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ORIGINAL ARTICLE |
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| Ahead of print
publication |
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Position of tonsillar tip in relation to foramen magnum in children with structurally normal MRI brain imaging
VenkataPravallika Putrevu, Unais Chettian Thodika, Yatin Vinodray Raiyani, Balamurugan Mangaleswaran
Department of Neurosciences, Apollo Hospitals, Chennai, Tamil Nadu, India
| Date of Submission | 11-Jun-2020 |
| Date of Decision | 27-Aug-2020 |
| Date of Acceptance | 01-Oct-2020 |
| Date of Web Publication | 02-Jul-2021 |
Correspondence Address:
VenkataPravallika Putrevu,
L-102, L Tower, The Metrozone, Anna Nagar, Chennai 600040, Tamil Nadu.
India
 Source of Support: None, Conflict of Interest: None DOI: 10.4103/JPN.JPN_154_20
 Abstract | | |
Introduction: The position of the tonsillar tip in relation to the foramen magnum varies from individual to individual, even after excluding all predisposing factors that might alter the tonsillar tip position; the mere presence of the tonsillar ectopia should not be misinterpreted as Chiari malformation. Aim: Our aim is to establish the normal range of the tonsillar tip position and the incidence of tonsillar ectopia and its relation to age and sex in asymptomatic Indian children with structurally normal MRI brain imaging. Materials and Methods: We have conducted a retrospective data collection comprising 301 structurally normal MRI brain images done for children younger than 14 years, in which we measured the level of the tonsillar tip from the foramen magnum by taking the Mc Rae line as constant. Measurements were taken in the right and left parasagittal sections of T1-weighted MRI brain images by using pre-existing software. Statistical Analysis Used: Data analysis was done by SPSS v 25.0; all P values <0.05 were considered statistically significant. Results and Conclusion: The incidence of tonsillar ectopia in Indian children was about 13.79%, and the median tonsillar position was 3.2 mm above the foramen magnum with a median range of 5.2 mm above to the level of the foramen magnum (0).
Keywords: Chiari malformation, foramen magnum, Mc Rae line, parasagittal sections, tonsillar ectopia
| Introduction | |  |
The position of the cerebellar tonsil tip in relation to the foramen magnum varies from individual to individual. It, in turn, depends on various other factors such as anatomical variations or malformations, size and shape of posterior fossa, cranio-vertebral junction anomalies, presence of any space-occupying lesion in the brain, hydrocephalus, etc. The purpose of our study was to establish the normal range of the tonsillar position and the incidence of tonsillar ectopia in asymptomatic Indian children and its relation to age and sex, after excluding all predisposing factors that might cause tonsillar herniation or ectopia, as the mere presence of the tonsillar tip below the level of the foramen magnum can be misinterpreted as Chiari malformation More Details.[1] There are no similar data available till date in the Indian population.
| Subjects and Methods | | |
We have conducted a retrospective data collection comprising 301 structurally normal MRI brain images done for children younger than 14 years in a tertiary care center, in which we measured the level of the tonsillar tip from the foramen magnum, by taking the Mc Rae line (the line extending from opisthion to basion, i.e. from the lowest point of the cortical bone seen at the posterior lip of the foramen magnum to the lowest point of the cortical bone of the clivus)[2],[3] as constant. Measurements were taken on both right and left parasagittal sections of T1-weighted MRI brain images. The presence of the tonsillar tip at the Mc Rae line, that is, at the level of the foramen magnum was marked as 0. The presence of the tonsillar tip above the Mc Rae line was measured in millimeters with a negative value and below the Mc Rae’s line it was measured in millimeters with a positive value. [Figure 1] and [Figure 2] demonstrate the Mc Rae line and measurement of tonsillar level from Mc Rae line. [Figure 1] being an example case for tonsillar level below foramen magnum, and [Figure 2] shows an example case with tonsillar level above foramen magnum. The position of the tonsil in about 602 tonsils was measured, including right and left tonsils for each subject. Overall, 301 MRIs that were reported to be structurally normal were randomly selected from about 1109 MRIs done in our institute during the past 1 year for children younger than 14 years, after excluding all those MRIs with anatomical or structural abnormalities and any predisposing factor that might alter the tonsillar position. The indications for doing an MRI for these children as obtained from the presenting complaint for which the MRI was done are, for example, seizures (116 subjects), developmental delay (65 subjects), behavioral abnormalities (30 subjects), headaches (60 subjects), birth asphyxia (14 subjects), and cerebral palsy (16 subjects). Though these children with developmental delay, birth asphyxia, or cerebral palsy had functional abnormalities, their MRI brain imaging did not show any structural / anatomical abnormalities or any predisposing factors that might lead to tonsillar descent; hence, they are considered to possess structurally normal brains and presumed to reflect the normal population. Measurements were taken to an accuracy of one decimal point in millimeters on digital images by using pre-existing software. | Figure 1: T1 sagittal section of the MRI brain of an 11-year-old boy who presented with headache. The MRI brain reported to be a normal study showing the presence of a tonsillar tip 5 mm below the Mc Rae line
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 | Figure 2: T1 sagittal section of the MRI brain of a 14-year-old girl who presented with headache. The MRI brain reported to be a normal study showing the presence of a tonsillar tip 4.1 mm above the Mc Rae line
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Statistical methods
All continuous variables were represented as mean + SD. Categorical variables such as sex, incidence, etc. are expressed as percentage. A comparison of continuous variables was done by the independent-sample t test; a comparison of categorical variables was done by the chi-square test. Data entry was done in an S Excel spreadsheet. Data analysis was done by SPSS v 25.0. All P values <0.05 were considered statistically significant.
| Results | | |
Out of the 602 measured values of the tonsillar position, 83 were below the level of foramen magnum, giving a total incidence of 13.79% of tonsillar ectopia. In our total study population, 18 children were younger than 1 year of age, 86 were between 2 and 5 years, 111 were between 6 and 10 years, and 86 were between 11 and 14 years. [Figure 3] shows a pie chart representing age group wise percentage among the total study population. Among them, the highest incidence of tonsillar ectopia was observed among the 11 to 14 years age group, that is, 20.9% and the least incidence was seen in infants, that is, 2.8%. [Figure 4] shows Turkey’s hinges showing age groupwise frequencies of tonsillar position. Among the 301 subjects, 116 were girls and 185 were boys. There is a slightly higher incidence of tonsillar ectopia in boys (14.5%) than in girls (12.6%).  | Figure 4: Turkey’s hinges showing age groupwise frequencies of tonsillar position
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The range of tonsillar position among the 301 subjects ranged from 19.1 mm above the foramen magnum to 14 mm below the foramen magnum. [Table 1] illustrates age group wise number of tonsils below and above foramen magnum. The median tonsillar position was 3.2 mm above the foramen magnum with a range of 5.2 mm above the level of the foramen magnum (0) [Table 2], and the mean tonsillar position in the study population was 3.06 mm above the foramen magnum (SD - 4.12).
As observed in our study, tonsillar position was seldom bilaterally symmetrical, with one of the tonsils being low-lying than the other or there can only be unilateral tonsillar ectopia. In our study, only 29 subjects had bilateral tonsillar ectopia whereas the remaining 25 subjects had only unilateral tonsillar ectopia. The mean difference in the position of two tonsils in patients who had unilateral tonsillar ectopia is 2.65 mm, with a range of 0.7 mm to 6.5 mm. The mean difference in the position of two tonsils in all the population studied is 1.47 mm, with a range of 0 mm to 8.8 mm. However, there is no observable clinical importance of having unilateral or bilateral tonsillar ectopia.
| Discussion | | |
The foramen magnum is an oval opening within the occipital bone: It has three parts, the squamosal part, the basal or the clival part, and the one that connects these two parts is the condylar part. It is divided into two parts: a narrow anterior part that sits above the odontoid process and a wider posterior part through which the contents of the foramen magnum pass.[4],[5]
The suboccipital surface of the cerebellum lies above the posterio-lateral edge of the foramen magnum. The lower part of the cerebellar hemisphere that is formed by the tonsils and the biventral lobules and the lowest part of the vermis that is formed by the nodule, uvula, and pyramid are related to the foramen magnum. Each tonsil is an ovoid structure that is attached along its superolateral border to the remainder of the cerebellum. These are the most common structures involved in herniation.[4]
Patients with tonsillar herniation through the foramen magnum may be asymptomatic or may present with signs of neural compression, raised intracranial pressure or even have a sudden unexpected death. The signs of neural compression include pain in the neck and upper arms, giddiness, ataxia, and disturbances in gait, double vision, tinnitus, hardness of hearing, difficulty in swallowing, nystagmus, weakness up to the degree of quadriparesis, and sensory deficits in the extremities. These symptoms were caused by dysfunction of the cerebellum, brain stem, and lower cranial and upper spinal nerves. Acute or chronic herniation can be seen with posterior fossa or any intracranial space-occupying lesions. Chronic herniation is seen with Arnold-Chiari malformation More Details.[4]
In our study, the incidence of tonsillar ectopia in children younger than 14 years of age was about 13.79%, and the median tonsillar position was 3.2 mm above the foramen magnum with a range of 5.2 mm above to the level of foramen magnum (0).
A similar study done in 1986 on US population, in which they studied 200 normal MRI brain images.[6],[7],[8],[9],[10],[11],[12],[13],[14],[15],[16],[17],[18] The results of both these studies were illustrated below.
Additional data on spinal screening of these children to look for spinal syrinx, which can lead to the diagnosis of Chiari malformation,[1] and clinical follow-up on these children to see whether they have developed any symptoms related to tonsillar ectopia in future, can provide additional help to confirm the observations of our study. However, there is no significant documented evidence that an asymptomatic tonsillar herniation in children causes fresh neurological symptom(s) in later life.
| Conclusion | | |
The incidence of tonsillar ectopia in Indian children was about 13.79%, and the median tonsillar position was 3.2 mm above the foramen magnum with a range of 5.2 mm above to the level of the foramen magnum (0). The mere presence of tonsillar ectopia cannot be concluded as Chiari malformation.[19],[20],[21]
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3]
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