Keywords
transtentorial herniation - tentorium cerebelli - brainstem herniation - notch classification
- cadaveric study
Introduction
The tentorium cerebelli is one of the four dural folds that separate the cranial cavity
into two compartments containing the forebrain and hindbrain. This dural fold is deficient
toward the center and anteriorly to produce a gap, the tentorial aperture.[1] The midbrain traverses through the tentorial orifice and this space provides the
only communication between the two chambers, supratentorial and infratentorial. The
space between the brainstem and the free edge of the tentorium cerebelli is divided
into three spaces, which are anterior, middle, and posterior. The anterior one lies
anterior to the brainstem and the third nerve is related to this space; the middle
space lies on each side of the brainstem and the hippocampus is related to this area;
and the posterior space is in posterior relation to the midbrain and in relation with
the region of the pineal gland and the vein of Galen.[2]
Transtentorial herniation ensues in a wide variety of neurosurgical disorders, including
traumatic brain injury, tumors, and cerebral edema. However, if it remains untreated,
transtentorial herniation steps forward swiftly to death. In the absence of direct
neuroradiological imaging invention, the lethal consequences of brainstem compression
due to uncal herniation were recognized and first described by Meyer in 1920.[3] Corsellis and Sunderland studied the anatomical variations of the tentorial notch,
which acted as a milestone for neurosurgeons.[4]
[5] After this, modern neuroimaging techniques had also emerged.[6] The tentorial aperture varies morphologically in dimensions and has a great importance
in the neurosurgical field. However, this aperture is defined by the free edge of
the tentorial fold, yet it remains anatomically indefinable because of its complicated
and variable anatomy in three dimensions, absence of blood vessels along its free
edge, and only infrequent calcification.[7]
Therefore, the present preliminary study was done with the objective of explicating
the morphology of the tentorial aperture in the Indian population in unfixed human
cadavers. This article provides a reference anatomical data for determination of dimensions
and type of notch on magnetic resonance imaging and computed tomography scan, which
may facilitate neurosurgical decision-making.
Materials and Methods
The preliminary study was conducted on unfixed human cadavers after getting an approval
from the ethical committee. It was performed during autopsies of cadavers, aged 20
to 74 years, performed within 24 hours of death to prevent any alteration in tentorial
notch morphology due to decomposition or putrefaction.[8]
Dissection Steps
The unfixed heads of the human cadavers were placed using a block to keep the head
at an angle of 45 to 60 degrees above the horizontal plane. Make a coronal incision
with the help of a scalpel from mastoid to mastoid and separate the subcutaneous attachments.
Retract the scalp anteriorly toward the eye and nose and posteriorly beyond the occipital
protuberance. Dissect the temporalis muscle and reflect it inferiorly. Evert the scalp
and cut the skull circumferentially just above the superciliary ridges and above the
inion with the help of an electric saw and remove the skull cap followed by cutting
the dura. Lift the frontal lobes in a gentle way with the help of the fingers and
cut the anterior falx. Dissect the diencephalon axially above the level of the optic
chiasma, through the third ventricle to the apex of the tentorial notch. The cerebrum
is removed, leaving intact a little part of the diencephalon, posterior part of the
falx, and tentorium cerebelli. Cut the optic nerve rostral to the pituitary fossa.
Follow with the contour of the tentorial edge to the point of the notch apex and the
midbrain was cut in the axial plane. Field was cleared with help of water and gauze.
Measurements
The following measurements were made during the autopsy with the help of a geometry
compass and vernier caliper to determine the morphometric variations of the tentorial
aperture ([Figs. 1] and [2]):
Fig. 1 Schematic diagram showing the measurements of the tentorial notch. ANW, anterior
notch width; AT, apicotectal distance; IC, interpedunculoclival distance; NL, notch
length; MNW, maximum notch width; PTL, posterior tentorial length.
Fig. 2 Measurements of the tentorial notch. AT, apicotectal distance; IC, interpedunculoclival
distance; MNW, maximum notch width.
-
Maximum notch length (MNL), the length between the apex of the notch to the posterosuperior
edge of the dorsum sellae.
-
Maximum notch width (MNW), maximum width of the tentorial notch.
-
Interpedunculoclival distance (IC), the distance between the posterosuperior edge
of the dorsum sellae and the interpeduncular fossa.
-
Apicotectal distance (AT), the distance from the apex of the notch to the tectum of
the midbrain.
-
Anterior notch width (ANW), the width of the tentorial notch in the axial plane through
the dorsum sellae posterior aspect.
Results
Out of 20 cadavers, 18 cadavers were males and 2 were females. The majority of the
cadavers were males. The mean MNL measured 52.13 ± 5.01 mm (range: 44.2–59.92 mm)
and the mean AT distance measured 19.09 ± 6.91 mm (range: 9.09–39.57 mm; [Table 1]). The correlation between these two values was significant (r = 0.66). However, the quantification of the cerebellar tissue located in the tentorial
aperture has not been done, but there was a positive correlation between cerebellar
tissue and the AT distance.
Table 1
Summary data for morphometric measurements of the anatomy of the tentorial notch
|
Value
|
Age at the time of death (y)
|
MNL (mm)
|
MNW (mm)
|
IC (mm)
|
AT (mm)
|
ANW (mm)
|
|
Minimum
|
20
|
44.2
|
24.4
|
7.32
|
9.09
|
22.4
|
|
1st quartile
|
24
|
47.3
|
28.11
|
11.30
|
14.22
|
24.39
|
|
Mean ± SD
|
36.3 ± 16.36
|
52.13 ± 5.01
|
29.12 ± 1.88
|
13.59 ± 4.35
|
19.09 ± 6.91
|
26.16 ± 2.45
|
|
Median
|
28
|
51.8
|
29.09
|
|
|
|
|
3rd quartile
|
54
|
56.26
|
29.7
|
14.27
|
24.71
|
27.29
|
|
Maximum
|
74
|
59.92
|
34.5
|
25.58
|
39.57
|
31.15
|
Abbreviations: ANW, anterior notch width; AT, apicotectal distance; IC, interpedunculoclival
distance; MNL, maximum notch length; MNW, maximum notch width; SD, standard deviation.
The mean values of the ANW and MNW measured 26.16 ± 2.45 mm and the mean distance
measured 29.12 ± 1.88 mm ([Table 1]). A strong correlation was found between these two values (r = 0.69). A mean IC distance of 13.59 ± 4.35 mm was found.
Notch Classification
The tentorial notches were classified into eight types by using two variables, MNL
and MNW ([Table 2]). The matrix formation used for tentorial notch classification is shown [Table 3] and the tentorial notch has been classified into eight types ([Table 2]).
Table 2
Various types of tentorial notches and the criteria used for typing
|
Type of notch
|
Dimension
|
Range (mm)
|
Percentage
|
|
Wide
|
MNW (wide)
|
29.80–34.30
|
20
|
|
MNL (midrange)
|
47.4–56.26
|
|
Narrow
|
MNW (narrow)
|
24.74–28.11
|
0
|
|
MNL (midrange)
|
47.4–56.26
|
|
Long
|
MNW (midrange)
|
28.20–29.70
|
5
|
|
MNL (long)
|
56.4–59.92
|
|
Short
|
MNW (midrange)
|
28.20–29.70
|
0
|
|
MNL (short)
|
44.20–47.30
|
|
Typical
|
MNW (midrange)
|
28.20–29.70
|
30
|
|
MNL (midrange)
|
47.4–56.26
|
|
Large
|
MNW (wide)
|
29.80–34.30
|
0
|
|
MNL (long)
|
56.4–59.92
|
|
Small
|
MNW (narrow)
|
24.74–28.11
|
15
|
|
MNL (short)
|
44.20–47.30
|
|
Mixed
|
MNW (narrow)
|
24.74–28.11
|
0
|
|
MNL (long)
|
56.4–59.92
|
|
MNW (wide)
|
29.80–34.30
|
|
MNL (short)
|
44.20–47.30
|
Abbreviations: MNL, maximum notch length; MNW, maximum notch width.
Table 3
Matrix preparation for classification of the tentorial notch
|
Type of notch (total number of cases), n = 20
|
|
MNW
|
Short MNL
|
Midrange MNL
|
Long MNL
|
|
Narrow
|
Small (3)
|
Narrow (0)
|
Mixed (1)
|
|
Midrange
|
Short (0)
|
Typical (6)
|
Long (1)
|
|
Wide
|
Mixed (0)
|
Wide (4)
|
Large (0)
|
Abbreviations: MNL, maximum notch length; MNW, maximum notch width.
The first quartile of the MNW ranging from 24.74 to 28.11 mm was labeled as narrow, and the middle two quartiles ranging from 28.2 to 29.7 and 29.8 to 34.3 mm were
labeled as midrange and the wider type, respectively. Out of 20 cases, 5 (25%) were narrow, 10 (50%) was midrange,
and 5 (25%) were wide. The first quartile of the MNL ranging from 44.2 to 47.3 mm
was labeled as short, the middle two quartiles ranging from 47.4 to 56.26 and 56.4
to 59.92 mm were labeled as midrange and long type, respectively. Out of 20 cases,
5 (25%) were short, 10 (50%) were midrange, and 5 (25%) were long ([Table 3]).
The tentorial notch having a long MNL and a midrange MNW was classified as long (5%). The tentorial notch having a midrange MNL and a wide MNW was classified as
wide (20%) as shown in [Fig. 3]. The tentorial notch having a midrange MNL and a narrow MNW was classified as narrow (0%). The tentorial notch having a short MNL and a midrange MNW was classified as
short (0%). The tentorial notch having a long MNL and a wide MNW was classified as large (0%). The tentorial notch having a midrange MNL and a midrange MNW was classified
as typical (30%), as shown in [Fig. 4]. The tentorial notch having a short MNL and a narrow MNW was classified as small (15%). Some notches were seen to be distributed under both wide and short or long
and narrow. Both types of notches were classified as mixed (30%).
Fig. 3 The wide type of notch (wide maximum notch width and midrange maximum notch length).
Fig. 4 The typical type of notch (midrange maximum notch width and midrange maximum notch
length).
Discussion
A wide spectrum of morphometric variability seen in the tentorial notch anatomy raises
a fundamental question of its relevance in the human body. Sunderland classified notches
into two groups, that is, broad and narrow; however, Corsellis gave a proposal that
patterns of herniation must be affected by the size and shape of the tentorial aperture.
Both authors clearly showed the orifice's anatomical variations and its relation with
the brainstem, but there was failure of the well-defined classification system.[4]
[5] There are few studies that have attempted to analyze the significance of variability
in tentorial notch anatomy and its association with herniation. They have demonstrated
a correlation between the radiological and autopsy findings, simply strengthening
the need for further research to validate this corroboration.[9]
[10] Klintworth proposed that the tentorium cerebelli and the tentorial aperture vary
considerably in positioning, size, and shape in different animal species. In some
animals like fish, reptiles, and amphibians, the tentorium cerebelli is absent, whereas
in some mammals like rats and guinea pigs, this tentorial partition is not complete.
But in humans and monkeys, this tentorial dural fold becomes a crescent-shaped fold
that acts as a partition and divides the cranial cavity into two compartments separating
the cerebral and cerebellar hemispheres.[11]
The various parameters of dimensions of the tentorial notch are compared with different
populations in [Table 4]. The MNL in our study (52.13 ± 5.01 mm) is less as compared with the studies by
Sunderland[5] who examined the Australian population (54.9 ± 6.93 mm) and Ono et al[2] who reported an MNL of 52 mm. The values of IC reported in our study are higher
than those observed by Ono et al,[2] which was 12.1 mm ([Table 4]). The range of AT distance in our study was found more (9.09–39.57) than that reported
by Adler and Milhorat[12] (4–32 mm) and Ono et al[2] (13–27 mm). This shows that there is greater AT distance in the Asian population
as compared with that of the western population.
Table 4
Comparison of data of tentorial notch with U.S., Australian, and Indian population
|
Parameters
|
Our study (Indian population)
|
Ono et al[2] (in U.S. population)
|
Sunderland[5] (in Australian population)
|
|
Mean ± SD (mm)
|
Range (mm)
|
Mean ± SD (mm)
|
Range (mm)
|
Mean ± SD (mm)
|
Range (mm)
|
|
MNL
|
52.13 ± 5.01
|
44.2–59.92
|
52
|
46–67
|
54.9 ± 6.93
|
44–75
|
|
MNW
|
29.12 ± 1.88
|
24.4–34.3
|
29.6
|
26–35
|
30.16 ± 3.21
|
23–39
|
|
IC
|
13.59 ± 4.35
|
7.32–25.58
|
12.1
|
7.8–15.6
|
|
|
|
AT
|
19.09 ± 6.91
|
9.09–39.57
|
19.8
|
13–27
|
|
|
|
ANW
|
26.16 ± 2.45
|
22.4–31.15
|
|
|
27.06 ± 3.53
|
19–35
|
Abbreviations: ANW, anterior notch width; AT, apicotectal distance; IC, interpedunculoclival
distance; MNL, maximum notch length; MNW, maximum notch width; SD, standard deviation.
Conclusion
This study provides the baseline data for the neurosurgeons about the anatomical variations
of the tentorial aperture. Moreover, this morphometric analysis elucidates its clinical
relevance in neurosurgery. The dimensions of the tentorial aperture may determine
the clinical sequelae and prognosis of many neurosurgical conditions.
