ct of the paranasal sinuses

120 CME Radiology 2001; 2(3):120-125

CT of the paranasal sinuses: normal anatomy, variants and pathology

How-Ming Tan & Vincent FH Chong

How-Ming TanMBBS FRCRRegistrar

Vincent FH ChongMBBS FRCRSenior Consultant

Department of DiagnosticRadiologySingapore General HospitalSingapore

Correspondence:Dr HM Tan Department of DiagnosticRadiologySingapore General HospitalOutram RoadSingapore 169608

Rila Publications Ltd.

Papers

AbstractThe place of computed tomography in the pre-operative

assessment of patients prior to functional endoscopic sinussurgery is well established. A good knowledge of the anatomyof the paranasal sinuses, the clinical significance of anatomicalvariants, and the terminology used in functional endoscopicsinus surgery is basic to the correct interpretation of imagingstudies. This article will review the anatomy of the paranasalsinuses, familiarise the reader with the common terminologyused in functional endoscopic sinus surgery and describe thepatterns of inflammatory changes.

KeywordsComputed tomography, anatomy, variants, sinusitis,

paranasal sinuses, CT.

IntroductionDuring fetal development, the paranasal sinuses

originate as invaginations of the nasal mucosa intothe lateral nasal wall, frontal, ethmoid, maxilla andthe sphenoid bones. This unique developmentexplains the enormous amount of anatomicalvariation. Computed tomography (CT) is anexcellent means of providing anatomicalinformation of this region, assessing disease extent,assisting endoscopic evaluation and guidingtreatment. The role of magnetic resonance imagingis limited but may provide further information onfungal infection and differentiating thickenedmucosa from f luid retention.1

Frontal sinuses The frontal sinuses are funnel-shaped cavities

that show marked individual variation. There isusually a central septum dividing the frontal sinusinto two parts but several septa may also be seen.The frontal recess, the drainage pathway of thefrontal sinus, usually drains into the middle meatus(62%) or into the ethmoid infundibilum (38%).2

This pathway is bordered by the agger nasi cellanteriorly, lamina papyracea laterally and middleturbinate medially. On coronal CT, the frontalrecess is seen superior and medial to the agger nasi

cell (Figure 1). This drainage pathway measures onthe average 13mm (range 2-20mm).2

Agger nasi cellsAnterior and inferior to the frontal recess are the

agger nasi cells (Latin for nasal mound). The aggernasi cells are extramural cells and represent the mostanterior ethmoid cells. On coronal CT, they appearinferior to the frontal recess and lateral to the middleturbinate (Figure 1). Thus, the agger nasi cells areimportant surgical landmarks and opening thesecells usually provides an excellent view of the frontalrecess.

The ostio-meatal unitThe ostio-meatal unit (OMU) comprises the

maxillary sinus ostium, the ethmoid infundibilum,anterior ethmoid cells and the frontal recess. Theethmoid infundibilum is bounded laterally by theinferomedial wall of the orbit, superiorly by thehiatus semilunaris and ethmoid bulla, and mediallyby the uncinate process (Figure 2). The maxillarysinus ostium and ethmoid infundibilum constitutethe common drainage for the anterior paranasalsinuses. One of the aims of FESS is to re-establishthe normal ventilation and the sinus drainage inthe OMU.

Figure 1. Coronal CT shows localised mucosal thickening inthe left frontal recess (arrow) as well as in the right frontalrecess. Note the relationship of the left frontal recess with thelamina papyracea (white arrow), the opacified agger nasi (A)and the middle turbinate (arrowhead).

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It is important to realise that the ethmoid infundibulumis a three-dimensional structure and not two-dimensionalas depicted on CT. As the maxillary sinus ostium opensinto the f loor of the ethmoid infundibulum, it is notpossible to see the ostium endoscopically withoutremoving the uncinate process. If an ostium is seenendoscopically, it is most likely to represent an accessoryostium or fontanelle.

The hiatus semilunaris gains its name from the archedappearance in the sagittal plane. It runs obliquely in aposteroinferior direction between the uncinate process andthe ethmoid bulla. It is best identified on parasagittalsections. On CT, it is bounded superiorly by the ethmoidbulla, laterally by the medial bony orbit, inferiorly by theuncinate process and medially the middle meatus. The hiatussemilunaris, the final segment of the drainage pathway fromthe maxillary sinus and ethmoid infundibulum,communicates medially with the middle meatus.

Uncinate process The relations of the uncinate process is again different

from the three-dimensional view through an endoscopeand the two-dimensional view portrayed on CT.Anteriorly, it is attached to the nasolacrimal apparatus;inferiorly to the inferior turbinate; posteriorly it has a freemargin; and superiorly, its attachment is variable.

On CT, the uncinate process can be seen attachedinferiorly to the inferior turbinate with the free edgerepresenting the posterior free margin. Anteriorly, theuncinate process may be attached to the lamina papyracea,the skull base or the middle turbinate. This variable superiorattachment results in different clinical implications.

If the uncinate process inserts into the laminapapyracea, the ethmoid infundibulum would be effectivelyclosed superiorly by a blind-ending pouch known as therecessus terminals. In this instance, the frontal recess and

the ethmoid infundibulum are separated and this explainswhy ethmoid infundibular inf lammation does not result inconcomitant frontal sinusitis. However, if the uncinateprocess is attached superiorly to the skull base or themiddle turbinate, the frontal sinus opens into the ethmoidinfundibulum and infection in the infundibulum mayaffect the frontal sinus, resulting in the involvement of thefrontal, ethmoid and maxillary sinuses (Figure 3).

The ethmoid bullaThe ethmoid bulla is a prominent anterior ethmoid

cell, constituting a reliable anatomical landmark (Figure 2).The degree of pneumatisation varies considerably(Figure 4) ranging from failure of pneumatisation (torusethmoidalis) to a giant ethmoid bulla insinuating betweenthe middle turbinate and uncinate process, displacing theuncinate process medially (Figure 5).3 The ethmoid bulla isbordered inferomedially by the infundibulum and hiatussemilunaris; laterally by the lamina papyracea andsuperoposteriorly by the sinus lateralis and basal lamina.

The Middle Turbinate The middle turbinate has a complex bony attachment.

Anteriorly, it is attached superiorly to the cribriform plate

Figure 2. Coronal CT shows the ostium of the right maxillary sinus(O), ethmoid bulla (B), uncinate process (white arrow), basal lamella(arrowhead) and sinus lateralis (asterisks). Note the left ethmoidinfundibulum (black arrow). The gap between the tip of the uncinateprocess and the ethmoid bulla constitutes the hiatus semilunaris(curved arrow).

Figure 3. Coronal CT shows inf lammation involving the left ostio-meatal unit which consists of the frontal recess (solid star), anteriorethmoid cells (black arrow), ethmoid infundibulum (white arrow) andmaxillary sinus (asterisk). Note the infundibular pattern on the rightinvolving the right maxillary sinus ostium (curve arrow) and theantrum (hollow star).

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(Figure 6). Posteriorly, it swings laterally into the coronalplane and attaches itself to the lamina papyracea (Figure 2).This coronal portion of the middle turbinate is called thebasal lamella or ground lamella. The lamella basalis dividesthe ethmoid cells into the anterior and posterior ethmoidcells. The surgical relevance is that the anterior ethmoidcells drain into the middle meatus while the posteriorethmoid cells drain into the superior meatus. The classicalanatomy of dividing the ethmoid sinus into anterior,middle, and posterior group of cells is no longer surgicallyrelevant.4

The middle turbinate continues posteriorly in an axialplane, forming the roof for the posterior portion of themiddle meatus. This three-dimensional orientation givesthe middle turbinate exceptional stability. Resection of theposterior portion may thus lead to anterior instability.5

Sinus lateralis The gap between the ethmoid bulla and the basal

lamina is known as the sinus lateralis and it opens into themiddle meatus (Figure 2). The relationships of the sinuslateralis are as follows: the ethmoid bulla anteriorly, theskull base superiorly, the basal lamina posteriorly, and thelamina papyracea laterally. Disease affecting the sinuslateralis is usually obvious radiologically but is oftendifficult to identify endoscopically.

Sphenoid sinus The sphenoid sinus is housed in the body of the

sphenoid bone and is related to the sella turcica superiorly.Its ostium is located medially in the anterosuperior portionof the anterior sinus wall and communicates with thesphenoethmoidal recess and the posterior portion of thesuperior meatus. The sphenoethmoidal recess is located

lateral to the nasal septum and although best demonstratedin the sagittal and axial planes, may also be seen on coronalimages. Important surgical relations of the sphenoid sinusinclude the carotid artery in its lateral walls (Figure 7), theoptic nerve superolaterally, and the Vidian canal in its f loor.The carotid artery may bulge into the sinus in 65% to 72%of patients and in 4% to 8% of cases, the thin sinus wallseparating the two may be absent (Figure 7).6 The

Figure 4. Coronal CT shows a small right ethmoid bulla (arrow) andbilateral concha bullosa (asterisks).

Figure 5. Coronal CT shows bilateral well-pneumatised ethmoid bul-lae (stars). Note the associated f lattening of the uncinate processes (whitearrow) medially which may potentially narrow the middle meatus.

Figure 6. Coronal CT shows the delicate attachment of the left mid-dle turbinate (arrow) to the cribriform plate. Note the inf lammatorychanges in the right frontal recess and anterior middle meatus (star).

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intersphenoid septum is often def lected to one side, andmay be attached to the bony wall covering the carotidartery. Hence, the artery may be injured when the septumis avulsed during surgery (Figure 8). Due to its relationswith the maxillary nerve, sphenoid sinusitis can producetrigeminal neuralgia (Figure 9).7

The posterior ethmoid has a variable relationship withthe sphenoid sinus and is intimately related to the opticnerve. The surgeon cannot assume that the sphenoid sinusis directly posterior to the posterior ethmoid sinus(Figure 10). In some cases, the posterior ethmoid cell mayextend laterally or superiorly beyond the anterior wall ofthe sphenoid sinus. This relationship, if not appreciated,may lead to the potential injury to the optic nerve by an

unsuspecting endoscopist. It was reported that the anterioropening of the optic canal may be located adjacent to themost posterior ethmoid cell (50%), at the junction of theposterior ethmoid and anterior sphenoid (25%) or adjacentto the sphenoid sinus (25%).8 However, a recent studybased on coronal CT showed that the optic nerve is mostlyrelated to the sphenoid sinus rather than the posteriorethmoid sinus.9 Complete bony dehiscence of the opticcanal exposing the nerve to injury may be present in4% -24 % of patients.9,10

Anatomical variants The nasal anatomy shows much individual variation.

These variations may predispose the patients to

Figure 7. Axial CT shows a dehiscent right carotid artery wall andassociated bulging of the artery into the sphenoid sinus (arrow). Notethe inf lammatory changes involving the left sphenoid sinus (star).

Figure 8. Axial CT shows the common wall between the right ante-rior sphenoid sinus wall and the posterior wall of the posterior eth-moid cell (arrows). The ostium (curved arrow) of right sphenoid sinus(star) opens into the sphenoethmoidal recess. Note the attachment ofthe sphenoid septum to the thin wall of the right carotid canal (blackarrow).

Figure 9. Coronal CT shows the relationship of the right maxillarynerve (straight arrow) with the sphenoid sinus. Note the inf lammatorychanges in the left sphenoid sinus (curved arrow) adjacent to the leftmaxillary nerve which may result in trigeminal neuralgia.

Figure 10. Axial CT shows the relationship between the right opticnerve (asterisks), posterior ethmoid sinus (small star) and sphenoidsinus (large star).

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inf lammatory disease because they may obstruct theinfundibulum or any part of the OMU resulting in theinterference of airf low or mucociliary clearance.

Middle turbinate variants A concha bullosa is a pneumatised middle turbinate and

has a reported prevalence of 34%.11 The presence of aconcha bullosa does not necessarily imply an abnormality.Indeed, concha bullosa is often noted in asymptomaticindividuals. However, a concha bullosa may be largeenough to cause obstruction in the middle meatus or theinfundibulum (Figure 4). The middle turbinate usuallycurves medially toward the nasal septum. However, in 26%of patients, the convexity is directed laterally resulting in aparadoxical middle turbinate (Figure 11).12

Uncinate process variants The free edge of the uncinate process may be deviated

medially (Figure 5), laterally, pneumatised or bent. Lateraldeviation may obstruct the infundibulum while medialdeviation may narrow the middle meatus. Pneumatisationmaybe seen in 4% of patients but this uncinate processvariant rarely compromises the infundibulum.13 A bentuncinate process may simulate a double middle turbinateon endoscopy. The term atelectatic uncinate processrefers to the situation where the edge of the uncinateprocess approximates the orbital f loor or the inferior aspectof the lamina papyracea. This phenomenon is usuallyassociated with a hypoplastic ethmoid bulla or maxillarysinus. Uncinectomy may therefore result in injury to theorbital contents.

Haller cells Haller cells are ethmoid cells that extend along the f loor

of the orbit. They vary in size and when large can narrow theostium of the maxillary sinus or the ethmoid infundibulum.Rarely, isolated inf lammatory disease may be noted within

the Haller cells.3 Inf lammatory disease involving the Hallercells is usually diagnosed on CT. Endoscopic evaluation isoften unremarkable in these patients.

Onodi cellsAn Onodi cell is a posterior ethmoid cell that extends

lateral and superior to the sphenoid sinus and abuts the opticnerve. Kainz and Stammberger defined an Onodi cell as aposterior ethmoid cell with an endoscopically visible bulgeof the optic canal.14 The vulnerability of the optic nerve withor without the presence of an Onodi cell is furthercompounded by the thin lamina papyracea in the posteriorethmoid area (Figure 10).

Ethmoid roofAsymmetry in the height of the ethmoid exposes the

lower side to inadvertent intracranial penetration duringendoscopy. The ethmoid roof is of critical importance fortwo reasons: Firstly, the bone is thin rendering this areavulnerable to cerebrospinal f luid leaks when breached(Figure 6). Secondly, the anterior ethmoidal artery isvulnerable to injury which may cause catastrophic bleedinginto the orbit. The anterior ethmoidal artery is a branch ofthe ophthalmic artery. From the orbit, it passes through acanal into the anterior ethmoid sinus just posterior to thefrontal recess. It then crosses the sinus and enters theanterior cranial fossa before exiting and re-entering thenasal cavity via the cribriform plate. This is the site wherethe artery is most liable to injury.

The roof of the ethmoid is formed by the foveaethmoidalis of the frontal bone laterally and the cribriformplate of the ethmoid bone medially. Due to the delicateattachment of the middle turbinate to the cribriform plateanteriorly, surgery in this area should be performed withcare as detachment of the middle turbinate may damage thedura, resulting in cerebrospinal f luid leak (Figure 6).15

Paranasal sinusitis Obstruction and impaired mucociliary drainage of the

paranasal sinuses results in sinusitis. Inf lammatory changesin the paranasal sinuses can be radiologically grouped intoseveral patterns of involvement. Isolated involvement ofthe maxillary sinus is often referred to as infundibularpattern (Figure 3). Involvement of the maxillary sinuswith inf lammatory changes in the ipsilateral frontal andanterior ethmoid sinuses is classif ied as the OMUpattern (Figure 3). Obstruction at the sphenoethmoidalrecess results in sphenoid and posterior ethmoid sinusitisgiving rise to the sphenoethmoidal pattern (Figure 12).However, in up to a third of patients with paranasalsinusitis, the pattern of inf lammation does not fit neatlyinto one of the above three groups. Patients who conformto one of the above-described patterns of obstruction farebetter following endoscopic surgery compared to the groupof patients with a random pattern of involvement.

Figure 11. Coronal CT shows paradoxical left middle turbinate(arrow). Note the ostium of the right sphenoid sinus (curve arrow)which is usually better demonstrated on axial images.

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Paranasal sinus mucocoeleParanasal sinus mucocoeles are mostly found in the

frontal sinus followed by the ethmoid sinus (Figure 13).The maxillary and sphenoid sinuses (Figure 14) are rarelyaffected. On CT, mucocoeles typically produce smoothexpansion of the involved sinus (Figure 13). Largemucocoeles may breach bone and extend into nasal cavity,orbit or intracranial cavity. Mucocoeles may becomeinfected and these pyocoeles are now frequentlydecompressed using endoscopic techniques. A delay in thetreatment of pyocoeles often leads to orbital abscess,

meningitis, subdural empyema or cavernous sinusthrombosis.

ConclusionA good knowledge of the complex CT anatomy of the

paranasal sinuses is crucial. This knowledge will provide anaccurate assessment of the normal variants and pathologicalchanges required for successful FESS. The radiologist bybeing familiar with the normal anatomy, variants andterminology used by the endoscopist can play an importantrole in the management of patients with paranasal sinusitis.

Figure 12. Axial CT shows sphenoethmoidal recess mucosal thicken-ing (curve arrow) resulting in left posterior ethmoid (asterisk) andsphenoid sinusitis (star).

Figure 14. a). Coronal CT shows expansion of the right sphenoid sinus (star) with multiple areas of erosion. Note the thickened sinus septum(arrow) indicating a long-standing lesion. b). Coronal T2-weighted MRI shows a high signal intensity mucocoele which was found to be infect-ed at surgery. Note the slight superolateral displacement of the internal carotid artery (arrow).

Figure 13. Axial contrast-enhanced CT shows a left posterior eth-moid mucocoele (solid star). The lesion has extended into the orbitalapex with displacement of the medial rectus muscle. Note the intimaterelationship with cavernous sinus (hollow stars) posteriorly.

14a 14b

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ct of the paranasal sinuses

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