Heeren, Tjebo Frédéric Chomé; (2022) Visual dysfunction in macular telangiectasia type 2. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Macular telangiectasia type 2 (macular telangiectasia type 2) is a bilateral neurodegenerative condition of the macula of the human eye which can lead to loss of central vision. There is evidence that metabolic dysfunction leads to slow degeneration of the retinal neuroglia, eventually leading to circumscribed loss of neuronal tissue (photoreceptor atrophy). A characteristic feature of macular telangiectasia type 2 is the temporal epicenter where the disease typically begins, and its limitation to a central oval shaped area of approximately five by ten degrees, called the macular telangiectasia type 2 area. Knowledge about visual function of people with macular telangiectasia type 2 was limited to visual acuity testing, investigations of reading performance, visual field testing with fundus-controlled perimetry (microperimetry), and scotopic perimetry (not fundus-controlled). This thesis summarises research aimed at exploring visual function in macular telangiectasia type 2 in more detail. In particular, visual acuity and reading performance are investigated in more detail, the (para)central scotomas are better characterised, and visual function in low light is elucidated by testing contrast sensitivity, low luminance visual acuity and dark-adapted microperimetry. Visual acuity data was collected as part of the international research collaboration The macular telangiectasia type 2 study, which started in 2005 and has since then accrued data of more than 3000 individuals with macular telangiectasia type 2. It was taken with Early Treatment of Diabetic Retinopathy Study (ETDRS) charts on a harmonised protocol. Distribution of visual acuity in the entire study cohort was investigated and eyes with low visual acuity were looked at in detail. It was found that only about half of eyes with very poor visual acuity showed evidence of neovascularisations, until recently still considered disease end stage, but nearly all eyes showed photoreceptor atrophy, which is therefore more likely to define the disease end stage. Scotomas were characterised further on retrospective analysis of microperimetry examinations from four large centers of the macular telangiectasia type 2 study. This analysis confirmed previous data which suggested mono-focality of the scotomas and the limitation to a specific size. Further microperimetry assessment was performed with a recently introduced new technology, allowing dark-adapted microperimetry with two wavelengths, aiding differentiation of cone and rod dysfunction. This test showed more general sensitivity reduction for blue light under low light conditions. This may be in keeping with the findings from contrast sensitivity testing in mesopic light conditions, showing strong impairment already in early disease stages, possibly indicating inner retinal dysfunction rather than photoreceptor dysfunction in those disease stages. Reading performance and the effect of binocularity was measured with Radner Reading charts. Reading was consistently slower when patients were using both eyes, strongly indicating binocular inhibition, in particular when arising from scotomas in left eyes. Based on the above, the findings resulted in new insights into visual function with implications on our understanding of the condition. Understanding visual impairment not only helps patient counselling, but also helps driving directions of future research.

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