Introduction

The sphenoid sinus is the deepest cell of all paranasal sinuses. Its correct opening becomes essential when there is sphenoid disease or when exposing the middle or posterior skull base. Care must be taken not to damage the optic nerves, the carotid arteries, or the pituitary gland, among other noble structures, to avoid major complications. This chapter concentrates on the surgical anatomy of the sphenoid sinus, preoperative imaging evaluation, surgical approaches detailed step by step, and potential complications and how to avoid them during surgery.

The intricacy of the anatomy of the paranasal sinuses lies in its great variability between subjects. In this sense, the sphenoid sinus is not an exception.

The sphenoid sinus (from modern Latin sphenoides, referring to “wedge”) is located in the middle of the skull. Every single sphenoid sinus wall is in touch with important structures: the lateral walls cover the internal carotid arteries, above these the optic nerves, the cavernous sinus, and all its relations with the third, fourth, fifth, and sixth cranial nerves.

5.1 Anatomy of the Sphenoid Sinus

The sphenoid sinus appears during the second to third years of life when the cartilage corresponding to the ossiculum Bertini attaches to the sphenoid bone and starts pneumatization. Progressively, the sphenoid sinus continues its aeration during the age of 9 to 12 years (and above) and expands laterally, posteriorly, and inferiorly until obtaining the ratio observed in the adult1 ( Fig. 5.1 ).

The sphenoid sinus is sculpted into the clivus and is enclosed superiorly and posteriorly by the sella turcica. Together with posterior ethmoid cells, the sphenoid ostium drains into the sphenoethmoidal recess located beneath the superior turbinate from which mucus passes to the nasopharynx.

A bony vertical septum divides the sinus into two, mostly asymmetric sinuses. This septum is highly variable and can be oriented not only on a vertical plane. When the septum extends more laterally, it usually ends up attached to the internal carotid arteries. This is the reason it is recommended to drill those septa and not fracture them.

It is not rare to discover extra or incomplete septations. Minor transversal septations are actually bony partitions between the posterior ethmoid and the sphenoid sinus and do not represent true sphenoid septations ( Fig. 5.2 ).

The current classification proposed by Lang in 1988 about the patterns of sphenoid pneumatization distinguishes the sphenoid aeration and the sphenoid wall contours in relation to the sella turcica.2 It is often used in the evaluation of sphenoid approaches. The four categories that this classification comprises are conchal type (5%), presellar type (5%), sellar type (25%), and postsellar type (65%) ( Fig. 5.3 ). The sphenoid sinuses may also have pneumatized extensions into the clinoid processes or the pterygoid plates. Aplasia can be seen in 1% and hypoplasia in approximately 13% of subjects.3

The sphenoid ostium is often located at the middle height of the sphenoid sinus. It has an average diameter of 3 mm and is located 7 mm (2–15 mm) above the choana and 4 mm from the midline, placing it in the upper half of the anterior wall of the sphenoid sinus.4 Adjacent to the superior turbinate, it is 7 cm from the nasolabial angle of the columella. The posterior sphenoid wall is approximately 9 cm from the base of the columella.

Sphenoid sinus dissection has been linked with unintentional carotid and optic nerve damage (not exceeding 0.3%).5,6 Damage of the internal carotid arteries or the optic nerves that adhere and lie behind thin bone structures may happen. In almost 90% of cases, the bone thickness is less than 0.5 mm7 or can even be dehiscent ( Fig. 5.4a,b ). Postmortem studies have demonstrated that the carotid artery is found dehiscent in up to 22%.

The pit of the posterior sphenoid wall caused by the respective protrusions of the optic canal superiorly and the internal carotid artery laterally and inferiorly is known as the lateral opticocarotid recess, and it is the projection of the anterior clinoid process of the sphenoid bone ( Fig. 5.5 ). Other recesses, such as the pterygoid recess, also vary depending on whether the sphenoid sinus is well pneumatized or not and constitute anatomic points of interest in the management of the lateral sphenoid wall (e.g., approaching encephaloceles comprising Sternberg′s canal, located laterally to V2).

The posterior sphenoethmoidal cell (or so-called Onodi cell) corresponds to the pneumatization of the most posterior ethmoid air cell that lies laterally and superiorly to the sphenoid sinus reaching the optic nerve. The incidence of sphenoethmoidal cells demonstrated by computed tomography (CT) scan ranges from 5 to 30% depending on the study population.8 Cadaveric dissections identified Onodi cells even in 30 to 60% of the subjects. Awareness of this fact may help reduce the risk of optic nerve trauma during surgery.9,10

The vidian nerve passes through the pterygoid canal from the cranium between the pterygopalatine fossa anteriorly (laterally in the fusion line of the pterygoid process with the greater wing of the sphenoid bone) and the lacerum segment of the internal carotid posteriorly. The position of the canal may vary from being totally or partially embedded into the sphenoid bone to protruding at the lateral aspect of the sphenoid sinus (when well pneumatized). The relation between left and right vidian canal sites is often asymmetric ( Fig. 5.6a ).

The foramen rotundum is embedded in the sphenoid bone laterally, containing the maxillary or second branch of the trigeminal nerve. It also brings sensitive innervation to the sphenoid.

5.2 Preoperative Evaluation

Related posts:

Chapter 4 Frontal Sinus and Draf Approaches Chapter 8 Sphenopalatine and Maxillary Arteries Chapter 6 Medial Maxillectomy Chapter 3 Endoscopic Lateral Nasal Wall and Anterior and Posterior Ethmoid Sinus Dissection Chapter 7 Anterior and Posterior Ethmoidal Arteries Chapter 28 Retrosigmoid–Transclival Approach

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