The primary goal of any surgical approach is to adequately visualize and treat the pathologic condition with minimal disruption to adjacent normal anatomy. The work of several researchers has revealed the promise of minimally invasive endonasal neurosurgery and paved the way for broader applications of the technology. This article discusses the current state of minimally invasive endonasal techniques to address the pathologic conditions of the anterior cranial fossa and parasellar region.
Historical perspective
The primary goal of any surgical approach is to adequately visualize and treat the pathologic condition with minimal disruption to adjacent normal anatomy. During the past 200 years, rapid advances have been made in endoscope technology and instrumentation. In 1806, Philipp Bozzini demonstrated the use of a device consisting of a tube that was illuminated by a candle and mirror to visualize structures within the human body. In the mid-1880s, the term endoscope was coined by the French urologist Antonin Jean Desormeaux. One of the key limitations of early endoscopes was poor illumination. The invention of the incandescent light bulb by Thomas Edison in 1879, followed by the development of fiber-optic technology beginning in 1926 by John Logie Baird, paved the way for greatly improved illumination systems. Revolutionary advances in rod-lens technology through the mid-1990s greatly expanded visualization. Finally, progress in video and display technologies has made it possible to view exquisite real-time images on large screens in high definition (HD) and, in some cases, in 3 dimensions, in addition to video documentation for teaching and cataloging purposes.
Whereas initial endoscopes were only used for visualization, concurrent advances were being made in the development of new instruments to allow endoscopic procedures to be performed as well. Maximilian Nitze was an early pioneer in endoscopic surgery. He published the Textbook of Cystoscopy in 1889 and was the first to use movable loops for urological procedures. The subsequent development of microinstruments and electrocautery made a wide range of endoscopic procedures possible. Although pioneers in urology and otolaryngology made rapid advancements in endoscopic approaches, it was many years later before neurosurgeons began to recognize its potential for the treatment of pathologic conditions of the anterior skull base and sella.
The first steps toward the eventual development of endonasal endoscopic neurosurgery came with the early work by Harvey Cushing, demonstrating the feasibility of transsphenoidal techniques for resection of the sellar lesion. In the late 1800s, attempts made by other researchers to access the sella through subfrontal or temporal approaches were associated with high perioperative mortality rates, with some reports approaching 80%. Cushing’s technique required first making a sublabial incision followed by exposing the submucosa and then resecting the nasal septum to allow direct access to the sphenoid sinus. The posterior wall of the sinus was opened with an osteotome to provide direct access to the sella. Between 1910 and 1925, Cushing used his transsphenoidal approach on 231 patients with a mortality rate of 5.6%. However, improvements in the safety and the greater exposure provided by intracranial approaches, compounded by problems with persistent spinal fluid leaks and meningitis, ultimately cooled the enthusiasm for the transsphenoidal approach, and the approach was all but abandoned.
However, one of Cushing’s students, Norman Dott, recognized the advantages of the transsphenoidal approach and continued to refine this technique. Gerard Guiot, a French neurosurgeon who worked with Dott, was impressed by the approach and went on to perform more than 1000 transsphenoidal pituitary adenoma resections. He and his fellow, Jules Hardy, further improved the technique. With the introduction of the operating microscope, Hardy reported his ability to treat pituitary adenomas, craniopharyngiomas, clival chordomas, and meningiomas with morbidity and mortality rates that were less than those of transcranial approaches.
With the revival and increasing popularity of the transsphenoidal approach and the development of the modern endoscope, neurosurgeons in the 1970s began to merge these techniques. Early applications used endoscopes to augment traditional microsurgical approaches to allow visualization of structures that were not within the direct line of sight. Apuzzo and colleagues provided some of the first reports using 70° and 120° endoscopes to inspect the sella after a traditional microsurgical transsphenoidal resection. Finally, in the 1990s, multidisciplinary teams including neurosurgeons and otolaryngologists began to report on purely endoscopic endonasal transsphenoidal approaches. In 1997, Jho and Carrau first published their experience with endoscopic endonasal transsphenoidal surgery on 50 patients. Their results revealed the promise of minimally invasive endonasal neurosurgery and paved the way for broader applications of the technology. This article discusses the current state of minimally invasive endonasal techniques to address the pathologic conditions of the anterior cranial fossa and parasellar region.
Anatomy
Detailed understanding of the normal anatomy is critical in any neurosurgical procedure; however, this fact becomes even more important with endoscopic approaches, in which one can easily become disoriented. Some of the key advantages of using an endoscope for transsphenoidal surgery are the ability to angle the view and inspect the tumor bed, and the superior illumination in a deep and dark cavity. This new degree of visualization is initially unfamiliar and can further contribute to surgeon disorientation. It is important to take full advantage of the improved visualization capability by having a detailed understanding of the anatomy of the sella and surrounding structures and to identify key landmarks early in the operation.
A careful review of preoperative imaging is crucial. Magnetic resonance imaging (MRI) provides critical details regarding the anatomy of both the lesion being addressed and the adjacent critical normal structures, such as the pituitary gland, carotid arteries, cavernous sinuses, and cranial nerves. Intrasellar lesions, such as pituitary adenomas or Rathke cleft cysts, can cause the sella to become greatly expanded. Also, as a sellar mass expands, the anatomy of the carotid arteries can become distorted. The cavernous portion of the carotid artery forms the lateral walls of the sphenoid sinus. In a recent cadaver study, the normal distance between the carotid arteries was 21 ± 2.5 mm. However, there can be great variability in the course of the carotid artery, particularly if the artery is displaced or encircled by the lesion. In a study of normal specimens, an extreme medial course of the carotid artery was identified in 8% of cases ( Fig. 1 ). The flow voids of the carotid arteries are easily identified on T2-weighted MRI, and understanding the relationship of the carotid arteries with the lesion is critical in avoiding complications. Another key structure is the optic chiasm and optic nerves. As a sellar mass expands, the diaphragma sellae is forced upward and displaces the optic chiasm. The diaphragm can act as a barrier from the optic apparatus during the resection and can be observed to come into the operative field as the lesion is resected. Alternatively, suprasellar masses, such as craniopharyngiomas, can extend around the superior surface of the chiasm, making difficult a gross total resection via a transsphenoidal approach alone. It is important to recognize this relationship preoperatively to avoid injury to the optic apparatus during resection. Finally, the location of the remaining normal pituitary tissue should be determined on preoperative imaging. The normal pituitary is often identifiable on dynamic gadolinium-enhanced T1-weighted MR images. The pituitary gland takes up the gadolinium early in the postinjection phase whereas the adenoma does not, and conversely, the gland shows no enhancement in the delayed scans whereas the adenoma does. Intrasellar masses often displace the normal pituitary posteriorly and to one side or the other. Understanding where the normal pituitary is located can help avoid intraoperative injury and prevent postoperative hormonal complications.
Whereas MRI provides important soft tissue detail, computed tomography (CT) provides information regarding the bony anatomy. The sphenoid sinus generally contains an intersinus septation (79% of cases) and sometimes additional accessory septations. These septa most often terminate in the midline but can be present along the internal carotid artery prominence (26.7%) or along the optic prominence (19.6%). Understanding the relationships between these septa and the carotid arteries preoperatively can help orient the surgeon ( Fig. 2 ).
In addition, several bony landmarks can be identified intraoperatively. Typically, the carotid arteries and optic nerves are shielded by readily recognizable prominences along the roof and lateral walls of the sphenoid sinus. As the carotid artery prominence curves posteriorly along the lateral wall of the sinus, it passes just inferior to the optic nerve prominence. The depression just lateral to this junction is the lateral opticocarotid recess, which corresponds to the pneumatization of the anterior clinoid process. The prominent bulge between the carotid prominences is the sella. Identifying these structures early in the case is critical to remaining oriented, and frameless stereotaxy can be a helpful confirmatory adjunct ( Fig. 3 ). An expansile mass can erode the overlying bone and obscure these landmarks, leaving them more susceptible to injury. In addition, bony dehiscences of the optic or carotid protuberances or anterior skull base are commonly identified on imaging, even without erosive pathologies. The presence of Onodi cells can also be identified on preoperative imaging, and these cells are present in approximately 8% of cases. Onodi cells are posterior ethmoid cells that can displace the sphenoid sinus posteriorly and interfere with the identification of common landmarks. More importantly, Onodi cells can contain the optic nerve, and it is important that these cells be recognized to avoid inadvertent optic nerve injury during ethmoidectomy.
Endonasal approaches were originally used to treat only sellar lesions, but now these approaches can be used to address a variety of midline skull base lesions. The nasally accessible boundaries extend from the cribriform plate anteriorly to the superior portions of the cervical spine posteriorly. The lateral boundaries are defined by the medial orbital walls anteriorly and the cavernous sinus and carotid arteries posteriorly. With proper training and experience, coupled with appropriate angled endoscopes and instruments, neuronavigation system, and preoperative planning, a wide variety of lesions can now be safely and successfully treated with the minimally invasive endonasal approach.
Anatomy
Detailed understanding of the normal anatomy is critical in any neurosurgical procedure; however, this fact becomes even more important with endoscopic approaches, in which one can easily become disoriented. Some of the key advantages of using an endoscope for transsphenoidal surgery are the ability to angle the view and inspect the tumor bed, and the superior illumination in a deep and dark cavity. This new degree of visualization is initially unfamiliar and can further contribute to surgeon disorientation. It is important to take full advantage of the improved visualization capability by having a detailed understanding of the anatomy of the sella and surrounding structures and to identify key landmarks early in the operation.
A careful review of preoperative imaging is crucial. Magnetic resonance imaging (MRI) provides critical details regarding the anatomy of both the lesion being addressed and the adjacent critical normal structures, such as the pituitary gland, carotid arteries, cavernous sinuses, and cranial nerves. Intrasellar lesions, such as pituitary adenomas or Rathke cleft cysts, can cause the sella to become greatly expanded. Also, as a sellar mass expands, the anatomy of the carotid arteries can become distorted. The cavernous portion of the carotid artery forms the lateral walls of the sphenoid sinus. In a recent cadaver study, the normal distance between the carotid arteries was 21 ± 2.5 mm. However, there can be great variability in the course of the carotid artery, particularly if the artery is displaced or encircled by the lesion. In a study of normal specimens, an extreme medial course of the carotid artery was identified in 8% of cases ( Fig. 1 ). The flow voids of the carotid arteries are easily identified on T2-weighted MRI, and understanding the relationship of the carotid arteries with the lesion is critical in avoiding complications. Another key structure is the optic chiasm and optic nerves. As a sellar mass expands, the diaphragma sellae is forced upward and displaces the optic chiasm. The diaphragm can act as a barrier from the optic apparatus during the resection and can be observed to come into the operative field as the lesion is resected. Alternatively, suprasellar masses, such as craniopharyngiomas, can extend around the superior surface of the chiasm, making difficult a gross total resection via a transsphenoidal approach alone. It is important to recognize this relationship preoperatively to avoid injury to the optic apparatus during resection. Finally, the location of the remaining normal pituitary tissue should be determined on preoperative imaging. The normal pituitary is often identifiable on dynamic gadolinium-enhanced T1-weighted MR images. The pituitary gland takes up the gadolinium early in the postinjection phase whereas the adenoma does not, and conversely, the gland shows no enhancement in the delayed scans whereas the adenoma does. Intrasellar masses often displace the normal pituitary posteriorly and to one side or the other. Understanding where the normal pituitary is located can help avoid intraoperative injury and prevent postoperative hormonal complications.
Whereas MRI provides important soft tissue detail, computed tomography (CT) provides information regarding the bony anatomy. The sphenoid sinus generally contains an intersinus septation (79% of cases) and sometimes additional accessory septations. These septa most often terminate in the midline but can be present along the internal carotid artery prominence (26.7%) or along the optic prominence (19.6%). Understanding the relationships between these septa and the carotid arteries preoperatively can help orient the surgeon ( Fig. 2 ).
In addition, several bony landmarks can be identified intraoperatively. Typically, the carotid arteries and optic nerves are shielded by readily recognizable prominences along the roof and lateral walls of the sphenoid sinus. As the carotid artery prominence curves posteriorly along the lateral wall of the sinus, it passes just inferior to the optic nerve prominence. The depression just lateral to this junction is the lateral opticocarotid recess, which corresponds to the pneumatization of the anterior clinoid process. The prominent bulge between the carotid prominences is the sella. Identifying these structures early in the case is critical to remaining oriented, and frameless stereotaxy can be a helpful confirmatory adjunct ( Fig. 3 ). An expansile mass can erode the overlying bone and obscure these landmarks, leaving them more susceptible to injury. In addition, bony dehiscences of the optic or carotid protuberances or anterior skull base are commonly identified on imaging, even without erosive pathologies. The presence of Onodi cells can also be identified on preoperative imaging, and these cells are present in approximately 8% of cases. Onodi cells are posterior ethmoid cells that can displace the sphenoid sinus posteriorly and interfere with the identification of common landmarks. More importantly, Onodi cells can contain the optic nerve, and it is important that these cells be recognized to avoid inadvertent optic nerve injury during ethmoidectomy.
Endonasal approaches were originally used to treat only sellar lesions, but now these approaches can be used to address a variety of midline skull base lesions. The nasally accessible boundaries extend from the cribriform plate anteriorly to the superior portions of the cervical spine posteriorly. The lateral boundaries are defined by the medial orbital walls anteriorly and the cavernous sinus and carotid arteries posteriorly. With proper training and experience, coupled with appropriate angled endoscopes and instruments, neuronavigation system, and preoperative planning, a wide variety of lesions can now be safely and successfully treated with the minimally invasive endonasal approach.
Indications
Endoscopic endonasal approaches to midline skull base lesions have several advantages when compared with traditional open approaches. The lesion can be directly accessed without retraction of the brain or neurovascular structures; the blood supply of the lesion can often be controlled early in the procedure; and visualization is improved, with better illumination, higher magnification, and a larger field of view than obtained by using the operating microscope. In addition, less morbidity, less blood loss, and shorter hospital stays have been reported with endoscopic approaches.
However, several critical issues need to be considered when determining whether an endoscopic approach is appropriate for an individual patient. The goal of the surgery needs to be clearly defined because achieving it will ultimately determine a successful outcome. This consideration should be the same regardless of whether an endonasal endoscopic or open approach is used. For example, some lesions may be amenable to biopsy alone, whereas in other cases patients would be better served with subtotal or gross total resection.
If the goal of the procedure is to achieve a gross total resection, several anatomic relationships should be studied on preoperative imaging to determine if this goal is feasible. The degree of pneumatization of the sphenoid bone determines the size of the operative corridor and how readily identifiable the bony landmarks within the sphenoid sinus will be. With a poorly pneumatized sphenoid sinus, the bone is thicker and superficial landmarks are more difficult to identify. Thus, more drilling will be required to gain adequate access and the risk of a complication may be higher. Endoscopic approaches are ideally suited for midline lesions with minimal lateral extension. In general, lesions that extend laterally beyond the medial orbital walls invade the cavernous sinus and encircle the carotid arteries or infiltrate the bone and soft tissue of the face, and these lesions may be best treated with an open approach. The presence of brain invasion or extensive intradural involvement is also an important consideration that may lead one more toward an open approach. A cerebrospinal fluid (CSF) cleft on T2-weighted magnetic resonance image, the amount of brain edema, and the size of the mass can all aid in determining if brain invasion has occurred. The relationship of the mass with large arteries and the visual apparatus also needs to be studied carefully. Often, intrasellar lesions, such as pituitary adenomas, displace the carotid arteries laterally and the optic chiasm superiorly. Even when pituitary adenomas have become large, they can often be resected using an endoscopic approach. However, predominantly suprasellar lesions can displace or encircle the optic nerves, carotid arteries, and anterior cerebral arteries in such a way as to potentially cause the endonasal approach to be more difficult and potentially more morbid. Recurrent disease with fibrosis and destruction of normal anatomic landmarks can also be viewed as relative contraindications. Finally, if an en bloc resection is desired based on oncologic principles, such as for nasopharyngeal carcinomas, an open craniofacial approach is likely to be superior because endoscopic resections are generally performed piecemeal.
Related posts:
The Concept of Minimally Invasive Neurosurgery
Application of Technology for Minimally Invasive Neurosurgery
Expanded Endonasal Approaches to Middle Cranial Fossa and Posterior Fossa Tumors
Minimally Invasive Neurosurgery for Cerebrospinal Fluid Disorders
Making the Transition from Microsurgery to Endoscopic Trans-Sphenoidal Pituitary Neurosurgery
Minimally Invasive Surgery for Movement Disorders
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