The nasal cavity and paranasal sinuses are intimately associated with the orbit, and as such frequently serve as an appropriate surgical corridor for endoscopic access to orbital pathology. For instance, the lamina papyracea, or the medial wall of the orbit, serves as the lateral boundary of a complete ethmoid sinus dissection, and inadvertent orbital entry is possible during routine endoscopic sinus surgery. A high nasal septal deviation may challenge the orbital surgeon during endoscopic orbital decompression or dacryocystorhinostomy. Finally, the superolateral wall of the sphenoid sinus or, in some cases, a posterolateral ethmoid air cell is indented by the optic canal, and recognition of these anatomic variants is important to avoid optic nerve injury. Thus a thorough understanding of the surgical anatomy of the nose, septum, and paranasal sinuses is critical for ensuring optimal outcomes in endoscopic orbital surgery.
Surgical Anatomy and Principles Relevant to the Orbital Surgeon
The orbital surgeon’s corridor begins at the nares (nostrils), or the entry point into the nasal cavity. The most anterior aspect of the nares, termed the nasal vestibule , is circumferentially lined by skin. The keratinized squamous epithelial lining abruptly transitions to respiratory mucosa (ciliated pseudostratified columnar epithelium) at the limen nasi, or mucocutaneous junction. From this point posteriorly, the entire nasal cavity, including within the paranasal sinuses, is lined by respiratory mucosa.
Airflow into the nasal passages is regulated at two levels of potential resistance. The external nasal valve is at the level of the nasal vestibule and is bordered by the columella medially, including the caudal nasal septum and medial crura, the nasal sill inferiorly, and the alar cartilage superolaterally. This area is also especially important for endoscopic surgeons, as effective use of the endoscope requires the ability to provide enough distraction and stability against the edges of the nares for maneuvering instruments. More posteriorly, the internal nasal valve, which is the major resistor of airflow and is located at the level of the limen nasi, is bounded by the upper lateral cartilage superiorly, the nasal septum medially, and the anterior head of the inferior turbinate laterally. Recognizing these potential areas for narrowing of the nasal passages, and the potential need to surgically address these areas before orbital surgery, is crucial.
The nasal septum divides the left and right nasal cavities ( Fig. 5.1 ). It is lined by mucoperichondrium anteriorly (covering the quadrangular cartilage) and mucoperiosteum posteriorly (covering the bony septum), and superiorly becomes continuous with the cribriform plate mucosa, and inferiorly with the nasal floor mucosa. In the absence of trauma or surgical manipulation, the posterior aspect of the quadrangular cartilage articulates neatly with the bony septum at the bony-cartilaginous junction. The bony septum consists of the perpendicular plate of the ethmoid bone superiorly, extending to the cribriform plate, and the vomer inferiorly, which borders the choana. The most inferior aspect of the nasal septum is the bony maxillary crest, which consists of the maxillary bone anteriorly and the palatine bone posteriorly.
The septum has a notably rich vascular supply and is the most common site of epistaxis (nosebleeds), accounting for more than 90% of cases ( Fig. 5.2 ). Specifically, the Kiesselbach plexus is a rich arcade of terminal arterial anastomoses located at the anterior septal mucosa bilaterally; it receives tributaries from the sphenopalatine artery, anterior ethmoidal artery, greater palatine artery, and superior labial artery.
Just anterior to the middle turbinates, the bilateral nasal septum may form a symmetrically protuberant zone known as the septal swell bodies. This is a specialized area of the nasal septum containing a higher proportion of venous sinusoids and may impede the surgeon from posterior surgical access. However, topical decongestion generally allows for vasoconstriction and transient shrinkage of the swell bodies, thereby permitting maneuverability around them.
In general, no septum is naturally straight, and there is always some degree of curvature or the presence of cartilaginous or bony spurs ( Fig. 5.3 ). Despite the presence of septal deviations, many patients do not experience clinically significant nasal obstruction in the absence of mucosal edema or inflammation, or a history of nasal trauma. In fact, many septal deviations are high and do not obstruct the nasal airway, which is lower down along the nasal floor. However, high septal deviations pose a unique problem for the endoscopic orbital surgeon, as they may preclude surgical access to the sinuses and thus limit the corridor to the orbit ( Fig. 5.4 ).
Septoplasty to correct the deviated nasal septum may be approached through a standard incision (hemitransfixion, along the caudal edge of the caudal septum, or Killian, more posteriorly, along the mucocutaneous junction) or a directed “spurectomy” (incision made right anterior to the area of deviation). Care must be taken to preserve a 1- to 1.5-cm L-shaped dorsal and caudal strut of cartilage anteriorly to preserve nasal tip support. Elevation of a submucoperichondrial flap, followed by incision of the quadrangular cartilage and elevation of a contralateral flap, allows for adequate exposure of the deviated bone and cartilage, which may then be removed. Superior dissection during septoplasty must be avoided, as cerebrospinal fluid leak, though rare, is possible.
The bilateral inferior turbinates are located along the inferior half of the lateral nasal wall and through the entire length of the nasal passage. These paired structures increase the overall surface area of the nasal mucosa and aid in humidification of inhaled air. The submucosa of the inferior turbinate is rich in venous supply and undergoes regular congestion and decongestion approximately every 90 minutes as part of the nasal physiologic cycle. For this reason, the inferior turbinates are very sensitive to inflammatory changes of the nasal mucosa (e.g., allergic, vasomotor), and commonly serve as an area of nasal obstruction. Similarly, they are very sensitive to decongestants (e.g., oxymetazoline, phenylephrine, epinephrine, cocaine), and tend to shrink in girth with topical application. The major blood supply of the inferior turbinate arises from branches of the sphenopalatine artery, which enters the turbinate from posteriorly.
A simple and high-yield procedure at the outset of any endonasal orbital surgery involves manipulating the inferior turbinates to create room in the nasal cavity for surgical dissection. Inferior turbinate infracture and outfracture is accomplished using a Freer elevator (Skylar Surgical Instruments, West Chester, PA), where it is first placed within the inferior meatus against the lateral edge of the anterior head of the inferior turbinate and fractured medially (should result in a palpable and/or audible “crack”), and is then followed with a lateral fracture against the medial surface of the turbinate head ( Fig. 5.5 ). This is carried along the entire length of the inferior turbinate and serves to create a working channel for instrument maneuvering, dissection, and creating of drip spaces. By performing both infracture and outfracture, a complete fracture through the inferior turbinate bone is created, which allows long-term remodeling of the nasal airway.