Several surgical options for repair of CSF leaks arising from the anterior skull base exist. There has been a paradigm shift over the last 30 years while choosing the best approach given the advancements made in endoscopic techniques.
Intracranial repair was frequently used (and is still used in select cases) for the routine repair of anterior cranial fossa CSF leaks. These leaks were typically approached via a frontal craniotomy. In rare situations, a middle fossa or posterior fossa craniotomy was required. Different repair techniques have been used, including the use of free or pedicled periosteal or dural flaps, muscle plugs, mobilized portions of the falx cerebri, fascia grafts, and flaps in conjunction with fibrin glue. Leaks arising from the sphenoid sinus are difficult to reach by means of an intracranial approach.
Advantages of the intracranial approach include the ability to inspect the adjacent cerebral cortex, directly visualize the dural defect and seal a leak in the presence of increased ICP with a larger graft. When preoperative localization attempts fail to reveal the site of a leak, intracranial approach with blind repair has been successful. In these situations, the cribriform and the sphenoid area, if necessary, are covered with the repair material.
Disadvantages of the intracranial approach include increased morbidity, increased risk of permanent anosmia, and trauma related to brain retraction, including hematoma, cognitive dysfunction, seizures, edema, and hemorrhage. In addition, the postoperative hospital stay is longer, adding to the overall cost of the procedure. Failure rates for this approach are 40% for the first attempt and 10% overall.
Defects in the posterior table of the frontal sinus may be approached externally via a coronal incision and osteoplastic flap. The osteoplastic flap provides the surgeon with a view of the entire posterior table of the frontal sinus and is especially useful for defects more than 2 cm above the floor and lateral to the lamina papyracea. In select cases, these defects may also be approached with a simpler eyebrow incision and an extended trephination of the frontal sinus in combination with an extended endoscopic frontal sinusotomy. Care must be taken to avoid unnecessary trauma to the surrounding mucosa and the frontal recess entirely.
External approaches to the skull base can also be obtained through various incisions or through nasal approaches for access to the ethmoid sinuses and sphenoid sinus. These include external ethmoidectomy, transethmoidal sphenoidotomy, transseptal sphenoidotomy, and the transantral approach to the skull base. These procedures are infrequently chosen in current practice, given the high success rates and low morbidity associated with the endoscopic approach. However, they should be part of every skull base surgeon’s armamentarium.
An external ethmoidectomy begins with a tarsorrhaphy on the ipsilateral eye in order to prevent corneal injury. The incision is made halfway between the medial canthus and the midline of the nose down to bone. Lateral elevation of the periosteum exposes the anterior lacrimal ridge and the lacrimal fossa. The lacrimal sac is elevated and retracted out of the fossa.
As the periosteum is elevated posteriorly along the lamina papyracea, the anterior ethmoidal artery will be encountered 2-2.5 cm posterior to the lacrimal crest. This artery needs to be ligated to increase exposure. The frontoethmoid suture line marks the level of the fovea ethmoidalis, thus dissection should never be superior to this line. The posterior ethmoidal artery is found approximately 1.2 cm posterior to the anterior ethmoidal artery in the frontoethmoid suture line. The optic nerve lies 5 mm posterior to the posterior ethmoidal artery.
The ethmoidal cells are then entered in the area of the lacrimal fossa, and the anterior two thirds of the lamina are removed. A complete dissection of the ethmoid labyrinth is performed. The skull base is then identified in the posterior ethmoids, and the anterior wall of the sphenoid is exposed.
To perform a transethmoidal sphenoidotomy, an external ethmoidectomy is carried out first as described above. The sphenoid sinus ostium is identified and opened first with a small curette or a beaded probe. A Kerrison punch can then be used to enlarge the opening. The anterior wall of the sphenoid is removed in a meticulous fashion to gain access to the sellar region.
The transseptal approach to the sphenoid can be carried out using a sublabial or transnasal incision. An external rhinoplasty incision is preferred by the authors.
The sublabial approach requires the use of a gingivobuccal sulcus incision to expose the pyriform aperture and free the nasal spine. The caudal septal cartilage is then identified, and a left (or right) septal mucoperichondrial flap is elevated. This mucoperichondrial flap is elevated laterally and inferiorly along the nasal floor in the subperiosteal plane. The cartilaginous septum is dislocated from the maxillary crest, and the contralateral nasal floor mucoperiosteal flap is elevated. The contralateral nasal septum is, therefore, not elevated off the cartilage. Once the bony-cartilaginous junction is reached, it is disarticulated and the contralateral posterior flap is elevated. The bony septum is removed to expose the sphenoid rostrum, which is widely removed via osteotomies or a drill to expose the entire sphenoid sinus.
A transantral approach to the skull base offers wider access to the anterior sphenoid, ethmoids, pterygopalatine fossa, and maxilla. An open anterior maxillary sinus antrostomy is known as the Caldwell-Luc procedure. A gingivobuccal sulcus incision is made, and the anterior wall of the maxilla is exposed. The periosteum is elevated superiorly as far as the infraorbital nerve, exercising extreme care to avoid injuring the nerve as it exits via the infraorbital foramen. A canine fossa osteotomy is performed to enter the maxillary sinus. Kerrison rongeurs are then used to extend the opening into the maxillary sinus. The ethmoidal bone can then be approached medially and superiorly through the maxilloethmoidal angle. A more posterior route is taken to expose the sphenoid sinus. When needed, exposure of the pterygopalatine fossa is achieved by creating an opening into the posterior wall of the maxillary sinus.
Compared with external techniques, endoscopic techniques have several advantages, including better field visualization with enhanced illumination and magnified as well as angled visualization. Another advantage is the ability to more accurately position the underlay or overlay grafts. Multiple studies demonstrate a 90-95% success rate with closure of skull base defects using the endoscopic approach.
General endoscopic concepts
As previously mentioned, the role of antibiotic prophylaxis has not been studied in a controlled fashion for iatrogenic and spontaneous CSF rhinorrhea. However, the authors believe that given the previously published rates of ascending meningitis in untreated CSF leaks, the administration of perioperative intravenous antibiotics is warranted.
Decongestion of the nasal cavity with topical 1:1000 epinephrine or 4% cocaine solution is recommended in order to maximize endoscopic visualization. Injection of 1% lidocaine with 1:100,000 epinephrine at the axilla of the middle turbinate and region of the sphenopalatine artery via a transoral or transnasal route causes vasoconstriction of the blood vessels and helps to minimize bleeding. The use of intravenous anesthesia with propofol and remifentanil has also been demonstrated to reduce intraoperative blood loss when compared with inhalational anesthesia. This is related to a decreased heart rate, which translates into decreased cardiac output, thus reducing the amount of peripheral circulatory volume.
Placement of a lumbar drain has not been demonstrated to decrease recurrence rates of CSF rhinorrhea after endoscopic repair. In theory, lumbar drain placement decreases the pressure exerted by the CSF at the site of the repair, thus allowing the tissues to heal. However, this theory has not been validated. In fact, a recent study found no difference in leak recurrence when patients who had a lumbar drain were compared to those who did not. This finding remained true when the patients were subdivided according to the etiology of the leak.
In general, lumbar drain placement remains institution and surgeon dependant. One must take into account that a lumbar drain can lead to headaches related to overzealous CSF drainage and limits patient mobility postoperatively. One of the benefits of lumbar drain placement is the ability to administer fluorescein to guide in the localization of the leak.
When a lumbar drain is used, fluorescein mixed with autologous CSF is injected slowly over several minutes. As previously discussed, fluorescein is not approved by the FDA for the diagnosis and treatment of CSF leaks. Precisely 0.1 mL of 10% fluorescein is mixed with 10 mL of CSF or bacteriostatic saline. The authors have found that injecting this mixture over 10 minutes has resulted in significantly fewer adverse events such as seizures when compared with early reports in the literature.
A study by Elmorsy and Khafagy of 31 patients with spontaneous CSF rhinorrhea indicated that skull base defects can be successfully closed endoscopically using a septal graft and a middle turbinate rotational flap. In a retrospective chart review, the investigators found that defect closure was obtained in 27 patients after one surgery, with closure achieved in two more after a second surgery, giving the procedure an overall success rate of 93.5%. Closure was unsuccessful in two of the 31 patients even after a third surgery, leading to referral for a shunt procedure.
A study by Lemonnier et al indicated that endoscopic endonasal eustachian tube closure is an effective management technique for refractory CSF rhinorrhea occurring after lateral skull base surgery. The surgery was successful in seven out of nine patients in the study, although one of the seven patients required a revision procedure.
Specific endoscopic approaches
Several different endoscopic approaches have been developed. Each is designed to gain access to the area of interest in the most efficient fashion. The transfrontal, transcribriform, transplanum, transsellar, transclival, and transpterygoid have all been well described.
The transfrontal approach allows access to the floor and posterior wall of the frontal sinus. Leaks originating from this area can be successfully repaired using this approach in the majority of the cases. The frontal sinus outflow tract must be carefully preserved in order to prevent mucocele formation in the long term. The main advantage of the transfrontal approach is that it avoids obliteration of the frontal sinus with an osteoplastic flap. This approach, however, may not effectively manage defects originating in the most lateral or superior aspects of the frontal sinus, since these regions may exceed the limitations of current instrumentation when the technique is performed endoscopically.
The approach begins by performing a complete ethmoidectomy. This is followed identification and dissection of the frontal recess. This area is then widened via a modified endoscopic Lothrop or Draf III procedure, which provides a panoramic exposure of the posterior table of the frontal sinus.
The transcribriform approach exposes the medial anterior cranial fossa from the medial aspect of the middle turbinate to the olfactory groove. Posteriorly, it extends to the anterior aspect of the planum sphenoidale. Removing the perpendicular plate of the ethmoid allows access to the crista galli. Extreme care must be used when dissecting near the area of the olfactory groove as damage to the olfactory fibers will cause anosmia.
Access to the lateral aspect of the anterior cranial fossa can be achieved by using the transfovea approach. The dissection extends from the middle turbinate laterally to the lamina papyracea. The frontal sinus marks the anterior limit, and the anterior wall of the sphenoid sinus defines the posterior limit. In some cases, the middle turbinate is removed and the transfovea and transcribriform approaches are combined.
The transplanum approach allows exposure of skull base defects along the planum sphenoidale and those with significant involvement of the suprasellar region. An anterior ethmoidectomy is performed first. This is followed by a posterior ethmoidectomy, which provides access to the most anterior aspect of the planum. The anterior wall of the sella is taken down to provide posterior exposure.
The transsellar approach is the route of choice for defects on the sella turcica with minimal suprasellar extension. It begins with a complete ethmoidectomy followed by identification and opening of the sphenoid ostia. The opening is then generously enlarged to provide wide exposure to the sella. If bilateral access is needed, the posterior bony septum and the intersinus septum can be removed.
The first steps to perform a transclival approach include a bilateral complete ethmoidectomy and a wide sphenoidotomy. The intersinus septum and rostrum are taken down. The dissection extends from carotid to carotid bilaterally and exposes the floor of the sella, the optic canals, and the upper clivus. Drilling the posterior wall of the sphenoid sinus permits exposure of the upper one third of the clivus. The abducens nerves define the lateral limit of the dissection. If access to the lower two thirds of the clivus is required, the nasopharynx is exposed via a transnasal route. The basopharyngeal fascia and prevertebral muscles are incised. The clivus is drilled down until the dura is exposed. The eustachian tubes mark the vertical segments of the carotid arteries and define the lateral extension of the dissection.
The transpterygoid approach begins by performing an endoscopic modified medial maxillectomy. This permits a wide view of the lateral extent of the maxilla and the posterior wall of the maxillary sinus. The infraorbital nerve is then identified and its trajectory followed. A complete sphenoethmoidectomy is then performed. The crista ethmoidalis is isolated, and the main branch of the sphenopalatine artery is identified.
At this point, the surgeon should decide whether a vascularized nasal-septal flap is going to be used to close the defect. If so, every effort to preserve the sphenopalatine artery and its more proximal supply is made. If free mucosal grafts are going to be used, the artery may be cauterized. In either situation, the bone of the posterior wall of the maxillary sinus is removed so the sphenopalatine artery can be dissected proximally to identify the (internal) maxillary artery and its ascending and descending branches. The sphenopalatine artery is also an important landmark since the pterygopalatine ganglion is situated directly posterior to the artery. Care must be taken to preserve the ganglion and its parasympathetic fibers, which contribute to lacrimation.
After the infraorbital nerve, maxillary artery and parasympathetic fibers are identified, the fat within the pterygopalatine fossa may be dissected or cauterized with bipolar cautery until the anterior wall of the lateral recess of the sphenoid sinus is identified. This bone is removed with a drill, thus exposing the contents of the lateral recess of the sphenoid sinus. Typically, any defect in the middle fossa floor occurs in this vicinity, lateral to the Sternberg canal and the foramen rotundum.