The approach to tumors of the pituitary gland has undergone a developmental evolution during the past century. While Sir Victor Horsley performed the first transcranial approach to the pituitary in 1899, the Viennese surgeon Herman Schloffer introduced the transnasal transsphenoidal approach to the pituitary gland in 1907. Historically significant modifications to this transsphenoidal approach include the sublabial incision proposed by Harvey Cushing’s in 1910, fluoroscopical guidance introduced by Gerard Guiot in 1958, the operating microscope demonstrated by Jules Hardy in 1965, and most recently the use of endoscopes as a less invasive alternative. Through this evolving surgical technique, it remains imperative that the neurosurgeon have a working familiarity with the nasal anatomy, its variances, and preoperative evaluation.
The Sphenoid Sinus
The pituitary gland resides in the sella turcica whose floor is surrounded by the body of the sphenoid bone. The sella is defined anteriorly by its lip—the tuberculum sella—and posteriorly by the dorsum sella. In one series of cadaveric dissections, the sella was found to measure, on average, 8.5 mm in depth, 10 mm in length, and 13.5 mm in width, with a volume of 575 mm . Moreover, in the majority of cases, the sella floor is less than 1 mm thick. This sella floor corresponds to the posterior aspect of the roof of the sphenoid sinus. It is this relationship between the floor of the sella and the roof of the sphenoid sinus that makes the transsphenoidal approach to the pituitary gland feasible.
The sphenoid sinus aerates the sphenoid bone and is the posterosuperior limit between the nasal cavity and the middle cranial fossa ( Figure 6-1 ). Sphenoid pneumatization begins around 10 months of age with a period of significant enlargement between 3 and 6 years of age. Recent data demonstrate that the sinus continues to pneumatize through the third decade of life, extending to the dorsum sella with an average maximum volume of 8 cc. Pneumatization of the sphenoid sinus has been classified by Hamberger based on its extent of aeration as either sellar, presellar, or conchal. In the sellar type, representing 73% to 86% of sphenoid sinuses, pneumatization extends fully into the body of the sphenoid and below the sella. In the presellar type, representing 11% to 27% of sphenoid sinuses, pneumatization does not extend past a plane perpendicular to the anterior sellar wall. In the conchal type, which represents 1% to 3% of sphenoid sinuses, there exists no pneumatization in the sphenoid bone. Variances in this normal anatomy have obvious implications in terms of transsphenoidal approaches to the pituitary gland.
The sphenoid sinus is usually divided into at least two asymmetrical cavities by a sagittally oriented intersinus septum, and in the majority of cases is also accompanied by accessory septas which may be oriented in any direction. A recent cadaveric study demonstrated that this intersinus septum terminates on the bulge of the internal carotid artery in 12% to 40% of cases, and on the optical canal in 5% to 7% of cases. Approximately one quarter of sphenoid sinuses have no intersinus septum and exist as a single undivided cavity. Preoperative study of the sphenoid septae must be accomplished radiographically to anticipate these deviations and to distinguish between a sinus septum versus posterior or lateral sphenoid wall.
The lateral wall of the sphenoid sinus is adjacent to the cavernous sinus. Located medially within the cavernous sinus, and abutting the sphenoid sinus, the internal carotid artery may be appreciated on the endonasal view residing in its carotid sulcus and indenting the lateral wall of the sphenoid sinus. This bony carotid sulcus is dehiscent in as many as 22% of cases. On endoscopic view into the sphenoid sinus, the optical nerve can be seen indenting the superolateral aspect of the sinus wall. As with the carotid artery, the bony covering of the optic nerve may be thin or dehiscent, although this occurs less frequently than with the carotid artery. How well the carotid artery and optic nerve are appreciated on the endonasal view into the sphenoid sinus is, in large part, because of the degree of sinus pneumatization as described by the Hamberger classification.
The sphenoid sinus drains via a natural ostium, which is located 1 cm superior to the posteroinferior end of the superior turbinate, and corresponds to the vertical middle of the anterior wall of the sphenoid sinus. This ostium usually drains medially into the sphenoethmoidal recess and less frequently through the superior turbinate into the posterior ethmoid sinus. This ostium is located on average 56 mm from the limen nasi at a 36-degree horizontal angle, or 63 mm from the cutaneous nasal sill at a 34-degree angle. Several studies have shown that the posteroinferior end of the superior turbinate is the most reliable endonasal landmark for identification of the natural ostium of the sphenoid sinus.
The External Nose
Transnasal approaches to the pituitary gland can be impacted by the external appearance of the nose, and a solid understanding of its supporting anatomy is important. The skin of the nose is often overlooked but can have an influence on nasal support. The upper half of the nasal skin is typically very thin and quickly reveals any structural irregularities that may be created from surgery. The skin of the nasal tip, on the other hand, tends to be much thicker and sebaceous, occasionally causing tip ptosis and nasal obstruction. The overall size of the nose can also be quite variable and impact the decision making in terms of route of access. Acromegalics, for example, have large noses that lend themselves well to any transnasal approach. Asian noses, on the other hand, tend to be smaller and may limit this approach. The congenitally small nose, traumatized nose, or twisted nose, all represent variations that should be part of the preoperative nasal evaluation.
The upper third of the nose consists of a pyramid of paired nasal bones, which articulate with the frontal bone at the nasofrontal suture. The soft tissue depression defines the nasion of the nose and creates the nasofrontal angle. Inferiorly, the nasal bones articulate with and support the superior border of the upper lateral cartilages. In the midline, it defines the rhinion and is the transition point to the middle nasal vault. It is in this area that most dorsal nasal humps occur.
Support for the middle one third of the nose is provided primarily by the dorsal septum and the upper lateral cartilages. The articulation between the upper lateral cartilages and the dorsal nasal septum has functional significance. Intranasally, these structures define the internal nasal valve , an important anatomical site for maintaining the laminar airflow and sensation of nasal patency, (e.g., “Breathe-Right strips” are designed to widen this area for functional purposes. If they become inadvertently separated during surgery, the upper lateral cartilages tend to collapse over time and lead to a pinched middle nasal vault with associated nasal obstruction. If separation does occur, the upper lateral cartilages should be resuspended to the dorsal septum with long-lasting sutures. Support and projection to the middle third of the nose can be lost from any compromise to the integrity of the dorsal septal strut. This can occur from external trauma or iatrogenic injury (e.g., septoplasty or transseptal pituitary surgery) and can lead to collapse of this area and a “saddle nose.” This deformity is not only associated with external stigmata but is also the cause of impaired function with nasal obstruction ( Figure 6-2 ).
The lower one third of the nose—the nasal tip—has the lower lateral (alar) cartilages as its primary support ( Figure 6-3 ). These cartilages are intimately attached to the caudal border of the upper lateral cartilages in a scroll-like fashion, and to the septum and overlying skin. Disruption of these support mechanisms will affect tip projection and rotation along with nasal function. The external nasal valve is defined by the alar lobule (i.e., the caudal edge of the lateral crus of the lower lateral cartilage), the soft tissue ala, the membranous nasal septum, and the nasal sill inferiorly. A postoperative complication from intranasal incisions is cicatricial scarring within the external nasal valve and consequent obstruction.
The nasal septum is the key midline support structure of the nose and is composed of the quadrilateral cartilage, perpendicular plate of the ethmoid bone, and vomer bone ( Figure 6-4 ). The anterior septal cartilage develops as the unossified portion of the perpendicular plate of the ethmoid. This cartilage attaches posteriorly to the perpendicular plate, posteroinferiorly to the vomer, and superiorly to the nasal bones and upper lateral cartilages. Anteroinferiorly, the cartilage resides within a canal along the anterior nasal spine and maxillary crest. The perpendicular plate of the ethmoid bone represents the posterosuperior portion of the septum. It has two important articulations within the nose. At its posterior edge, the perpendicular plate attaches to the crest of the sphenoid and is a useful landmark for staying in the midline of the nose; despite septal deviations and variations to the intersinus septae, the posterior septum and sphenoid keel make a consistent reference. Secondly, the superior attachment of the perpendicular plate is to the thin, perforated, horizontal cribriform plate at the roof of the nasal cavity. The cribriform plate supports the olfactory bulb and transmits olfactory nerves through multiple foramina. Attached to the cribriform plate is the dura of the anterior cranial fossa. This posterior and superior portion of the septum is rarely dissected but if exposure is needed in this area, tremendous care must be observed when removing this ethmoid portion of the septum in order not to fracture the cribriform or to tear the attached dura. Overly aggressive resection of this portion of the septum may lead to anosmia or to a postoperative cerebrospinal fluid leak.