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Skull Base Brain Tumor Research




The Endoscopic Supraorbital Approach to Tumors of the Middle Cranial Base
By Mohamed S. Kabil, M.D. and Hrayr K. Shahinian, M.D.

Background

Access to tumors of the middle cranial base has traditionally required wide surgical exposures via open craniotomies. These open techniques often require the use of potentially disfiguring skin incisions and are often associated with a significant degree of brain retraction and potential morbidity. We report our experience with the use of a minimally invasive supraorbital endoscopic approach through the eyebrow for excision of middle cranial base tumors in two cases.

Methods

We describe two patients, with large sized middle cranial fossa tumors (a medial sphenoid wing meningioma measuring 6x4 cm and a recurrent right cavernous sinus meningioma measuring 4 x 3.5 cm) that were entirely removed via a fully endoscopic supraorbital approach utilizing a 1.5 cm keyhole craniotomy.

Conclusion

These cases demonstrate how the application of endoscopic techniques to surgery of the middle cranial base can eliminate the need for traditional open techniques without compromising surgical success.

Introduction

Access to tumors of the middle cranial base has traditionally required aggressive transcranial approaches such as the frontotemporal or pterional. These wide exposures, however, often come at the expense of a significant degree of frontal and/or temporal brain retraction and involve unnecessary surgical dissection, which in turn may result in undesirable perioperative morbidity.

To minimize the injurious effects of frontal and temporal lobe retraction and to avoid the use of potentially disfiguring skin incisions, approaches to these tumors have become progressively less invasive. The strategic placement of "keyholes" eliminates the need for excessive surgical manipulation without sacrificing exposure or outcome. Furthermore, with the introduction of the fully endoscopic approaches to the skull base, these tumors have now become amenable to endoscopic resection with a minimum of deleterious consequences.

Our group adopted a previously developed supraorbital keyhole approach [3,4,13,14] in our surgical practice. The approach (although minimal alterations have been described by different aurthors) and its different utilities have been previously reported by many authors [1,2,8-12]. The different areas of the skull base that could be accessed via the supraorbital approach have also been previously summarized [11]; these mainly include: basal parts of the frontal lobe, medial parts of the sylvian fissure, temperomesial area, anterior clinoid proceess, sphenoid ridge,orbital roof, optic canal, olfactory froove, parts of the anterior circulation, both optic nerves, epsilateral optic tract, pituatary stalk region, some areas of the posterior circulation such as the basilar tip and P1 segment of the posterior cerebral artery, and the anterior surface of the upper pons with both cranial nerves number III.

Since late 2002, our group began routinely using the endoscope in performing the supraorbital approach and have published our results [5,6]. Utilizing this technique we found that the use of endoscopy as the sole imaging modality was ideally suited for exploration of the anterior cranial fossa and parasellar area obviating the need for a bicoronal scalp flap or an extensive bifrontal craniotomy without diminishing visualization of the median and paramedian skull base. Additionally, we realized that the adaptation of rigid endoscopy to the supraorbital approach broadens the available surgical exposure, thus providing extended access to the middle and not only the anterior cranial fossa without additional dissection or retraction. Utilizing the endoscopic supraorbital approach to access the middle cranial base, we found that the endoscope enhanced our ability to respect the anatomy of this area due to its superior visibility, therefore enabling a more complete excision of middle cranial base tumors.

Surgical technique

The patient is placed supine on the operating room table and the head of the bed is slightly raised to improve venous drainage. Following the induction of general anesthesia, the patient's neck is extended approximately 15°-30° so that the frontal as well as the temporal lobes will relax and retract downward with gravity away from the orbital roof and the floor of the skull base once cerebrospinal fluid (CSF) is drained. This position facilitates access to middle cranial fossa tumors from an anterior trajectory. Thus positioned, the head is fixed in place using a three-pin clamp and the frontal and para-nasal areas are cleansed with an aqueous antiseptic solution and then draped.

The base of a pneumatically powered endoscope holding arm (Mitaka Kohki Co., Tokyo) is fastened to the operating room table opposite the surgeon; the arm extends to hold the endoscope firmly. A 4.0 mm 0° rigid endoscope (Karl Storz of America, Culver City, CA, USA) is attached to the holding arm.

A standard skin incision is placed within the hair of the eyebrow few mms above the orbital rim according to each patient's individual anatomy that has to be respected. The incision is bounded medially by the supraorbital notch and laterally by the lateral end of the eyebrow just anterior to the frontozygomatic suture. There is no standard position for the skin incision as it has to vary slightly according to each patient's individual anatomy and skull shape. Subsequent to skin and soft tissue incision, a 1.5cm supraorbital craniotomy is performed with its lower end flush with the skull base. The dura is incised and CSF is slowly drained. A combination of mild hyperventilation, positioning and CSF drainage opens a space as the frontal lobe "relaxes" away from the anterior cranial base downwards facilitated by gravity. At this point the monitor becomes literally the "surgeon's eye". The endoscope is introduced through the keyhole and advanced between the frontal lobe and the floor of the anterior cranial base, closely sliding over the superior wall and the lesser wing of sphenoid bone all the way to the middle cranial fossa.

A panoramic view of the tumor is displayed on a flat screen. Endoscopic survey reveals the degree of intracranial tumor spread. Using a combination of custom designed bipolar electrocoagulation and a micro cavitron ultrasonic surgical aspirator (CUSA) the tumor is gradually resected. Once tumor is thought to be completely removed the zero-degree endoscope is withdrawn and the thirty-degree endoscope is introduced and rotated in a clockwise and anticlockwise direction to conduct a survey of the middle cranial fossa and look for any residual tumor out of the straight view of the zero-degree endoscope. Any tumor remnants are removed assuring that the tumor has been totally excised. Of note that the sphenoid ridge and/or the superior orbital roof could all be drilled away, to either give a better basal view or better access to the tumor.

Following tumor removal, the keyhole bone flap is repositioned using absorbable microplates and screws. The skin incision is closed with careful attention to the aesthetic repair. If the frontal sinus is accidentally opened during the bony work, the mucosa is stripped, the nasofrontal duct is obliterated and the sinus is cranialized. The majority of patients undergoing these procedures are monitored in the intensive care unit overnight and thereafter transferred to the ward for 24 hours until discharge from the hospital, typically within 48 hours.

Patients and Methods: History, Patient 1

A 32-year-old woman, that developed headaches two weeks prior to admission. Headaches resolved with intravenous steroids; there was no history of seizures, visual disturbances or cranial nerve deficits.

Examination/Preoperative Testing

On examination the patient's vital signs were within the normal limits. The patient was awake and conversant with no focal neurological deficits; the rest of her examination was insignificant.

Magnetic resonance imaging (MRI) revealed a left sided 6x4 cm extra-axial enhancing mass located anteriorly in the middle cranial base along the greater wing of the sphenoid bone, consistent with medial sphenoid wing meningioma. White matter vasogenic edema was also noted in the left frontotemporal region.

Surgery

Under general anesthesia, after positioning, prepping, draping and incising the skin in the previously described manner, a 1.5 cm keyhole left supraorbital craniotomy was performed, the dura was opened and, CSF was drained from the anterior cranial fossa till adequate relaxation of the left frontal lobe was obtained. A zero-degree endoscope was then introduced and was advanced along the floor of the anterior skull base, taking great care to avoid any injuries to the left frontal lobe. Upon advancing the endoscope to the middle cranial fossa, a large sphenoid wing meningioma within the middle cranial fossa was immediately visualized. The surface was electrocoagulated and a small piece was sent to pathology and confirmed to be a meningioma. Following this, using a combination of the bipolar electrocautery and the micro- ultrasonic aspirator (CUSA), the central most portion of the tumor was initially debulked. The capsule was then gently dissected free of the medial temporal lobe whiles the superficial and small vessels were electrocoagulated. The lateral most aspect followed by the posterior and superior aspects of the capsule were then a gradually resected and gently peeled off the normal brain tissue. The only portion of the capsule remaining at this point was the medial most aspect abutting the left internal carotid artery and it was gently peeled off this vital structure. The zero-degree endoscope was then withdrawn and an angled endoscope was introduced, revealing additional tumor at the bottom of the middle fossa. Again, using a combination of the bipolar electrocautery and micro- CUSA and angled ring curettes, the remainder and final portions of the most inferior aspect of the tumor were removed until a gross total resection of the tumor was achieved.

Finally, hemostasis was obtained and the area was copiously irrigated. Several pieces of Gel-foam were placed in the tumor bed, and the endoscope was gradually withdrawn. The frontal lobe was inspected and no contusions or bleeders were noted. The dura was re-approximated using 4-0 Nurolon sutures. The craniotomy was reconstructed using absorbable bone plate and fibrin glue, following which the subcutaneous tissues were approximated using 4-0 Vicryl sutures and the skin incision was approximated using a 5-0 Monocryl subcuticular stitch. Steri-strips were applied and a small band-aid dressing was similarly applied to the incision and the patient was extubated. The patient was assessed and was moving all four extremities to command and had intact vision and ocular movement in both eyes. She was taken to the recovery room in a stable condition.

History, Patient 2

A 31-year-old lady with a long history of bilateral cavernous sinus meningiomas and neurofibromatosis. She presented to our service as status post multiple doses of radiation therapy and multiple previous surgical resections, now presenting with a large 4 x 3.5 cm right sphenoid wing and cavernous sinus enlarging meningioma and another stable tumor in the left cavernous sinus. The patient complained of occasional blurring of vision which may be secondary to dry eyes and occasional double vision.

Examination/Preoperative Testing

On examination the patient's vital signs were within the normal range. Right facial numbness was noted as a result of the fifth nerve being damaged. Also diplopia was noted. Other wise she was intact and the rest of her neurological examination was unremarkable.

An MRI was performed which revealed a medial right temporal lobe enhancing mass, measuring 4 x 3.5 cm with extension into the right trigeminal cavern and mild mass effect upon the right anterolateral aspect of the pons. Its superomedial aspect bordered the suprasellar cistern. Inferiorly, the mass extended to the skull base lying along the lesser wing of the sphenoid bone. A smaller enhancing 1.2 x 1.4 cm mass at the medial aspect of the left temporal lobe also extending into the left trigeminal cavern and was also compatible with a meningioma. Post-surgical changes in the medial temporal lobe and the frontal lobe along the cavernous sinus were noted. The masses abutted the carotid arteries but neither completely surrounded nor markedly compressed them.

Surgery

Under general anesthesia, proper positioning, prepping and draping a right eyebrow incision and a 1.5-cm supraorbital craniotomy were performed. Dissection and access to the middle cranial base and tumor was then carried out in the previously described manner. A large portion of the tumor was directly visible from over the right sphenoid ridge with the rest of the tumor hiding at the base of the anterior middle cranial fossa. The tumor was noted to be well circumscribed and extremely fibrous. Initially, its surface was electrocoagulated and a piece of it was sent to pathology, confirming a meningioma. Using a combination of the micro-CUSA and the bipolar electrocautery, the central portion of the tumor was electrocoagulated and aspirated. Attention was then shifted to the capsule of the tumor where a nice margin was found between the tumor and the medial right temporal lobe. The tumor was dissected circumferentially and gradually resected off of the medial temporal lobe and the frontal lobe.

Attention was then shifted to the lateral inferior most aspect of the tumor which was extending all the way to the floor of the middle cranial fossa. Therefore, the superior orbital roof and the sphenoid ridge had to be drilled off using a combination of cutting and diamond burs in order to be able to access the floor of the middle cranial fossa. Once this step was completed good access was obtained to the floor of the middle cranial fossa and the inferior most aspect of the tumor was dissected and resected off its floor.

The medial aspect of the tumor was dealt with last as this portion of the tumor was felt to be the most dangerous due to the location of the intracavernous carotid artery. Using microdissection techniques and microinstruments and with great care not to injure the internal carotid artery the tumor was gradually resected from lateral to medial until the carotid artery was identified. Using a combination of the bipolar electrocautery and sharp dissection with angled scissors, the tumor was shaved off of the internal carotid artery from its most medial anterior and inferior aspect until the internal carotid artery was cleared.

At this point the zero-degree endoscope withdrawn, and a thirty-degree endoscope was introduced in order to ascertain the completeness of the resection. The thirty-degree endoscope was turned clockwise and counterclockwise 360 degrees and further minimal tumor remnants were identified and removed. After gross total resection of the tumor and hemostasis, the area was copiously irrigated and closure was carried out in the previously described fashion.

Results

In both cases, the fully endoscopic supraorbital approach allowed excellent visualization of the tumor and the surrounding vascular and neural structures. A macroscopic complete removal was obtained in both cases. The skin scar was virtually invisible in both patients at one month postoperatively and they were both discharged home within 48 hours of their operation.

Discussion

Several approaches have been developed to decrease the size of craniotomies, minimize brain retraction, and promote adequate exposure of the cranial base. In 1913, Frazier [3] reported the first case in which he resected the supraorbital ridge to operate on a pituitary tumor through the anterior cranial fossa. Jane et al. [4] described a supraorbital exploration, which they considered to be the approach of choice for orbital tumors and for anterior communicating artery aneurysms, pituitary tumors, craniopharyngiomas, olfactory grove meningiomas and parasellar lesions.

In our series of patients we found the adaptation of rigid endoscopy to the supraorbital approach is a useful technique for tumors of the middle cranial fossa. Due to its thin caliber, wide range of view and ability to navigate and access remote areas of the skull base, the endoscope broadens the available surgical exposure without the introduction of additional dissection or retraction. Additionally, endoscopes of varying angles of view provide a panoramic perspective of the relevant surgical anatomy and allow for thorough evaluation of the extent of intracranial tumor extension. The maneuverability of the endoscope allows the surgeon to position it directly at the level of dissection, effectively reducing the viewing and operating distances.

In our experience, despite the small size of the craniotomy, the exploration followed by slow CSF drainage provided a space large enough for safe intracranial access to the middle cranial fossa via an entirely anatomical route, thus reserving the integrity of as much normal tissue as possible. Unnecessary manipulation or brain retraction was not needed.

The advantages that the described approach offers over conventional craniotomy are significant including a hidden skin incision completely within the eyebrow; the superficial temporal artery, the frontal branch of the facial nerve and temporalis muscle are not involved in the surgery thus avoiding their injury, and assuring a good blood supply to the region with excellent wound healing; a faster and more direct way to reach the surgical field and obtain an excellent surgical exposure of the field with elimination of unnecessary steps. The lens of the endoscope being at its tip and just in front of the pathology, minimizes unnecessary brain retraction while the greater part of the brain remains protected with the dura and bone; and finally a smoother, more pleasant postoperative course and an earlier return to work for the patient.

The advantages and disadvantages of the endoscope versus the microscope have been well discussed by King et al [7]. The main disadvantages attributed to the use of the endoscope as the sole imaging modality include repeated soiling of the endoscope's lens with debris and blood from the surgical field, the potential injurious threat from the endoscope's shaft, and the biplanar imaging capabilities of the endoscope versus the 3-dimensional view provided by the surgical microscope. These disadvantages have to be weighed against the mentioned advantages. The endoscope may be used to complement the operating microscope in visualization but this may require modification of the approach to include more bony work and brain retraction; whether the endoscope may replace the operating micoscope is controversial and subject to debate.

Moreover, many of the potential risks attributed to the use of rigid endoscopy in skull base surgery could be minimized or even avoided. An irragation sheath surrounding the shaft of the endoscope avoids the necessity of repetitive withdrawal of the endoscope out of the surgical field in order to clean its lens. Additionally, the use and proper positioning of modern pneumatically powered endoscopic arm holders with a "lock" minimizes the injurious risk attributed to sudden accidental or jerky movements of the endoscope. Three dimensional endoscopy and other continuous advancements and refinements occurring in this rapidly growing field will definitely add to the usefulness and popularity of utilizing endoscopes in skull base and brain surgery.

The disadvantages related to the approach itself are minimal and to a great extent avoidable with meticulous and careful performance of the surgical steps. These include scalp anesthesia, due to section or stretch of the supraorbital and supratrochlear nerves; transient postoperative frontalis muscle palsy, or fistula through an occult frontal sinus opening leading to rhinorrhea. To avoid these complications the mini-craniotomy should be far enough from the anticipated location of the frontal sinus and the nasofrontal duct should be obliterated and the sinus cranialized in case of accidental opening. The frontal branch of the facsial nerve virtually never crosses in this area when the skin incision is created in the previously described manner, therefore it is not likely to be directly injured, however, overstretching of the skin is unnecessary with the fully endoscopic technique and should be avoided as transient palsy of the frontalis muscle, due to transmitted stretch from the skin, can occur. The medial limit of the incision should always be kept lateral to the supraorbital notch, bearing in mind that palsy of the supraorbital and supratrochlear nerves may be a handicap for the patient.

Conclusion

We believe that access to anterior and middle cranial base tumors with virtually no brain retraction provided by the fully endoscopic supraorbital approach minimizes the risk of injury to brain tissue, brainstem, cranial nerves, and vascular structures. Furthermore, it results in a more complete tumor removal due to the superior visibility provided by the endoscope with its different angles of view. This minimally invasive technique allowed for rapid recovery of the patients and resulted in minimal postoperative discomfort.

We conclude that the fully endoscopic supraorbital keyhole approach is safe, effective and ideally suited for treatment of tumors of the anterior and middle cranial base. A precise knowledge of anatomical landmarks is however important to avoid unnecessary injury to neural or vascular structures within the anterior and middle skull base. We believe that this technique represents an interesting addition to the field. The significance of this is providing an effective, minimally invasive alternative to access the middle cranial base rather than traditional large craniotomies that seem to be unnecessary and could be avoided.

References
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