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Editors Selection IGR 9-3

Glaucoma and Systemic Diseases: POAG and Dementia

Comment by Francesca Cordeiro on:

59457 Associations between primary open angle glaucoma, Alzheimer's disease and vascular dementia: record linkage study, Keenan TD; Goldacre R; Goldacre MJ, British Journal of Ophthalmology, 2015; 99: 524-527

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For a number of years, there has been speculation that there may be a link between Alzheimer's Disease (AD) and POAG. However, despite increasing evidence of possible common molecular mechanisms underlying the two diseases, epidemiological studies have shown conflicting results. Recently, Keenan and Goldacre have explored this further using the method of record linkage that the authors have previously applied to AMD and AD, as also for assessing trends in trabeculectomy and cataract surgery in England.

In record linkage, data relating to successive care episodes are brought together per patient, enabling analysis across multiple episodes of care across specialties. Data in this study came from English national hospital episode statistics (HES) through the NHS Information Centre and the Oxford record linkage group. In this analysis, the authors used a complete data set from all hospitals in England from 1999 to 2011, looking specifically at hospital admissions.

Eighty-seven thousand six hundred and fifty-eight POAG, 251 703 AD and 217 302 vascular dementia cohorts were compared to a reference cohort (> 2.5 million people). The findings suggested that the risk of AD following a diagnosis of POAG was not elevated (rate ratio 1.01), whilst that of vascular dementia after POAG was modestly elevated (with rate ratio 1.10). The likelihood of a hospital record of POAG following AD or vascular dementia was very low, with rate ratios 0.28 and 0.32.

The question as to whether or not there is a definitive link between AD and POAG remains unanswered

Whilst this is an interesting result, it is important to consider several factors. Firstly, although there are major advantages in the use of record linkage not least of which is related to cost, there are real drawbacks, the most important in this study being inclusion only of hospital admissions. Hence, only patients who were admitted into hospital or for day-case surgery were included. As the authors themselves admit, 'this methodology will not capture all patients with POAG in England, particularly those treated over a long period exclusively with medical therapy' - in other words, the majority of patients attending glaucoma follow-up. Secondly, record linkage is dependent on the reliability and appropriateness of HES-entered diagnostic codes, and this has long been an issue, especially given the over-burdening of the NHS with administrative duties. Moreover, sub-coding problems such as with NTG or even Glaucoma Suspect may occur, as well as those associated with AD and other forms of dementia. Mis-coding would therefore represent a significant issue in the analysis.

POAG and AD are neurodegenerative, chronic and progressive diseases associated with irreversible neuronal cell loss. Both primarily affect the elderly, with a strongly age-dependent incidence, and as life expectancy increases, their burden will escalate. As they share similar pathological processes, and potential treatments, the question as to whether or not there is a definitive link between AD and POAG remains unanswered. This study highlights the need for further good and prospective epidemiological data in the field.

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