Embryology and anatomy of nasal sinuses

1. Dr Ahlam Alzuway

2. (a)Ethmoid sinus:
The ethmoid sinus is thought to start out as multiple invaginations from the
lateral wall of the nasal capsule around the fifth month of development
- Most developed at birth ( in number of cells but not in size)
- Anterior ethmoid originates from middle meatus
-Posterior ethmoid originates from superior meatus
Other structures originating from ethmoid bone include middle, superior and
supreme turbinate, cribriform plate, and perpendicular plate of ethmoid

4. (b) Maxillary sinus:
-The maxillary sinus develops as an outpouching between the middle and inferior
turbinates.
-It is the first sinus to develop, beginning its invagination process during the third
gestational month.
- In early childhood, floor of sinus is situated above nasal floor due to presence of
unerupted dentition
(c) Sphenoid sinus:
-The sphenoid sinus originates from an outpouching from the posterior aspect of
the nasal capsule during the third month of gestation.
- Pneumatization begins postnatally (~1 year of age)
-Adult size reached by age 12 beyond which size remains constant though shape
may change

5. (d) Frontal sinus:
- The development of the frontal sinus starts with the anterior pneumatization of
the
frontal recess into the frontal bone around week 16 of gestation. Several folds and
furrows develop within the frontal recess that eventually give rise to the agger
nasi
cell (first frontal furrow), frontal sinus proper (second frontal furrow), and anterior
ethmoid cells (third and fourth frontal furrows).
-Its Last sinus to begin and complete development
-Pneumatization into the frontal bone does not start until 6 ms to 2 yrs after birth,
and radiologic evidence of the sinus is not usually seen until the age of 6 or 7.
-The two frontal sinuses are typically asymmetric, with 10 to 12% of the adult
population displaying
only one pneumatized frontal sinus. Up to 4% of the population lacks both frontal
sinuses.

6. -Only ethmoid and maxillary sinuses are present at birth
-Order of paranasal sinuses according to size (from largest to smallest)
-Maxillary sinus
-Frontal sinus
-Sphenoid sinus
-Ethmoid sinus
There are a variety of congenital malformations that can occur as a result of
abnormal nasal and paranasal sinus development. Notable abnormalities include
congenital midline masses such as encephaloceles, nasal gliomas, and dermoid
cysts. Also observable at the midline of the posterior nasal airway in the
nasopharynx are Thornwaldt cysts.

7. >>Paranasal sinus anatomy:
1- Ethmoid sinus:
(A) Anterior ethmoid sinus (anterior to MT ground lamella)Anterior ethmoid cells are
generally smaller but greater in number
a. Agger nasi:
-Most anterior ethmoid cell
-Projects anterior to axilla of the MT creating a bulge in the lateral nasal wall
-Posterior limit forms the anterior border of the frontal recess
-Anteroposterior distance of the frontal recess largely determined by degree of
pneumatization of the agger nasi cell
-A large, well-pneumatized agger nasi cell confers a small frontal beak which gives
rise to a large frontal anteroposterior distance

10. (b). Ethmoid bulla:
-Largest anterior ethmoid cell
-Attached laterally to LP
(c). Sinus lateralis:
-Made up of the suprabullar and retrobullar recesses
-Site of suprabullar recess = above EB in the absence of a suprabullar cell
-Site of retrobullar recess = posterior to EB, anterior to MT basal lamella
-Boundaries include EB anteriorly, basal lamella of MT posteriorly, MT medially,
LP
laterally and fovea ethmoidalis superiorly
(d). Suprabullar cell:
-Ethmoid cell located above EB without pneumatizing into frontal sinus
-Fovea ethmoidalis forms roof of this cell

12. (e). Frontal bulla cell:
-Suprabullar cell which pneumatizes into frontal sinus along posterior wall of
frontal sinus
(f). Supraorbital ethmoid cell:
-Ethmoid cell located posterolateral to the frontal sinus ostium with
pneumatization lateral to the LP and superolateral to the orbital roof
(orbital
plate of the frontal bone)
-Anterior ethmoid artery typically located within the posterior wall of the cell
along or immediately beneath the skull base

14. (g). Haller cell (infraorbital ethmoid cell):
-Most common anatomic variation within the maxillary sinus
-Most commonly originates from the anterior ethmoid sinus
-Pneumatizes along the inferomedial orbit and may obstruct the natural drainage
pathway of the maxillary sinus
(B) Posterior ethmoid sinus (posterior to basal lamella of MT)Posterior ethmoid cells are
generally larger but fewer in number
. Onodi cell (sphenoethmoidal cell):
-Posterior ethmoid cell located over the superolateral aspect of the sphenoid sinus
-When present, the optic nerve and the internal carotid artery can project along
the superolateral wall of the Onodi cell placing them at increased risk of
iatrogenic injury
-Incidence is approximately 30%
-Onodi cell located superolateral
-Sphenoid sinus located inferomedial

17. -Fovea ethmoidalis (ethmoid roof):Formed by orbital plate of frontal bone
-Slopes downward (~15°) from anterior to posterior and lateral to medial
-Posteromedial region of fovea ethmoidalis theoretically at greater risk of injury
during ESS given lower height
-Attaches to lateral lamella medially
-Lateral lamella:Formed by ethmoid bone
-Forms lateral surface of cribriform fossa
-Thinnest and weakest bone in the skull base
-Cribriform plate:Forms floor of cribriform fossa
-Perforated by multiple olfactory nerve fibers
-Also slopes downward as it passes posteriorly

19.  Keros classification: assessing the depth of the olfactory fossa (corresponds to the
length of lateral lamella)
-Lateral lamella of cribriform plate is the thinnest bone in the skull base
Type 1: 1 to 3 mm (second most common configuration)
Type 2: 4 to 7 mm (majority of cases)
Type 3: 8 to 16 mm (rare)

20. Maxillary sinus:
(a) Boundaries of maxillary sinus
- Superior: orbital floor
- Inferior: alveolar and palatine process of maxilla
- Lateral: zygoma
- Medial: lateral nasal wall
- Posterior: pterygopalatine (PPF) and infratemporal fossa (ITF)
Anterior: facial surface of maxilla
(b) Endoscopic landmarks associated with the maxillary sinus
- Roof of maxillary sinus: approximates height of sphenoid sinus floor
- Posterior wall of maxillary sinus: approximates depth of sphenoid rostrum
- Medial maxillary wall of maxillary sinus:With normal maxillary sinus
pneumatization, it is located in line with a vertical line drawn tangential to LP
When maxillary sinus is hypoplastic or atelectatic, medial maxillary wall may be lateral
to LP

23. (c) Foramen of maxillary bone:
- Infraorbital foramen:Contains infraorbital nerve, artery, and vein
-Runs along maxillary sinus roof within infraorbital canal
-Iatrogenic injury possible if canal is dehiscent (incidence of 14%)
- Incisive foramen:Contains greater palatine artery and nerve
-Maxillary os:Located within posterior one-third of ethmoid infundibulum
-Superior alveolar foramen:Contains posterior, middle and anterior superior
nerve, artery, and vein
• Sphenoid sinus: Classification of sphenoid pneumatization
-Conchal
-Presellar
-Sellar
-Postsellar

25. (b) Walls and recesses of the sphenoid sinus:
-Planum sphenoidale:Forms sphenoid roof
-Contiguous anteriorly with fovea ethmoidalis
-Sphenoid rostrum: Forms sphenoid face and anterior floor
-Articulates anteriorly with the vomer and perpendicular plate of ethmoid
-Sella turcica:
-Rounded projection along posterosuperior wall in a well-pneumatized sphenoid sinus
-Forms floor of hypophyseal fossa (containing pituitary gland; middle cranial fossa)
-May be attenuated or anteriorly displaced in the presence of a pituitary
macroadenoma
-Bounded anterosuperiorly by tuberculum sella and posteriorly by dorsum sella
-Lateral pterygoid recess (lateral recess):
-Inferolateral pneumatization of the sphenoid sinus
-Common location for spontaneous CSF leak and encephalocele
-Lateral wall of sphenoid:Forms medial wall of cavernous sinus
-When well-pneumatized, bony impressions of the internal carotid artery (partially
dehiscent in 25%) and optic nerve (dehiscent in 6%) can be visualized

28. Sphenoid intersinus septum:May asymmetrically divide the sphenoid sinus
 Must be removed with caution in transnasal endoscopic approaches to the skull base
when inserts onto or in the vicinity of the carotid and/or optic canal
Clival recess:Forms posteroinferior wall of the sphenoid sinus if well-pneumatized
 Separates sphenoid sinus from posterior cranial fossa
Choanal arch:Forms the floor of the sphenoid
 Corresponds to roof of nasopharynx
 Bordered laterally by the medial pterygoid process
(c) Landmarks for sphenoid ostium:
Most reliable landmark = between the nasal septum and posterior insertion of the
superior turbinate
 One-third of the way up from choana to skull base
 1.5 cm superior to the bony choanal arch
 7 cm at a 30 degree angle from the anterior nasal spine

31. Frontal sinus:
(a) Made up of two frontal sinuses, frequently asymmetric, separated by an intersinus septum
(b) Thick anterior wall, thin posterior wall
(c) Frontal beak: thick bone of the frontal process of maxilla, anterior to the agger nasi that
projects posteriorly into the frontal recess, thereby limiting its anteroposterior distance
(d) Frontal recess :Hourglass space with the narrowest portion corresponding to the frontal os
Communicates with the frontal sinus superiorly and anterior ethmoid inferiorly
-Boundaries:
-Anterior = frontal beak/agger nasi
-Medial = lateral lamella
-Lateral = LP
-Posterior = EB/suprabullar recess
-Posterosuperior = fovea ethmoidalis

34.  Significant variation accounts for the complexity of frontal sinus dissection
 Frontal sinus cells:
- Frontal ethmoidal cells: Anterior ethmoidal cells in contact with the anterior
wall of the frontal recess (frontal process of maxilla)
-Three classification schemes exist-
1. Kuhn classification.
2. Modified Kuhn classification (Wormald)
3. International frontal sinus anatomy classification (IFAC)

35. 1. Kuhn classification
 Type 1: single cell above agger nasi cell
 Type 2: tier of cells above agger nasi cell
 Type 3: single-cell pneumatizing into the frontal sinus
 Type 4: isolated cell within the frontal sinus

36. 2. Modified Kuhn classification (Wormald):
 Type 1 and 2: no change from previous classification
 Type 3: cell pneumatizing into the frontal sinus but less
than 50% of the vertical height of the sinus
 Type 4: cell pneumatizing into the frontal sinus greater
than 50% of the vertical height of the sinus

37. 3. International frontal sinus anatomy classification (IFAC)
 Supra agger cell: anterior-lateral ethmoidal cell, located above the agger nasi cell
(not pneumatizing into the frontal sinus)
 Supra agger frontal cell: anterior-lateral ethmoidal cell that extends into the frontal
sinus
 Supra bulla cell: cell above the bulla ethmoidalis that does not enter the frontal sinus
 Supra bulla frontal cell: cell that originates in the supra bulla region and pneumatizes
along the skull base into the posterior region of the frontal sinus
 Intersinus septal cell:Pneumatization of the interfrontal sinus septum
Originates medially thereby displacing frontal sinus drainage pathway laterally
 Other relevant frontal recess cells include agger nasi cell, suprabullar cells, frontal
bulla cells, and supraorbital ethmoid cells