Keywords
uncal herniation - neuroimaging - novel ratio - risk prediction - CT head - traumatic brain injury
Introduction
Management of traumatic brain injury (TBI) has been a subject of research for thousands of years with the first documented head injury dating back to 1600 BC when Edwin Smith papyrus took note of it and officially documented it in his surgical treatise.[1] Uncal herniation, also known as transtentorial herniation, suggests the substantial displacement of uncus, a small in-turned continuation of the parahippocampal gyrus in the anteromedial aspect of the temporal lobe,[2] toward the midbrain causing its compression along with that of the brainstem and subsequently resulting in respiratory and circulatory center failures and death eventually. It was originally Meyer[3] who identified and recognized transtentorial coning as a cause of brainstem compression. Clinically uncal herniation manifests as Cushing's reflex, presenting with high blood pressure and reflexive bradycardia. The hallmark of uncal herniation is a fixed and dilated ipsilateral pupil due to compression of the ipsilateral oculomotor nerve.[4]
[5]
On head computed tomography (CT), early signs of uncal herniation include encroachment on the suprasellar cistern, displacement of the brainstem, enlargement of the ipsilateral crural subarachnoid space, and compression of the contralateral cerebral peduncle.[6] It has been observed that tentorial anatomy, even though varying from person to person, has a clear influence on brainstem distortion due to herniation from TBI.[7] Since there are multiple variables in morphometry of tentorial anatomy, we propose a method of calculation to standardize the difference in variation by evaluating a novel ratio that we hope can offer an actual range of defined measurements and help us predict the risk of uncal herniation in patients with TBI.
Materials and Methods
This was a prospective study done in our department over 2 months (August to October 2024). The brains of all patients with severe TBI (Glasgow coma scale [GCS] <9) who were admitted under neurosurgery and died in the hospital were removed during autopsy to see for uncal herniation. The head CT at admission of these patients was also evaluated and the inter-clinoidal distance (ICD) and the inter-tentorial distance (ITD) was measured using the following landmarks:
Measure the ITD at the 50% mark on the line between the dorsum sellae and the basilar artery. This usually coincides with the level of the uncus.
The ICD/ITD ratio was calculated for all patients based on the above measurements ([Fig. 1]).
Fig. 1 Diagrammatic representation showing sphenoid bone, brainstem with basilar artery, and tentorium. The purple arrow shows the inter-tentorial distance, which is measured from the 50% point on a line drawn from the dorsum sella to the basilar artery (green arrow) on the axial slice of the computed tomography (CT) of the head. The dark blue arrow shows the inter-clinoidal distance (between the tips of two anterior clinoid processes).
Results
A total of 17 patients with severe TBI died during the study period and autopsy was performed on them. Of these, 12 patients did not have any uncal herniation. The majority of these patients were males (91.6%) with only one female (8.3%). Five patients had uncal herniation, and in this group, three (60%) were males and two (40%) were females.
ICD/ITD Ratio in the “No Uncal Herniation” Group
In patients without uncal herniation, the average IACD was 2.37 ± 0.15 ([Fig. 2]; [Table 1]). The average ITD was 2.42 ± 0.17. The average IACD-to-ITD ratio was 0.97 ± 0.08.
Fig. 2 Ratio of the inter-clinoidal distance (ICD) to the inter-tentorial distance (ITD) in patients without uncal herniation. IACD, inter-anterior clinoid process distance.
Table 1
Age, sex, ICD, ITD, and ICD/ITD ratio in patients without uncal herniation
|
Case no.
|
Age (y)
|
Sex
|
ICD (cm)
|
ITD (cm)
|
ICD/ITD
|
|
1
|
45
|
M
|
2.20
|
2.33
|
0.93
|
|
2
|
45
|
M
|
2.04
|
2.51
|
0.81
|
|
3
|
32
|
M
|
2.41
|
2.58
|
0.93
|
|
4
|
38
|
M
|
2.42
|
2.30
|
1.05
|
|
5
|
16
|
M
|
2.39
|
2.49
|
0.95
|
|
6
|
27
|
M
|
2.22
|
2.14
|
1.03
|
|
7
|
24
|
M
|
2.6
|
2.40
|
1.08
|
|
8
|
59
|
F
|
2.35
|
2.61
|
0.90
|
|
9
|
57
|
M
|
2.56
|
2.68
|
0.95
|
|
10
|
21
|
M
|
2.31
|
2.12
|
1.08
|
|
11
|
27
|
M
|
2.48
|
2.33
|
1.06
|
|
12
|
12
|
M
|
2.49
|
2.62
|
0.95
|
Abbreviations: ICD, inter-clinoidal distance; ITD, inter-tentorial distance.
ICD/ITD Ratio in the Uncal Herniation Group
The average IACD was 2.49 ± 0.069 ([Fig. 3]; [Table 2]). The average ratio of IACD to ITD was 1.67 ± 0.42. Examples of some of these measurements can be seen in [Fig. 4].
Fig. 3 Axial slice, head computed tomography scan showing the inter-clinoidal distance (ICD) and inter-tentorial distance (ITD) along with measurements in nonuncal herniation patients. (A) Inter-anterior clinoidal distance (red arrow), ITD (green arrow), and basilar artery (yellow arrow). (B) The ICD at the tip of anterior clinoid (yellow line). (C) Measuring the ITD at the midpoint of the line joining the mid-dorsum sellae to the basilar artery.
Table 2
Age, sex, ICD, ITD, and ICD/ITD ratio in patients with uncal herniation
|
Case no.
|
Age (y)
|
Sex
|
IACD
|
ITD
|
ICD/ITD
|
|
1
|
22
|
F
|
2.47
|
1.61
|
1.53
|
|
2
|
39
|
M
|
2.41
|
1.59
|
1.59
|
|
3
|
26
|
M
|
2.62
|
1.04
|
2.5
|
|
4
|
52
|
M
|
2.51
|
1.7
|
1.47
|
|
5
|
65
|
F
|
2.48
|
1.9
|
1.30
|
Abbreviations: IACD, inter-anterior clinoid process distance; ICD, inter-clinoidal distance; ITD, inter-tentorial distance.
Fig. 4 Pictures demonstrating similar measurements in patients with uncal herniation. (A, C) Measuring the inter-tentorial distance (between the visible tentorial border and the herniated uncus). (B, D) Measuring the inter-anterior clinoidal distance.
Discussion
TBI, whether mild or severe, has the capacity to initiate a cascade of functional processes that can have a direct effect on the anatomical domain, thereby leading to outcomes such as contusions, edema or infarcts, and distort the otherwise normal spaces in the brain. Knowing the “who” to treat and “when” to treat is the foundation to a sound neurosurgical treatment. The present study represents a proposed method to predict risk of uncal herniation in patients with TBI.
The variation in tentorial anatomy has been long considered to have some role in the large spectrum of variable presentations of uncal herniation. Although controversial and still debated, tentorial anatomy and its association with risk of transtentorial herniation is not well understood and only few studies have evaluated the tentorial hiatus size and its clinical implication.[8]
[9]
[10] We could find only one study that analyzed the morphometric variation in tentorial notch anatomy in autopsy and CT of the head injury patients to see for a correlation between tentorial notch anatomy and uncal herniation and changes over the brainstem surface in case of uncal herniation. The authors did not find any statistically significant difference in tentorial notch parameters in autopsy and on CT of the head in the uncal and nonuncal herniation groups.[11]
In this study, we have used a novel ratio (ICD/ITD) to identify and recognize patients with tentorial anatomy relatively favoring a higher risk of uncal herniation should they be predisposed to cranial trauma and therefore help in predicting the risk of herniation in such patients based on a basic neuroimaging modality such as CT of the brain to formulate an apt management strategy for the benefit of the patients.
Conclusion
Our method provides a simple and practical way to assess the tentorial hiatus on the head CT using the novel ICD/ITD ratio. An ICD/ITD ratio above 1.2 indicates higher risk of uncal herniation in patients with TBI.
