Endoscopic Skull Base Surgery I: A New Animal Model for Pituitary Surgery
By Reza Jarrahy, M.D., John Young, V.M.D., M.S., George Berci, M.D., Hrayr K. Shahinian, M.D., FACS
Endoscopy has emerged as a new means to perform minimally invasive surgery of the skull base. Specifically, endoscopic techniques and instruments can be used to safely and effectively approach and resect tumors of the pituitary gland in humans. No animal model currently exists to serve as a template upon which to refine and develop endoscopic surgical technique in this region of the anatomy. We operated on two purpose-bred Hampshire-Yorkshire-Duroc hybrid swine to demonstrate the application of endoscopy to pituitary surgery. Based upon similar anatomical relationships in humans and swine between the oropharynx, nasopharynx, and skull base, we used a transoral, transpalatal approach to access the vomer of the swine. Under endoscopic exposure, we resected the vomer, entered the sphenoid sinus, and then resected the sphenoid septum, sella turcica, and adenohypophysis. Clear visualization of the pituitary, hypophyseal stalk, cavernous sinuses, and carotid prominences was achieved and documented with digital photography. Benefits and limitations of the technique were noted. These results have pertinent implications both for the study of the surgical anatomy of the swine craniofacial skeleton, and for future development of endoscopic surgical manipulation of the skull base.
Endoscopic surgery of the skull base is an alternative to microsurgery. To date no animal model has been described for endoscopic techniques in this region of the anatomy. We describe an endoscopic surgical approach for hypophysectomy. The swine is a useful animal model for this and other skull base procedures.
Skull base surgery, Endoscopy, Hypophysectomy, Pituitary surgery, Porcine skull, Comparative anatomy
Surgical technique over the past century has seen a general trend toward less invasive means of achieving desired surgical goals. The development of new endoscopic techniques is currently at the forefront of this pattern. The role of endoscopy in modern surgical practice cannot be understated: its introduction has provided a reliable means of minimizing patient morbidity and complication rates, decreasing length of hospital stay, and significantly contributing to lower hospital costs.7
Widely adapted to the practice of general surgery early on, endoscopy has made inroads to the surgical subspecialties as well: procedures once thought to have little application for endoscopy have been revolutionized by it. The field of skull base surgery is in the midst of such a movement. Transseptal transsphenoidal hypophysectomy, for example, is a procedure that has evolved from an open skull technique with gross manipulation of the brain to a microscopic dissection via a translabial approach.2, 5 Currently, we are developing a new means of performing resection of pituitary tumors entirely endoscopically. With endoscopy, superlative optical exposure and surgical maneuverability are realized when compared to conventional operative microscopy. Furthermore, the endoscopic approach to the pituitary is transnasal, obviating the oral and nasal dissection of the microscopic procedure.4
Suitable animal models for surgical approaches to and manipulations of the skull base, however, have not been described. Having chosen the swine as a template due to its analogous relationships in the craniofacial skeleton, we were unable to locate appropriate references describing surgical anatomy of the area, other than classical anatomic descriptions.3 Therefore, in consultation with the attending veterinary staff at our institution, we devised an original protocol for surgery of the swine pituitary gland using endoscopic technique and materials. The goal of this work was to document the surgical anatomy of the swine skull base and to offer an in vivo animal model for continued development of this technique.
Materials and Method
The following procedures were performed on two 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 Institutional Animal Care and Use Committee prior to experimentation.
The subjects were 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 through which intravenous fluids and thiopental 2.5% (approximately 6-8 cc) was administered to effect. The animal was then intubated and an inhalational anesthetic agent (isofluorane 1-2%) was titrated according to vital signs to maintain anesthesia throughout the surgery.
The swine were placed in a supine position and prepped and draped in the standard surgical fashion. We first attempted visualization of the anterior wall of the sphenoid sinus by advancing Storz™ 4.0-mm zero-degree 18-cm long endoscopic telescopes through the nostrils. The ventral nasal concha and endoturbinates of the porcine ethmoid bone were visualized. However, the snouts of the subjects were prohibitively long: the telescopes could not be advanced far enough to reach the back of the nasal cavity. The endoscope was therefore removed and our attention was turned to exposure of the sella via a transpalatal approach.
The oral cavity was manually opened and held in place with an oral retractor. The labial commisures were dissected using electrocautery to achieve maximum possible retraction. Also using the electrocautery dissector, the entire mucosa of the palate was removed, exposing the palatine bones underneath. Next using the Jed-Med™ cutting drill, the posterior palatine bone was dissected and removed, revealing the underlying pterygoid bone, which was also drilled. The anterior palatine bones were also dissected to demonstrate the anterior relationship of the endoturbinates of the ethmoid bone to the sphenoid sinus. The vomer was thus exposed. Under endoscopic visualization, microsurgical rongeurs were used to fracture and remove the vomer from its attachment to the anterior wall of the sphenoid sinus. The Jed-Med™ cutting drill was again employed to drill through this wall and access the cavity of the sphenoid sinus; both right and left halves of the sinus, including the bony septum separating them, were viewed.
As in humans, the supero-posterior wall of the sphenoid sinus of the swine makes up the infero-anterior wall of the sella turcica. This wall was fractured and removed with the rongeur to reveal first the dura overlying the pituitary gland, and then the gland itself in its native position. The endoscope was then advanced into the sella for an exploration of the gland, the hypophyseal stalk, and the surrounding structures. These included the cavernous sinuses and carotid artery prominences bilaterally. (The locations of the carotid prominences were identified by their pulsations lateral to the cavernous sinuses. These dynamic motions did not reproduce in the static digital images.)
At this point, the zero-degree endoscope was replaced with the thirty-degree endoscope to note differences in the optical qualities of the two instruments. An entirely new panoramic view of the surgical field was provided. Specifically, the view of the suprasellar area (hypophyseal stalk) was much clearer, and a greater dimension of depth was added to the imaging of the parasellar spaces (cavernous sinuses and carotid prominences).
At the completion of the exploration, a partial hypophysectomy was performed to gain experience with the use of dissecting curettes within the confines of the sella. The visualization provided by the endoscopes allowed for a thorough resection without any gross compromise of the surrounding structures.
Ultimately, each animal was euthanized while under surgical anesthesia by the assistant veterinary staff with a high-concentration intravenous dose of potassium chloride, as per our I.A.C.U.C.-approved protocol.
The length of the pig snout precluded transnasal access to the sphenoid sinus and sella turcica. Modification of our surgical plan, however, allowed us to expose the sphenoid sinus via a direct intraoral, transpalatal, and transpterygoidal technique that exposed the sphenoid sinus and sella turcica extremely well.
Via gross dissection, the relationships of the palatine, pterygoid, and endoturbinate sections of the ethmoid bone were identified. Exposure of the latter demonstrated, from an intraoral perspective, the extent of our ability to advance the endoscopes through the nostrils.
Dissection of the vomer, sphenoid sinus, sinus septum, and sella turcica was as effective under the endoscope as, in our experience, under the microscope in human patients. Anatomic relationships were defined and documented. We were able to obtain wide enough resections of the bony landmarks to adequately expose our ultimate target. Once in the sella, the survey of the anatomy provided by the endoscopes was unparalleled. Intra-operatively, the pituitary gland was easily manipulated with a high degree of sensitivity to the very critical structures surrounding it. While the pulsations of the carotid arteries were not captured by the static means of imaging at our disposal, their visualization was instrumental in avoiding an area of the anatomy of the parasellar spaces that causes potentially lethal complications.
The subjects tolerated the procedures well, without any gross evidence of hemorrhage, of cerebrospinal fluid leak, or of damage to any of the major neighboring structures.
We did encounter certain technical difficulties using the endoscopes and surgical instruments in such confined spaces. For one, during the endoscopic dissection, a piece of equipment was carried in each hand of the operating surgeon at all times, making the process intermittently cumbersome. A holding arm would have enabled stable positioning of the endoscope so that the surgeon could use both hands to operate the dissecting instruments. An additional technical challenge was manifest in the pollution of the endoscope lens with blood and other debris in the surgical site. By necessity, the endoscopes were repeatedly removed from the surgical site to be cleaned. Repetitive repositioning of the scopes into such a confined space was a painstaking task. Again, at the time of surgery we were without a mechanism whereby the surgeon could clean the surface of the endoscope lens without removing it from the surgical field.
Animal experimentation has been invaluable to progress in surgical technique in general, and to the evolution of endoscopy in particular. Multiple animal models have been described and tested in the collective effort to train endoscopic surgeons and to create new endoscopic techniques.1, 6 Surgeons and researchers, in turn, have relied upon knowledge of the surgical anatomy of their animal subjects to render these efforts worthwhile.
While the most broad and large-scale efforts at developing these models have been put forth by general surgeons interested in advances in laparoscopy, surgical subspecialists are now putting forth similar efforts as they adapt minimally invasive endoscopic procedures to their craft. Specifically, skull base surgeons are developing techniques that may obviate the need for operative microscopy in the resection of pituitary tumors.
To this end, we have looked for an animal system to help increase our knowledge of the comparative anatomy of the skull base, and to help develop new techniques for endoscopic surgery of the pituitary gland. We focused on the swine due to the homology between the porcine and human craniofacial skeletal relationships. Existing resources that define the surgical anatomy of this area, however, are few.
Our procedures have helped demonstrate and image anatomical relationships between the bony and soft tissues of the porcine oral cavity, nasal cavity, and skull base. We have been able to implement applications of endoscopy heretofore unused in this animal model in order to define the relevant surgical anatomy. Our results highlight the usefulness of endoscopy in providing superior exposure of the structures that are critical to hypophysectomy. Further endoscopic exploration of the architecture of porcine facial cavities and bony architecture will complement existing illustrative and radiographic records of this anatomy. Our experience with the benefits of endoscopic technique in this procedure highlights its utility. The limitations we encountered identify the next stages in development of adjunctive endoscopic paraphernalia: holding arm and irrigation system prototypes have already been developed for the purpose of hands-free, clean lens endoscopic surgery.
We therefore conclude that additional study in the use of the swine as a model for endoscopic skull base surgery is warranted, both in the area of the sella turcica and in other regions of the skull base currently being surgically managed exclusively under the operative microscope.
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