ReviewMetabolic syndrome and eye diseases
Introduction
Metabolic syndrome is becoming a worldwide medical and public health challenge as it has been seen increasing in prevalence over the years and has a strong association with cardiovascular diseases [1]. Metabolic syndrome is increasingly being recognised as a distinct entity which comprises of the following components: central obesity, hyperglycaemia, hypertension and dyslipidaemia [2].
Vision impairment and age-related eye diseases are also increasing due to aging populations, increasing life expectancy and impact of risk factors such as smoking, diabetes and hypertension [3], [4], [5], [6]. A recent World Health Organisation study estimated that 186 million people aged 50 and above are visually impaired globally, of which 32 million are blind [7]. The leading causes of blindness globally are cataract, age-related macular degeneration (AMD), diabetic retinopathy (DR) and glaucoma, which account for the majority of vision loss unrelated to refractive error [8], [9], [10], [11].
Many of the individual components of the metabolic syndrome have well established associations with these eye diseases [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. However, the consistency of these associations have not been clearly examined. There were two recent reviews on similar topic, but both were brief. They evaluated the associations of ocular diseases, in particular cataract and glaucoma with metabolic syndrome [26], [27], but the associations of DR and AMD with metabolic syndrome were hardly evaluated. In this review, we summarize current evidence supporting associations between metabolic syndrome and age-related eye diseases from observational studies focusing on DR, cataract, AMD, and glaucoma, which account for the majority of vision loss in middle aged and elderly populations. We also reviewed current understanding in the pathophysiological mechanisms underlying these associations. Although our main focus is to review the ocular associations of metabolic syndrome proper, we also reviewed the associations of individual metabolic syndrome components with ocular diseases because of the paucity of research in this area.
Section snippets
Methodology
We conducted a systematic review of association of metabolic syndrome with ocular diseases. Bibliographical searches were performed in the medline database (PUBMED) up to February 2015 using the following controlled vocabulary (Mesh) search terms: āmetabolic syndrome xā[Mesh] AND (ādiabetic retinopathyā[Mesh] OR ācataractā[Mesh] OR āglaucomaā[Mesh] OR āmacular degenerationā[Mesh] OR āeye diseasesā[Mesh]). Our inclusion criteria were articles written in English language and either
Definition of metabolic syndrome
Currently, there are several definitions of metabolic syndrome, and this led to some difficulties in comparing the results of different studies. Three of the most commonly used definitions are those defined by National Cholesterol Education ProgramāAdult Treatment Panel III (NECP-ATP III) [28] and its revised definition in 2005 [29], International Diabetes Federation (IDF) [2] and World Health Organisation (WHO) [30] (Table 1). All these definitions agree on the key components including
Prevalence of metabolic syndrome
The IDF estimates that one quarter of the world population has metabolic syndrome [31] with worldwide prevalence in adults ranging from <10% amongst Greeks [32], Japanese [33] and Koreans [34] to 44% in Americans [35]. The wide variation in prevalence is speculated to be due to the differences in population characteristics and also the definition of the syndrome used for that particular study [36]. As the prevalence of all components of metabolic syndrome increases with age, the prevalence of
Conclusion
From the literature thus far, we have gathered consistent evidence from prospective and cross-sectional studies demonstrating a significant and expected association between metabolic syndrome and cataract. We also showed that there is sufficient evidence, at least cross-sectionally, to support an association between metabolic syndrome and DR in persons with type 2 diabetes. Prospective studies are warranted to confirm the association. There are fewer studies that have examined the association
Conflict of interest statement
The authors have declared that no conflict of interest exists.
Funding
No funding support.
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