• CT of the paranasal sinuses : normal anatomy, variants and ...

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    • Abstract: mucosa into the lateral nasal wall, frontal, ethmoid, maxilla and the sphenoid bones. ... Obstruction at the sphenoethmoidal recess results in sphenoid and posterior ethmoid sinusitis ...

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Journal of Optoelectronics and Biomedical Materials Vol.2 Issue 4, October-December 2010, p. 281 – 289
Department of Radio Diagnosis, Subharti Medical College Meerut
Department of Otorhinolaryngology, Sarswati Institute of Medical Sciences
The place of computed tomography in the pre-operative assessment of patients prior to
functional endoscopic sinus surgery is well established. A good knowledge of the anatomy
of the paranasal sinuses, the clinical significance of anatomical variants, and the
terminology used in functional endoscopic sinus surgery is basic to the correct
interpretation of imaging studies. This article will review the anatomy of paranasal
sinuses, Jami Karise the reader with the common terminology used in functional
endoscopic sinus surgery and describe the patterns of inflammatory changes.
(Received December 9, 2010; accepted December 22, 2010)
Keywords: Computed tomography, anatomy, variants, sinusitis, paranasal sinuses, CT.
1. Introduction
During fetal development, the paranasal sinuses originate as invagination of the nasal
mucosa into the lateral nasal wall, frontal, ethmoid, maxilla and the sphenoid bones. This unique
development explains the enormous amount of anatomical variation. Computed tomography (CT)
is an excellent means of providing anatomical information of this region, assessing disease extent,
assisting endoscopic evaluation and guiding treatment. The role of magnetic resonance imaging is
limited but may provide further information on fungal infection and differentiating thickened
mucosa from fluid retention.1
2. Frontal sinuses
The frontal sinuses are funnel-shaped cavities that show marked individual variation.
There is usually a central septum dividing the frontal sinus into two parts but several septa may
also be seen. The frontal recess, the drainage pathway of the frontal sinus, usually drains into the
middle meatus (62%) or into the ethmoid infundibilum (38%).2 This pathway is bordered by the
agger nasi cell anteriorly, lamina papyracea laterally and middle turbinate medially. On coronal
CT, the frontal recess is seen superior and medial to the agger nasi cell (Figure 1). This drainage
pathway measures on the average 13mm (range 2-20mm).
Corresponding author: [email protected]
Fig. 1. Coronal CT shows localised mucosal thickening in the left frontal recess (arrow)
as well as in the right frontal recess. Note the relationship of the left frontal recess with
the lamina papyracea (white arrow), the opacified agger nasi (A) and the middle turbinate
Agger nasi cells
Anterior and inferior to the frontal recess are the agger nasi cells (Latin for "nasal
mound"). The agger nasi cells are extramural cells and represent the most anterior ethmoid cells.
On coronal CT, they appear" inferior to the frontal recess and lateral to the middle turbinate
(Figure 1). Thus, the agger nasi cells are important surgical landmarks and opening these cells
usually provides an excellent view of the frontal recess.
The ostio-meatal unit
The ostio-meatal unit (OMU) comprises the maxillary sinus ostium, the ethmoid
infundibilum, anterior ethmoid cells and the frontal recess. The ethmoid infundibilum is bounded
laterally by the inferomedial wall of the orbit, superiorly by the hiatus semilunaris and ethmoid
bulla, and medially by the uncinate process (Figure 2). The maxillary sinus ostium and ethmoid
infundibilum constitute the common drainage for the anterior paranasal sinuses. One of the aims of
FESS is to re-establish the normal ventilation and the sinus drainage in the OMU.
Fig. 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 ethmoid infundibulum (black arrow). The gap between the tip of the uncinate
process and the ethmoid bulla constitutes the hiatus semilunaris (curved arrow).
It is important to realise that the ethmoid infundibulum is a three-dimensional structure
and not two-dimensional as depicted on CT. As the maxillary sinus ostium opens into the floor of
the ethmoid infundibulum, it is not possible to see the ostium endoscopically without removing the
uncinate process. If an ostium is seen endoscopically, it is most likely to represent an accessory
ostium or fontanelle.
The hiatus semilunaris gains its name from the arched appearance in the sagittal plane. It
runs obliquely in a posteroinferior direction between the uncinate process and the ethmoid bulla. It
is best identified on parasagittal sections. On CT, it is bounded superiorly by the ethmoid bulla,
laterally by the medial bony orbit, inferiorly by the uncinate process and medially the middle
meatus. The hiatus semilunaris, the final segment of the drainage pathway from the 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 endoscope and the two-dimensional view portrayed on CT. Anteriorly, it is attached to
the nasolacrimal apparatus; inferiorly to the inferior turbinate; posteriorly it has a free margin; and
superiorly, its attachment is variable.
On CT, the uncinate process can be seen attached inferiorly to the inferior turbinate with
the free edge representing the posterior free margin. Anteriorly, the uncinate process may be
attached to the lamina papyracea, the skull base or the middle turbinate. This variable superior
attachment results in different clinical implications.
Fig. 3. Coronal CT shows inflammation involving the left ostiomeatal unit which consists
of the frontal recess (solid star), anterior ethmoid cells (black arrow), ethmoid
infundibulum (white arrow) and maxillary sinus (asterisk). Note the infundibular pattern
on the right involving the right maxillary sinus ostium (curve arrow) and the antrum
(hollow star).
If the uncinate process inserts into the lamina papyracea, the ethmoid infundibulum would
be effectively closed superiorly by a blind-ending pouch known as the recessus terminals. In this
instance, the frontal recess and the ethmoid infundibulum are separated and this explains why
ethmoid infundibular inflammation does not result in concomitant frontal sinusitis. However, if the
uncinate process is attached superiorly to the skull base or the middle turbinate, the frontal sinus
opens into the ethmoid infundibulum and infection in the infundibulum may affect the frontal
sinus, resulting in the involvement of the frontal, ethmoid and maxillary sinuses.
The ethmoid bulla
The 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 (torus ethmoidalis) to a giant ethmoid bulla insinuating between the
middle turbinate and uncinate process, displacing the uncinate process medially (Figure 5).' The
ethmoid bulla is bordered inferomedially by the infundibulum and hiatus semilunaris; laterally by
the lamina papyracea and superoposteriorly 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 6). Posteriorly, it swings laterally into the coronal plane and attaches
itself to the lamina papyracea (Figure 2). This coronal portion of the middle turbinate is called the
basal lamella or ground lamella. The lamella basalis divides the ethmoid cells into the anterior and
posterior ethmoid cells. The surgical relevance is that the anterior ethmoid cells drain into the
middle meatus while the posterior ethmoid cells drain into the superior meatus. The classical
anatomy of dividing the ethmoid sinus into anterior, middle, and posterior group of cells is no
longer surgically relevant.
The middle turbinate continues posteriorly in an axial plane, forming the roof for the
posterior portion of the middle meatus. This three-dimensional orientation gives the middle
turbinate exceptional stability. Resection of the posterior portion may thus lead to anterior
Fig. 4. Coronal CT shows a small right ethmoid bulla (arrow) and bilateral concha bullosa (asterisks).
Sinus lateralis
The gap between the ethmoid bulla and the basal lamina is known as the sinus lateralis and
it opens into the middle meatus (Figure 2). The relationships of the sinus lateralis are as follows:
the ethmoid bulla anteriorly, the skull base superiorly, the basal lamina posteriorly, and the lamina
papyracea laterally. Disease affecting the sinus lateralis is usually obvious radiologically but is
often difficult 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 portion of the anterior sinus
wall and communicates with the sphenoethmoidal recess and the posterior portion of the superior
meatus. The sphenoethmoidal recess is located lateral to the nasal septum and although best
demonstrated in the sagittal and axial planes, may also be seen on coronal images. Important
surgical relations of the sphenoid sinus include the carotid artery in its lateral walls (Figure 7), the
optic nerve superolaterally, and the Vidian canal in its floor. The carotid artery may bulge into the
sinus in 65% to 72% of patients and in 4% to 8% of cases, the thin sinus wall separating the two
may be absent (Figure 7).° The intersphenoid septum is often deflected to one side, and may be
attached to the bony wall covering the carotid artery. Hence, the artery may be injured when the
septum is avulsed during surgery (Figure 8). Due to its relations with the maxillary nerve,
sphenoid sinusitis can produce trigeminal neuralgia (Figure 9).
Fig. 5. Coronal CT shows bilateral well-pneumatised ethmoid bullae (stars). Note the
associated flattening of the uncinate processes (white arrow) medially which may
potentially narrow the middle meatus.
Fig. 6. Coronal CT shows the delicate attachment of the left middle turbinate (arrow) to
the cribriform plate. Note the inflammatory changes in the right frontal recess and
anterior middle meatus (star).
The posterior ethmoid has a variable relationship with the sphenoid sinus and is intimately
related to the optic nerve. The surgeon cannot assume that the sphenoid sinus is directly posterior
to the posterior ethmoid sinus (Figure 10). In some cases, the posterior ethmoid cell may extend
laterally or superiorly beyond the anterior wall of the 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 anterior opening of the optic canal may be located adjacent to the most
posterior ethmoid cell (50%), at the junction of the posterior ethmoid and anterior sphenoid (25%)
or adjacent to the sphenoid sinus (25%).8 However, a recent study based on coronal CT showed
that the optic nerve is mostly related to the sphenoid sinus rather than the posterior ethmoid sinus.'
Complete bony dehiscence of the optic canal exposing the nerve to injury may be present in 4% -
24 % of patients."
Fig. 7. Axial CT shows a dehiscent right Fig. 8. Axial CT shows the common wall
carotid artery wall and associated bulging of between the right anterior sphenoid sinus wall
the artery into the sphenoid sinus (arrow). and the posterior wall of the posterior ethmoid
Note the inflammatory changes involving the cell (arrows). The ostium (curved arrow) of
left sphenoid sinus (star). right sphenoid sinus (star) opens into the
sphenoethmoidal recess. Note the attachment of
the sphenoid septum to the thin wall of the right
carotid canal (black arrow).
Fig. 9. Coronal CT shows the relationship of Fig. 10. Axial CT shows the relationship
the right maxillary nerve (straight arrow) between the right optic nerve (asterisks),
with the sphenoid sinus. Note the posterior ethmoid sinus (small star) and
inflammatory changes in the left sphenoid sphenoid sinus (large star).
sinus (curved arrow) adjacent to the left
maxillary nerve which may result in
trigeminal neuralgia.
Anatomical variants
The nasal anatomy shows much individual variation. These variations may predispose the
patients to inflammatory disease because they may obstruct the infundibulum or any part of the
OMU resulting in the interference of airflow or mucociliary clearance.
Middle turbinate variants
A concha bullosa is a pneumatised middle turbinate and has a reported prevalence of
34%." The presence of a concha bullosa does not necessarily imply an abnormality. Indeed,
concha bullosa is often noted in asymptomatic individuals. However, a concha bullosa may be
large enough to cause obstruction in the middle meatus or the infundibulum (Figure 4). The middle
turbinate usually curves medially toward the nasal septum. However, in 26% of patients, the
convexity is directed laterally resulting in a paradoxical middle turbinate (Figure II).
Fig. 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
Uncinate process variants
The free edge of the uncinate process may be deviated medially (Figure 5), laterally,
pneumatised or bent. Lateral deviation may obstruct the infundibulum while medial deviation may
narrow the middle meatus. Pneumatisation may be seen in 4% of patients but this uncinate process
variant rarely compromises the infundibulum." A bent uncinate process may simulate a double
middle turbinate on endoscopy. The term "atelectatic uncinate process" refers to the situation
where the edge of the uncinate process approximates the orbital floor or the inferior aspect of the
lamina papyracea. This phenomenon is usually associated with a hypoplastic ethmoid bulla or
maxillary sinus. Uncinectomy may therefore result in injury to the orbital contents.
Haller cells
Haller cells are ethmoid cells that extend along the floor of the orbit. They vary in size and
when large can narrow the ostium of the maxillary sinus or the ethmoid infundibulum. Rarely,
isolated inflammatory disease may be noted within the Haller cells.' Inflammatory disease
involving the Haller cells is usually diagnosed on CT. Endoscopic evaluation is often
unremarkable in these patients.
Onodi cells
An Onodi cell is a posterior ethmoid cell that extends lateral and superior to the sphenoid
sinus and abuts the optic nerve. Kainz and Stammberger defined an Onodi cell as a posterior
ethmoid cell with an endoscopically visible bulge of the optic canal. The vulnerability of the optic
nerve with or without the presence of an Onodi cell is further compounded by the thin lamina
papyracea in the posterior ethmoid area (Figure 10).
Ethmoid roof
Asymmetry in the height of the ethmoid exposes the lower side to inadvertent intracranial
penetration during endoscopy. The ethmoid roof is of critical importance for two reasons: Firstly,
the bone is thin rendering this area vulnerable to cerebrospinal fluid leaks when breached (Figure
6). Secondly, the anterior ethmoidal artery is vulnerable to injury which may cause catastrophic
bleeding into the orbit. The anterior ethmoidal artery is a branch of the ophthalmic artery. From
the orbit, it passes through a canal into the anterior ethmoid sinus just posterior to the frontal
recess. It then crosses the sinus and enters the anterior cranial fossa before exiting and re-entering
the nasal cavity via the cribriform plate. This is the site where the artery is most liable to injury.
The roof of the ethmoid is formed by the fovea ethmoidalis of the frontal bone laterally
and the cribriform plate of the ethmoid bone medially. Due to the delicate attachment of the
middle turbinate to the cribriform plate anteriorly, surgery in this area should be performed with
care as detachment of the middle turbinate may damage the dura, resulting in cerebrospinal fluid
leak (Figure 6).'5
Paranasal sinusitis
Obstruction and impaired mucociliary drainage of the paranasal sinuses results in sinusitis.
Inflammatory changes in the paranasal sinuses can be radiologically grouped into several patterns
of involvement. Isolated involvement of the maxillary sinus is often referred to as "infundibular
pattern" (Figure 3). Involvement of the maxillary sinus with inflammatory changes in the
ipsilateral frontal and anterior ethmoid sinuses is classified as the "OMU pattern" (Figure 3).
Obstruction at the sphenoethmoidal recess results in sphenoid and posterior ethmoid sinusitis
giving rise to the "sphenoethmoidal pattern" (Figure 12). However, in up to a third of patients with
paranasal sinusitis, the pattern of inflammation does not fit neatly into one of the above three
groups. Patients who conform to one of the above-described patterns of obstruction fare better
following endoscopic surgery compared to the group of patients with a random pattern of
Paranasal sinus mucocoeles
Paranasal sinus mucocoeles are mostly found in the frontal sinus followed by the ethmoid
sinus (Figure 13). The maxillary and sphenoid sinuses (Figure 14) are rarely affected. On CT,
mucocoeles typically produce smooth expansion of the involved sinus (Figure 13). Large
mucocoeles may breach bone and extend into nasal cavity, orbit or intracranial cavity. Mucocoeles
may become infected and these pyocoeles are now frequently decompressed using endoscopic
techniques. A delay in the treatment of pyocoeles often leads to orbital abscess, meningitis,
subdural empyema or cavernous sinus thrombosis.
Fig. 12. Axial CT shows sphenoe-thmoidal Fig. 13. Axial contrast-enhanced CT shows a left
recess mucosal thickening (curve arrow) posterior ethmoid mucocoele (solid star). The
resulting in left posterior ethmoid (asterisk) lesion has extended into the apex with
and sphenoid sinusitis (star). displacement of the medial rectus muscle. Note
the relationship with cavernous sinus (hollow
stars) posteriorly.
Fig. 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 infected at surgery. Note the slight superolateral
displacement of the internal carotid artery (arrow).
3. Conclusion
A good knowledge of the complex CT anatomy of the paranasal sinuses is crucial. This
knowledge will provide an accurate assessment of the normal variants and pathological changes
required for successful FESS.
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