The anatomy of the paranasal sinuses is complex with multiple anatomic variants that can predispose patients to disease. Knowledge of this complex anatomy is important not only for successful treatment but to also avoid complications at the time of surgery. This article will review the anatomy with emphasis on a variety of clinically important anatomic variants.
Key points
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This article reviews the complex anatomy of the paranasal sinuses and the multiple anatomic variants.
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It is important for the radiologist and surgeon to have knowledge of the anatomy of the paranasal sinuses to help determine the cause of disease and increase success of surgical treatment.
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There are many anatomic variants that can predispose the patient to complications during endoscopic sinus surgery, which are presented in this article.
Introduction
The nasal cavity is pyramidal-shaped, located midline between the frontal sinuses and the oral cavity, surrounded by the air-filled paranasal sinuses. The nasal cavity receives and conditions air before passage to the more distal respiratory tract. The function of the paranasal sinuses includes immunological defense, decreasing the weight of the head, buffering against facial trauma, and increasing voice resonance. The paranasal sinuses subdivide into air-filled cavities lined by ciliated mucosa, which communicate directly with the nasal cavity.
A thorough understanding of the anatomy of the paranasal sinuses, nasal cavity, and many anatomic variants is essential in achieving successful and safe functional endoscopic sinus surgery (FESS) outcomes. Many anatomic variants are incidental but some can obstruct sinonasal drainage pathways, whereas others present potential for iatrogenic injury during FESS. This article will provide a detailed review of the anatomy and anatomic variants of the nasal cavity and paranasal sinuses.
Imaging techniques
Noncontrast computed tomography (CT) is the primary imaging modality used to evaluate inflammatory mucosal disease of the paranasal sinuses. CT precisely defines anatomy and potential surgical hazards before FESS. Preoperative CT also provides image-guided navigation during FESS. Thin slice high-resolution CT with multiplanar reconstructions in the axial, coronal, and sagittal planes in both bone and soft tissue kernels are useful to define anatomy and mucosal disease. MRI of the sinuses obtained without and with contrast is necessary to evaluate sinonasal masses. MRI can also be helpful in the evaluation of intracranial and soft tissue extension of pathologic condition, skull base encephaloceles, and aggressive infectious processes such as invasive fungal sinusitis.
Nasal cavity and skull base
The paranasal sinuses drain into the nasal cavities. The nasal cavity contains the superior, middle, and inferior turbinates, which divide the nasal cavity into the superior, middle, and inferior meatus, respectively. Some patients also have a supreme turbinate. The posterior ethmoid air cells and sphenoid sinuses drain through the sphenoethmoidal recess into the superior meatus. The ostiomeatal unit (OMU) is the anatomical complex that drains the frontal sinuses, anterior ethmoidal air cells, and maxillary sinuses into the middle meatus. The OMU comprises the maxillary sinus ostium, ethmoid infundibulum, uncinate process, hiatus semilunaris, middle meatus, ethmoid bulla, and frontal recess ( Fig. 1 ). The nasolacrimal duct drains into the inferior meatus, beneath the inferior turbinate. The nasal cavity is divided at the midline by the nasal septum.
Knowledge of the many anatomic variants that occur in the nasal cavities is essential for successful FESS and to reduce the risk of intraoperative complications. A deviated nasal septum is one of the most common variants. Nasal septal deviations can be unilateral or have an S-shaped configuration with deviations along both sides of the midline. A prominent deviated septum can displace the middle turbinate laterally and obstruct the OMU. It may also interfere with surgical access to the middle meatus. Nasal septal spurs, particularly those that contact either the turbinates or lateral wall of the nasal cavity, can lead to obstruction and cause contact point headaches. Pneumatization of the posterior bony nasal septum can narrow the sphenoethmoidal recess and impede access to the sphenoid ostium.
The ethmoid infundibulum is a thin passage, which connects the maxillary ostium to the hiatus semilunaris and is the primary drainage pathway of the maxillary sinuses. The uncinate process is a thin bone that forms the medial wall of the ethmoid infundibulum and is typically resected during FESS. It attaches anteriorly to the lacrimal bone and posteriorly has a free concave margin. The inferior attachment is to the inferior turbinate insertion of the lateral nasal cavity wall. The superior attachment of the uncinate process can be variable. It may attach to the anterior skull base, middle turbinate, or lamina papyracea. The attachment determines the location of frontal sinus drainage. When the uncinate process attaches to the lamina papyracea, the frontal sinus drains into the middle meatus and creates a blind pouch called the recessus terminalis. If it attaches to the anterior skull base or the middle turbinate, then the frontal sinus drains into the ethmoid infundibulum. An atelectatic uncinate process as occurs in maxillary sinus hypoplasia or acquired atelectasis can efface the infundibulum and increase the risk of orbital penetration during FESS. The uncinate process can be pneumatized, potentially leading to infundibulum narrowing.
The middle turbinate has a complex shape and attaches to the nasal cavity in all 3 planes. Anatomic variants of the middle turbinate include concha bullosa, paradoxical middle turbinate, and pneumatized basal lamella. A concha bullosa, a pneumatized middle turbinate, can occur with nasal septal deviation, narrowing of the nasal air passages, or narrowing of the ethmoid infundibulum ( Fig. 2 ). The middle turbinate usually forms a medial convex curvature. A paradoxical lateral convexity can occur and may result in obstruction/narrowing of the middle meatus or ethmoid infundibulum (see Fig. 2 ). The basal lamella of the middle turbinate separates the anterior from the posterior ethmoidal air cells and is an important landmark during FESS. Pneumatization of the basal lamella may be mistaken for an anterior ethmoidal air cell at the time of surgery. Hypertrophy of the nasal turbinates can also lead to obstruction of nasal passages. During normal physiologic nasal cycling, the mucosa lining the turbinates on one side engorges with blood to become reversibly larger than the contralateral side. The nasal cycle is the periodic alternating pattern of unilateral congestion and decongestion of the erectile tissue within the turbinates and should not be mistaken for pathologic condition.
The lamellae course from the nasal cavity through the ethmoid air cells and attach to the anterior skull base. They are critical surgical landmarks and provide organizational structure for the sinonasal cavity. The lamellae are best understood and visualized in the coronal and parasagittal planes. From anterior to posterior, they include the uncinate process, anterior margin of the ethmoid bulla, basal lamella, lamella of the superior turbinate, occasionally the lamella of the supreme turbinate, and the anterior wall of the sphenoid sinus (see Fig. 1 ).
Understanding the anatomic variants of the anterior skull base is essential to decreasing the risk of surgical complications. The anterior skull base is composed of the fovea ethmoidalis and cribriform plate. The cribriform plate is made up of the medial and lateral lamellae. The medial lamellae forms the floor of the olfactory fossa. The lateral lamella is vertically oriented, forms the lateral wall of the olfactory fossa, and is contiguous with the fovea ethmoidalis superolaterally and medial lamella inferomedially. The olfactory recess is the superomedial portion of the nasal cavity, which abuts the cribriform plate and is medial to the skull base insertion of the middle turbinate. In 1962, Keros classified differences in the heights of the olfactory fossae ( Table 1 ) ( Fig. 3 ). The height of the olfactory fossa is determined by the length of the lateral lamella. The lateral lamella is at risk of injury during FESS, which can result in a cerebrospinal fluid (CSF) leak or an encephalocele ( Fig. 4 ). , Risk of injury is greatest with Keros type 3 because of the increased length of the thinness of the bone. There can be asymmetry in the depths of the bilateral olfactory fossae, with the risk of injury being higher on the lower side.
| Keros Classification of the Olfactory Fossa | |
|---|---|
| Type | Length of Lateral Lamella (mm) |
| Type 1 | 1–3 |
| Type 2 | 4–7 |
| Type 3 | >7 |
Determining skull base height and evaluating for osseous dehiscence of the cribriform plate preoperatively will decrease the risk of injury. Cribriform plate defects can occur following surgery, traumatic insult, or can be congenital (see Fig. 4 ). Multiple classification systems have been developed to assess the ethmoid skull base height. A low-lying anterior skull base can mimic a posterior ethmoid air cell during surgery, resulting in unintended penetration. The methodology proposed by Rudmick and Smith is depicted in Fig. 5 . The height of the ethmoid skull base is determined on a coronal CT image at the level of the anterior ethmoidal artery. A line is drawn through the midorbital plane and used as the inferior extent of the measurement. The height of the ethmoid skull base is measured from the midorbital plane to the midportion of the fovea ethmoidalis. Their study found that the average height was 8.5 mm, and 70% of patients were greater than 7 mm, which is considered safe for surgery. A moderate height of 4 to 7 mm was seen in 25% of patients and is moderately safe. A low height of less than 4 mm was seen in 5% of patients and poses the highest risk for complication.
Maxillary sinus and ostiomeatal unit
The maxillary sinus has several recesses, the most prominent being the alveolar recess, which points inferiorly, and the zygomatic recess, which points laterally. Mucosal thickening or small cysts are common in the alveolar recess and often incidental. The maxillary alveolar ridge forms the floor of the maxillary sinus. The sinuses’ natural mucous drainage pattern is the basis for FESS. The anterior group of paranasal sinuses, which include the maxillary sinuses, anterior ethmoid air cells, and frontal sinus, drain toward the OMU. The cilia within the maxillary sinus propel mucous and debris along the sinus walls toward the maxillary ostium, even though it is at the highest point of the medial maxillary wall ( Fig. 6 ). Before the development of FESS, the Caldwell-Luc procedure was performed to create a window along the anterior maxillary sinus wall assuming that gravity would assist in mucosal drainage (see Fig. 6 ). However, the mucociliary apparatus drains upwards toward the maxillary ostium and bypasses the surgically created fenestrations. The goal of modern FESS is to reestablish normal ventilation and drainage, remove foci of disease, and preserve the mucosa and mucociliary clearance.
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