Trigeminal Neuralgia: Fluoroscopically-Assisted Laser Targeting of the Foramen Ovale:
Technical Note.

Michael K. Landi, M.D., Walter Grand, M.D., Department of Neurosurgery, State University of New York at Buffalo.

Introduction:

A new method of localizing skull base foramen was developed as an alternative to conventional methods using anatomic landmarks (2,3,8,13,14) computed tomography (1,7) and plain fluoroscopy (4,5,9,1112). The technique uses the Dual Radiation Targeting System (DRTS - MINRAD Inc, Orchard Park, NY), a laser targeting system for fluoroscopy. The DRTS™ Platform attaches to a standard fluoroscope (figure 1).

The DRTS™ identifies a line of x-ray radiation from the x-ray source to the image intensifier and positions a laser beam collinear with that line. A cross hair target symbol displayed on the TV monitor represents a point on the line of radiation defined by the laser beam. When the image of the target symbol is positioned over the x-ray image of a deep tissue structure, the laser beam indicates the surface point of entry and angle of approach to the target on the patient. The system has a +/- 1 mm accuracy at the center of the laser spot.

Materials and Methods:

The technique for laser localization, targeting and performing the cannulation of the foremen ovale was developed using 20 adult human skulls and two fresh cadaver specimens with the DRTS™ mounted on a OEC-Diasonics Series 9400 (OEC- Diasonics, Salt Lake City, UT). The dimensions of the foramen ovale were measured on the skulls. The visual and radiographic appearance of the trajectory utilized in described freehand foramen cannulation was evaluated.

The DRTS™, mounted on a standard C-Arm fluoroscope (OEC 9400 or OEC 9600), was used to localize and provide guidance information for the cannulation of the foremen ovale in five patients in an operating room (OR). The DRTS™, mounted on a Toshiba DFP-2000 bi-plane system, was used to localize and provide guidance information for the cannulation of the foramen ovale in two patients in a radiology suite. Patients were positioned supine with their neck in slight extension, the head was rotated approximately 35-40 degrees away from the painful side. The C-Arm was positioned at the head of the bed such that the C-Arm arc could easily be rotated between an anterior-posterior view of the skull to a lateral view. Using fluoroscopy, the foramen ovale was located within the window to the infratemporal fossa bordered by the inferior edge of the zygomatic bone, lateral wall of the maxilla, superior edge of the body of the mandible and the anterior border of the mandible coronoid process. If the foramen was difficult to identify, a submental view of the skull was used to locate the foramen (figure 2).

Once identified, the C-Arm was rotated, keeping the image of the foramen ovale in a view under direct fluoroscopy, until the image of the foramen was located within the window to the infratemporal fossa, with a suitable surface point of entry indicated by the laser beam. Once the foramen ovate was localized by the cross-hairs (figure 3), x-ray radiation was turned off. After adequate sedation with propophol, the needle tip was then positioned at the laser spot and advanced along the axis of the laser beam through the foramen oval into the retrogasserian portion of the trigeminal nerve. Fluoroscopy time was recorded for 5/7 patients treated in the OR.

Results:

The dimensions of the foramen oval were Left: average length: 0.8 mm, width: 3.8 mm (range length: 5.0 - 10.0 mm, width: 2.5 - 6.0 mm) Right: average length 6.9 mm, width: 3.4 mm (range length: 6.0 - 9.0 mm, width 2.55 - 4.0 mm). The foramen oval was successfully localized and cannulated on the first attempt in all six patients using the DRTS™. The mean fluoroscopy time for localization was 6.1 seconds (range 3.4-10.2 sec). Fluoroscopy time decreased as experience increased. One patient who underwent glycerol injection had previously been treated by microvascular decompression and a percutaneous balloon compression, failed to get relief from the procedure, six of the seven patients obtained good relief from the procedure described in this paper. There were no complications.

Discussion:

The anterior approach to the foramen ovale has been used for over 80 years since its description by Hartel in 1914 (6). Since then the method has been improved by a combination of new technology and modifications by several authors using numerous radiographic and free hand techniques. The anterior approach has been used for injections, thermocoagulation, balloon compression of the gasserian ganglion as well as a window to the cavernous sinus for biopsy (10).

Use of this technique protects against inadvertent cannulation of other skull base foramen or injury to cranial nerves or vascular structures. Foramen that may be mistakenly penetrated during the free-hand technique include the, the superior orbital fissure (anterior superior), the jugular foramen (posterior inferior), foramen Vesalii (anterior medial) and Innominant canal of Arnold (posterior) (13), foramen magnum, foramen lacerum. Lateral fluoroscopy is used once the foramen ovale is entered to ensure proper depth of the electrode beyond the edge of the clivus.

Limitations:

The principle limitation of the technique of the quality of fluoroscopic image, Older model fluoroscopes required longer fluoro time for target identification. With correct positioning and knowledge of the physical and radiographic anatomy, fluoroscopic localization is rapid.

Sources of Error:

The accuracy of fluoroscopically assisted laser targeting with the DRTS™ is +/- 1 mm on bench testing. How this accuracy translates in actual clinical applications has yet to be established. Potential sources of error include patient movement during the procedure and bending of the needle during advancement (finer gauge needles).

Advantages:

Although several authors report easy localization of the foramen ovale using anatomic landmarks, it has been the experience of the authors that blind cannulation of the foramen ovale can be challenging. The DRTS™ utilizes fluoroscopy to identify the foramen ovale, allows the operator to turn off x-ray radiation and follow laser guidance to the target. This minimizes patient discomfort, trauma, and procedure time.

Summary:

In this limited experience, fluoroscopically-assisted laser targeting was found to be an inexpensive simple alternative method for defining a surface point of entry and angle of approach to the foramen oval compared to anatomical landmark based methods. The outlined technique can be applied to other foramen. Further studies are needed to compare the accuracy of this method to other techniques.

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(This abstract was accepted for the Congress of Neurological Surgeons 1998 Annual Meeting, October 3-8, 1998, Seattle, Washington. )