Introduction
Tentorial incisura is the hiatus through which supra- and infratentorial spaces communicate. Knowledge about anatomy of tentorium and tentorial incisura is more important for various surgical approaches—both for supra- and infratentorial lesions.
The important neural structures related to tentorial incisura include the cerebrum, cerebellum, olfactory, optic, oculomotor, trochlear, trigeminal, abducent nerve, and midbrain.[1]
Size of the tentorial incisura varies from person to person even in same-age groups. Its size plays an important role in tentorial herniation that occurs due to supratentorial hematoma or supratentorial space-occupying lesion. There are only few studies in the literature analyzing the size of the tentorial incisura and the correlation between the tentorial incisura size and tentorial herniation. There are hardly any studies available in the literature analyzing the tentorial size of Indian population and its clinical importance. In this study, the authors analyzed the size of tentorial incisura in Indian population both in cadaver and in autopsy cases and tried to correlate the size with the outcome in patients with head injury. In autopsy cases, antemortem clinical conditions were recorded, and the relationship between tentorial incisura size and tentorial herniation was analyzed.
Aim of the Study
Aim of the study is to measure the size of tentorial notch in Indian population, compare it with the western population from the literature available, and analyze the relation between size of tentorial notch and the outcome in patients with head injury.
Materials and Methods
The study was initially started with cadavers, as the number of cadavers during the study period was less. Autopsy samples were analyzed, and the tentorial notch anatomy was found to be comparable to that of cadaver samples. Hence autopsy cases were also included in the study.
For cadaveric study, 10 cadavers in the Institute of Anatomy, Madras Medical College and Rajiv Gandhi Government General Hospital, Chennai, were studied.
For autopsy study, 15 specimens in the Institute of Forensic Medicine, Madras Medical College and Rajiv Gandhi Government General Hospital, Chennai, were studied.
Clearance (No. 03022013, dated March 11, 2013) from institutional ethical committee was obtained.
Inclusion Criteria
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Ten cadavers from the Institute of Anatomy, Madras Medical College, during the study period
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Fifteen autopsy specimen from the Institute of Forensic Medicine, Madras Medical College, during the study period
Exclusion Criteria
In all the specimens, tentorial incisura was measured using vernier caliper after opening the skull and cutting the midbrain at the level of tentorial edge carefully without damaging the tentorium ([Figs. 1], [2]). Measurements were done in millimeters.
Fig. 1 Autopsy specimen showing tentorial notch (arrow).
Fig. 2 Cadaver specimen showing tentorial notch (arrow).
Parameters studied were ([Fig. 3]) as follows:
Fig. 3 Various parameters measured in tentorial notch. The black line (1) measures the anterior notch width, the pink line (2) the maximum notch width, the blue line (3) the notch length, the green line (4) the posterior tentorial length, and the red line (5) the interpeduncular clival distance.
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Anterior notch width (ANW) is the width of the tentorial notch in the axial plane through the posterior aspect of the dorsum sellae.
-
Maximum notch width (MNW) is the maximum width of the notch in the axial plane.
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Notch length (NL) is the length of the tentorial notch from the superoposterior edge of the dorsum sellae to the apex of the notch of tentorium.
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Posterior tentorial length (PTL) is the shortest distance between the apex of the notch and the most anterior part of the confluence of sinuses.
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Interpedunculoclival (IC) distance is the distance from the interpeduncular fossa to the superoposterior edge of the dorsum sellae.
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Apicotectal (AT) distance is the distance from the tectum in the median plane to the perpendicular line dropped from the apex of tentorial notch to the cerebellum.
Among the 10 cadavers studied, the authors could measure all the six parameters only in six cadavers. As the brain was liquefied in the remaining four cadavers, IC and AT distances could not be measured.
In autopsy specimens, only ANW, MNW, NL, and PTL were measured as IC and AT distances could not be measured because the midbrain was not rigid as in cadaveric specimens.
To study the association between tentorial size and incidence of coning, demographic details of all the 15 autopsy cases were noted and they were retrospectively analyzed for admission Glasgow coma scale (GCS), computed tomographic (CT) findings, degree of midline shift, and rate of deterioration of GCS. Out of the 15 autopsy cases, 4 deaths were nontraumatic without antemortem brain imaging and 11 deaths were trauma related with positive findings in the antemortem brain imaging.
Out of the 11 traumatic cases,
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Four cases underwent surgery immediately after admission in view of their poor admission GCS with intracranial hematoma with significant midline shift (> 5 mm). These cases succumbed to severe head injury even after adequate management.
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Four cases were initially managed conservatively in view of good GCS and less midline shift (< 5 mm) on admission, but these four cases deteriorated in spite of appropriate medical management, and all these four cases developed midline shift of more than 5 mm on deterioration.
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Three cases were managed conservatively throughout the period because there were no indications for surgery even in follow-up imaging.
Observations
The following analyses were performed from the data collected:
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Mean values of all the parameters in all the 25 cases were calculated and compared with western population in the literature.
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Age-dependent mean value calculated for all the four values: ANW, MNW, NL, and PTL.
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25th percentile (1st quartile), 50th percentile (2nd quartile), and 75th percentile (3rd quartile) values calculated for all the parameters except AT and IC distance because this value was available only for five cadavers not for other cases.
Classification of Tentorial Incisura
Based on MNW and NL values, tentorial incisura was classified into wide, narrow, long, short, typical, large, small, and mixed. Cases of large tentorial incisura have MNW of more than 33 mm and NL of more than 59 mm in contrast to the smaller type with MNW values less than 27 mm and NL values less than 48 mm, as detailed in [Table 1].
Table 1
Classification of tentorial incisura based on MNW and NL
|
S. No.
|
Types
|
Criteria for MNW and NL
|
|
Abbreviations: MNW, maximum notch width; NL, notch length.
|
|
1
|
Wide
|
MNW more than 3rd quartile and NL between 1st and 3rd quartile
|
|
2
|
Narrow
|
MNW less than 1st quartile and NL between 1st and 3rd quartile
|
|
3
|
Long
|
NL more than 3rd quartile and MNW between 1st and 3rd quartile
|
|
4
|
Short
|
NL less than1st quartile and MNW between 1st and 3rd quartile
|
|
5
|
Typical
|
MNW between 1st and 3rd quartile and NL between 1st and 3rd quartile
|
|
6
|
Large
|
MNW more than 3rd quartile and NL more than 3rd quartile
|
|
7
|
Small
|
MNW less than 1st quartile and NL less than 1st quartile
|
|
8
|
Mixed
|
MNW more than 3rd quartile and NL less than 1st quartile or NL more than 3rd quartile and MNW less than 1st quartile
|
The autopsy cases were further divided into five age groups as shown in [Table 2], to study the effect of age on tentorial size. Cases in each group were further classified according to the tentorial incisura size in which they fall, based on previously described tentorial incisura classification.
Table 2
Distribution of cases among various age groups
|
S. No.
|
Age group (y)
|
No. of cases
|
|
1
|
20–25
|
2
|
|
2
|
45–50
|
3
|
|
3
|
51–55
|
5
|
|
4
|
56–60
|
2
|
|
5
|
61–65
|
3
|
For 11 trauma-related deaths in autopsy cases, average deterioration of GCS per day was calculated during the treatment period, and this was compared with the tentorial size to study the correlation between tentorial size and patient deterioration. For this purpose, these cases were divided into two groups based on the tentorial size as
Rate of deterioration of GCS per day was recorded and compared for these two groups.
Statistical Analysis and Results
Mean, median, and standard deviation (SD) values of ANW, MNW NL, PTL, IC distance, and AT distance of all 25 cases were calculated; 25th, 50th, 75th percentile values were calculated for ANW, MNW, PTL, and NL ([Table 3]). Mean values of all the measurements are comparable with western population from the available studies in the literature. There was no significant variation in the size of tentorial incisura found in our part of Tamil Nadu, India.
Table 3
Mean values of ANW, MNW, NL, PTL, IC distance, AT distance, and percentiles of ANW, MNW, NL, and PTL of 25 cases
|
ANW
|
MNW
|
PTL
|
NL
|
AT
|
IC
|
|
Abbreviations: ANW, anterior notch width; AT, apicotectal; IC, interpedunculoclival; MNW, maximum notch width; NL, notch length; PLT, posterior tentorial length; SD, standard deviation.
|
|
Mean
|
27.7024
|
30.7496
|
39.2604
|
53.8484
|
15.7475
|
11.815
|
|
Median
|
28.1000
|
32.0500
|
38.9800
|
56.0000
|
16.42
|
14.36
|
|
SD
|
2.82120
|
3.89572
|
9.49341
|
6.67189
|
11.21627
|
7.580
|
|
25%
|
25.6900
|
27.1400
|
32.3200
|
48.6700
|
|
|
|
50%
|
28.1000
|
32.0500
|
38.9800
|
56.0000
|
|
|
|
75%
|
30.2000
|
33.0750
|
44.3400
|
59.6250
|
|
|
Groups’ Frequency Percent
Frequency distribution of various types of tentorial incisura and percentage of various types of incisura were analyzed in the samples studied ([Table 4]).
Table 4
Frequency and percentage of various types of incisura
|
Groups
|
Frequency
|
Percent
|
|
Abbreviations: MNW, maximum notch width; NL, notch length.
|
|
Wide (MNW more than 3rd quartile and NL between 1st and 3rd quartile)
|
2
|
8
|
|
Narrow (MNW less than 1st quartile and NL between 1st and 3rd quartile)
|
1
|
4
|
|
Short (NL less than 1st quartile and MNW between 1st and 3rd quartile)
|
1
|
4
|
|
Large (MNW more than 3rd quartile and NL more than 3rd quartile)
|
4
|
16
|
|
Typical (MNW between 1st and 3rd quartile and NL between 1st and 3rd quartile)
|
10
|
40
|
|
Long (NL more than 3rd quartile and MNW between 1st and 3rd quartile)
|
1
|
4
|
|
Small (MNW less than 1st quartile and NL less than 1st quartile)
|
5
|
20
|
|
Mixed (MNW more than 3rd quartile and NL less than 1st quartile or NL more than 3rd quartile and MNW less than 1st quartile)
|
1
|
4
|
Out of 25 cases, nearly 10 (40%) of cases had typical incisura type, 5 had small, 4 had large, 2 had wide, and the incisural types narrow, short, long, and mixed type are seen in one specimen each.
Age-dependent mean value was calculated for all the four values (ANW, MNW, NL, PTL) ([Table 5]). It shows that the tentorial size is significantly smaller in the age group between 20 and 25 years (mean value of both ANW and MNW is less than 1st quartile). However, after the age of 45 years, there is no significant age-dependent variation in tentorial size ([Table 6]) and majority are typical. Large incisura was noted in the 45–50 and 51–55 years age groups.
Table 5
Age-dependent mean value
|
Age group (y)
|
ANW mean
|
MNW mean
|
PTL mean
|
NL mean
|
|
Abbreviations: ANW, anterior notch width; MNW, maximum notch width; NL, notch length; PLT, posterior tentorial length.
|
|
20–25
|
22.1800
|
24.0500
|
43.1600
|
41.5000
|
|
40–45
|
29.0333
|
32.2667
|
32.7667
|
57.0667
|
|
51–55
|
29.1760
|
32.1880
|
30.6880
|
60.7380
|
|
61–65
|
28.2333
|
31.7400
|
32.8467
|
54.2167
|
Table 6
Age-wise distribution of tentorial incisura
|
Age group (y)
|
Total no. of cases
|
Wide
|
Narrow
|
Short
|
Large
|
Typical
|
Long
|
Small
|
Mixed
|
|
20–25
|
2
|
|
|
|
|
|
|
2
|
|
|
45–50
|
3
|
|
|
|
1
|
2
|
|
|
|
|
51–55
|
5
|
|
|
|
3
|
|
1
|
|
1
|
|
56–60
|
2
|
|
|
|
|
2
|
|
|
|
|
61–65
|
3
|
|
|
|
|
3
|
|
|
|
All the 11 trauma cases were divided into two groups. All the trauma patients who had large tentorial incisura (MNW more than 3rd quartile and NL more than 3rd quartile) were considered as group 1; they were four patients. Others were considered as group 2; they were seven patients. Mean value of rate of deterioration of GCS per day for each group was recorded and compared ([Table 7]). For the group with wide tentorial incisura, rate of deterioration of GCS was rapid with fall of more than 2 points per day, as compared with other tentorial types.
Table 7
Rate of deterioration of GCS in wide and other tentorial groups
|
Group
|
No. of patients
|
Mean
|
SD
|
|
Abbreviations: GCS, Glasgow coma scale; SD, standard deviation.
|
|
Rate of deterioration of GCS
|
1.00
|
4
|
2.7300
|
0.69995
|
|
2.00
|
7
|
1.0329
|
1.11476
|
t-Test
Statistical analysis was done using t-test that showed that rate of deterioration of GCS is higher in patients who had large tentorial incisura compared with other groups, which is statistically significant (p = 0.024) ([Table 8]).
Table 8
t-Test to assess the significance
|
t
|
Df
|
p Value
|
Mean difference
|
Lower
|
Upper
|
|
Abbreviation: GCS, Glasgow coma scale.
|
|
Rate of deterioration of GCS
|
−2.719
|
9
|
0.024
|
−1.69714
|
−3.10918
|
−0.28511
|
Discussion
The tentorial incisura is the opening that connects the supra- and infratentorial spaces. The incisura is triangular in shape, base is situated anteriorly over dorsum sellae, and apex is situated posteriorly, dorsal to the midbrain and pineal gland. The incisural space is divided into anterior incisural, middle incisural, and posterior incisural spaces.[2] First one is anterior incisural space that is situated anterior to the brainstem, second is paired middle incisural space that is located lateral to the brainstem, and posterior incisural space that is located posterior to the brainstem. Some variations may be there in the tentorial incisura for some people because of the developmental defects[3] of tentorium.
Tentorial herniation is the most common and important form of brain herniation. In descending herniation caused by the supratentorial mass lesions, uncus and parahippocampal gyri herniate downward through the incisura, and in ascending herniation,[4] superior part of cerebellum herniates upward through the tentorial incisura. This brain herniation causes compression of vessels and stretching and compression of the brainstem and cranial nerves.[5] Compression of arteries causes arterial infarction and compression of vein causes venous infarction, increasing the edema and elevation of intracranial pressure (ICP). In herniation of the tentorial incisura and foramen magnum, it is believed to be caused by the progressive obliteration of subarachnoid space with the mass lesion. Thus, the important factor in preventing or treating herniation of the brain is maintenance or reestablishment of the subarachnoid space at the site of the herniation.[6] Type of tentorial herniation depends on the position and rate of expansion of the lesion and size and shape of the incisura. Signs appear early when structures are deformed rapidly, but in chronic herniation, clinical signs develop very slowly. Wide tentorial notch, while not only increasing the chances of herniation, also causes more brain tissue to impact on the brainstem. Descending herniation is subdivided into anterior, posterior, and complete types. In the anterior type, uncus herniates to the interpeduncular and crural cisterns. This shift pushes the brainstem to opposite side and thus increases the space between free edge and the brainstem, thus facilitating a further shift of tissue through the tentorial aperture. As a result, the parahippocampal gyrus, from the splenium to the uncus, forces through the opening and the incisura becomes packed with herniated temporal lobe, deformed hypothalamus, and compressed midbrain. Compression of the midbrain reticular activating pathways causes decreased level of consciousness. Compression of ipsilateral cerebral peduncle causes the contralateral pyramidal signs, and if the lateral displacement of brainstem is very severe, contralateral cerebral peduncle is forced against the free edge, thus producing a groove on the peduncle called a Kernohan's notch, with the ipsilateral pyramidal signs. In the terminal stage, deformation of the mid-brain causes decerebrate rigidity. Distortion and compression of the posterior hypothalamus cause cardiovascular, respiratory, and thermoregulatory[7] disturbances. Pituitary stalk may stretch and compress against the dorsum sellae, thus causing diabetes insipidus. Oculomotor nerve courses between medial border of the uncus and the posterior petroclinoidal fold and may be kinked or pressed here or between the posterior cerebellar artery (PCA) and superior cerebellar artery (SCA), or it may be stretched as the hernia pushes the midbrain posteriorly. First, the pupilloconstrictor fibers, which are concentrated over the superior surface of the nerve, are compressed. Later, somatic nerve fibers to the extraocular muscles are disturbed. In the initial stages, irritation of pupilloconstrictor fibers causes pupillary constriction, but later this usually gives way to a paralytic effect with pupillary dilation when the hernia enlarges. Optic tract is displaced medially and below, but the resulting visual loss is often masked by the deepening coma. Compression of the uncus, amygdala, parahippocampal gyrus, and hippocampal formation against the free edge of tentorium may cause memory, behavioral, and personality changes. Scarring of the hippocampal formation may cause seizures. Trochlear nerve usually escapes involvement in such herniation, but downward displacement of the brainstem may result in a palsy of abducent nerve by stretching it. Uncal herniation at the level of the notch was characterized and termed as “temporal pressure cone,” “tentorial pressure cone,” and “transtentorial herniation of the brainstem”[8]
[9]
[10] that were applied to this pathologic process. Experimentally, the group of signs and symptoms associated with herniation were reproduced in the monkey[11] and pathologically examined by Scheinker,[10] Munro and Sisson,[12] and Schwarz and Rosner.[13] Neuropathologic sequelae of tentorial herniation were found and demonstrated grooving and contralateral displacement of the brainstem, medial shift and herniation of the hippocampal gyrus, compression of the third cranial nerves, and brainstem hemorrhages. In 1958, Sunderland[14] classified tentorial notches as broad or narrow, and Corsellis[15] suggested that the patterns of herniation should be affected by notch size and shape. Although Sunderland and Corsellis demonstrated anatomical variations in the tentorial notch and the relation to the brainstem, well-defined classification system was not developed and no hypotheses were presented to explain patterns of transtentorial herniation and the variable clinical sequelae that arise after concussive and inertial brain injury. There are only a few studies in the literature, analyzing the tentorial incisura and classification of incisura, and none of those are from Indian population. Keeping in mind that there may be little demographic variations in the anatomy, the authors decided to analyze tentorial incisura in the Indian population. The studies available in the literature analyzed only the cadaveric tentorial incisura, but no studies were available in the literature analyzing fresh autopsy specimen. This study analyzed both cadaveric and autopsy specimen's tentorial incisura.
Notch Width and Transtentorial Herniation
The clinical manifestations and neuroanatomical distortions occurring in tentorial herniations are well established by several studies in the literature. Neuropathologic sequelae of tentorial herniation were found and demonstrated grooving and contralateral displacement of the brainstem, medial shift and herniation of the hippocampal gyrus, compression of the third cranial nerves, and brainstem hemorrhages. Despite the clinical pathologic understanding of transtentorial herniation, the reasons for the different signs of neurologic deterioration that appear among patients remain unclear. Similarly, the anatomical mechanism for the occurrence of the false localizing sign is understood, but its incidental occurrence has been difficult to explain. Dissimilarity in tentorial notch morphology and regional anatomy may help elucidate this phenomenon. It has been demonstrated that long and wide notches contain a greater amount of cerebellar tissue than short and narrow notches. Some variations may be there in the tentorial incisura for some people because of the developmental defects[3] of tentorium. The relationship between tentorial incisura type and other herniations is not studied.
Tentorial incisura can be studied in detail with magnetic resonance imaging (MRI) noninvasively. The authors can anticipate sudden deterioration in patients having large tentorial incisura as per the observations in this study and in the literature. That way it will benefit the management of patients with head injury.
The authors also noticed in this study a strong relationship between large apertures and fast deterioration of patients because of tentorial herniation that is supported by the study done by Rhoton.[2] These findings may have implications regarding the propensity for transtentorial herniation of cerebellar or cerebral parenchyma in the rostral or caudal direction, respectively. Thus, the important factor in preventing or treating herniation of the brain is maintenance or reestablishment of the subarachnoid space at the site of the herniation occurrence.[16] For the surgery of the lesions around the incisura, incising the tentorium or retraction of tentorium is necessary most times. For the incision of the tentorium, incision should be posterior to the point of tentorium where trochlear nerve joins the tentorial edge. Sectioning of the tentorium also can be used rarely for reducing the supratentorial pressure and the pressure over the brainstem in the situation where the original pathology cannot be removed. This procedure was first used by Cushing for the control of ICP.
From this study, the authors have noticed that patients with head injury admitted with good GCS (GCS > 13) who had large tentorial incisura subsequently deteriorated faster compared with others who have lesser tentorial size, which is statistically significant.
From this study, the authors found that patients with good GCS and midline shift of less than 5 mm deteriorated fast and severely (implying massive brain tissue herniation) because of the availability of large tentorial notch. Hence it is worthwhile to ponder whether the accepted guidelines of more than 5 mm as the surgical indicator for decompression will hold good for the patients with large tentorial notch. Perhaps they require surgical intervention for the midline shift of less than 5 mm. This theory has to be validated with larger study and fast and reliable method of measuring the tentorial notch radiologically. This may produce new algorithm for managing patients with midline shift.
