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Abstract

Primary open angle glaucoma is a progressive and relatively common disease. Damage to the retinal ganglion cells causes characteristic structural changes in the optic nerve head, progressive loss of visual function and eventually blindness. Early detection of this disease is therefore desirable. It has been suggested that the assessment of impaired visual function in glaucoma can be made more sensitive by the selective isolation of specific stimulus attributes. An important aim of this investigation was to establish how the processing of colour or luminance contrast signals is affected differently in glaucoma.

Chromatic sensitivity was measured using a computerised colour display that employs isoluminant colour-defined stimuli buried in dynamic luminance contrast noise, stimulus conditions that isolate the use of colour signals and reveal loss of chromatic sensitivity. The characterisation of chromatic sensitivity loss in a group of POAG and at risk subjects was determined by measuring chromatic displacement (CD) thresholds along 12 directions in CIE - xy chromaticity chart, which allowed fitting CD ellipses. The analysis of the pattern of CD loss at different stages of the disease, both at the fovea and 7 deg, reveals the earliest signs of damage to be found at the paracentral location (7 deg). A non-selective CD loss with a characteristic increase in variance is observed early in the disease. As damage progresses foveal sensitivity is also progressively affected showing a greater B/Y relative loss, which becomes similar and even exceeded by the R/G loss when the disease is established. For moderate and advanced stages of the disease, the relative loss becomes greater for the R/G thresholds at both foveal and 7 deg. The test showed an overall success rate of 71% in identifying cases correctly.

In addition, moving, colour-defined stimuli designed to isolate either the transient luminance channel or the R/G and B/Y chromatic colour opponent mechanisms were used to find evidence of preferential damage for detection of one or the other of these stimulus attributes. Findings in patients with POAG and normal subjects of similar and younger age were compared. The relative increase of chromatic (R/G and B/Y) and achromatic thresholds was determined with ageing and glaucomatous damage. Although both mechanisms were significantly affected by POAG damage, the chromatic thresholds showed greater relative increase. The R/G and B/Y chromatic thresholds increased differently as a function of the severity of visual field loss. The B/Y sensitivity showed the greatest loss in early glaucoma subjects, however the R/G mechanism showed a faster rate of threshold increase and a better correlation with severity of field loss than B/Y thresholds. Ageing affected significantly both the chromatic (R/G and B/Y) and achromatic mechanisms. Chromatic threshold increases were on average twice the increase observed in the achromatic mechanism for foveal and parafoveal measurements. The rate of increase, as a function of age, in chromatic thresholds (along R/G and B/Y mechanisms) showed an accelerated fashion from the 5th-6th decade onwards. Meanwhile the achromatic thresholds increased at a slower, linear rate throughout life. In spite of the initial, more rapid increase along the B/Y colour opponent system (particularly for foveal thresholds), there were no significant differences between the rate of ageing for R/G and B/Y mechanisms. The largest age-related loss of chromatic sensitivity occurs in the lower hemifield for the B/Y mechanism.

In conclusion, both the glaucomatous damage and the ageing process lead to reduction in both R/G and B/Y chromatic sensitivity. The relative B/Y versus R/G chromatic loss in POAG depends on the severity of glaucomatous damage. The first signs of chromatic discrimination loss in the patients at risk of suffering POAG appear in the paracentral locations (7 deg) and are characterised by a non-selective loss of chromatic sensitivity and an increased inter- and within- subject variability. The relative age-related loss of R/G and B/Y sensitivity is mostly non- selective and independent of stimulus location.

Publication Type:	Thesis (Doctoral)
Subjects:	R Medicine > RZ Other systems of medicine
Departments:	School of Health & Psychological Sciences > Optometry & Visual Sciences School of Health & Psychological Sciences > School of Health & Psychological Sciences Doctoral Theses Doctoral Theses

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