Endoscopic sinus surgery (ESS) has significantly advanced since it was first applied. The introduction of endoscopic examination of the sinuses took place in 1902. However, for most of the last century and until the 1970s, ESS was not performed regularly.[1][2] In fact, sinus pathologies were addressed using external approaches using a headlight.[3] Since the 1970s, the techniques applied in endoscopic sinus surgery have been in a constant race with advances in technology with new surgical instrumentation, imaging, simulation, and navigation.

The concept behind sinus surgery stems from Messerklinger's studies on mucociliary clearance and its role in the pathogenesis of sinusitis. The goals of functional endoscopic sinus surgery (FESS) in the treatment of sinusitis are to enlarge sinus ostia, restore adequate aeration of sinuses, improve mucociliary transport, and provide a better route for topical therapies. The notion behind FESS may seem straightforward.  However, the anatomical variance and the broad range and severity of diseases addressed in every FESS remain challenges for the surgeon in every case.  Pre-operative planning for sinus surgery is the crucial step to obtain optimal results and to avoid all possible complications.  

Endoscopic sinus surgery targets sinus pathology and is the gold standard for treating chronic rhinosinusitis (CRS). The boundaries of ESS are continually expanding with technological advances.  At this point, the indications of ESS have surpassed the field of rhinosinusitis. The application of this procedure marked its place in the management of sinus tumors and pathologies beyond the sinuses.

A thorough understanding of the nasal and paranasal anatomy is the key to safe endoscopic sinus surgery. There is a significant variance in the paranasal anatomy among different individuals and different sides. Therefore, studying computerized tomography and other radiologic studies before proceeding with surgery is of utmost importance. 

The external nose consists of nasal bones, upper later cartilages, and lower lateral cartilages. The internal part of the nose is divided into two nasal cavities by the septum.

Each nasal cavity has a medial wall, which is the vertical septum and a lateral wall. The crista galli, cribriform plates, and the sphenoid body form the roof of the nasal cavity.  The floor consists of the palatine process of the maxilla and the horizontal process of the palatine bone.[4]

The septum is a rigid structure and consists of bony and cartilaginous parts. The septal cartilage forms the anterior part of the septum. The perpendicular plate of the ethmoid is found postero-superiorly, the vomer postero-inferiorly, and the crest of the maxillary and palatine bones form the base of the septum. The septum is covered by mucosa and has an extensive blood supply that runs between the mucosa and the perichondrium. The arteries that supply the septum consist of the sphenopalatine artery, which is a branch of the maxillary artery, the anterior and posterior ethmoidal arteries originating from the ophthalmic artery, the septal branch of the superior labial artery deriving from the facial artery, and the septal branch of the ascending and greater palatine arteries branching from the maxillary artery. Little’s area is a vascular area where several arteries anastomose and form the Kiesselbach’s plexus; it is a common area of anterior epistaxis.

The lateral nasal wall has bony outgrowths, which are the turbinates. There are three to four turbinates on each side. The superior, middle, and if present, the supreme turbinate derive from the ethmoid bone and can, therefore, contain conchas.  As for the inferior turbinate, it derives from a separate bone. The turbinates are covered by mucosa, and they play a vital role in the filtration, humidification, and regulation of the flow of inhaled air.

Inferior to every concha, there is a nasal passage, also called the meatus. The inferior meatus is the passage where the nasolacrimal duct drains through the Hasner’s valve. The middle meatus is the most complex and is the main passage through which most paranasal sinuses ( frontal, maxillary, and anterior ethmoid sinuses) drain. Within the middle meatus, the first lamella that is identified is the uncinate process, which is a projection from the ethmoid and is attached to the lacrimal bone anteriorly, the inferior turbinate inferiorly, and has a two-dimensional opening posteriorly, also identified as the hiatus semilunaris. Superiorly, the uncinate can have three different attachments: the lamina papyracea, the middle turbinate, or the ethmoid sinus roof. The hiatus semilunaris is the crescent-shaped space between the ethmoid bulla superiorly and the uncinate inferiorly, and is the area where the secretions drain from the sinuses into the nasal cavity; this structure expands antero-superiorly into the ethmoidal infundibulum.

The ethmoid sinuses are bordered by lamina papyracea laterally, fovea ethmoidalis superiorly, and by nasal cavity medially. The ethmoid sinuses are divided into anterior and posterior cells by a basal lamella. The anterior cells drain in the middle meatus while the posterior cells drain in the sphenoethmoidal recess of the superior meatus. The most anterior cells, the agger nasi, are located at the superior attachment of the middle turbinate. The ethmoidal bulla is the largest ethmoidal cell and is always found posterior to the hiatus semilunaris. Posterior to the basal lamella, the posterior ethmoidal cells are found; these are larger in size and smaller in number compared to the anterior cells. The blood supply to the ethmoid sinuses derives from the anterior and posterior ethmoidal arteries; while the venous drainage is directed to the superior ophthalmic vein or pterygopalatine plexus.

The maxillary sinuses are located between the floor of the orbits superiorly, and the alveolar processes of the maxilla inferiorly. The maxillary sinus ostium is located in the medial wall of the sinus and most commonly opens into the posterior third of the ethmoid infundibulum. It is also usually located on the superior aspect of the medial wall; therefore, it is important not to penetrate the sinus superior to the ostium to avoid violation of the medial orbital wall.[5] In up to 43% of cases, accessory maxillary ostium can be seen and can be located in either the anterior nasal fontanelle or the posterior nasal fontanelle. It is crucial to differentiate the maxillary ostium from accessory ostia during FESS and to enlarge the true maxillary ostium. Occasionally, ethmoidal cells can enlarge and expand laterally into the maxillary sinus wall; these cells are also known as Haller cells. The blood supply of maxillary sinuses derives from branches of the maxillary and facial arteries. Venous return that drains into the facial vein or the pterygoid plexus.

The sphenoid sinuses are paired spaces, located within the body of the sphenoid bone, and separated by a septum. The sphenoid ostium is located in the anterior wall of the sinus and drains into the sphenoethmoidal recess within the superior meatus. The sphenoid sinus is surrounded by several structures, including the internal carotid arteries, the cavernous sinuses, and the optic, vidian, maxillary, oculomotor, trochlear, and abducent nerves. The sphenoid sinus provides a pathway to endoscopically access the skull base, pituitary, optic nerve, and many other structures.[6] Occasionally, sphenoethmoidal cells, also known as Onodi cells, can be found superolateral to the sphenoid and can be closely associated with the optic nerve. Identifying the presence of these sphenoethmoidal cells and their association with the optic nerves, and optic nerve dehiscence and dehiscence of the carotid is extremely important to steer clear from catastrophic complications. Blood supply is derived from the sphenopalatine artery, and venous drainage is via the maxillary vein.

The frontal sinuses are the most superior sinuses and are situated between the inner and outer tables of the frontal bones. Both frontal sinuses are separated by a septum. The frontal sinus ostium is located in the medial aspect of the floor of the sinus; it drains into a frontal recess. The drainage of the frontal recess can flow into the ethmoidal infundibulum or medial to it based on the superior attachment of the uncinate.

The recent classification of frontal sinus anatomy includes:[7]

• Anterior cells: agger nasi, supra agger cells (superior to agger nasi cells), supra agger frontal (extend into the frontal sinus)
• Posterior cells: supra bulla (above bulla ethmoidalis), supra bulla frontal (above bulla ethmoidalis extending into frontal sinus), supraorbital ethmoid (above the roof of the orbit).
• Medial cells: frontal septal ( attached or in the frontal septum) 

The blood supply derives from the supraorbital and supratrochlear arteries (branches of the ophthalmic artery), the venous drainage is via the superior ophthalmic vein.

Since functional endoscopic sinus surgery was introduced, the indications for performing this procedure have been expanding. The advances in the endoscopes, the camera, instrumentation, and navigation laid the groundwork for a limitless world of endoscopic surgery to access the skull base, optic nerve, cavernous sinus, pituitary, orbit, pterygopalatine fossa, and many other spaces and structures. 

The initial and most common indication for FESS is chronic rhinosinusitis. Rhinosinusitis is an inflammatory process of the paranasal sinuses; it is categorized into different groups based on the duration of the inflammatory process:

• Acute rhinosinusitis: less than four weeks
• Subacute rhinosinusitis: between 4 to 12 weeks
• Chronic rhinosinusitis: longer than 12 weeks

These pathologies are some of the most commonly seen conditions by physicians.  Sinus disease is also a significant financial burden on the health care system. For instance, there is, on average, a yearly expenditure of $8.3 billion for chronic rhinosinusitis in the United States.[8]

In addition to cost, CRS has a major negative implication on the quality of life of patients emotionally and physically.[9] Diagnosis of CRS should be based on symptoms and objective findings on physical examination using anterior rhinoscopy, or nasal endoscopy, or on computed tomography scans.[10] Based on the clinical practice guidelines, CRS is initially treated with saline irrigation and/or topical intranasal steroids.[10] When maximal medical therapy fails, the next step is endoscopic sinus surgery. There is still no clear consensus on the criteria for maximal medical therapy vary or on the timing of surgery.[11] The role of surgery in the treatment of CRS with or without polyposis was studied for many years, and FESS has proved its role in significantly improving the quality of life of patients with CRS.[12]

In addition to CRS, FESS plays a role in the management of complicated acute rhinosinusitis (ARS). Extracranial and intracranial complications of ARS are often categorized according to Chandler's classification. They include in order of increasing severity: pre-septal cellulitis, orbital cellulitis, subperiosteal abscess (SPA), orbital abscess (OA), and cavernous sinus thrombosis. In cases of pre-septal and orbital cellulitis, FESS is considered when there is visual impairment or increased intraocular pressure, and in situations that do not improve with medical treatment. 

It also plays a fundamental role in the management of SPA (especially those that are larger than 1 cm) and of OA. The abscess is drained, the sinuses are opened and drained to restore patency, and cultures are taken for targeted antibiotic therapy.[13] 

Given the intimate association between the ethmoid sinuses and the orbit, Endoscopic Sinus Surgery has also been used to access certain orbital pathologies trans-nasally while avoiding skin incisions. Some of the indications for an endonasal approach to the orbit include decompression of the orbit and optic canal in Graves disease or post-traumatic optic neuropathy, lesions of extraconal medial orbital apex or space, benign sinonasal tumors invading the orbit medially, and medial orbital wall fractures.[14]

ESS has also become the standard procedure for mucoceles, invasive and non-invasive fungal sinusitis, silent sinus syndrome, pituitary tumors, cerebrospinal fluid leaks, benign and malignant sinonasal tumors, and ventral skull base lesions, lesions of the petrous apex, or pterygomaxillary fossa. Expanded sinus surgery plays a role in cases of malignancies in the nasal and paranasal spaces, even for those that extend through the anterior skull base.[15]

Navigation-guided endoscopic sinus surgery has gained much recognition in the world of rhinology. It guides the surgeon intraoperatively based on imaging obtained before surgery. The tracking system in image-guided surgery helps achieve a more thorough and complete dissection of the sinuses, better visualization of tumor borders to obtain negative margins, lower risk of complications. 

The indications for using the navigation system include:[16]

• Revision sinus surgery
• Distorted sinonasal anatomy
• Benign or malignant sinonasal tumors
• Repair of the cerebrospinal fluid leak; skull base defects or lesions
• Pathologies close to the optic nerve, orbit, carotid artery, or skull base
• Pathologies involving frontal, sphenoid, or ethmoid sinuses
• Extensive polyposis

Contraindications for FESS include patients who have general contraindications for general or local anesthesia.

Also, contraindications for purely endoscopic surgery include lesions/ pathologies extending into the palate, skin/soft tissues, laterally into or above the orbit, lateral recesses of the frontal sinus, or advanced intracranial involvement.[15] In cases with significant extensions, a combined endoscopic and open approach would be a safer and more appropriate procedure.

Equipment required in the OR includes a television monitor, navigation system, camera, sinus endoscopy tray including varied curettes, downbiters, backbiters, elevators, ball-tip probes, through-cut instruments, Kerrison rongeurs, giraffe instruments, sinus forceps with different angulations, punch instruments, endoscopes ( 0, 30, 45, 70 degree scopes and reverse scopes), and a powered debrider with straight and angled blades.