Endoscopic Skull Base Surgery II: A New Animal Model for Surgery of the Posterior Fossa
By Reza Jarrahy, M.D., John Young, V.M.D., M.S., George Berci, M.D., Hrayr K. Shahinian, M.D.
The field of skull base surgery has been influenced by a general philosophy that currently exists in modern surgical practice favoring less invasive means of managing surgical disease. Adapting techniques developed by general surgeons and other surgical subspecialists, skull base surgeons are now experimenting with endoscopy to resect tumors, manage vascular lesions, and manipulate critical intracranial structures. Lesions formerly requiring significant soft tissue dissection and craniotomy for exposure are now potentially amenable to treatment via a keyhole approach. As in other surgical specialties, however, a reliable animal model is necessary for experimentation with and development of new endoscopic techniques in the skull base. The swine provides just such a model, primarily due to craniofacial and skull base relationships that are analagous to humans. We have focused on the posterior skull base of the swine in this experiment: via a retrosigmoid craniotomy we opened the dura of the posterior fossa and used endoscopes to visualize and manipulate the critical structures in this area. The cerebellum and midbrain were appreciated, as were cranial nerves V, VII, VIII, IX, X, and XI. Blood vessels on the surface of the midbrain were also identified. This experience further supports the use of the swine as an appropriate animal model for endoscopic skull base surgery.
There is currently no accepted animal model for endoscopic skull base surgery, despite the introduction of endoscopic techniques to this area. We have used the swine to demonstrate the efficacy of endoscopic exposure in surgery of the posterior fossa and to document previous unpublished images of the swine midbrain ultrastructure.
Skull base surgery, Endoscopy, Posterior fossa, Vascular loops, Acoustic neuroma, Comparative anatomy
The development of operative microscopy in the 1960s revolutionized the field of skull base surgery. The operative microscope provided accurate and detailed imaging of very restricted intracranial spaces, obviating the need for extensive craniotomy in many cases.4, 8 Current progress in the field of endoscopy heralds another era in the evolution of surgical management of the skull base: endoscopic telescopes and instruments have improved upon the optical and technical limitations of the microscope, allowing more complete and thorough navigation within confined anatomical areas.10 Compared to the static imaging provided by conventional microscopy, endoscopic lenses of varying width and angulation allow surgeons to dynamically rotate and alter their perspective of the surgical field. This amounts to a greater appreciation of the three-dimensional relationships between the anatomical targets of therapy and the surrounding structures.
A viable and reliable animal model is a prerequisite for performing endoscopic skull base procedures in human patients. Such models have long existed and are continuously developed for innovative endoscopic surgical technique in other surgical fields.1, 8 Intimate knowledge of anatomical relationships within the craniofacial skeleton and familiarity with the technical abilities and limitations of the endoscopic equipment involved is paramount to surgical success in the area of the skull base. We have previously described our experience in performing endoscopic skull base surgery – specifically, on the pituitary gland – in swine. Prior to this experimentation we were unable to find documentation of other suitable animal models for this type of surgery. Choice of the swine is appropriate due to existing homology between porcine and human skull base relationships and due to similar comparative anatomy regarding the anatomy of upper and lower cranial nerves.2, 5
We have turned our attention to the posterior fossa of the swine to further demonstrate the usefulness of the swine as a model for endoscopic skull base surgery. The posterior fossa is an anatomical area that is implicated in many lesions, which, while extremely debilitating, are very susceptible to surgical resection. Acoustic neuromas of the vestibulocochlear nerve and vascular loops impinging upon cranial nerves as they emerge from the midbrain and enter their respective skull base foramina have been managed by operative microscopy for years.3, 7 The potential for endoscopic imaging to facilitate more extensive tumor resection or to decrease surgical complication rates calls for experimentation with endoscopy in this area. We therefore devised an original surgical protocol for performing endoscopic surgery of the posterior fossa of the swine in order to document the surgical anatomy of this region and to offer an in vivo animal model for continued development of this technique.
The following procedure was performed on a live 30-kg purpose-bred Hampshire-Yorkshire-Duroc hybrid swine at the animal research facility of Cedars-Sinai Medical Center in Los Angeles, California. Operative goals and methods were planned in consultation with attending veterinary staff prior to surgery. The surgical protocol was reviewed and approved by the Cedars-Sinai Research Institute I.A.C.U.C. prior to experimentation.
The subject was chemically immobilized using the following intramuscular agents: ketamine (20 mg/kg), acepropamine (0.5 mg/kg), and atropine (0.05 mg/kg). An intravenous catheter was placed in a large superficial vein of the ear via which intravenous fluids and thiopental 2.5% (approximately 6-8 cc) were administered to effect. The pig was then intubated and an inhalational anesthetic agent (isofluorane 1-2%) was titrated according to vital signs to maintain anesthesia throughout the surgery.
The pig was placed in the left lateral decubitus position and the right retroauricular area was shaved. The surgical site was prepped and draped in the standard surgical fashion. A semilunar incision was carried down through the soft tissue posterior to the auricle down to the temporal bone. Once bone was identified, dissection was continued in a subperiosteal plane to expose an area of bone surface approximately 5 cm in diameter.
Using cutting and diamond bits, the Jed-Med™ drill was used to perform a retrosigmoid craniotomy down to the level of the dura mater, resulting in a 2-cm burr hole. Up-biting and down-biting Kerrison rongeurs were then used to slightly expand the diameter of the burr hole, while carefully leaving the dura intact. Bone wax and bipolar electrocautery were used to control bleeding from the bony surfaces and from the dura.
Following hemostasis, the dura was opened via a cruciate incision and the brain tissue was exposed. Cerebrospinal fluid was gently suctioned from the field. A Storz™ 3.3 mm zero-degree endoscope was introduced through the dural incision into the posterior fossa. The cerebellum was immediately identified and retracted posteriorly. As the scope was advanced caudally the architecture of the midbrain came into view. The facial/vestibulocochlear nerve complex (CN VII/VIII) was first identified emanating from the medulla; the nerve bundles separated from each other anteriorly as CN VII entered the facial canal and CN VIII entered the internal acoustic meatus.
Rotation of the zero-degree endoscope in a transverse axis exposed the trigeminal nerve (CN V) rostrally as it emerged from the lateral aspect of the pons. As the exposure was not optimal, however, the zero-degree endoscope was replaced with the thirty-degree endoscope. The nerve was clearly brought into the view and the endoscope was advanced rostrally toward its base at the pons. Care was taken not to manipulate CN VII/VII during this exploration. Attention was then turned to the more caudal regions of the posterior fossa. First the zero-degree and then the thirty-degree endoscopes were advanced caudally to expose the lower cranial nerve bundles--CN IX, X, and XI--which emerged from the midbrain in close proximity to one another. Again, care was taken not to disturb the more rostral structures during this exploration. Here as well the perspective offered by the thirty-degree endoscope complemented the imaging of the zero-degree scope; with the use of both we were able to appreciate a dynamic panorama of the region.
Proceeding further caudally, we identified several prominent blood vessels and their tributaries running along the lateral surface of the medulla. At this level, they likely represented the anterior and posterior inferior cerebellar arteries and various branches thereof.
At this point, having defined the relationships of the major nervous and vascular anatomical structures that give rise to surgical disease in the posterior fossa in humans, we completed the surgery without further exploration or manipulation. The animal subject was ultimately euthanized while under surgical anesthesia using an intravenous injection of high-concentration potassium chloride, as per our I.A.C.U.C.-approved protocol.
Similarities between porcine and human skull ultrastructure allowed analogous landmarks to be used throughout the surgery. Soft tissue dissection was carried out posterior and inferior to the external auditory canal, as in humans, to arrive at the retrosigmoid portion of the temporal bone overlying the posterior fossa. Once within the fossa, the relationships of the cranial nerves, blood vessels, and bony structures of the skull base were readily apparent and similar to those we have observed in humans using microscopy. The flexibility of the different endoscopes was instrumental in identifying these structures and in accurately depicting their spatial relationships to one another.
Throughout the procedure, we encountered intermittent technical difficulties associated with the endoscopic equipment relating to visualization and manipulation. Working within a very confined and fluid-filled cavity, the endoscope lenses were vulnerable to obscuring elements, such as blood, cerebrospinal fluid, and floating debris. Conditions of poor visibility were exacerbated by variations in intracerebral pressure attributable to the respiratory cycle: for optimal visualization of structures, ventilator respirations were strategically held for a few moments, then resumed.
In addition to altering the respiratory pattern to promote optimal imaging, the endoscopes had to be occasionally removed and cleaned. The suction devices at our disposal were too cumbersome for the more limited dimensions of the porcine posterior fossa.
Another problem we encountered was reminiscent of a similar difficulty in our original hypophysectomy procedures: manipulating an endoscope in one hand and a dissecting instrument in the other is challenging even to the most dexterous surgeon. Encouragingly, however, even the limited experience we have gained in performing our original procedures made us more proficient in this one.
Endoscopic skull base surgery is fast becoming a viable alternative to intracranial microscopic techniques. The dynamic optical qualities of the endoscope should allow for more careful navigation through the very constrained cavities of the skull base, for more detailed appreciation of critical surgical anatomy, and for more thorough resection of tumor with fewer complications due to better visualization. To realize these goals, however, a reliable animal model must be available for surgeons hoping to perfect this technique. The swine provides just such a model.
This experience demonstrates how well the surgical anatomy of the porcine skull base mimics what we see in human patients. Anatomical relationships between cranial nerves and vessels in the posterior fossa of the swine offer surgical landmarks that are directly analogous to those we have experienced in our microsurgical experience. Acoustic neuromas and vascular loops impinging upon the upper and lower cranial nerves are lesions that are particularly susceptible to endoscopic visualization and manipulation. The swine offers a modeled surgical environment in which endoscopy of such a constrained and vital area of the skull base can be developed safely and effectively.
Our experiment has highlighted the optical and surgical benefits of using endoscopy in the skull base, as well as some of the current limitations. New endoscopic instruments are becoming available to address the issue of rendering the endoscope "hands-free" and of cleaning the lens of the endoscope in situ. This procedure also underscores the importance of tailoring instrument size to the dimensions of the cavity in which the surgery is being performed. This experience can be interpreted as an outline for a useful and reproducible model to proceed with the work of further developing endoscopic skull base techniques.
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