Structural changes in the anterior visual pathway and its correlation with functional visual outcome: a case-control study of children with optic pathway gliomas
Higgins, Bethany E.
(2018)
Structural changes in the anterior visual pathway and its correlation with functional visual outcome: a case-control study of children with optic pathway gliomas.
MRes thesis, University of Nottingham.
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Abstract
Background
Optic Pathway Gliomas (OPGs) are a type of benign tumour found in paediatric cohorts that are typically located in the anterior visual pathway and can result in irreversible visual impairment. Suffering from low vision has a significant negative impact on physical and emotional health, leaving many patients vulnerable and dependent on others (1). Research has been conducted that links structural changes in optic nerves to decreased levels of visual function in diseases such as multiple sclerosis (2,3), but to the author’s knowledge, no investigation has been completed into the correlation of optic nerve size and visual function in children with OPGs.
Methods
A systematic literature review was conducted to investigate what method of assessing optic nerve size was the most reliable. This highlighted the need for three distinct, observational studies. The first retrospective, observational experiment investigated if cross-sectional area assessment of optic nerve size was reliable in measuring the optic nerves of 22 children with OPGs (aged 2-15 years). Optic nerve size was assessed from retrospective, structural MRI scans. The second observational experiment analysed the variation of optic nerve sizes in 30 healthy children (aged 5-16 years) in relation to their age, visual acuity and height. Optic nerve size was assessed from structural MRI scans and visual acuity scores were collected using the Sheridan Gardiner test. The third case-control, observational experiment investigated the correlation between optic nerve size, average retinal nerve fibre layer (RNFL) thickness and visual acuity in 27 children with OPGs (aged 2-15 years). Moreover, the variation of optic nerve size and visual acuity according to age was compared between children with OPGs and 30 healthy controls (aged 5-16 years). Data from the previous studies were utilised, coupled with retrospective visual function assessments of visual acuity and OCT results.
Results
A measure of optic nerve size using a cross-sectional area technique was established to be reliable using a coefficient of variation and an intraclass correlation coefficient measure. Mean optic nerve size in the healthy paediatric cohort was 8.798mm² and it was found to negatively correlate with both age (R²=0.396) and height (R²=0.393). This is the first recording of maturation in a paediatric cohort demonstrating axonal decline, to the author’s knowledge. In children with OPGs, optic nerve size was found to not correlate with RNFL thickness, age or visual acuity across all data in this dataset. However, the data for the left eyes did exhibit a significant negative correlation between these variables, and both optic nerve size and RNFL thickness were found to significantly predict visual acuity (R² = 0.402). RNFL thickness was also confirmed to correlate with visual acuity in both the left (R²=0.380) and the right eyes (R²=0.394). When optic nerve size and visual acuity of children with OPGs and the healthy cohort were compared, the OPG group was found to have significantly smaller nerves and poorer visual function. Lastly, despite issues with small sample sizes throughout the studies due to the difficulties in collecting usable MR images, information regarding faster or more motion resistant sequences have been accrued which will guide future paediatric imaging of awake cohorts.
Conclusion
A measurement method of optic nerve size has been validated as reproducible and has aided the researcher in demonstrating a previously unrecorded, negative correlation between optic nerve size with height and age in a healthy cohort. In the OPG cohort, this measurement method was used to find that optic nerve size was significantly smaller than in the healthy cohort, and that RNFL thickness correlated with visual acuity which was supported by the literature, while optic nerve size did not correlate with visual acuity. Moreover, RNFL thickness and optic nerve size were found to predict visual acuity, with RNFL thickness being the more significant predictor. Although, as this study was limited by a small sample sizes, these provide the preliminary findings on which to base a larger study.
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