Saunders, L. J. (2015). Studies on real world visual field data in glaucoma. (Unpublished Doctoral thesis, City, University of London)
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Glaucoma is a leading cause of blindness. As a progressive condition, it is important to monitor how the visual field (VF) changes over time with perimetry in preventing vision from deteriorating to a stage where quality of life is affected.
However, there is little evidence of how clinical measurements correlate with meaningful quality of life landmarks for the patient or, by extension, the proportion of patients in danger of progressing to these landmarks. Further, measurement variability associated with visual fields make it difficult to monitor true change over time. The purpose of this thesis was to use large-scale clinical data (almost 500,000 VFs) to address some of these issues.
The first study attempted to relate clinical measurements of glaucoma severity to UK legal fitness to drive status. Legal fitness to drive (LFTD) was estimated using the integrated visual field as a surrogate of the Esterman test, which is the approved method by the UK DVLA of defining LFTD, while the mean deviation (MD) was used to represent defect severity. An MD of -14dB or worse in the better eye was found to be associated with a 92% (95% Confidence Interval [CI]: 87-95%) probability of being legally unfit to drive.
The second study used a statistical model to estimate the number of patients progressing at rates that could lead to this landmark of significant visual impairment or blindness in their predicted remaining lifetime. A significant minority of patients were progressing at rates that could lead to statutory blindness, as defined by the US Social Security Administration, in their predicted remaining lifetime (5.2% [CI: 4.5-6.0%]) with a further 10% in danger of becoming legally unfit to drive (10.4% [CI: 9.4-11.4%]). More than 90% (CI: 85.7-94.3%) of patients predicted to progress to statutory blindness had an MD worse than -6dB in at least one eye at presentation, suggesting an association between baseline VF damage and risk of future impairment.
The next section investigated whether choice of testing algorithm, SITA Standard or SITA Fast, affected the time taken to detect progression in VF follow-up. The precision of the tests was measured using linear modelling techniques and the impact of these differences was analysed using simulations. Though SITA Fast was found to be slightly less precise, no evidence was found to suggest that this resulted in progression being detected later.
The final study evaluated a validated and published risk calculator, which utilised baseline risk factors to profile risk of fast progression. A simpler model using baseline VF data was developed to have similar statistical properties for comparison(including equivalent R2 statistics). The results suggested that risk calculators with low R2 statistics had little utility for predicting future progression rate in clinical practice.
Together these results contribute a variety of novel findings and demonstrate the benefit of using large quantities of data collected from the everyday clinical milieu to extend clinical knowledge.
|Item Type:||Thesis (Doctoral)|
|Subjects:||R Medicine > RE Ophthalmology|
|Divisions:||School of Health Sciences > Department of Optometry & Visual Science|
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