Endoscopic Transsphenoidal Surgery: Anatomy, Instrumentation, Techniques

Anatomy

If a thin endoscope (1 to 2 mm in diameter) is inserted into the nasal cavity and through the sphenoid ostium, it can reach the sellar floor without touching or altering any anatomical structure along its pathway. The absence of anatomical barriers between the vestibule of the nasal cavities and the sellar floor explains why the endonasal approach represents a logical, direct, and minimally traumatic route for the removal of sellar lesions.

The use of the endoscope through an endonasal route requires detailed knowledge of the anatomical structures (and their variations) that form the walls of the nasal cavities and that with such an approach are not hidden by the blades of the sphenoidal retractor, as in traditional transsphenoidal microsurgical approaches. Furthermore, the wider endoscopic vision into the sphenoid sinus and sellar cavity provides a number of useful landmarks for the surgeon and their recognition is absolutely required to prevent complications.

Nasal Landmarks

The two nasal cavities resemble truncated pyramids, wider below than above. They are enclosed by the cribriform plate (roof), hard and soft palate (floor), nasal septum (medial wall), ethmoid, maxillary bones, and inferior nasal turbinate (lateral wall), body of the sphenoid bone (posterior wall), and the external nose (anterior wall). From a surgical point of view the endoscopic knowledge of the lateral and posterior wall of the nasal cavity needs major consideration.

The lateral wall of each nasal cavity contains important landmarks, useful for the orientation of the surgeon. It usually accommodates three nasal turbinates, has a very irregular surface, and is characterized by depressions and ostia that represent the communications between the nasal cavities and the various sinuses that are incorporated into the cranial and facial bones. The inferior one is the most anterior turbinate and is the first to be encountered when the endoscope is inserted into the nasal vestibule ( Figure 12-1, A ). The inferior turbinate is a distinct and completely independent structure. The progression of the endoscope in an anteroposterior direction, between the tail of the inferior turbinate and the nasal septum, permits the endoscopist to reach the choana, where the meatus of the eustachian tube is located. The choana represents a main landmark of the endoscopic transsphenoidal approach because it provides a defined limit between the rhinopharynx and the anterior wall of the sphenoid sinus. Furthermore the ostium is not always immediately visible and the choana gives the surgeon an idea of the height of the clivus and of the sella even before seeing them both.

The middle and superior turbinates are parts of the ethmoid. The anterior margin of the middle turbinate is about 2 cm posterior to the anterior margin of the inferior turbinate. It can be identified by tilting the zero degree endoscope about 30 degrees upward with respect to the floor of the nasal cavity ( Figure 12-1, B ). In some patients, the head of the middle turbinate can be more or less pneumatized and in such cases is defined as a concha bullosa . A wide concha bullosa can severely narrow the nasal cavity, precluding the further progression of the endoscope. Nevertheless, it can be opened and its medial bony wall removed, thus obtaining adequate space between the middle turbinate and the nasal septum. The tail of the middle turbinate lies approximately at the level of the sphenopalatine foramen; through this foramen the sphenopalatine artery enters into the nasal cavity. It is an important landmark for identifying potential sources of arterial bleeding. The sphenopalatine artery is the terminal branch of the internal maxillary artery, which in turn is a branch of the external carotid artery. In the nasal cavity, the sphenopalatine artery has two ramifications: the medial one is the nasopalatine artery, which passes above the choana and goes to the nasal septum; the lateral ramification is the posterior nasal artery, which reaches the lateral wall of the nasal cavity and supplies the middle and inferior turbinates. The branches of the nasopalatine artery can be easily injured during the removal of the anterior wall of the sphenoid sinus ( Figure 12-1, C ).

The superior turbinate is located posteriorly and superiorly to the middle turbinate; sometimes it is accompanied by a small supernumerary turbinate, also known as the supreme turbinate.

The posterior wall of each nasal cavity is formed by the sphenoethmoid recess, located above and behind the superior nasal turbinate and in front of the anterior aspect of the sphenoid sinus. The sphenoethmoid recess is the site of the sphenoid ostium, which is the communication between the nasal cavity and the sphenoid sinus ( Figure 12-1, D ). This ostium, of variable dimensions, sometimes is hidden by the tail of the superior turbinate and may not be readily recognizable; in such cases, the sphenoid cavity can be identified by ascending with the endoscope from the superior aspect of the choana for 1.5 cm along the sphenoethmoid recess. In elderly subjects, the sphenoid ostium may be quite wide: in such cases, the simple introduction of the endoscope through the ostium permits exploration of the sphenoid cavity and visualization of the sellar floor with all its surrounding landmarks.

If the sphenoid sinus and the sphenoid rostrum are well pneumatized, the ostium can be situated laterally, covered by the superior or supreme turbinate, and thus is not initially visible. In these cases, the superior and/or supreme turbinate can be gently lateralized or removed, taking care to protect the lateral lamella of the cribriform plate on which these turbinates are inserted. In fact, it is important to keep in mind that the roof of the ethmoid has two components: a superolateral roof (frontal bone) made of quite thick bone, and a superomedial wall (ethmoid bone) composed of thin bone, the so-called lateral lamella of the cribriform plate. The more elongated the lateral lamella is, the greater is the risk of damaging the cribriform plate during the removal or the lateral luxation of these turbinates, thus causing an ethmoid CSF leak.

Sphenoid Landmarks

After identifying the sphenoid ostium and the sphenoethmoid recess, the nasal septum is detached from the sphenoid bone at that level to expose the whole anterior wall of the sphenoid sinus, which resembles the prow of a boat seen from the front ( Figure 12-2, A ). The anterior wall of the sphenoid is removed in a circular fashion, preventing extension of the bony removal too far in the inferolateral direction, behind the tail of the middle turbinate where the sphenopalatine artery and its principal branches are located.

The degree of pneumatization of the sphenoid bone is an important factor in the endoscopic procedure. The greater the degree of pneumatization of the sphenoid sinus, the easier is the identification of the bony protuberances and depressions inside the sphenoid sinus. In cases of presellar or conchal type of sphenoid sinus, most of the advantages offered by the wider endoscopic view are reduced by the absence of bony protuberances and depressions inside the sphenoid sinus.

The space within the sphenoid sinus is ordinarily subdivided by one or several septa. Single septa often are not located in the midline and in 20% of cases the posterior attachment of a sphenoid septum is on the carotid protuberance, thus becoming an important landmark for preventing injury to the carotid artery.

After the removal of the septa, the sphenoid sinus roughly resembles a cube :

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The roof is part of the anterior segment of the anterior cranial fossa and extends from the planum sphenoidale to the limbus of the sphenoid. Behind it is the sella turcica with the prechiasmal sulcus in between.
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The floor is formed by the roof of the choanae anteriorly and by the roof of the nasopharynx posteriorly. In well-pneumatized sphenoid sinuses, the pterygoid canal, where the vidian nerve runs, is recognizable as a bulge of the floor in a paramedian position. It should not be mistaken for a septal remnant. Laterally, at the junction with the lateral wall, the maxillary nerve may form another bulge and is best recognized with an angled endoscope turned laterally.
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The superior aspect of the lateral wall also forms the bony medial wall of the cavernous sinus. A recess of variable size, the opticocarotid recess, is located between the bulge of the optic nerve and that of the internal carotid artery. This recess may be quite deep and extends to the anterior clinoid process.
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The posterior wall consists of the clivus.

The panoramic view provided by the endoscope of the bony prominences and depressions inside the sphenoid sinus allows one to see a sort of “fetal face,” where the forehead corresponds to the sphenoid planum, the eyes to the two opticocarotid recesses, the eyebrows to the two optic nerves, the nose to the sella, and the mouth to the clivus, laterally limited by the two paraclival carotid arteries, representing the cheeks ( Figure12-2 B and C ).

The sella and the surrounding structures: the planum sphenoidale, the cavernous sinus, and the clivus

The pituitary gland is situated within the hypophyseal fossa, a fibro-osseous compartment near the center of the cranial base. This fossa is limited laterally and superiorly by reflections of the dura mater and anteriorly, posteriorly, and inferiorly by the sella turcica, a depression in the body of the sphenoid bone.

The hypophyseal fossa is not an enclosed compartment. The diaphragma sellae, a fold of dura with a central aperture, forms an incomplete roof above the sella turcica. The diaphragma separates the anterior lobe from the overlying optic chiasm. The central opening of the diaphragma is of variable size and transmits the pituitary stalk and its blood supply ( Figure 12-3 ). The subarachnoid space of the chiasmatic cistern can extend through the aperture of the diaphragma and into the sella turcica for varying distances above the gland.

When approaching a pituitary lesion, it is fundamental for the surgeon to keep in mind what anatomical structures are above the sella. The pituitary gland is an extra-arachnoid structure, situated below a plane composed of a dural ring (diaphragma sellae) and the overlying suprasellar cistern. All the surgical maneuvers must respect these structures to prevent postoperative CSF leaks and other major complications.

To expose the suprasellar region and the planum sphenoidale with the endoscope, it is not sufficient to simply enlarge the sphenoid ostium and to remove the rostrum, as commonly done in the standard endoscopic approach to the sellar region. The attainment of the suprasellar region and the sphenoid planum is achieved through a more anterior trajectory as compared with the one employed for reaching the sellar region. This route requires a wider opening of the superior portion of the anterior wall of the sphenoid sinus, which is obtained by removing the superior and/or supreme turbinates and the posterior ethmoid cells localized laterally to these turbinates. The superior and/or supreme turbinates are removed along their base on the lamina of the turbinates, paying attention not to damage it. After removing the turbinates, the posterior ethmoid cells are opened in a posterior direction, exposing the anterior wall of the sphenoid sinus, in a lateral direction, up to the posterior portion of the medial wall of the orbit and to the apex of the orbit ( Figure 12-4, A ), and in a superior direction, up to the ethmoid portion of the planum. In the course of such maneuvers, particular attention must be paid to preventing damage to the posterior ethmoidal artery, a branch of the ophthalmic artery that passes through a thin, bony channel along the roof of the ethmoid ( Figure 12-4, B ). It is also important not to extend the removal of the nasal septum and the ethmoid too anteriorly to prevent damaging the olfactory nerve endings or the lamina cribrosa of the ethmoid. Once the sphenoid cavity is exposed, all the bony protuberances and depressions inside it must be recognized. The bony protuberances over the anterior loop of the intracavernous portion of the internal carotid arteries are identified laterally to the sellar floor. Above the sellar floor, the angle formed by the convergence of the sphenoid planum with the sellar floor is recognizable; this, from the intracranial view, corresponds to the tuberculum sellae. As one moves in an anterior direction, the sphenoid planum is visible, laterally delimited by the protuberances of the optic nerves that diverge toward the apices of the orbits. The removal of the tuberculum sellae and/or of the sphenoid planum is performed after extensive removal of the mucosa that lines the sphenoid cavity ( Figure 12-4, C ). The opening can be extended in a posteroanterior direction up to 1.5 to 2 cm and laterally not beyond the posterior ethmoidal artery; the lateral extension of the opening is limited by the protuberances of the optic nerves.

The approach to the cavernous sinus is performed through the removal of the bone that covers the intracavernous internal carotid arteries (carotid bony protuberances) ( Figure 12-5, A ). This maneuver permits exposure of both the medial and lateral compartments of the cavernous sinus. The intracavernous carotid artery inside the sphenoid sinus looks like a shrimp and it is possible to identify the various segments on the basis of relationships with the surrounding structures. It is possible to distinguish a parasellar and a paraclival segment. The parasellar segment is C-shaped opened posteriorly and ends at the level of the distal dural ring. The paraclival segment can be subdivided into an extracavernous lacerum segment, more caudal, and an intracavernous trigeminal segment, which is more cranial and has relationships with the gasserian ganglion and the first two trigeminal branches. From the medial aspect of the parasellar segment of the intracavernous ICA, it is possible to see the origin of the inferior hypophyseal artery ( Figure 12-5, B ). Laterally to the intracavernous ICA, it is possible to see the nervous structures contained within the cavernous sinus ( Figure 12-5, C ). With the enlargement of the bony opening in the direction of the opticocarotid recess, it is possible to expose the segment of the intracavernous ICA between the proximal and distal dural rings ( Figure 12-5, D ).

Access to the clivus takes place through a lower trajectory with respect to that necessary for the sellar region. Along such a trajectory we will find the vomer and the inferior wall of the sphenoid sinus that, based on its degree of pneumatization, will divide in various measures the sphenoid portion of the clivus from the rhinopharyngeal segment. Entry into the posterior cranial fossa through the clivus is most easily gained in retrosellar type sphenoid sinus.

Once the choana is localized, the nasal septum is incised, approximately 0.5 to 1 cm posteriorly to the sphenoid rostrum and extending up to the inferior margin of the vomer. The nasal septum is detached from the rostrum. The sphenoid rostrum and the posterior portion of the vomer are removed ( Figure 12-6, A ). If necessary, the rhinopharyngeal portion of the clivus can be exposed by means of a midline incision of the mucosa of the rhinopharynx, extending downward to the level of the eustachian tube.

Once the sphenoid portion of the clivus is exposed, the sphenoid mucosa is removed and the clivus is drilled. The limits of the clival fenestration are represented by the paraclival tracts of the intracavernous carotid artery laterally, the sellar floor superiorly, and the floor of the sphenoid sinus inferiorly.

The clivus contains the basilar plexus, which is the most extensive series of intercavernous venous connections across the midline. The superior and inferior petrosal sinuses join the basilar plexus. The abducent nerve enters the cavernous sinus by passing through the basilar sinus close to the inferior portion of the paraclival tract of the intracavernous carotid artery; therefore particular attention should be paid during bone removal in this area. The dorsal meningeal artery, a branch of the meningohypophyseal artery, provides the arterial blood supply to the dura of the clival region.

After removal of the periosteum and dura, the basilar artery in the basal cistern and its branches, and the neighboring cranial nerves, are well seen along almost their entire courses ( Figure 12-6, B ).

Instrumentation

The two main characteristics of the endoscopic approach, when compared with the standard microsurgical operation, arise from the use of the endoscope itself and from the absence of the transsphenoidal retractor. These two characteristics determine the need for proper endoscopic equipment to look within the surgical field and specially designed surgical tools to work effectively where the endoscope allows us to see.

Endoscopic Equipment

The endoscopic picture on the television monitor is not the direct transposition of the real image, as is seen looking through the eyepiece of a microscope, but it is the result of a microprocessor’s interpretation and of the interconnections of different devices in the system.

The endoscopic equipment is composed of four main parts:

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The generator of the light (light source)
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The transmission of the light (endoscope and cable)
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The image acquisition (camera)
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The image display (video monitor).

Each part is necessary to obtain the endoscopic picture on the video monitor that represents the only view of the surgical field for the surgeon and others involved in the surgical procedure. If one of the components of the endoscopic equipment does not work well, it will influence all the others; therefore it is mandatory to check the optimal function of every link of the chain before the surgical operation starts.

Concerning the light source , cold xenon light sources have a lower degree of heat dispersion as compared with halogen light sources, with subsequent reduced risk of damage to the neurovascular structures. Furthermore, the xenon light source has a color temperature of 6000 K, the same as solar light: it is as if we use the sun to light the operating field, with a chromatic spectrum similar to normal. Even though cold xenon light sources have lower heat dispersion, care must be applied because today’s cold light is still powerful enough to burn tissue.

The endoscope gives a wide and adjustable view of the surgical field and brings the vision and the light inside it close to the relevant anatomy, thus permitting the inspection of fine details. Because of the very large field of view (FOV) of the endoscope, it usually has a large depth of view (DOV) and therefore allows focus both from very close to and far from the object. The degree of magnification in endoscopy depends on the distance of the object to the lens: the closer to the target the bigger the magnification. Concerning the bidimensional vision provided by the endoscope, it does not lack in depth of field since many landmarks are identified with repeated in-and-out movement. The endoscopic image is like a painting, where light and shadows and the relative dimensions of multiple objects (i.e., the bigger, the closer) give precise location perspective and depth of field of the surgical target area in the mind of the surgeon. The endoscopic view is slightly deformed by the “barrel effect.” This is appreciable, especially with the endoscope close to the target and is characterized by an image that in the periphery appears too far away and in the center is too close. Nevertheless, modern digital cameras automatically calculate and correct for “barrel” distortion, thus minimizing the problem.

The endoscope employed in endoscopic transsphenoidal surgery is a rigid scope, 4 mm in diameter, 18 cm in length, and with zero degree lens face ( Figure 12-7 ). Endoscopes 2.7 to 1.9 mm in diameter can also be used, especially in children or in very narrow nostrils, but the smaller the diameter of the lens, the less light it can transport. On the contrary, for each 10% of increase of diameter there is a 46% increase in light transmitted. The endoscope should be introduced in a irrigation sheath, connected to a cleaning-irrigation system; the latter is controlled by a manual or foot switch. The irrigation system permits cleaning of the distal lens, thus avoiding repeated entrances and exits from the nostril. The use of angled endoscopes (30 and 45 degrees) is advised in the sellar phase of the procedure to explore the residual cavity, after lesion removal, or to inspect the suprasellar and parasellar compartments.