Sinus Anatomy

Overview and Drainage

The paranasal sinuses are a variable system of drainage corridors bounded by the orbit, anterior skull base, optic apparatus, carotid arteries, and pterygopalatine-region neurovascular structures. For CT/MR interpretation and endoscopic-surgical planning, the most useful framework is the anterior versus posterior sinonasal compartment, divided by the basal lamella of the middle turbinate.

The nasal cavity extends from vestibule to choanae and is divided by the septum. Superior, middle, and inferior turbinates create the corresponding meatuses. The middle meatus receives the anterior sinonasal compartment; the superior meatus receives posterior ethmoid drainage; and the sphenoid sinus opens into the sphenoethmoidal recess above and posterior to the superior turbinate.

Preoperative CT Purpose

Preoperative CT is central to FESS planning. A useful report defines disease distribution and impaired drainage, identifies variants that narrow the frontal recess, ostiomeatal complex, or ethmoid infundibulum, and calls attention to vulnerable adjacent structures. Major FESS complications have been reported in the 0–2.25% range and include CSF leak, hemorrhage, orbital injury, optic-nerve injury, meningitis, and lacrimal-duct injury.

Nasal Septum and Mucosa

Nasal septum

• Septal deviation is common and may be accompanied by a bony spur.
• Deviation or spurring may restrict access to the middle meatus and contribute to mucosal contact symptoms.
• Report septal deviation/spur when it affects access or produces meaningful airway narrowing.

Nasal cycle

Asymmetric turbinate, septal, or ethmoid-region mucosal thickening may be physiologic rather than inflammatory. This periodic nasal-cycle change typically spares the frontal, maxillary, and sphenoid sinuses and should not be mistaken for sinonasal inflammatory disease on CT.

Lateral Nasal Wall and OMC

The ostiomeatal complex (OMC/OMU) is a functional bottleneck rather than a single discrete anatomic structure. It is the common drainage region for the maxillary sinus, frontal sinus, and anterior ethmoid cells. Focal mucosal disease here can create the recognizable ipsilateral anterior OMC pattern of maxillary, anterior ethmoid, and frontal sinus disease.

Structures to recognize

• Uncinate process: Thin, curved bony plate of the lateral nasal wall; forms the anterior/medial boundary of the ethmoid infundibulum and separates it from the middle meatus.
• Ethmoid bulla: Usually the largest and most constant anterior ethmoid cell; forms the posterior boundary of the hiatus semilunaris and contributes to the posterior infundibular boundary.
• Hiatus semilunaris: Curvilinear cleft between the free edge of the uncinate anteriorly/inferiorly and ethmoid bulla posteriorly/superiorly.
• Ethmoid infundibulum: Narrow three-dimensional passage lateral to the uncinate and medial/anterior to the bulla; receives the natural maxillary ostium.
• Middle meatus: Final common drainage channel for the anterior compartment.
• Frontal recess: Funnel-like, obliquely oriented drainage pathway immediately inferior to the frontal sinus floor/ostium; not synonymous with the frontal ostium and not necessarily the same channel as the ethmoid infundibulum.

Uncinate attachment

• Attachment to the lamina papyracea tends to route frontal drainage into the infundibulum.
• Attachment to the skull base or middle-turbinate region generally separates frontal drainage from the infundibulum, directing it more directly to the middle meatus.

This relationship explains why maxillary and frontal disease may coexist or be dissociated despite adjacent anatomy.

Coronal CT mental map

1. Find the agger nasi region and frontal recess.
2. Follow the uncinate process inferiorly and posteriorly.
3. Identify the maxillary ostium opening into the infundibulum.
4. Identify the ethmoid bulla posterior to the hiatus semilunaris.
5. Follow the middle turbinate superiorly to the skull-base attachment, then posteriorly to the basal lamella.
6. Use the basal lamella as the transition from anterior to posterior ethmoid anatomy.

Individual Sinuses

Maxillary sinus

• Base/medial wall: Lateral nasal wall.
• Roof: Orbital floor; inspect for infraorbital canal protrusion/dehiscence and orbital-floor defects.
• Floor: Alveolar process and maxillary dentition; assess for odontogenic disease, oroantral communication, dental material, implants, and periapical pathology.
• Anterior wall: Facial surface of maxilla.
• Posterolateral wall: Infratemporal surface of maxilla.
• Posterior wall: Adjacent to pterygopalatine fossa; transgression can mark aggressive pathology.
• Apex: Extends laterally toward the zygomatic process.

The natural ostium lies high on the medial wall, limiting gravity-dependent clearance. An accessory ostium may occur in the posterior fontanelle. When natural and accessory ostia are both patent, mucus recirculation along the medial maxillary wall is a useful CT clue in chronic disease. Potential recesses include alveolar, zygomatic, infraorbital, and palatine recesses.

Ethmoid labyrinth

The ethmoid complex is a variable honeycomb of air cells. The third basal lamella (the basal lamella of the middle turbinate) separates anterior from posterior ethmoid cells: anterior cells drain to the middle meatus and participate in OMC disease, whereas posterior cells drain to the superior meatus. “Middle ethmoid” terminology varies but generally behaves with the anterior compartment in drainage and surgical practice.

• Lateral boundary: Lamina papyracea and orbit.
• Superior boundary: Ethmoid roof/fovea ethmoidalis and lateral lamella of cribriform plate.
• Medial boundary: Upper nasal cavity and turbinates.
• Posterior boundary: Sphenoid sinus and sphenoethmoidal recess.
• Anterior boundary: Frontal process of maxilla, lacrimal bone, agger nasi region, and frontal recess.

The ethmoid bone includes the perpendicular plate, cribriform plate, and paired ethmoidal labyrinths. In the Killian basal-lamella framework, the uncinate, ethmoid bulla, middle turbinate, superior turbinate, and sometimes supreme turbinate form sequential partitions. Frontal and maxillary drainage lies between the first and second lamellae; the ethmoid bulla drains between the second and third; the third separates anterior from posterior ethmoids; and the fourth separates posterior and supreme meatuses.

Frontal sinus and drainage pathway

The frontal sinus varies widely in size, shape, septation, and laterality. The interfrontal septum is often eccentric and should not be assumed to mark the true midline. The posterior table separates the sinus from the anterior cranial fossa/dura; the floor has medial and lateral orbital components.

The frontal beak is a bony ridge formed by superior extension of the frontal process of the maxilla. The frontal ostium lies at its level; the beak separates the frontal sinus above from its drainage pathway below.

The frontal drainage pathway is bounded laterally by lamina papyracea, medially by lateral lamella of the olfactory fossa, anteriorly by frontal beak and agger nasi cell, and posteriorly by bulla lamella. It can be conceptualized as a superior frontal recess and an inferior frontal sinus outflow tract. The latter may be formed by the ethmoid infundibulum or by the middle meatus, based on uncinate attachment and adjacent anterior ethmoid cells.

Assess it as a three-dimensional channel. Enlarged agger nasi cells, frontal cells, a prominent bulla, unusual uncinate attachment, and other adjacent cells can narrow or redirect it. More than one frontal-sinus opening may be present. Lateral frontal sinus disease can be challenging to address endoscopically.

Frontal-region cells

• Agger nasi cell: Most anterior ethmoid cell; extends into the lacrimal bone, lies posteromedial to the lacrimal sac, anterior to the anterior skull-base attachment of the middle turbinate, and anteroinferior to the frontal recess. It forms the anterior frontal-recess margin and drains to the middle meatus.
• Frontal cells: Variable anterior ethmoid cells extending into or around the frontal recess; report descriptively when they alter drainage.
• Suprabullar cells: Posterior frontal-recess cells along the skull base that do not extend into the frontal sinus.
• Frontal bullar cells: Posterior cells along the skull base that extend into the frontal sinus.
• Supraorbital ethmoid cells: Extend posterolaterally from the frontal recess over the orbit; axial CT may show a bony septum separating them from the frontal sinus.
• Interfrontal sinus septal cells: Located along the frontal sinus septum.

Sphenoid sinus

The sphenoid sinus should be assessed as a map of adjacent critical structures, not simply as an air cavity. The anterior-wall ostium is usually near the midline and opens to the sphenoethmoidal recess.

• Superior: Sella turcica, pituitary gland, optic chiasm, and planum sphenoidale.
• Lateral: Cavernous sinuses, cavernous/paraclinoid ICA, and parasellar CN III/IV/V1/V2 depending on pneumatization.
• Superolateral: Optic canals and optic nerves.
• Inferolateral: Vidian canals and foramen rotundum/V2, especially with lateral-recess pneumatization.
• Posterior: Clivus and prepontine/brainstem-related structures beyond the clival cortex.
• Anterior: Sphenoethmoidal recess and posterior ethmoid/Onodi cells.

Sphenoid pneumatization

Greater pneumatization can expose critical structures through thin bone or apparent dehiscence. Sphenoid septa are frequently eccentric and may insert on an ICA or optic-canal prominence; do not assume that a septum is a safe midline landmark.

Middle Turbinate

The middle turbinate and basal lamella are parts of the same embryologic structure—the third basal lamella. Its sequential skull attachments are anterior vertical (to cribriform plate, sagittal), middle oblique (to basal lamella or lamina papyracea, coronal), and posterior horizontal (forming the roof of posterior middle meatus, axial). This configuration stabilizes the turbinate; excessive posterior resection can result in anterior instability.

• Concha bullosa: Pneumatization of the middle turbinate. It does not intrinsically cause sinusitis but may narrow the middle meatus or ethmoid infundibulum. It can itself contain inflammatory disease, polyp, or mucocele. Preserve the skull-base attachment during surgery.
• Paradoxical middle turbinate: Lateral rather than medial convexity; usually insignificant but may contribute to infundibular or middle-meatal narrowing when prominent.
• Turbinate sinus: Marked middle-turbinate curvature with an internal concavity; a morphological variant.
• Superior-turbinate pneumatization can occur but is less common.

Skull Base and Orbital Danger Zones

Olfactory fossa and lateral lamella

The cribriform plate forms the medial anterior skull base. The lateral lamella connects it to the fovea ethmoidalis and is the most vulnerable portion of the anterior skull base during ethmoid surgery. It can be extremely thin.

Side-to-side asymmetry may be more important than the absolute Keros category because it can create a unilateral low point or oblique ethmoid roof. Assess both sides independently.

Lamina papyracea

• Recognize congenital or acquired dehiscence.
• Assess for old trauma, orbital-fat or medial-rectus prolapse, and contour irregularity.
• Note the proximity of uncinate process, Haller cell, enlarged ethmoid cells, or disease to the orbit.
• With ethmoiditis, a subperiosteal abscess typically accumulates along the medial orbit.

Anterior ethmoidal artery

The anterior ethmoidal artery arises from the ophthalmic artery and traverses the anterior ethmoid near the roof. On coronal CT, determine whether it is closely applied to the skull base in a canal, partially suspended below it, or freely coursing in a mesentery across an ethmoid cell. A low-lying or dehiscent-appearing artery is vulnerable during ethmoid dissection and may retract into the orbit if injured.

Variants Worth Reporting

The reporting threshold should be lower for preoperative studies, disease in the corresponding drainage route, or a variant that changes risk to orbit, skull base, optic nerve, or ICA.

Middle-meatus variants

• Concha bullosa and paradoxical middle turbinate.
• Septal deviation and spur.
• Uncinate variants: lateralized, medially bent, pneumatized, or variant superior attachment; describe if associated with infundibular/frontal-recess narrowing.
• Haller cell: Infraorbital ethmoid cell along the inferomedial orbital floor, lateral to the infundibulum/maxillary ostium; may narrow the infundibulum and increase orbital surgical relevance.
• Accessory maxillary ostium, especially in chronic maxillary disease because of possible recirculation.

Frontal-recess variants

• Agger nasi-cell enlargement.
• Supraorbital ethmoid cell.
• Frontal, suprabullar, frontal bullar, or interfrontal septal cells that constrict or redirect the drainage tract.
• Marked frontal asymmetry, hypoplasia/aplasia, or extensive lateral/posterior pneumatization.
• Low or asymmetric frontal floor/posterior-table anatomy.

Posterior ethmoid and sphenoid variants

• Onodi cell: Posterior ethmoid cell extending superiorly, laterally, or posteriorly relative to the sphenoid; often closely related to the optic nerve and may mimic a sphenoid compartment.
• Optic-nerve protrusion or absent/imperceptible bony interface, particularly with anterior-clinoid pneumatization or Onodi cells.
• ICA protrusion or absent/imperceptible bony interface, especially with sellar or lateral-recess pneumatization; avoid overcalling partial-volume thinning as definite dehiscence.
• Anterior-clinoid pneumatization and opticocarotid recess.
• Pterygoid-process/lateral-recess pneumatization with increased relevance of vidian canal and V2/foramen rotundum.
• Eccentric sphenoid septum inserting on ICA or optic canal.

CT and MRI Strategy

CT

• Thin-section bone-algorithm CT with multiplanar reformations is the primary examination for drainage pathways, OMC/frontal-recess obstruction, pre-FESS mapping, chronic inflammatory osseous remodeling/osteitis, bony erosion, and orbital/skull-base/optic-canal/carotid-canal relationships.
• Coronal images are particularly useful for OMC, ethmoid roof, orbit, and frontal recess.
• Axial images are useful for front-to-back drainage pathways, posterior ethmoids, sphenoid anatomy, pterygopalatine fossa, and anterior skull base.
• Sagittal images are especially helpful for frontal outflow and sphenoid-sellar relationships.

MRI

• Orbital, intracranial, dural, cavernous-sinus, or vascular complications.
• Tumor extent, perineural spread, marrow invasion, and skull-base foraminal involvement.
• Differentiation of retained secretions from mass.
• Fungal disease, particularly with hyperattenuating CT contents and markedly low T2 signal.
• Mucocele characterization and relationship to orbit/anterior cranial fossa.
• Encephalocele or CSF-leak assessment, usually with CT correlation for the osseous defect.

When sinus opacification is present, consider whether the process is centered outside the sinus with secondary invasion, such as from the nasal cavity, orbit, anterior skull base, pterygopalatine fossa, maxillary alveolus, or sinonasal neurovascular corridors.

Practical Pre-FESS Checklist

A radiology-oriented adaptation of the CLOSE mnemonic:

• C — Cribriform plate/olfactory fossa: Keros depth, asymmetry, low fovea, focal defect, prior fracture or surgery.
• L — Lamina papyracea: Dehiscence, prior blowout injury, orbital fat/medial rectus prolapse, and relevant air-cell or disease proximity.
• O — Onodi cell: Presence, laterality, and optic-nerve relationship.
• S — Sphenoid sinus: Pneumatization, lateral recesses, optic-canal/ICA prominence or absent bony interface, and septal attachment to critical structures.
• E — Ethmoidal arteries: Especially the AEA course and apparent canal dehiscence.

Also assess frontal sinus aeration and frontal recess narrowing; agger nasi and supraorbital ethmoid cells; uncinate attachment and infundibular patency; Haller cells and maxillary ostial obstruction; concha bullosa/paradoxical middle turbinate; access-limiting septal deviation/spur; odontogenic maxillary source; and prior surgical changes such as antrostomy, ethmoidectomy, skull-base reconstruction, or retained partitions.

Report-Ready Framework

Illustrative report language: Adapt the findings and bracketed fields to the examination. The preoperative paragraph is a supplied example.

Routine sinus CT:

Sinonasal drainage pathways: Bilateral maxillary ostia/infundibula are patent. Frontal recesses are patent. No obstructing Haller cell, enlarged agger nasi cell, or substantial concha bullosa.

Ethmoid/skull base: Mild asymmetric depth of the olfactory fossae, deeper on the [side]; no focal skull-base osseous defect. Laminae papyraceae are intact.

Sphenoid region: [Sellar/presellar/conchal] sphenoid pneumatization. [No/left/right] Onodi cell. The optic canals and cavernous ICAs [do/do not] protrude into the sphenoid sinus; sphenoid septum [does/does not] attach near the [right/left] carotid canal.

Other: [Septal deviation/spur]. [Accessory maxillary ostium/Haller cells/prior surgery/odontogenic disease].

Dedicated preoperative study—example final paragraph:

Preoperative anatomic considerations: Deep left olfactory fossa with asymmetric low left ethmoid roof; left AEA suspended below the skull base. Left Onodi cell abuts the optic canal. Sellar sphenoid pneumatization with eccentric septum inserting on the right carotid canal. Small right lamina papyracea dehiscence with orbital-fat prolapse.

Key Interpretive Questions

The core practical goal is to answer: Where is the obstructed drainage pathway? What is the pattern of inflammatory spread? Where are the vulnerable orbit, skull-base, optic-nerve, carotid, and lacrimal landmarks?