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.