Keywords
temporal bone - mastoidectomy - semicircular canals - endolymphatic sac - cerebellopontine
angle
Introduction
The anatomy of the sigmoid sinus (SS) is highly variable, and it is of utmost importance
to have detailed knowledge about these variations while performing mastoid surgeries/transmastoid
approaches, such as cochlear implantations, approaches to the cerebellopontine angle
(CPA), and the infratemporal fossa approach (Fisch type-A) for the excision of glomus
jugulotympanicum tumors.[1]
[2]
[3]
[4]
The aim of the present study was to report the anatomical variations of the SS in
cadaveric temporal bones (TBs) by microdissection, and to propose a new classification/grading
system of these SS variations that is relevant from a surgical viewpoint. The proposed
classification emphasizes the hindrance posed by the SS to the visualization of the
posterior semicircular canal (PSCC) as a landmark. The extent of visualization of
the PSCC works as a surrogate indicator of the space constraint while carrying out
the transmastoid approaches (including the retrolabyrinthine [RL] and translabyrinthine
[TL] approaches). The objectification of the space constraint classified in this fashion
will help in predicting the surgical restraints while accessing various structures
via a transmastoid approach and help with more objective documentation of the intraoperative
anatomical structures as correlated with varied SS positions. The secondary aim was
to assess the relationship of the SS anatomy with the pneumatization of the TB and
with the volume of the mastoid cavity.
Methods
Cadaveric TB dissections were performed in the surgical skills laboratory of our department
after approval from the Ethical Committee of our Institute (File No.F.8–522/A-522/2017/RS
(Project code number: A-522; dated September 19th 2017). These TBs were harvested from unclaimed cadavers and preserved in 10% formaldehyde
for sterilization before dissection. The microdissections were performed using a Leica
M320 F12 microscope with LED (Leica Biosystems, Wetzlar, Germany), which has an inbuilt
3-megapixel camera plus a HD video system (Leica microscope) under 6.4 magnification.
A Forte 200 electric microdril (Saeshin Precision CO. Ltd. #93-15 Paho-Dong, Dalseo-Gu,
Daegu, Korea) with 25,000 rpm was used for dissection.
After the removal of the soft tissues from the TB, the dissections were performed
following the standard procedure. The TB was fixed in the House-Urban TB holder and,
after the identification of the Macewen triangle, a cortical mastoidectomy was performed.
Each TB dissection procedure was recorded, and important pictures were taken separately
under the microscope for analysis.
The bones were categorized as well-pneumatized or under-pneumatized by the dissecting
surgeon intraoperatively, depending upon the cellularity of the mastoid cell system.
The SS was classified according to the visibility of the structures viewed parallel
to the thinned out posterior wall of the external auditory canal (essentially a viewing
axis perpendicular to the cortical bone of the mastoid). The volume of the mastoid
cavity was assessed conventionally by filling it with a measured volume of water after
blocking the mastoid aditus.
The positioning of the SS was classified into three grades (I, II, and III; favorable,
intermediate, and unfavorable, respectively) by Irugu DVK & Singh A, based on its
anatomical location, with special reference to the presigmoid bony plate (defined
as the bone between the anterior margin of the SS and the PSCC) and to the dome of
the PSCC. The grading of the SS with the previously mentioned reference landmarks
is detailed below and is represented in [Fig. 1].
Fig. 1 (A) Dissected temporal bone with marked anatomical reference points for anatomical
grading of SS (B) SS grade I: The SS, PSBP, and PSCC are in the same vertical plane.
All of the three structures are fully visible with good exposure of the Trautmann
triangle and a voluminous mastoid cavity. (C) SS grading II: The SS is anteriorly
placed obscuring the PSBP, but the PSCC can be seen. With the anterior placement of
the SS, narrowing of Trautmann triangle and the reduction in the volume of the mastoid
cavity can be appreciated. (D) SS grading III: Further anteriorly placed SS obscuring
the PSBP and the PSCC with a significantly compromised volume of the mastoid cavity
and significant reduction of the exposure of the Trautmann triangle. Abbreviations:
AD, aditus; DR, digastric ridge; EAC, external auditory canal; LSCC, lateral semicircular
canal; PMW, posterior meatal wall; PSBP, presigmoid bony plate; PSCC, posterior semicircular
canal; SDA, sinodural angle; SS, sigmoid sinus; SSCC, superior semicircular canal.
-
Grade I (favorable): The SS was not hindering the view of the presigmoid bony plate
and of the PSCC. All of these structures were clearly visible with a flat surface
and good volume of the mastoid cavity with a wide Trautmann triangle. These factors
were considered to impart good accessibility for the RL and TL approaches to the CPA.
-
Grade II (intermediate): Anterior placement of the SS, obscuring the view of the presigmoid
bony plate but not of the PSCC. The volume of the mastoid cavity was reduced and the
Trautmann triangle became narrower compared with grade I. It imposes some hindrance
and eventual risk to the approach to the CPA by the RL approach. The TL approach is
preferred in such cases.
-
Grade III (unfavorable): The SS was situated so far anterior as to impede the visualization
of the presigmoid bony plate and of the PSCC. The Trautmann triangle was severely
narrowed and it was associated with a significant reduction in the volume of the mastoid
cavity. It was considered to be an unfavorable situation to approach the CPA through
the RL or the TL approaches, and the retrosigmoid (RS) approach is advisable in such
cases.
Statistical analysis: The volumetric data of the left and of the right sides was compared
by using the student-t test. The comparison of volume among the three grades of the
SS (with respect to pneumatization) was performed using the one-way analysis of variance
(ANOVA) followed by the Bonferroni test correction for multiple comparisons. The Stata
14.1 (StataCorp, College Station, TX, USA) software was used to analyze the data,
and p < 0.05 was considered statistically significant.
Results
A total of 94 cadaveric temporal bones were dissected in our surgical skill laboratory,
comprising 47 bones each, of right and left sides. A total of 59 bones were well-pneumatized,
and the other 35 bones were underpneumatized ([Table 1]).
Table 1
Distribution of bones according to pneumatization
|
Pneumatisation
|
Bone side
|
Sigmoid sinus grading
|
|
Right
|
Left
|
I
|
II
|
III
|
|
Well-pneumatized
|
31
|
28
|
34
|
23
|
2
|
|
Underpneumatized
|
16
|
19
|
3
|
16
|
16
|
|
Total
|
47
|
47
|
37
|
39
|
18
|
According to the SS grading described above, the distribution of the dissected temporal
bones is described in [Table 1]. Grade I consisted of 37 bones (39.36%), of which 16 were right-sided (17.02%) and
21 were left-sided (22.34%) bones. Grade II consisted of 39 bones (41.49%) comprising
23 right-sided (24.47%) and 16 left-sided (17.02%) bones. Grade III had 18 bones (19.15%)
comprising 8 (8.51%) right-sided and 10 (10.64%) left-sided bones. The level of pneumatization
between the two sides was statistically comparable.
On comparing the SS grading with the volume of the mastoid cavity, grade I was found
to have a mean volume of 6.6 ml (standard deviation [SD] of 0.90 ml), grade II had
a mean volume of 4.4 ml (SD of 0.84 ml), and in grade III, the mastoid volume was
2.0 ml (SD of 0.69 ml). The volume difference in different SS grades was statistically
significant, with a p-value of 0.001 ([Table 2]) ([Fig. 2]).
Table 2
Comparative results of sigmoid sinus grading with the volume of the mastoid cavity
|
Sigmoid sinus grading (Irugu & Singh)
|
Mean Volume (SD); (ml)
|
Number of bones (%)
|
|
I
|
6.58 (0.90)
|
37 (39.4)
|
|
II
|
4.41(0.84)
|
39 (41.5)
|
|
III
|
1.95 (0.69)
|
18 (19.1)
|
Abbreviations: ml, milliliters; SD, standard deviation.
p-value = 0.001.
Fig. 2 Box and whiskers plot of sigmoid sinus grading with the volume of the mastoid cavity.
(x-axis: sigmoid sinus grade; y-axis: volume of the mastoid cavity in ml; Vol: volume).
(The boxes represent the interquartile range [the middle 50% of volume distribution],
and the whiskers encompass the entire range of volume distribution. The lines dividing
the boxes represent the median, and the dots outside the boxes are the outliers).
The distribution of the bones regarding the SS grading and pneumatization and its
correlation with the volume of the mastoid cavity is described in [Table 3] and is graphically represented in [Fig. 3]. A clear trend of reduction in the volume of the mastoid cavity with an increase
in the SS grade can be appreciated. Within a particular SS grade, the higher the degree
of pneumatization, the higher the volume of the mastoid. The p-value for the ANOVA between the SS gradings with pneumatization and the volume of the mastoid
cavity was 0.0001.
Table 3
Distribution of bones with respect to sigmoid sinus grading and pneumatization with
the summary of the volume of the mastoid cavity
|
Sigmoid sinus grading and pneumatisation
|
Summary of the volume of the mastoid cavity; mean (SD); (ml)
|
Number of bones (%)
|
|
SSG I WP
|
6.64 (0.91)
|
34 (36.2)
|
|
SSG I UP
|
5.93 (0.51)
|
3 (3.2)
|
|
SSG II WP
|
4.69 (0.87)
|
23 (24.5)
|
|
SSG II UP
|
4.01 (0.62)
|
16 (17.0)
|
|
SSG III WP
|
3.25 (0.35)
|
2 (2.1)
|
|
SSG III UP
|
1.79 (0.53)
|
16 (17.0)
|
Abbreviations: ml, milliliters; SD, standard deviation; UP, underpneumatized; WP,
well-pneumatized.
p-value = 0.0001.
Fig. 3 Box and whiskers plot of the correlation of results between the volume of the mastoid
cavity with the sigmoid sinus grading and pneumatization (x-axis: sigmoid sinus groups; y-axis: volume of the mastoid cavity in ml). (The boxes represent the interquartile range {the middle 50% of volume distribution}
and the whiskers encompass the entire range of volume distribution. The line dividing
the box represents the median and the dots outside the box are the outliers). Abbreviations:
NP, underpneumatized; P, well-pneumatized; SS, sigmoid sinus; Vol;, volume.
Discussion
The knowledge of variations in the anatomical location of the SS in the TB is very
important for performing mastoid surgeries, especially transmastoid approaches to
the CPA. The anatomical variations in the location of the SS affects the exposure
of the Trautmann triangle and the volume of the mastoid cavity.[1]
[2]
[3]
[4]
The Trautmann triangle is bound superiorly by the superior petrosal sinus, posteriorly
by the SS, anteriorly by the PSCC, and anteroinferiorly by the jugular bulb (JB).[4]
[5]
[6] There have been many attempts to classify the SS in the past based on the location
of the SS and on the exposure of the Trautmann triangle, but none of them classify
the SS in relation to the surrounding vital structures with consideration to the volume
of the mastoid as a function of the position of the SS. The various classifications
are listed in [Table 4].[1]
[2]
[4]
Table 4
Lliterature review of classifications of sigmoid sinus variations
|
S. No.
|
Author and Year
|
Basis of the Classification [reference]
|
|
1
|
Ichijo et al (1993)
|
Shape of the sigmoid sinus[7]
|
|
2
|
Kayalioglu et al (1996)
|
Anatomical position of the sigmoid sinus in the mastoid cavity[2]
|
|
3
|
Sarmiento et al (2004)
|
Based on the narrowing of the Trautmann triangle[4]
|
|
4
|
Sun et al (2009)
|
Three imaginary radiological (computed tomography) reference lines theory[1]
|
In 1993, Ichijo et al[7] proposed a classification of shapes of the SS. It was a classification of the cross-sectional
appearance of the SS based on the study of high-resolution computed tomography (CT)
scans of TBs. This classification, however, does not provide information about the
surrounding structures and is of limited practical importance from an otologic surgical
point of view. The classification of Kayalioglu et al has similar limitations.[2]
[8]
Sarmiento et al[4] classified the SS into three types based on its position in relation to the Trautmann
triangle. They designated a posteriorly displaced SS as type 1, which was associated
with a large Trautmann triangle. Type 2 and type 3 SSs were anteriorly and medially
placed sinus morphologies, associated with a tight Trautmann triangle, more so with
the medially placed SS.
In 2009, Sun et al[1] proposed a radiological classification of the SS in four groups in relation to three
lines drawn on CT scan, reflecting important surgical structures. The authors emphasized
the ability of the classification system to reflect the SS as a landmark of surgical
significance for transmastoid surgeries. However, the radiological data was not backed
by the real-time intraoperative dissection findings validating the data.
The position of the SS significantly affects the exposure of the endolymphatic sac
via a transmastoid approach, and of the CPA/petroclival fissure area via a presigmoid
retrolabyrinthine approach.[5]
[6] The exposure of the posterior cranial fossa via a presigmoid retrolabyrinthine approach
depends on the positional relationship of the SS and of the PSCC in an anteroposterior
plane and of the Superior Petrosal Sinus (SPS) and of the JB in a superoinferior plane.[5]
[6]
[9] The more forward placed the SS is, the more constricted the exposure via this corridor
gets. This leads to twofold consequences. It impedes the direct access to the area
of pathology and risks the surrounding vital structures, including the facial nerve
and the posterior labyrinth. Even if the access to the area of concern is obtained,
venturing into it via this constrained corridor may hamper maneuvering in the target
region and may result in difficulty in achieving complete disease removal, adequate
dural closure, and sealing of the cerebrospinal fluid (CSF) leak with the attendant
adverse outcomes.[5]
[6]
[9]
The ideal characteristics of the lesion accessible via this route would be a lesion
placed superficially and not based on the fundus of the internal auditory canal, a
low JB and an SS placed posteriorly with reference to the posterior semicircular canal.
With the lesion not conforming to these attributes, the surgeon may need to convert
to a transcrusal/ translabyrinthine approach, to skeletonization/posterior displacement
of the SS, or to undertake a retrosigmoid approach.[6]
[10]
Through our study, we have proposed to classify the ease of transmastoid approaches
according to the location of the SS in relation to the PSCC. We have classified the
SS into three categories (grade I, II and III; favorable, intermediate and unfavorable
grades, respectively). The most common pattern was that of the grade II SS (41.5%),
followed by grade I SS (39.4%); grade III being the least common pattern (19.1%).
A grade I SS affords a large exposure of the Trautmann triangle, as well as an ample
surgical space to execute various transmastoid approaches. The exposure gets progressively
diminished as we move from grade II to grade III SSs. The volume of the mastoid correlates
significantly with the location of the SS in TBs. The prediction of the volume of
the mastoid cavity was significantly affected by the location of the SS and by the
pneumatization of the TB (6.6 ml with well-pneumatized grade I SS versus 1.8 ml in
cases of underpneumatized grade III SS; p = 0.0001).
The present study emphasizes the difficulties in performing the surgeries in the distorted
anatomy, with special reference to mastoid surgeries, implantation surgeries, and
transmastoid approaches, such as endolymphatic sac decompression, CPA tumor excision,
and the infratemporal fossa approaches to glomus tumors. Every patient needs to be
evaluated preoperatively thoroughly with a suitable radiological method, especially
high-resolution CT of the TB, which provides knowledge of the anatomy and of the extent
of the disease involving the TB, to avoid intraoperative complications, as well as
to enable better counseling of the patient.[11]
[12]
[13]
Conclusion
-
The present study presents a clinically oriented classification of the positioning
of the SS in three grades.
-
Sigmoid sinus grades I, II and III were noted to be present in 39.4, 41.5 and 24.4%
of the cases, respectively.
-
Access to the posterior fossa for the transmastoid, TL and RL approaches is easier
and wider in grade I SS.
-
Well-pneumatized bones had a higher probability of having grade I SS (favorable configuration),
while underpneumatized bones have a more unfavorable configuration of the SS.
-
The volume of the mastoid impacts the location of the SS. The more favorably placed
SSs were found to be associated with a larger volume of the mastoids.
