Anatomy

The ethmoid sinuses arise from a series of evaginations of the nasal mucosa into a lateral ethmoid mass during the sixth fetal month. Pneumatization will continue until 7 years, and these cells will reach their adult size at 12 years of age.

Superomedially, the ethmoid sinus is bordered by the thinner bone of the lateral lamella of the cribriform plate, and superolaterally by the thicker bone of the fovea ethmoidalis. The sphenoid sinus or skull base forms its posterior border. Superolaterally, the ethmoid sinus is bordered by the thinner bone of the lateral lamella of the cribriform plate, and superomedially by the thicker bone of the fovea ethmoidalis. The length of the lateral lamella has been categorized into three types, as defined by Keros. In Keros type 1, the fovea ethmoidalis lies 1 mm to 3 mm above the level of the cribriform plate; thus the lateral lamella is shortened. In type 2, the difference in height is 4 mm to 7 mm, and in type 3, the difference in height is 8 mm to 16 mm. The longer the lateral lamella, the greater chance there is for inadvertent entry through the skull base.

Additional important anatomic structures of the skull base include the anterior and posterior ethmoidal arteries. The anterior ethmoid artery (AEA) originates from the ophthalmic artery in the orbit, enters the ethmoids through the frontoethmoidal suture, traverses the skull base, and ends in the nasal cavity. By following the face of the ethmoid bulla superiorly to the ethmoid roof and examining the region a few millimeters posterior to this point, the artery often can be found. The canal runs anywhere between 1 mm inferior to the height of the cribriform plate to 4 mm superior, and it often can be dehiscent. It is because of this variability that special care must be taken when dissecting bony septae projecting down from the ethmoid roof. Damage to the artery can cause bleeding, and if it retracts into the orbit, a retrobulbar hematoma can develop, which left untreated, can lead to blindness. This danger also exists in regards to the posterior ethmoid artery (PEA). But with its more posterosuperior location, entering the bony roof of the posterior ethmoid cells at the junction between the lamina papyracea and the frontal bone 2 to 8 mm anterior to the optic nerve, there is less risk for intraoperative injury.

The ethmoid sinuses are divided into anterior and posterior cells by the basal lamella of the middle turbinate. The anterior cells drain into the ethmoid infundibulum, and the posterior cells drain into the sphenoethmoidal recess. The ethmoid infundibulum is a three-dimensional space, running anterosuperiorly to posteroinferiorly, bounded by the ethmoid bulla, the lateral nasal wall, and the uncinate process. The uncinate process is a thin semilunar piece of bone that acts as the anteromedial border of the infundibulum. The superior edge of this bone, although most often free, can attach to either the skull base or the lamina papyracea.

The largest of the anterior ethmoid cells is called the ethmoid bulla. This cell sits posterosuperiorly to the infundibulum. The space behind the bulla is referred to as the sinus lateralis. If the bulla does not reach the skull base superiorly, the resulting space above the bulla is called the suprabullar recess. The most anterior of the ethmoid cells is termed the agger nasi cell, a pneumatization of the lacrimal bone. It is found anterosuperior to the attachment of the middle turbinate to the lateral wall. The posterior wall of the agger nasi usually forms the anterior wall of the frontal recess, and it is present in 93% to 98% of cases.

Ethmoid cells that extend into the maxillary sinus above the ostium are called infraorbital ethmoid cells. Infraorbital ethmoid cells often have been found incidentally, with one study finding these cells in 45.9% of patients undergoing CT scan for sinus complaints and in 41.6% of patients scanned for nonsinus reasons. These cells are often not clinically significant; however, they sometimes can cause obstruction of the ostiomeatal complex.

Ethmoid cells that grow posteriorly and superolaterally to the sphenoid sinus have the potential for pneumatizing around the optic nerve or carotid artery. These cells are called Onodi cells, and they occur in approximately 9% to 12% of the general population. Dissection in and around these cells holds the possible danger of injuring the optic nerve or carotid artery.

Preoperative planning

If a patient fails maximal medical therapy and the decision is made to go ahead with functional endoscopic sinus surgery (FESS), there are important preoperative steps that will make the surgery not only easier for the surgeon, but safer for the patient.

Reviewing the radiology before surgery is the most important of these steps. A patient undergoing sinus surgery should have a computed tomography (CT) scan of the paranasal sinuses to identify any and all anatomic variations. The ethmoid labyrinth is very complex, and without prior knowledge of the individual patient’s anatomy the surgery can be fraught with potential complications. Knowing preoperatively the location of ethmoid variants like agger nasi cells, infraorbital ethmoid cells, and Onodi cells allows the surgeon to predict areas where important structures like the lamina papyracea, optic nerve, and carotid artery may be at risk ( Figs. 1 and 2 ). The lamina papyracea may be dehiscent or medially deviated, and the lateral lamellae of the cribriform plate can be extremely deep, asymmetric, or even eroded from chronic disease ( Figs. 3 and 4 ). It is also important to identify the location of the anterior ethmoid artery as it traverses the ethmoid roof ( Fig. 5 ).

Bilateral Haller cells ( white arrows ). Note that there are multiple Haller cells on the patient’s left side.

Left Onodi cell ( light grey arrow ) with optic nerve ( white arrow ) at lateral aspect. The sphenoid sinus lies below ( star ).

Left lamina papyracea with dehiscence. Note fragment of bone in ethmoid cavity ( arrow ).

Bilateral lamina lateralis of the fovea ethmoidalis showing signs of early erosion from polypoid disease ( arrows ).

Bilateral demonstration of bony nipple marking entrance of anterior ethmoid arteries into the nasal cavity ( white arrows ). Note that the vessels course below the skull base.

The preoperative medical clearance should proceed just as indicated for all surgeries, and for FESS in particular, the cardiovascular status of each patient becomes an important factor in how much controlled hypotension can be used intraoperatively. Proper positioning, the request for controlled hypotension, and total intravenous anesthesia (TIVA) should be a collaborative effort with the anesthetic team and discussed preoperatively.

The patient also should have preoperative placement of pledgets soaked in a vasoconstrictive agent, such as cocaine, phenylephrine, or oxymetazoline, followed by transoral or transnasal injection of 1cc of 1% lidocaine with epinephrine (1:100,000) to region of the greater palatine or sphenopalatine foramen respectively.

All patients at the authors’ institution are given a course of antibiotics and steroids to be started 1 week preoperatively. Lastly, all patients are given a long list of medications that have been implicated in increased bleeding during or after surgery, both traditional and naturopathic, to avoid 2 weeks before and 2 weeks after surgery.

The authors have found all these measures work together to allow for more meticulous dissection by controlling mucosal inflammation and subsequent intraoperative bleeding.

Preoperative planning

If a patient fails maximal medical therapy and the decision is made to go ahead with functional endoscopic sinus surgery (FESS), there are important preoperative steps that will make the surgery not only easier for the surgeon, but safer for the patient.

Reviewing the radiology before surgery is the most important of these steps. A patient undergoing sinus surgery should have a computed tomography (CT) scan of the paranasal sinuses to identify any and all anatomic variations. The ethmoid labyrinth is very complex, and without prior knowledge of the individual patient’s anatomy the surgery can be fraught with potential complications. Knowing preoperatively the location of ethmoid variants like agger nasi cells, infraorbital ethmoid cells, and Onodi cells allows the surgeon to predict areas where important structures like the lamina papyracea, optic nerve, and carotid artery may be at risk ( Figs. 1 and 2 ). The lamina papyracea may be dehiscent or medially deviated, and the lateral lamellae of the cribriform plate can be extremely deep, asymmetric, or even eroded from chronic disease ( Figs. 3 and 4 ). It is also important to identify the location of the anterior ethmoid artery as it traverses the ethmoid roof ( Fig. 5 ).

Bilateral Haller cells ( white arrows ). Note that there are multiple Haller cells on the patient’s left side.

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