Computed tomography morphometric analysis of the central clival depression and petroclival angle for application of the presigmoid approach in the pediatric population.

AIMS: Lateral transtemporal approaches are useful for addressing lesions located ventral to the brainstem, especially when the pathologic diagnosis of the tumor dictates that a gross or near total resection improves outcomes. One approach, the presigmoid approach receives little attention in the pediatric population thus far. We sought to characterize morphometric changes, particularly the clival depth and the petroclival Cobb angle, that occur in the temporal bones of children and draw implications about doing a presigmoid approach in children. SETTINGS AND
DESIGN: This study was a retrospective study performed at John Sealy Hospital, a level-one trauma center that takes care of pediatric injuries as well. SUBJECTS AND METHODS: We performed a morphometric analysis of noncontrast computed tomography head studies in 96 boys and 67 girls. Central clival depth and petroclival angle were obtained in the axial plane at the level of the internal auditory meatus using the method described by Abdel Aziz et al. STATISTICAL ANALYSIS USED: Descriptive statistics and Student's t-test to compare groups were calculated using Microsoft Excel.
RESULTS: We found no gender difference in mean central clival depth or petroclival angle (P = 0.98 and P = 0.61, respectively). However, when we broke our cohort by age into those younger than 9 years of age and those 10 years or older, we found the petroclival angle decreased by 6.2° which was statistically significant (P < 0.000000006).
CONCLUSIONS: These findings suggest that a presigmoid retrolabyrinthine approach is useful for children 9 years of age and younger as the petroclival angle appears to decrease resulting in a shallower clival depression in these patients.

Introduction

A majority of pediatric brain tumors occur in the posterior fossa.[1] Although uncommon, some tumors such as ependymomas can have ventral prepontine extension.[2] There is a relative paucity of literature and experience using lateral transtemporal approaches that could directly access these ventral brainstem lesions in the pediatric population.[34] Considering that the extent of surgical resection is the most important prognostic factor in many tumors such as medulloblastomas[5] and ependymomas, the need appears to be more pressing.

Subjects and Methods

Morphometric analysis was performed on the skull base in 163 children. Patients were selected from all children who underwent a computed tomography scan of the head at John Sealy Hospital between July and October 2015. Exclusion criteria included patients >18 years of age, craniosynostosis, other congenital deformities of the skull base, and radiographic evidence of trauma, neoplastic, or inflammatory disease, infection, or previous surgery at the region of interest.

The central clival depth was measured by the method described by Abdel Aziz et al.[6] Briefly, the central clival depth is measured first by drawing a line between the floor of both internal auditory canals (the intermeatal line). Next, a perpendicular line is taken to the intermeatal line is drawn ending at the apex of the clivus. The distance of this line was recorded.[6] The petroclival angle was measured by taking the creating one limb extending through both posterior clinoids and another limb being parallel to the course of the petrous ridge. We also created a new reference angle that we have termed the, “petrous Cobb angle.” This is measured taking a line parallel to the left petrous ridge and comparing with a similar right

Angular measurements were obtained as described in Figure 1 with the standard measurement palette in our picture archiving and communications system (Phillips iSite; Foster City, CA, USA). Angular measurements were automatically rounded to the nearest degree.

Figure 1

Demonstrates how measurements were obtained on noncontrast computed tomography head. The clival depth is measured by first creating a horizontal line between the internal auditory canals of each petrous bone, then dropping a perpendicular line to the clivus which is shown in orange. The petroclival angle, shown in green, is an angular measurement created using a horizontal line between the posterior clinoids and another line parallel with the petrous ridge. The petrous Cobb angle, seen in yellow, is also an angular measurement created by drawing parallel lines to the petrous ridge on each side

Comparisons based on age, year by year, were recorded. Descriptive statistics and Student's t-test to compare groups were calculated using Microsoft Excel.

Results

There were 96 boys and 67 girls included in our analysis. There was no gender difference in mean central clival depth or petroclival angle (P = 0.98 and P = 0.61, respectively). Males and females both had a mean central clival depth of 17.3 mm. Males had a petroclival angle of 127°, whereas females had a petroclival angle of 126° [Figure 2]. Males had a petrous Cobb angle of 101°, whereas females had an angle of 100°.

Figure 2

Clival depth measured in millimetres of both males and females plotted against age. A Pearson's coefficient for this graph was 0.59

The mean age of the study group was 10 years (range, 1 day to 18 years). When we broke our cohort by age into those younger than 9 years of age and those 10 years or older, we found the petroclival angle decreased by 6.8° from 129.9° to 123.1°. This was a statistically significant result (P < 0.000000006). The petroclival Cobb angle also decreased significantly in those 10 years and older compared with those 9 years and younger (P < 0.0000017). There was no difference central clival depth between those 9 years of age and those 10 years or older (P = 0.12) [Figure 3]. A Pearson's correlation of clival depth and age was performed which demonstrated an r value of 0.59 [Figure 4].

Figure 3

Petroclival angle measurement as described by Abdel Aziz of both males and females plotted against age with a line of best fit (r = −0.52)

Figure 4

A combined petrous Cobb angle of all males and females plotted against age. Pearson's coefficient or R was – internal auditory canals 0.44

Discussion

The presigmoid retrolabyrinthine approach uses the transtemporal route to address lateral pathology located between the internal acoustic meatus and the upper border of the jugular tubercle. The medial portion of this zone termed by some authors' as the, “central clival depression,” cannot be exposed due to remaining temporal bone structures such as labyrinthine and cochlea which obscures the anatomy.[6] While resectability of a tumor is multifactorial depending on the nature, location, and extension of the tumor, visualization afforded by an approach is critical. To the best of our knowledge, no studies have been published analyzing changes of central clival depression, petroclival angle, and petrous Cobb angle all of which are critical features in determining resectability of a tumor, in the pediatric age group.

We hypothesize the reason for the changes are due to continuous growth of petrous bone throughout childhood. Unlike the cranial vault and facial bones, which are purely membranous in origin, the skull base is composed of chondrocranium and has endochondral growth, the process which drives lengthening of bones. This endochondral growth continues throughout the body until adolescence. The petrous bone starts off with six ossification centers that appear at the 5th fetal month. They are located above the round window, ampulla of the semi-circular canal, vestibule, internal acoustic meatus, petrous apex, and the arcuate eminence. The ossification of the facial canal starts at the 6th fetal month, although the geniculate and tympanic portions are not complete until 1 year of age.[7]

Conclusion

A presigmoid retrolabyrinthine approach is useful for approaching ventral tumors located between internal acoustic meatus and jugular tubercle in the children. Its main limitation is a lack of visualization of the volume in the central clival depression. However, this does not appear to be the case in children 9 years of age and younger as the petroclival angle appears to decrease resulting in a shallower clival depression in these patients.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.