17 Clinical Pearls in Endoscopic Pituitary Surgery: An Otolaryngologist’s Perspective

Dharambir S. Sethi and Beng Ti Ang

Pituitary surgery has been traditionally performed using a sublabial transseptal transsphenoidal approach and the operating microscope.¹ Tumor dissection has been performed with the aid of the operating microscope, which provides excellent magnification, binocular vision, and a good depth of field essential for tumor removal, and transsphenoidal surgery has proven to be a safe and effective first-line therapy for most patients with sellar and suprasellar lesions. Since the description of endonasal resection of pituitary adenomas by Jankowski et al,² there has been a surge of interest in endonasal pituitary surgery. In the past decade, endonasal surgery for pituitary tumors has gained acceptance worldwide.^3–5 We started endonasal pituitary surgery at our institution in 1994. A cadaver study, in which 30 fresh frozen cadavers were endoscopically dissected to study the anatomy of the sphenoid sinus, sella turcica, cavernous sinus, and the parasellar region, formed the basis of this operation.⁶ Working as a combined rhinology-neurosurgery team, we performed our first endoscopic pituitary surgery in October 1994. This has been a longstanding partnership in which almost 600 pituitary tumors have been operated on using direct endonasal techniques. Our initial approach was transseptal.⁷ The transseptal approach had the major limitation of having to work through a narrow channel created by elevating bilateral mucoperichondrial flaps on the nasal septum. Instrumentation was often difficult, particularly when the situation required the combined efforts of two surgeons. To overcome these limitations, we modified our approach to a direct endonasal transsphenoidal approach, avoiding a septoplasty and its inherent complications. A common transnasal corridor is thus created, allowing the use of both hands and at times having four instruments in the operative field.^8,9 This chapter discusses the practical surgical aspects for those venturing into endonasal pituitary surgery.

Relevant Surgical Anatomy

The importance of cadaveric dissection for learning the endoscopic anatomy and becoming familiar with the endoscopic instrumentation cannot be overemphasized. The sphenoid sinus is subject to considerable variation in size and shape and in the degree of pneumatization.¹⁰ Based on the extent of its pneumatization, the sphenoid sinus has been classified into three types: conchal, presellar, and sellar. In the conchal type the area below the sella is a solid block of bone without an air cavity. In the presellar type, the air cavity does not penetrate beyond a plane perpendicular to the sellar wall. The sellar type is the most common, occurring in 76% of individuals, and the air cavity extends into the body of the sphenoid below the sella and may extend as far posterior as the clivus. The conchal type is common in children under the age of 12 years, after which pneumatization begins within the sphenoid sinus. In the conchal type, which is infrequent in adults, the thickness of bone separating the sella from the sphenoid sinus is at least 10 mm.¹¹ A sellartype pneumatized sphenoid sinus can be compared with a pyramid-shaped six-sided box, the larger side of which is facing forward and forms the anterior wall. The anterior wall is shaped like the keel of a ship and is termed the sphenoid rostrum. The sphenoid ostia are located in the anterior wall. The roof of the sphenoid sinus is flat and termed the planum sphenoidale. It is limited posteriorly by the sellar floor, laterally by the optic nerve prominences, and anteriorly by the anterior wall of the sphenoid sinus. The width of the planum sphenoidale anteriorly, between the lamina papyracea, has been measured in cadaver studies at 26 ± 4 mm and narrows to 16 ± 3 mm posteriorly at the posterior aspect of the tuberculum sella.¹² The posterior wall of the sphenoid sinus is made up of the sellar floor in the upper part and by the clivus in the lower part. It is limited laterally by the carotid prominences, superiorly by the roof or the planum sphenoi dale, and inferiorly by the floor of the sphenoid sinus. The depth of the sphenoid sinus, defined as the distance from the ostium of the sphenoid sinus to the closest part of the sella, averages 17 mm (range 12–23 mm).^10,13 The lateral wall of the sphenoid sinus shows the prominence of the optic nerve anterosuperiorly and the cavernous carotid artery postero-inferiorly. The floor of the sphenoid sinus is formed by the clivus posteriorly and the sphenoid rostrum anteriorly. The extent to which the clivus participates in formation of the posterior wall or the floor of the sphenoid sinus depends on the pneumatization of the sphenoid sinus.

The ostium of the sphenoid sinus is located in the sphenoethmoid recess and varies in size from 1 to 4 mm. In most cases it can be identified medial to the nasal septum and lateral to the superior turbinate approximately 1.5 cm superior to the posterior choana. The septa within the sphenoid sinus vary greatly in size, shape, thickness, location, completeness, and relation to the sellar floor. The cavities within the sinus are seldom symmetrical and are often subdivided by irregular minor septa. A sphenoid sinus that has presellar pneumatization on one side and sellar type on the other side is not uncommon. A single major septum separating the sinus into two large cavities has been reported in only 68% of specimens and may be off the midline by as far as 8 mm.11 The most common type of sphenoid sinus has multiple small cavities in the large paired sinuses. The smaller cavities are separated by septa oriented in all directions. These intersinus sinus septa or the accessory septa may terminate onto the carotid canal in 40% and onto the optic nerves in 4% of individuals.^10,13

The lateral wall of the sphenoid sinus is related to the cavernous sinus that extends from the orbital apex to the posterior clinoid process. The cavernous sinus contains delicate venous channels; the cavernous part of the internal carotid artery; the third, fourth, and sixth cranial nerves; and fibrofatty tissue. The internal carotid artery is the most medial structure within the cavernous sinus and forms a discernible prominence on the posterolateral aspect of the lateral wall of the sphenoid sinus. This prominence is well identified when the sphenoid sinus is well pneumatized. The carotid prominence can be divided into three parts: the retrosellar, infrasellar, and presellar segments. The first part, the retrosellar segment, is located in the posterolateral part of the sinus. This segment of the prominence is present only in well-pneumatized sellar-type sinuses in which the air cavity extends laterally in the area below the dorsum. The second part, the infrasellar segment, is located below the sellar floor. The third part, the presellar segment, is located anterolateral to the anterior sellar wall. Of the 50 specimens examined, Rhoton¹³ reported 98% had presellar, 80% had infrasellar, and 78% had retrosellar prominences. Any part of the prominence may be present and the others absent. If all three parts are present and connected, they form a serpiginous bulge marking the full course of the carotid artery.

The bone separating the artery and the sphenoid sinus is thinner over the anterior than the posterior parts of the carotid prominence and is thinnest over the part of the artery just below the tuberculum sellae. A layer of bone less than 0.5 mm thick separates the artery and sinus in nearly 90% of sinuses, and areas of absence of bone between the artery and the sinus may be present in nearly 10%. The proximity of the carotid prominences to the midline is an important consideration in pituitary surgery. The shortest distance between both carotid bulges into the sphenoid sinus is usually located at the level of the tuberculum sellae in 72%, at the level of the floor of the sella in 20%, and at the clivus in 8%.¹⁰ On the anterosuperior aspect of the lateral wall of the sphenoid sinus is another bulge formed by the optic nerve as it traverses the optic canal from the optic chiasma to the orbital apex. There are areas where no bone separates the optic sheath and sinus mucosa. In nearly 80% of the optic nerves, less than 0.5 mm of bone separates the optic nerve and sheath from the sinus mucosa. In more than half the cases, both the optic nerve and the cavernous carotid artery are separated from the sphenoid sinus mucosa by bone 0.5 mm or less in thickness. The incidence of dehiscence of these structures has been noted to be 4% and 8%, respectively.¹⁰

The absence of such bony protection within the walls of the sinus may explain some of the cases of cranial nerve deficits and carotid artery injury after transsphenoidal operations.¹⁴ The prominence of the internal carotid artery is separated from the bulge of the optic nerve by a pneumatized diverticulum on the lateral wall of the sphenoid sinus called the opticocarotid recess. The extent of pneumatization may vary and in some cases extend through the optic strut into the anterior clinoid process. In a well-pneumatized sphenoid sinus, the pterygoid canal and a segment of the maxillary division of the trigeminal nerve may be identified in the lateral recess of the sphenoid sinus. The trigeminal ganglion and the first and third trigeminal divisions are separated from the lateral wall of the sphenoid sinus by the carotid artery.

The roof of the sphenoid, the planum sphenoidale anteriorly, is in continuum with the roof of the ethmoid sinus. At the junction of the planum sphenoidale and the posterior wall of the sphenoid, the sphenoid bone is thickened to form the tuberculum sella. Inferior to the tuberculum sella, on the posterior wall, is the sella floor, which forms a midline bulge. The thickness of the anterior sellar wall ranges from 0.1 to 0.7 mm (mean, 0.4 mm) as compared with 0.3 to 1.5 mm (mean, 0.7 mm) for the presellar type.^11,15 Often the dura may be visible through this thin bone, imparting a bluish hue to the sellar floor that aids in its recognition. Removal of the sellar floor provides access to the sella turcica.

The main portion of the pituitary gland lies in the sella turcica and is connected to the brain by the infundibulum. The diaphragma sella forms the roof of the sella turcica. It covers the pituitary gland except for a small central opening in the center, which transmits the pituitary stalk. The diaphragma is more rectangular than circular, tends to be convex or concave rather than flat, and is thinner around the infundibulum and somewhat thicker at the periphery. It is frequently a thin, tenuous structure that would not be an adequate barrier for protecting suprasellar structures during transsphenoidal surgery.¹¹ The opening in the center of the diaphragm is large when compared with the size of the pituitary stalk. In front of the infundibulum, the upper aspect of the gland is related directly to the arachnoid and pia mater. An outpouching of arachnoid may protrude through the central opening in the diaphragma into the sella in about half the patients and may be inadvertently opened by probing instruments during tumor removal, causing cerebrospinal fluid leakage.14 A deficiency of the diaphragma sella is assumed to be a precondition to formation of an empty sella.

Venous sinuses that interconnect the paired cavernous sinuses may be found in the margins of the diaphragma and around the pituitary gland. The intercavernous connections within the sella are named on the basis of their relationship to the pituitary gland; the anterior intercavernous sinuses pass anterior to the hypophysis, and the posterior inter-cavernous sinus pass behind the gland. These intercavernous connections may occur at any site along the anterior, inferior, or posterior surface of the gland, or all connections between the two sides may be absent. The anterior intercavernous sinus may cover the whole anterior wall of the sella. The anterior sinus is usually larger than the posterior sinus, but either or both may be absent. If the anterior and posterior connections coexist, the whole structure constitutes the “circular sinus.”^11,13 Inadvertent entry into the anterior intercavernous connection that extends downward in front of the gland during transsphenoidal operation may produce brisk bleeding. However, this usually stops with temporary compression of the channel with hemostatic foam or with light coagulation, which serves to glue the walls of the channel together.

Working as a Multidisciplinary Team

Pituitary surgery lies within the realm of the neurosurgeon. For an otolaryngologist wishing to partake in pituitary surgery, it is extremely important to work closely with neurosurgeons. In the current era, the accumulated experience of endoscopic trained otolaryngologists may be used by neurosurgeons in providing endonasal access to the sphenoid sinus and the floor of the sella. The otolaryngologist’s prime role during surgery is to provide a relatively bloodless unimpeded access to the sphenoid sinus through a wide midline sphenoidotomy and assist the neurosurgeon in tumor removal. Although it is possible for a single surgeon to perform this surgery, a team approach is preferred. Once the initial approach is complete, both the neurosurgeon and otolaryngologist can work together. In some situations it is important for the two surgeons to work together, the otolaryngologist providing manual manipulation of the endoscope to optimize visualization, particularly when angled scopes are used and the field of the view is constantly changing.

Working as Part of a Pituitary Service

Management of pituitary tumors requires a dedicated multidisciplinary treatment team. Inappropriate pituitary hormone secretion and visual field deficits are the most characteristic presenting features of pituitary adenomas. Therefore, our referral base comes mainly from departments of endocrinology and ophthalmology. Specialists from these departments are actively involved in perioperative care of patients with pituitary tumors. In addition, the neuroradiologist, neuroanesthesiologist, interventional radiologist, neuropathologist, and radiation oncologist form important members of the team. In our institution, we have formed a pituitary service that consists of dedicated specialists involved in the care of patients with pituitary tumors. In addition to the neurosurgeon, otolaryngologist, endocrinologists, ophthalmologist, radiologist, radiation oncologist, and interventional radiologist, the board also includes representatives from the service quality department. Pituitary board meetings are held on a 3-month or ad hoc basis. Complex cases, postoperative results, and complications are audited during these meetings.

Indications for Surgery

Surgery for pituitary tumors has proven to be effective therapy for both endocrine active and nonfunctioning pituitary adenomas. Indications for surgery include all nonsecreting and most secreting pituitary tumors except for prolactinomas, which are usually well controlled by medical therapy with a dopamine antagonist.¹⁶ Indications for surgery also include failure of or resistance to medical treatment or in-tolerable side effects of medical therapy and recurrences, in combination or in association with the other therapeutic options, medical or radiotherapeutic.

Nonsecretory tumors may vary in size, expanding the sella and extending along the paths of least resistance, laterally into the cavernous sinuses, superiorly into the suprasellar cistern, and anteriorly into the sphenoid sinus. Some nonsecretory tumors may have very large suprasellar extension. These tumors are best managed surgically with a combined endonasal and transcranial approach either in the same setting or as staged operations. Residual tumor following maximal endonasal surgical debulking may be dealt with by a repeat endoscopic procedure or craniotomy or adjuvant stereotactic radiation therapy, depending on the evaluation of the patient. Most secretory tumors, presenting with features of acromegaly and Cushing’s disease, are an indication for surgery. For prolactin-secreting tumors, surgery is considered for those tumors that do not respond to medical therapy or for patients who are unable to tolerate medical treatment or who have cystic tumors. Pituitary apoplexy may require emergent surgery, as these patients usually present with sudden and rapid deterioration of vision. For a surgeon embarking on pituitary surgery, it is recommended to start with small and soft tumors and after attaining adequate experience to advance to larger tumors with significant suprasellar extension or cavernous sinus invasion.

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