” jl THE ANATOMY AND PHYSIOLOGY OF THE

SPHENOID SINUS
JONATHAN Z. BASKIN, MD, M. ABRAHAM KURIAKOSE, MD, DDS,
RICHARD A. LEBOWITZ, MD

Anatomically, the sphenoid sinus is more complicated than the other paranasal sinuses as a result of its close
proximity to surrounding vital structures. It is directly related to the anterior cranial fossa, hypophysis, brain stem,
optic nerves, carotid artery, as well as the cavernous sinuses and its contents. Additionally, of the three single
cavity paranasal sinuses (ie, frontal, maxillary, sphenoid), it is the sinus with the most anatomic variability. This
complexity creates the need for a comprehensive understanding of the anatomy before surgery in and around the
sinus. This article reviews the embryology, physiology, and anatomy of the sphenoid sinus.

EMBRYOLOGY The “mucous blanket” is made up of 2 layers: an inner

layer and an outer layer. The inner “sol” layer is composed
In actuality, the sphenoid sinus is an extension of the of thin mucous, while the outer “gel” layer mucous is
ethmoid sinuses into the sphenoid bone. The precursor to more viscous and tenacious. The mucous is composed of
the sphenoid sinus emerges in the fourth month of fetal 96% water and 3% to 4% glycoproteins, and is produced
development as an evagination of the posterior nasal cap- by a combination of mucous and serous glands. It contains
sule into the sphenoid bone. The sphenoid sinus does not immunologically active substances and is responsible for
really begin to grow significantly until the third year of trapping foreign particles as small as 2 pm and facilitating
life. The body of the sinus is usually pneumatized by the immunologic processing.
age of 7 years. The sinus grows throughout childhood, The clearance of the mucous blanket, from the anterior
reaching its peak size of 14 X 14 X 12 mm1 to 20 X 23 X nasal cavity to the nasopharynx and within the paranasal
17 mm2 at about the age of 20 years. This corresponds to a sinuses, occurs approximately every 10 to 15 minutes in
reported average adult sinus volume of 7.4 cm (range 0 healthy individuals. The movement of mucous occurs as a
to 14).3 result of ciliary activity. Cilia are long, thin organelles, 0.3
Frn in diameter and 0.7 Frn in length. When functioning
optimally, the ciliary beat frequency is 10 to 15 per second,
PHYSIOLOGY with an average flow rate of about 0.84 cm per minute.
The function of the paranasal sinuses is not yet com- Cilia function best under warm humid conditions, with
pletely understood. It has been suggested that the sinuses function significantly impaired when the humidity is less
assist with the warming and humidification of inspired than 50%, or the temperature is less than 18°C. Ciliary
air, and contribute to mucous production. It has also been motility is also hampered by apposition of mucosal sur-
noted that the paranasal sinuses are important for contrib- faces, which results in the stasis of the mucous blanket.
uting to increased vocal resonance, and are instrumental In general, normal sinus drainage is dependent on ostial
for buffering increased pressure in the upper aerodiges- patency, adequate mucous production, and viable ciliary
tive tract. On a more theoretical level, pneumatization of function. The flow of the mucous blanket in the sphenoid
the bones of the face and skull base may have evolved as sinus is directed towards the natural ostium. The mucous
a way of decreasing the weight of these bones. Physiolog- then drains into the sphenoethmoid recess where it is
ically, there appears to be nothing that separates the sphe- directed towards the nasopharynx.
noid sinus from the rest of the paranasal sinuses.
The sinuses are lined by respiratory (ie, pseudostratified
columnar) epithelia. The paranasal sinuses produce only a ANATOMY
small amount of the mucous present in the nasal cavity.
The sphenoid sinus has been classified into three gen-
eral types that occur at different frequencies and reflect the
From the Department of Otolaryngology and Head and Neck Surgery, degree of pneumatization of the sphenoid bone: sellar,
New York University School of Medicine, New York, NY.
presellar, and conchal (Fig 1),4 The sellar type of sphenoid
Address reprint requests to Richard Lebowitz, MD, NYU Medical Cen-
ter, Department of Otolaryngology, Suite 3C, 530 First Avenue, New
sinus occurs in approximately 86% of individuals, and
York, NY 10016. represents a well pneumatized sphenoid body with full
0 2003 Elsevier Inc. All rights reserved. indentation of the sella into the sinus. The presellar type
1043-l 81 O/03/1403-0002$30.00/O (11%) has a moderate amount of sphenoid pneumatization
doi:lO.l053/S1043-1810(03)00027-7 with no sellar indentation, while in the conchal type (3%),

168 OPERATIVE TECHNIQUES IN OTOLARYNGOLOGY-HEAD AND NECK SURGERY, VOL 14, NO 3 (SEPT), 2003: PP 168-172
FIGURE 1. Sagittal views of the three major classifications of
the sphenoid sinus: sellar (A); presellar (6); conchal (C).

there is minimal pneumatization of the sphenoid bone.

Lang adds a fourth classification that he refers to as post-
sellar.5 This designation indicates the extension of pneu-
matization posterior to the hypophysial indentation. In a
small number of people, there is no pneumatization of the
sphenoid bone. There is, of course, additional variability in
younger people who have not undergone complete pneu-
matization of the sphenoid bone.
Pneumatization of the sphenoid sinus may extend ante-
riorly into the vomer (sphenovomerine bulla) and rarely
into the ethmoid bone. Extensions into the maxilla, pala-
tine bone, and supraorbital regions have been reported,
but these are also very rare. Laterally, the sphenoid sinus
can extend into the lesser or greater wings of the sphenoid
bone, and inferolaterally into the pterygoid processes. Pos-
teriorly, there can be extension into the basilar process of
the occipital bone (ie, clivus).
Most relevant to the surgeon is sphenoid pneumatiza-
tion that extends to those segments of bone that cover vital
structures. Depending on the pneumatization process, the
bone covering the sella, carotid arteries, optic nerves,
branches of the trigeminal nerve, and the vidian nerve can
FIGURE 3. (A and B) Parasagittal view of the sphenoid sinus
either be very thin or even absent, causing these structures
illustrating a sellar sinus (SS) with the hypophysis (HP) making a
vulnerable to injury during surgery.
central indentation. The brain and optic chiasm are retracted
The right and left sphenoid sinuses are divided by a
superiorly showing the optic nerve as it enters the optic canal.
sagittal intersinus septum (Fig 2). It is located in the mrd-
The tuberculum sella and planum sphenoidale are readily seen
line, producing symmetric paired sinuses in only 27% of anterior to the hypophysis. The middle (MT) and superior turbi-
specimens, and it is fully vertical 25% of the time.’ In the
nates (ST) are clearly visible with the nasal septum reflected
majority of sinuses, the intersinus septum is situated in the
inferiorly. MM, middle meatus; PG, pituitary gland; PS, planum
midline anteriorly and then deviates to either side as it
sphenoidale.

extends posteriorly (43%). Furthermore, there may be mul-

tiple septa and, in some cases, none at all.
The size of the sinus is highly variable. Schaeffer re-
ported general dimensions of 14 x 14 x 12 mm.7 In elderly
patients, Lang showed an average width of 13.5 mm su-
periorly, 16.9 mm in the middle, and 18.7 mm inferiorly.5
The length was 19.4 mm superiorly, 4.8 mm in the center,
and 18.5 mm inferiorly.

INTERNAL SINUS ANATOMY

An understanding of the internal anatomy of the sphe-
noid sinus is particularly important because of the central-
ity of the sinus relative to vital neural and vascular struc-
tures. Centrally, in the roof of the sphenoid sinus lays the
sella turcica, which contains the pituitary gland (hypoph-
ysis) (Fig 3). Th e average distance between the sphenoid
ostium and the sella is 17.1 mm according to Fujii et al*
and 14.6 mm according to Lang.5 The roof of the sphenoid
FIGURE 2. Axial (horizontal) view of the sphenoid sinuses (SS) sinus anterior to the sella, which separates the sinus from
showing the midline septum and relationship of the sinus to the the dura of the anterior cranial fossa, is referred to as the
hypophysis, orbital apices, and ethmoid sinuses (ES). CS, cav- planum sphenoidale.
ernous sinus; MT, middle turbinate; NLD, nasolacrimal duct; ON, The degree of sellar exposure in the sphenoid sinus is
optic nerve. dependant on the degree of pneumatization, as described

BASKIN ET AL 169
 the cavernous and anterior clinoid segments of the inter-

4,I’ 3TJn
/ ..-f - / /)
i nal carotid artery (ICA) (Fig 5). The degree of bony cov-
I /' -/--- i
ering and the length of the carotid that comes into relief in
the sinus are highly variable. Fujii et al reports the pres-
ence of lateral (cavernous) ICA prominence in the vast
majority of cases,8 while Lang noted the lateral ICA prom-
inence in a minority.5 Most articles have identified the
anterior (clinoid) ICA creating a carotid prominence in
approximately 50% of specimens. The bone overlying the
carotid artery is often thinner than that overlying the sella
and can, like the bone of the optic canal, be dehiscent. The
reported incidence of ICA dehiscence is similar to the optic
nerve, ranging from 0% in some cadaveric specimens to
23% in radiographic studies.5,9 Although the prominence
of the ICA is generally in the lateral wall of the sphenoid,
it can be in a more medial position. Together with the
relatively thin bony covering, this underscores the impor-
tance of caution during transsphenoidal surgery of the
hypophysis. Mean distance between the internal carotid
arteries tends to be shortest at the tuberculum sella, with
an average distance of 13.9 mm (range of 10 to 17).8 The
ICA in this particular location is relatively immobile, mak-
ing it more susceptible to injury than in other locations.
The dura of the cavernous sinus is in contact with a
large part of the lateral wall of the sphenoid sinus. Lang
reported the mean distance between the medial walls of

FIGURE 4. (A and B) Parts of the roof and lateral wall of the

sphenoid sinus have been removed to show the close relation-
ship of the optic nerve (ON) to the sinus. CA, carotid artery; OA,
orbital apex; PG, pituitary gland; V2, second division trigeminal
nerve.

previously. The thickness of the bone overlying the hy-

pophysis varies. In a sellar sphenoid sinus, anteriorly, the
area most crucial for surgical access to the pituitary gland,
the average bone thickness is 0.4 mm (range 0.1 to 0.7).’
The bone on the undersurface of the sella tends to be
slightly thinner.
Intracranially, just anterosuperior to the sella turcica
lays the tuberculum sella and the chiasmatic sulcus, which
corresponds to the junction of the planum sphenoidale
and the sella turcica. Within the sinus, this produces the
tuberculum recess. Although the chiasmatic sulcus does
not create a visible impression within the sinus, the prom-
inence created by the optic nerves, as they extend anteri-
orly in the optic canal, can be seen on the lateral wall of the
sphenoid sinus (Fig 4). The bone covering the optic nerves
is thin and can frequently be dehiscent. Reports of dehis-
cence have been as high as 23% in radiographic studies,’
while some cadaveric studies have shown a very low
incidence of optic nerve dehiscence. Lang reports that
within the sinus, the presence of supraoptic and infraoptic
recesses 33% and 38% of the time, respectively.5 The dis-
tance between the optic nerves as they enter the optic
canals is 14 mm (range 9 to 24).l” The opticocarotid recess FIGURE 5. (A and B) The floor and lateral wall of the sphenoid
is the small space on the lateral wall of the sphenoid sinus, sinus have been removed to illustrate the course of the internal
between the optic canal, superiorly, and the carotid prom- carotid artery (CA) and its close proximity to the sinus. Also, note
inence, inferiorly. the maxillary division of the trigeminal nerve as it traverses the
Just lateral to the sella extending into the lateral wall is lateral wall of the sinus in a position lateral to the internal CA. PG,
the carotid prominence. This prominence corresponds to pituitary gland; V2, second division trigeminal nerve.

170 ANATOMY AND PHYSIOLOGY OF THE SPHENOID SINUS

 FIGURE 6. (A and 6) Sagittal view of the nasal cavity and
sphnnoid sinus (SS). The probe extends from the limen nasi to
the sphenoid sinus ostium. The angle between the nasal floor
and a line joining the sphenoid ostium with the vestibule is
usually approximately 30”. ETO, eustachian tube oriface; NP,
nasopharynx; PG, pituitary gland.

FIGURE 7. (A and B) The na-

sal septum is retracted, and a
probe has been passed
through the sphenoid sinus
(SS) ostium. As is generally
the case, the ostium is ap-
proximately 1 cm superior to
the posterior tip of the supe-
rior turbinate. FS, frontal si-
nus; HP, hypophysis; PG, pi-
tuitary gland.

BASKIN ET AL 171
the cavernous sinuses to be 14.9 mm (range 10.1 to 18.2).5 of the superior nasal turbinate is the most reliable land-
The carotid artery is generally medial to the nerves within mark. The ostium is generally 1 cm superior to the tip (Fig
the cavernous sinus. However, the maxillary division of 7). It is medial to the superior turbinate in the majority of
the trigeminal nerve, which runs along the lateral and specimens studied (83%) and lateral in only a minority
inferior aspect of the cavernous sinus, produces a visible (17%), according to Kim et all3 Lang reported the ostium
ridge in 29% to 40% of specimens (Fig 5). The prominence to be an average of 4.8 mm from the nasal midline, with
produced by the second division of cranial nerve (V2) is slight differences between male and female subjects5
known as the trigeminal prominence. The position of the natural sphenoid ostium in the an-
In the floor of a well pneumatized sphenoid sinus infe- terior sinus wall has been in the upper third (52%) of the
rior and medial to the trigeminal prominence, the vidian wall, the middle third (34%) and the lower third (14%).5
nerve in its canal will occasionally come into relief. The Using the inferior border of the ostium as a reference
vidian canal measures an average of 16.2 mm (range 11.5 point, Kim et al found the ostium in the vertical midline in
to 23). Lang reported that the nerve ran below the floor of the majority of specimens. i3 The ostium itself is round 70%
the sinus in 38% of specimens, at the floor in 34%, within of the time, and the remainder is elliptical. The diameter of
the sinus in 18%, and was dehiscent 10% of the time.5 The the ostium ranges from “pinpoint” to 3.5 mm (mean 2.4).5
posterior wall of the sphenoid sinus separates the sinus
from the brain stem and basilar artery. The bone here
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172 ANATOMY AND PHYSIOLOGY OF THE SPHENOID SINUS