**5. Physiopathology of AD manifestations in the eye and the visual pathway**

Changes in the visual system associated with AD have been the focus of the scientific com‐ munity over many years, with some extensive reviews focus on different aspects of the problem [7, 47, 175–188]. All this evidence emphasises that visual changes may in medical practice help in the assessment of these patients and may even provide a predictive value potentially useful in diagnosis.

#### **5.1. The lens**

β‐Amyloid deposits in the brain are a pathologic marker for AD. Amyloid β‐peptides Aβ1–42 and Aβ1–40 have been identified in the human lens. Aβ1–40 was found in the aqueous humour, and its concentration is comparable to that found in the cerebral cortex and in the cerebrospinal fluid of AD patients [189]. On the one hand, it was recently discovered that there is an increase in β‐amyloid deposits in the supranuclear lens fibres, which may be linked to the equatorial supranuclear cataracts more frequently found in these patients [1]. On the other hand, the study by Bei et al. determined that the measuring of the lens opacity was unlikely to provide a noninvasive measure of the risk of developing AD [190].

#### **5.2. Retina**

although the VA is not affected [158–160], there is a decrease in CS [158–160] and the RNFL thickness in MS patients in comparison with healthy individuals [160, 161]. RNFL thickening occurs in both the peripapillary [60, 160, 162, 163] and the macular area [54, 60]. Furthermore, RNFL thinning in MS patients directly correlates with the progression of neurological

PD is a chronic neurodegenerative disorder that is associated mainly with motor dysfunction, although it can also involve some non‐motor symptoms, including visual deficits. These deficiencies may manifest as decreased CS [59, 163], impaired colour vision (the tritan axis is altered first) [57], and abnormal electrophysiological responses [57, 164]. The retinas of PD patients show photoreceptor and RGC dysfunction, morphological deterioration of the perifoveal dopaminergic plexus [165], and thinning of the RNFL [52, 53, 164, 166, 167]. According to the hypothesis that the disease results from a dopamine imbalance, it seems that visual deficits in PD could also be caused by dopamine depletion. In fact, some of the visual

The first abnormalities in the visual system for AD, observed in the 1970s, were regarded strictly as a dysfunction at the cortical level. Subsequent studies over the past 25 years have revealed that all parts of the visual system, including the ON and the retina, may be affected in AD. Some aspects of this involvement are still not well understood and are still the subject of recent research. Anatomical changes along the visual pathways and their corresponding functional changes have been detected and analysed by psychophysical procedures. AD can affect different aspects of the visual processing in line with the impact of the disease in the dorsal and ventral regions of the brain. Patients with dorsal‐region damage suffer alterations in functions such as discrimination and angular‐motion perception [169–172]. Those with damage in the ventral region show difficulty in discriminating faces, colours, and shapes [37,

**5. Physiopathology of AD manifestations in the eye and the visual pathway**

Changes in the visual system associated with AD have been the focus of the scientific com‐ munity over many years, with some extensive reviews focus on different aspects of the problem [7, 47, 175–188]. All this evidence emphasises that visual changes may in medical practice help in the assessment of these patients and may even provide a predictive value potentially useful

β‐Amyloid deposits in the brain are a pathologic marker for AD. Amyloid β‐peptides Aβ1–42 and Aβ1–40 have been identified in the human lens. Aβ1–40 was found in the aqueous humour,

deficits experienced by PD patients can be improved by levodopa treatment [168].

impairment and disease duration [160].

*4.2.3. Parkinson's disease (PD)*

390 Update on Dementia

*4.2.4. Alzheimer's disease (AD)*

173, 174].

in diagnosis.

**5.1. The lens**

#### *5.2.1. Retinal ganglion cells (RGCs)*

The first histopathological studies on human‐donor retinas of AD patients were made in the 1980s. Hinton et al. [8] examined four eyes from AD patients, finding a loss on the number of RGC, but a shortcoming of the study was that they did not provide numerical values in their results. However, Curcio et al. [27] found no significant difference in the number of RGC between the AD group and the age‐matched controls. In the mid‐1990s, Blanks et al. [23, 24] confirmed the initial observations of Hinton et al. These researchers compared the postmortem number of RGC from 12 retinas of nine patients with severe AD and 15 retinas from 12 age‐ matched controls. These studies found a 25% decrease in the number of RGC (*p* < 0.001) and an 82% increase in the astrocyte ratio per neuron in the retina (*p* < 0.001). However, a study with AD transgenic mice did not show a significant difference in the number of CGR compared to controls [191].

All the histopathological studies carried out so far involve a relatively small number of subjects. Therefore, it would be advisable to undertake more studies with larger numbers of subjects in order to verify the RGC decline in AD.

#### *5.2.2. Vascularisation and retinal blood flow*

Recent data suggest the vascular involvement of the retina in AD patients. Vascular changes in the retina are thought to share similar pathogenic mechanisms with cerebral vasculature [15, 192]. In fact, it is known that cerebral vascular insufficiency is one of the earliest patho‐ logical signs in the development of AD [193, 194].

Currently, there are few studies on vascularisation and retinal blood flow in AD. In 2007, Berisha et al. [15] studied the retinal vascularisation and blood flow in patients with AD. These researchers used Doppler laser in nine patients with probable mild (*n* = 6) and moderate AD (*n* = 3) plus eight age‐matched controls. They detected a significant narrowing in the diameter of the retinal veins and decreased blood flow in AD patients, compared with controls. Mroczkowska et al. analysed the dynamic retinal vascularisation, noting that there were some signs of microvascular dysfunction that were correlated with the extent of cognitive impair‐ ment [195]. However, the study published by Tsai et al. found no differences either in the vascular structure or the calibre of retinal vessels in AD transgenic mice, compared to control animals [191].

#### *5.2.3. Amyloid plaques, neurofibrillary tangles and vascular angiopathy*

The first unsuccessful attempts to find amyloid plaques, neurofibrillary tangles or vascular retinal angiopathy in eyes of AD patients were performed in 1989 by Blanks et al. [21]. Although none of these typical AD signs were detected in the retina, their findings showed different levels of degeneration in RGC correlated with the degree of impairment of the patient.

In the last few years, β‐amyloid deposits and hyperphosphorylated tau proteins have been detected in elderly retinas in a model of AD in double transgenic mice [196–198]. These β‐ amyloid plaques are distributed from the ganglion‐cell layer to the inner plexiform layer. Some of these are also located in the outer nuclear layer, in the outer segments of the photoreceptors and in the ON [197]. These deposits, analysed with immunohistochemical techniques, are found to be accompanied by an increase in MCP‐1+ immunoreactivity and F4/80+ in RGC layer. These results suggest that β‐amyloid deposits cause neurodegeneration in the retina of these mice. This idea is further supported by the presence of TUNEL+ immunostaining in the RGC layer, so that there is some histological evidence of apoptosis in this layer [196]. In 2009, a β‐ amyloid vaccine was tested in the experimental mice model mentioned above, resulting in a lower number of retinal β‐amyloid deposits. Nevertheless, there was a marked increase in retinal microvascular β‐amyloid deposits as well as local neuroinflammation due to microglial infiltration and astrogliosis linked to a disorder in the organisation of the retina [197].

A postmortem study in human retinas showed for the first time the presence of β‐amyloid plaques in AD patients [199]. Subsequently, other authors have confirmed the presence of β‐ amyloid deposits, which were more prevalent in perivascular and perimacular areas, both in AD patients and in those with mild cognitive impairment (MCI) [191, 200]. Campbell et al. recognised β‐amyloid plaques in the retina by observing its polarisation properties and proposing it as a new diagnostic method [201].

#### **5.3. Choroid**

The latest improvement in OCT technology has enabled us to study the thickness of the choroid in vivo in patients with mild to moderate AD. A statistically significant generalised loss of foveal choroid thickness was found in these patients [191, 202, 203]. According to the authors, this choroidal thinning in AD may be associated with hypoperfusion and/or atrophic changes in this vascular layer. Several immunohistochemical studies in AD transgenic animals have reported a higher frequency of RPE hypertrophy and binucleated cells, but these changes were not seen in human retinas [191]. Previous studies have shown an Aβ accumulation in the choroidal vasculature in ageing mice and in a transgenic mouse model of AD [196, 204]. As happens in the brain, Aβ accumulation in the choroid may induce an inflammatory response and complement activation, which would lead to progressive vasoregression of the choroidal vasculature (and subsequent retinal neurodegeneration), through the same pathological cascade that has already been described in AD brains [191, 205–207].

#### **5.4. The optic nerve**

*5.2.3. Amyloid plaques, neurofibrillary tangles and vascular angiopathy*

The first unsuccessful attempts to find amyloid plaques, neurofibrillary tangles or vascular retinal angiopathy in eyes of AD patients were performed in 1989 by Blanks et al. [21]. Although none of these typical AD signs were detected in the retina, their findings showed different levels of degeneration in RGC correlated with the degree of impairment of the patient.

In the last few years, β‐amyloid deposits and hyperphosphorylated tau proteins have been detected in elderly retinas in a model of AD in double transgenic mice [196–198]. These β‐ amyloid plaques are distributed from the ganglion‐cell layer to the inner plexiform layer. Some of these are also located in the outer nuclear layer, in the outer segments of the photoreceptors and in the ON [197]. These deposits, analysed with immunohistochemical techniques, are found to be accompanied by an increase in MCP‐1+ immunoreactivity and F4/80+ in RGC layer. These results suggest that β‐amyloid deposits cause neurodegeneration in the retina of these mice. This idea is further supported by the presence of TUNEL+ immunostaining in the RGC layer, so that there is some histological evidence of apoptosis in this layer [196]. In 2009, a β‐ amyloid vaccine was tested in the experimental mice model mentioned above, resulting in a lower number of retinal β‐amyloid deposits. Nevertheless, there was a marked increase in retinal microvascular β‐amyloid deposits as well as local neuroinflammation due to microglial

infiltration and astrogliosis linked to a disorder in the organisation of the retina [197].

proposing it as a new diagnostic method [201].

**5.3. Choroid**

392 Update on Dementia

A postmortem study in human retinas showed for the first time the presence of β‐amyloid plaques in AD patients [199]. Subsequently, other authors have confirmed the presence of β‐ amyloid deposits, which were more prevalent in perivascular and perimacular areas, both in AD patients and in those with mild cognitive impairment (MCI) [191, 200]. Campbell et al. recognised β‐amyloid plaques in the retina by observing its polarisation properties and

The latest improvement in OCT technology has enabled us to study the thickness of the choroid in vivo in patients with mild to moderate AD. A statistically significant generalised loss of foveal choroid thickness was found in these patients [191, 202, 203]. According to the authors, this choroidal thinning in AD may be associated with hypoperfusion and/or atrophic changes in this vascular layer. Several immunohistochemical studies in AD transgenic animals have reported a higher frequency of RPE hypertrophy and binucleated cells, but these changes were not seen in human retinas [191]. Previous studies have shown an Aβ accumulation in the choroidal vasculature in ageing mice and in a transgenic mouse model of AD [196, 204]. As happens in the brain, Aβ accumulation in the choroid may induce an inflammatory response and complement activation, which would lead to progressive vasoregression of the choroidal vasculature (and subsequent retinal neurodegeneration), through the same pathological

cascade that has already been described in AD brains [191, 205–207].

Hinton et al. [8] described widespread axonal degeneration of the optic nerves in 8 out of 10 AD patients that were analysed postmortem. In an additional study, a morphometric analysis of the ON suggested a predominant loss of magnocellular neurons that contribute large‐calibre fibres. In a study published in 2005 by Syed et al. [208], some significant differences in axonal density were found by dyeing axon contours with toluidine blue, both in the central and in the peripheral areas of the ON. The analysis was performed on 12 AD patients, compared with 13 advanced‐age control subjects. It should be pointed out that Syed's study found a decrease in smaller axons, with transversal section areas measuring less than 1.99 μm2 . Studies using magnetic resonance imaging (MRI) have found a statistically significant decrease in the ON volume in AD patients. However, this reduction does not correlate with brain volume [209].

Neurofibrillary tangles of tau protein have also been observed in ON [211], although this may not be specific for AD. Low‐density lipoprotein receptor‐related protein (LRP) is involved in the pathogenesis of AD by mediating the transport of amyloid‐β (Aβ) out of the brain into the systemic circulation. Recently, Cuzzo et al. [210] found a decrease in the expression of LRP in the optic nerves of 11 patients with AD, compared to 10 control subjects. This would support the theory that LRP may play a role in the physiopathology of the optic neuropathy in AD. In the same study, the group of Cuzzo observed a decrease in neurofilament immunostaining in AD patients in comparison with control, thus confirming the previous findings by Hinton et al. [8]. Also, an increase in the receptor expression of advanced glycation end‐products in the astrocytes near microvasculature has been reported in ON samples from AD patients [211].

Different studies have also pointed out some differences between the control group and the AD patients in the appearance of the ON head, both with red‐free photography [13, 212, 213] and with scanning laser ophthalmoscope [214]. However, other studies failed to find such differences between AD patients and controls using the latter technique [31].

#### **5.5. Lateral geniculate nucleus (LGN)**

Scholtz et al. [215] noted a myelin loss and reduced function of neurons in the LGN in AD patients. The presence of β‐amyloid plaques and neurofibrillary tangles of tau protein are also shown to be more abundant in the parvocellular than in the magnocellular layers of the LGN [218, 219]. On the contrary, a recent study found that tau pathology was scarce in the LGN and it did not differ significantly with age‐matched control patients [216].

#### **5.6. Other brain nuclei**

#### *5.6.1. Superior coliculus (SC)*

The SC, sometimes referred to as the optic tectum, is a paired and laminated structure with a retinotopic organisation in the medium brain, which receives about 10% of RGC axons and is involved in the control of eye movements. Numerous amyloid plaques were found in the SC of AD patients [217]. Furthermore, subsequent studies identified abundant neuropathological neurofibrillary tangles [216, 218, 219] in this nucleus. These pathological changes in the SC may explain the problems of ocular motility frequently found in patients with AD [220].

#### *5.6.2. Suprachiasmatic nucleus*

The suprachiasmatic nucleus (SCN) is a paired structure formed by a group of neurons in the medial hypothalamus, above the optic chiasm, which receives direct RGC input through a retinal‐hypothalamic tract [221]. In addition, it is a primary control centre of circadian rhythms by stimulating melatonin secretion by the pineal gland. It has been seen that there may be marked changes in the SCN in AD, such as decreases in the volume and number of cells (including specific neuronal subpopulations, such as vasopressin and neurotensin neurons) and the formation of neurofibrillary tangles [222–225]. These neuropathological findings may be correlated with the misalignment of the circadian rhythm in AD [226].

#### *5.6.3. Pulvinar nuclei*

Pulvinar nuclei, are a collection of nuclei located in the pulvinar thalamus, have reciprocal connections with association areas of the cerebral cortex, in the parietal, occipital and temporal lobes. They consist of several divisions that receive multiple inputs from the visual cortex subdivisions, from the SC and the retina (to a limited extent) [227]. This area is involved in visual attention and the control of eye movements.

Numerous amyloid plaques and some neuritic plaques (extracellular deposits within the brain grey matter which are a mixture between amyloid and death neuronal processes) were found along the various subdivisions of the pulvinar nuclei in nine AD patients compared with younger patients and age‐matched controls [228]. This could explain the deterioration of visual attention in AD patients [174].

#### **5.7. Visual cortex**

The visual cortex is located in the occipital lobe. It comprises the striate cortex or V1 (primary visual cortex) and visual cortical extrastriate areas such as V2, V3, V4, V5, etc. (secondary visual cortex). Together, the primary and secondary visual cortex consists of a mosaic of several dozen visual areas occupying a large part of the cerebral cortex, approximately 20–25% in humans [229, 230].

In AD, the primary visual cortex is affected after the involvement of other cortical regions [231– 233], except in a variant that manifests with early visual symptoms [234, 235]. Nevertheless, the accumulation of amyloid plaques and neurofibrillary tangles, the decrease in the number of neurons and capillary density, and the reduction of certain enzymes in V1 of AD patients are well documented [191, 233–236].

Although some preliminary studies claimed that there are minimal neurofibrillary tangles and amyloid plaques in primary and secondary visual cortex [237], subsequent studies have found abundant neurofibrillary tangles and plaques in the secondary visual cortex (mostly in the extrastriate 18 and 19 areas) [238, 239]. In both areas 17 and 18, the average neuronal density decreases to a similar degree (∼30%) [240]. However, the difference between the two areas was the concentration of neurofibrillary tangles (2% of the neurons present tangles in area 17 vs. 10% in area 18) [240]. The reason for such discrepancy in neuronal loss could be related to the vulnerability of some neurons to the presence of neurofibrillary tangles or to the possibility of some cell loss unrelated to the degeneration of neurofibrillary tangles [47]. In addition to amyloid plaques and tangles, astrocytic gliosis was found in the primary visual cortex [241]. A likely associated dendritic pathology has also been observed in AD: dystrophic dendrites, loss of dendritic branches, and pathological alteration of dendritic spines [242, 243].
