Ocular manifestations of Alzheimer's disease
Alzheimer disease (AD) is a primary progressive neurodegenerative disease affecting approximately 4.7% of individuals age 60 years. AD is typically characterized by deterioration of memory and other neurocognitive functions that ultimately interferes with daily life and is can be fatal. AD is the most common form of dementia, accounting for over 50% of cases. In the United States, 5.5 million people are affected and and up to 35 million worldwide have the disease. There is currently no cure for AD but there are treatments with variable and modest efficacy .
AD is believed to arise from the accumulation of misfolded proteins that induces oxidative and secondary inflammatory damage on the aging brain leading to cognitive decline. Specific pathological features of AD include β-amyloid (Aβ) peptide plaques and tau protein neurofibrillary tangles (NFT). According to the predominant ‘amyloid hypothesis,’ Aβ precipitates secondary to an imbalance between production and clearance such that there is an accumulation and aggregation of Aβ2. NFT are formed from the intracellular aggregation of abnormal tau proteins. The number of NFT correlates with disease severity. Secondary inflammation and oxidative stress may interfere with synaptic and neuronal activity, leading to neuronal loss and brain atrophy.
As visualization of Aβ and NFT via brain imaging is of limited specificity and resolution, AD diagnosis is primarily clinical with reliance on a range of testing modalities ranging from cognitive assessments and physical exam to brain imaging (MRI, CT, PET). Definitive diagnosis is only through postmortem histological exam with visualization of NFT and Aβ.
|Ocular Structures||Pathological Changes in AD|
|Retina||- Deposition of proteins tau, Aβ and pTAu- Impaired metabolism of amyloid β precursor protein (APP)|
|Retinal and choroidal vasculature||- Retinal and choroidal vascular β-amyloid deposits in transgenic mouse model of AD lead to retinal degeneration- Impaired vascularization|
|Retinal vascular blood flow||- Blood flow disturbances can lead to neurodegeneration|
|Optic nerve||- Axonal degeneration in the axonal segments- Loss of optic nerve thickness
- Papillary paleness due to axonal loss and perfusion alterations
|Lens||- Correlation between AD and supranuclear cataract- Presence of abnormal protein deposits|
|Tears||- Changes in the chemical barrier composition of tears|
|Cornea||- Reduced corneal sensitivity|
|Pupil||- Pupillary response, possible biomarker|
|Choroid||- Attenuation of choroidal thickness|
|Visual Function||Pathological Changes in AD|
|Visual fields||- Inferior hemifield loss|
|Visual acuity||- Decreased visual acuity in low luminance|
|Sensory perception||- Clinically important symptoms of visuospatial disorientation|
|Visual processing||- Deficits of visual motion perception|
|Contrast sensitivity||- Contrast sensitivity disturbances and motion perception|
|Color vision||- Color discrimination error inversely proportional with mini-mental state examination (MMSE) score|
|Stereopsis||- Reduced stereoscopic depth perception|
|Circadian rhythm||- Alterations in the circadian rhythm and β-amyloid deposits inside and surrounding degenerating mRGCs|
Of note, the visual variant of AD (VVAD), also known as posterior cortical atrophy (PCA), is characterized by prominent visual symptoms due to localized atrophy of the parieto-occipital lobe. VVAD is For more information, please see corresponding EyeWiki article.
Patients with PCA typically present with difficulty reading or problems with visuospatial and visual processing. A homonymous hemianopsia or cortical visual loss with a negative structural imaging study (e.g., MRI) or neuroimaging showing only posterior cortical atrophy may suggest the diagnosis of AD.
Previously, retinal OCT imaging in AD patients noted NFL layer atrophy and ganglion cell death . However, these findings were not specific to AD and were also reported in other neurodegenerative disorders (e.g., Parkinson disease). Reduced RNFL thickness on OCT has been associated with memory deficits. Recent studies have reported detection of hallmark Aβ plaques in the retina in AD patients via scanning laser ophthalmoscope using the natural fluorochrome curcumin that binds to the plaques . Fluorescence is quantified via an automated calculation of the retinal amyloid index (RAI). Compared to healthy controls, AD patients had a 2.1-fold increase in fluorescent intensity over baseline. Furthermore, the fluorescence pattern is consistent with histological data showing Aβ deposits clustered in the peripheral superior quadrant, often along blood vessels. Retinal vascular parameters (RVPs) may also serve as a tool for early AD diagnosis . The decreased retinal microvascular network density noted in AD patients suggests retinal vessel reaction to flicker stimulation, delayed in AD patients, could be another potential non-invasive biomarker.
Optical coherence tomography angiography (OCTA)
OCTA has also been used on AD patients to detect decreased retinal vasculature density as well as reduced retinal and choroidal flow rates . The metabolic hyperspectral retinal camera is another device currently being studied for its biomarker potential via measurement of regional retinal vessel oxygen saturation. This device utilizes 225 contiguous spectral bands collected at high speeds to localize biomolecules and structures based on their respective spectral signatures. Another study evaluating the association between type 2 diabetes and AD employs microperimetry to measure retinal neurodegeneration. Microperimetry assesses retinal sensitivity by measuring the minimum light intensity which the patients can perceive by stimulating specific areas of retina with spots of light.
Another target for AD detection is the pathologic hyperphosphorylated Tau protein (NFT), which has been detected in postmortem human retinas and AD human models in the absence of tau aggregation, suggesting a pre-symptomatic disease stage.
Studies have detected β-amyloid in the lens of human AD patients as well as correlation between cortical cataracts and AD degeneration. A clinical trial is currently evaluating the use of aftobetin hydrochloride, a topical amyloid-binding compound to detect β-amyloid in patients with mild cognitive impairment and mild AD (ClinicalTrials.gov Identifier: NCT02928211).
AD patients have been reported to have increased tear flow rate and protein levels. The presence of lipocalin-1, dermicidin, lysozyme C and lactritin had an 81% sensitivity and 77% specificity for AD35.
Pupillary Light Response and Eye Movement
Pupillary light reflex amplitude has been noted to be decreased in AD. There is also an ongoing study to track eye movements as an AD diagnostic tool, with previous studies showing delayed saccadic eye movement and smooth ocular pursuit (ClinicalTrials.gov Identifier: NCT01434940).
While still in development, this wide range of AD detection modalities are promising as a specific, early and noninvasive method of objective AD diagnosis, with potential for prognostication as well.
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