular clip is placed at the proximal end of the ventricular catheter, and the catheter is manually manipulated until the tip is placed at the ipsilateral foramen of Monro, typically 5.8 cms from the inner table of the skull. Catheter lengths greater than 6 cms usually suggest improper catheter positioning often in the contralateral caudate nucleus, basilar cisterns, or even contralateral Sylvian fissure. Fluoroscopy can help redirect the catheter into the proper trajectory in real time. Having placed the catheter at the desired target, the catheter is cut and secured to a right angle connector on the Ommaya reservoir with a 2-0 silk tie. The right angle connector is then secured to the skull through a drill hole in the posterior aspect of the burr hole with a 2-0 silk stitch. The wound is irrigated with antibiotic solution and closed in two layers.

For frameless stereotaxy, i.e., without intraoperative fluoroscopy, patients undergo pre-operative CT scans with fiducial markers. Once in the operating room, they are placed into a rigid frame using skull pins, followed by registration to the stereotactic system. The opening of the skull and dura is identical to the prior technique. However, the placement of the catheter is then directed using the ventricular catheter probe to the indicated depth by aligning the trajectory in two planes using the trajectory mode and guidance views on the stereotactic system (Fig. 4). Once positioned correctly, the catheter and reservoir are secured as described above. Either method should ensure catheter placement at the foramen of Monro. Our preferred method remains intraoperative pneumoencephalography, which allows one to achieve a real time intraoperative image that also demonstrates patency of the ventricular catheter.

Pneumoencephalography

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Figure 4. Frameless Stereotaxy for Ommaya Placement: Intraoperative Stealth Station screen demonstrating real-time three-dimensional view of the catheter trajectory necessary to place the tip at the Foramen of Monroe

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Figure 4. Frameless Stereotaxy for Ommaya Placement: Intraoperative Stealth Station screen demonstrating real-time three-dimensional view of the catheter trajectory necessary to place the tip at the Foramen of Monroe

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