Abstract
Alzheimer’s Disease (AD) is one of the main neurodegenerative diseases worldwide. Unfortunately, AD shares many similarities with other dementias at early stages, which impedes an accurate premortem diagnosis. Therefore, it is urgent to find biomarkers to allow for early diagnosis of the disease. There is increasing scientific evidence highlighting the similarities between the eye and other structures of the CNS, suggesting that knowledge acquired in eye research could be useful for research and diagnosis of AD. For example, the retina and optic nerve are considered part of the central nervous system, and their damage can result in retrograde and anterograde axon degeneration, as well as abnormal protein aggregation. In the anterior eye segment, the aqueous humor and tear film may be comparable to the cerebrospinal fluid. Both fluids are enriched with molecules that can be potential neurodegenerative biomarkers. Indeed, the pathophysiology of AD, characterized by cerebral deposits of amyloid-beta (Aβ) and tau protein, is also present in the eyes of AD patients, besides numerous structural and functional changes observed in the structure of the eyes. Therefore, all this evidence suggests that ocular changes have the potential to be used as either predictive values for AD assessment or as diagnostic tools.
Highlights
Alzheimer’s Disease (AD) is a degenerative disorder of the nervous system with a slow and progressive onset
The retina is a light-sensitive tissue lining the inner surface of the eye, and it is composed of 10 different layers: the inner limiting membrane, retinal nerve fiber layer (RNFL), retinal ganglion cell layer (RGCL), inner plexiform layer (IPL), inner nuclear layer, outer plexiform layer, outer nuclear layer, external limiting membrane, photoreceptor cell layer, and retinal pigment epithelium (Figure 2) [38]
Retina and Optic Nerve The retina is a light-sensitive tissue lining the inner surface of the eye, and it is composed of 10 different layers: the inner limiting membrane, retinal nerve fiber layer (RNFL), retinal ganglion cell layer (RGCL), inner plexiform layer (IPL), inner nuclear layer, outer plexiform layer, outer nuclear layer, external limiting membrane, photoreceptor cell layer, and retinal pigment epithelium (Figure 2) [38]
Summary
Alzheimer’s Disease (AD) is a degenerative disorder of the nervous system with a slow and progressive onset. NIBS approaches are currently getting attention as they show promising results by stimulating different brain regions simultaneously, which improves memory and specific cognitive functions [12,13,14,15,16] These NIBS approaches could offer a reliable therapeutic option for those AD patients not responding to drug treatments [14]. It is important to identify neurodegenerative biomarkers that detect cognitive decline or progression from MCI to dementia [17,18] In this regard, ophthalmological assessments have detected several ocular changes in patients with central nervous system (CNS) disorders [19]. Ophthalmological assessments have detected several ocular changes in patients with central nervous system (CNS) disorders [19] In many of these disorders, ocular manifestations often precede brain symptoms, suggesting that eye exams could offer an early diagnosis of the underlying disease [19]. We will highlight the caveats of the studies in order to unify consensual criteria that could facilitate the comparison of results between different reports in the future
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