Percutaneous Intraspinal Navigation (PIN)

What is PIN?

PIN stands for percutaneous intraspinal navigation. Just as we’ve been using the arteries as a conduit to navigate the vascular system for a long time, we also believe it may be possible to use the space surrounding the brain and spinal cord — the so-called sub-arachnoid space — to navigate in and around the brain and spinal cord. We've known for a long time that you can access that space by putting a needle in the back. Currently, people visualize the spinal cord via myelograms and also perform lumbar punctures to draw some of the spinal fluid. Another way that people have accessed the subarachnoid space is through the brain. Neurosurgeons put in ventriculostomies, or shunt catheters, to treat hydrocephalus. So, the concept is not new. It's probably a little more developed now, but PIN is essentially the concept of putting catheters in the subarachnoid space to treat that space just like it was the inside of an artery, using tools that we use in interventional radiology, like catheters and guidewires, to navigate that area and also move around the surface of the brain.

How do the catheters and guidewires that you are using compare to the standard interventional equipment?

What I've been doing so far is using off-the-shelf technology — basically the exact same devices that we use for angiographic procedures, but adapted for these procedures. I personally believe that there is a whole new family of technologies which could emerge if PIN develops and people start incorporating it into more routine activities. For instance, there are structures that we may want to access where we can use a combination of x-rays and direct visualization. We've been working with some different types of endoscopes on catheter-type devices in order to look at the brain or spinal cord from inside the subarachnoid space. These devices also have channels where we can pass guidewires and other things through to be able to perform surgical procedures. However, the types of procedures that endoscopes are now developed for are things like bronchoscopy. There are a few endoscopes that have been developed for looking inside the blood vessels, but none really have the type of flexibility and the field of view that we would like to have for managing the subarachnoid space, so there are some adaptations that could be made in endoscopic technology. We also have experimented with the use of MR imaging to guide the navigation and to watch as we are passing catheters and devices up alongside the spinal cord and the brain. However, the types of tools that are available for interventional MR procedures are really pretty primitive in terms of resolution, as well as any use of those tools to interact with the MR scanner technology to generate better information about your location with respect to the surrounding structures.

What types of conditions do you believe this procedure could address?

When you put a catheter or device into the lumbar space, the first thing that you encounter is the nerve roots coming off of the spinal cord. Working with cadavers, we've done endoscopic procedures which look at nerve roots and experimented with the manipulation of these nerve roots from a percutaneous approach. The typical way this area is approached surgically, such as with a laminectomy, is where you actually remove bone to open up and visualize the area. We also believe that there are some ways that we can use electrophysiology to test for nerve damage or nerve malfunction, or to possibly treat pain syndromes. Right now, they use neurostimulation of electrodes as a form of treatment, and with PIN, you can get more precise placement against nerve roots or adjacent to nerve roots, since we can look at them directly. As you move up towards the brain, you reach the area alongside the spinal cord. There's a whole lot of work being done right now with stem cell implantation for spinal cord injury. Also, different places are working with spinal cord hypothermia. Devices that are introduced through the PIN technique should be really ideal for these two types of treatments, because it saves a whole operative procedure along with the attendant risks and discomforts associated with it. Moving on up towards the head, you can have abnormalities in the subarachnoid space, or the space around the brain, where blood vessels sometimes press on nerves or press against nerve roots and cause pain sensations in the face. These are called Tic Doloreaux, or facial tic. Neurosurgeons will put devices between the blood vessels and the nerve roots to keep the blood vessels from pulsing against the nerves, which is another procedure that might be feasible via the PIN technique. We know for sure that it is possible to pass a catheter up into the ventricular system, because we’ve done that and reported it in the literature.¹ We believe that there may also be some patients who have increased pressure in their brain, hydrocephalus, who could be treated with this type of a surgical approach. Neurosurgeons currently do a procedure that they call a third ventricular fenestration, where they will take a piece of skull off and go down between the right and left hemispheres of the brain to the corpus collossum, and go into the ventricular system that way, in order to make a hole in the bottom of the third ventricle. We’ve shown that PIN can do essentially the same thing, without doing any kind of brain surgery. There are other procedures that we believe will have applicability for PIN, but again, it’s all pretty early in the development stages. We’re testing some of these theories by working with cadavers and working with some other companies to try to develop further techniques. We’ve been working primarily with Boston Scientific on the basic techniques and with Olympus to look at endoscopes. We have defined clearly that we can routinely put a catheter in a lot of different spaces, such as what is called the posterior fossa, or the area around the brain stem and cerebellum. There are a lot of neurosurgical procedures that are newly emerging, such as physiological electrode implantation for Parkinson’s disease and stem cell implantation for different disorders. Ultimately, all of these procedures may be able to be done through the PIN approach, without subjecting the patient to a more significant type of brain surgery procedure. There have been some reports in the Japanese literature regarding patients with a ruptured aneurysm and a hemorrhage around the brain, where they used a technique similar to PIN. These physicians put a catheter up alongside the spinal cord and at the base of the skull to basically wash the blood out. This helps cut down on the incidence of vasospasm following a subarachnoid hemorrhage.² We also think that there may be applicability for PIN regarding the use of hypothermia in the spinal cord for spinal cord injury. There’s also a fair amount of literature looking at hypothermia for stroke. Ultimately, if we can develop the right device, this may be a technique that we can use.

Technique-wise, is working in the lumbar space and subarachnoid space different from working in blood vessels?

It actually involves a lot of the techniques that interventional neuroradiologists use for blood vessels and cardiologists use for blood vessels, it’s just that it’s not inside a blood vessel.

What are some of the challenges you're trying to overcome?

There is a real concern, which is what is making people a little nervous about getting into this technology more deeply, regarding injury to the nerve tissue that we are passing with the catheter or device. We don’t have any experience that says this concern is a real problem, but it is certainly a theoretical problem. Our studies with animals suggests that catheters up to at least 2 mm diameter are well-tolerated by the spinal cord, which gets us close to the diameters we have available in commercial endoscopes. Concerns about injury to the spinal cord or the brain during the course of the procedure are why we are moving slowly in getting into the patient care type of arena. We're trying to start simple, and the thing that's closest and easiest is the spinal cord. We recently did an animal study on a dog, and we were able to successfully put in a device that we’ve developed for hypothermia. We were able to develop a significant amount of cooling of the spinal fluid around the spinal cord. The dog had no obvious clinical or gross pathologic injury to the spinal cord.

Are there advantages to the percutaneous procedure beyond the fact that it's easier on the patient?

Well, that’s certainly one of the advantages. But another one of the advantages is that for some of these procedures, in the spine for instance, they have to take bone off in order to access it. That is not just discomfort; it’s a significant operation. To access the structures in the posterior fossa, the neurosurgical procedure involves taking part of the skull off, exposing the tissue and essentially pulling it out of the way in order to access the other structures. You can have injury to the lower cranial nerves and you can have injury to the brain stem. The neurosurgical approach to these areas is not a trivial thing, and if this different approach is less traumatic it may have fewer complications.

What kind of background would be best for someone interested in performing PIN procedures?

My own background is in interventional neuroradiology, but there’s a large number of neurosurgeons who have training in these interventional techniques — the endovascular neurosurgeons. They are really well-equipped from the standpoint of neurosurgical experience, and they’re well-equipped from the standpoint of technical experience with the devices. They would be the ideal candidates.

Your facility is collaborating with Toshiba. Can you tell us about your experience?

I’ve worked with Toshiba since 1989, and it's been an excellent collaboration. I know that we’ve benefited from it, and I think they have too. Our deepest collaboration with Toshiba is in angiography equipment. We have an experimental angiography lab where we collaborated with Toshiba, and we've been using it for the x-ray component of the PIN procedures. At our hospital, we have two Toshiba biplane angiography labs. I have worked with Toshiba in the development of three-dimensional angiography, and also at a clinical level regarding flat-panel image intensifiers, which are in development right now. Our facility has two regular fluoroscopy rooms that are used for myelograms (spinal cord imaging exam) or GI procedures, and we also have a Toshiba CT scanner. Toshiba is in a collaboration with Vital Images (Plymouth, MN) with their Vitrea^® workstation for 3-dimensional imaging, and we’ve worked with them in CT and angiography. Also, our research lab (animal lab) is an angiography lab with Toshiba.

Dr. Purdy discloses that he has received research support via devices and equipment, but not direct financial support, from Toshiba, Boston Scientific Corporation, and Olympus. Dr. Purdy can be contacted at: phillip.purdy@utsouthwestern.edu