Academic journal article General Psychiatry

Meta-Analysis of the Relationship of Peripheral Retinal Nerve Fiber Layer Thickness to Alzheimer's Disease and Mild Cognitive Impairment

Academic journal article General Psychiatry

Meta-Analysis of the Relationship of Peripheral Retinal Nerve Fiber Layer Thickness to Alzheimer's Disease and Mild Cognitive Impairment

Article excerpt

1. Introduction

Alzheimer's disease (AD) is one of the most common types of dementia. [1] In 2015 there were more than 46 million individuals with AD in the world and, based on expected increases in life expectancy, by 2050 the number of individuals with AD will surpass 100 million. [2] AD has an insidious onset that makes it difficult to diagnose in the early stages. There are, moreover, no effective treatments once the disease has become clinically evident, [3] so early recognition, diagnosis, and preemptive treatment is one of the best potential options for delaying the development of AD. However, there is, as yet, no biomarker that can accurately identify prodromal AD. [4]

The visual cortex is the part of the brain that controls some of the earliest clinical manifestations of AD. [5,6] Abnormalities in visual-related functions, such as difficulty with reading and searching for targets, depth perception disabilities, and decline of spatial contrast sensitivity, are evident in individuals in the early stages of AD. [7] Moreover, throughout the course of AD, the retina experiences neurodegenerative changes, including loss of retinal ganglion cells and nerve fibers, macular degeneration, and increases in optic disc cupping. [5] The retina and the central nervous system (CNS) are homologous during embryonic development, so the retina is the only part of the CNS that can be directly observed in living individuals. Thus, the thickness of the retinal nerve fiber layer (RNFL) may act as a window to assess ongoing changes in the CNS. [8]

Previous methods for measuring RNFL thickness, such as fundus photography, scanning laser ophthalmoscopy (SLO), and nerve fiber analysis (NFA), depended to some extent on subjective factors, so results tended to be unreliable between different raters. The current gold standard measurement method, optic coherence tomography (OCT), can accurately delineate the 10 layers of fiber structures in the optic nerve, and it is fast, noninvasive, and economical. In the past decade studies using OCT have generally reported that the thickness of the peripheral RNFL among individuals with AD and mild cognitive impairment-which is often an early transitional state prior to dementia-were significantly thinner than the peripheral RNFL thicknesses of age-matched normal controls. [9-11] But there were significant differences in results for the four different quadrants of the optic nerve. Some studies showed a significant change in the superior quadrant; [11-15] others showed significant changes in the inferior quadrant, [11,14,15] while still others showed significant changes in the temporal quadrant. [14,16,17] Moreover, there were inconsistent findings about differences of RNFL thickness between individuals with AD and those with MCI; some studies showed that individuals with AD had significantly thinner RNFL than individuals with MCI, [14,17] but other studies did not support this finding. [11,15,18]

To clarify the relative importance of RNFL in different optic nerve quadrants and to summarize results comparing RNFL results for persons with AD versus those for persons with MCI, this systematic review and meta-analysis pools data about RNFL thickness from studies that meet rigorous inclusion criteria. The results are used to assess the potential usefulness of RNFL thickness as a biomarker for early AD.

2. Methods

2.1 Data retrieval

The article retrieval process is depicted in Figure 1. English databases considered include PubMed (1977-2015), Embase (1971-2015), ISI web of knowledge (1955-2015), Ovid/MEDLINE (1967-2015), Science Direct (1965-2015), and the Cochrane Library (1967-2015). Chinese databases considered include the Chinese National Knowledge Infrastructure (CNKI) database (1980-2015), the Chongqing VIP database for Chinese Technical Periodicals (VIP) (1996-2015), WANFANG DATA (1977-2015), and the China BioMedical Literature Service System (SinoMed) (1995-2015). …

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