An eye’s look unmasks the mystery: correlation between serum amyloid beta peptide, hippocampal volume and retinal thickness in Alzheimer`s disease

Alzheimer’s disease (AD) is a neurodegenerative disorder representing a major health problem worldwide [1]. It accounts for 60–80% of all types of dementias with subsequent diverse socioeconomic implications [2]. About 23% of initially diagnosed patients with AD turn to warrantee another alternative diagnosis at a later stage of the illness. Even in cases diagnosed with AD with confidence, usually the majority of them are diagnosed when their disease has already reached an advanced stage [3].

Unsurprisingly and in the context of this disease mysterious aspect, cognitively intact elderly persons could have uncountable neuritic plaques and neurofibrillary tangles at autopsy. On the other hand, people with the clinical picture of probable AD may lack the well-known defining postmortem findings [4].

Rather than being merely a clinical syndrome, AD is a continuing biological process endorses progressive neuroanatomical, biochemical, neurophysiological changes with subsequent neurocognitive irreversible declining. Pathologically, oligomerization of amyloid beta (Aβ) and neurofibrillary tangles (NFTs) accumulation in neurons with subsequent localized dendrites, axons and synapses dysfunctions are the hallmark of such a biological disease. Although much of the pathological secrets of AD had largely been uncovered and explored thoroughly, yet the dilemma of in vitro early detection of the AD remains problematic issue that—undoubtedly reflected on the availability of promising effective treatment strategies [5‐7].

Because the neuronal injury inflicted by the described pathological strikes can surpass the edge of threshold beyond which any therapy will be unsuccessful, it has been suggested that treatment should has the primary target to halt the neurodegeneration in the silent or—at least—very early stage of AD otherwise any therapeutic intervention become a worthless mirage [8‐10]. That is why there is urgent need for verifying new tools for diagnosis of AD at presymptomatic stages [11].

Several tools were used to reflect the pathological condition in brains of Alzheimer`s patients, including different radiology and laboratory studies. To date, the spearhead radiological technique used for a confident diagnosis of AD is positron emission tomography (PET) amyloid imaging which considered the most direct tool to assess living brain amyloid deposition. Unfortunately, because of high cost and complexity it cannot be widely popularized [12].

Moving to an alternative biological marker, one of the best ideal biomarkers that have been available for 35 years, is cerebrospinal fluid (CSF) assessment of tau and amyloid proteins that reflect the direct pathological changes that occurs in brain tissue.

However, invasive maneuver of lumber puncture in addition to increasing the risk of inducing infection to nervous system make majority of patients feel unease or even refuse to perform this test [13]. As mentioned in literature, there is the considerable laboratory-to-laboratory range of reference measurement variability that makes the standardization questionable. Knowing this fact also add more hesitancy for both the physician and the patient before deciding to go through this procedure [14].

For all of the aforementioned reasons, we suppose that we are in a dire need to label a new promising approach that should be cheaper and less invasive to improve the diagnostic accuracy of AD. We are targeting to combine different known biomarkers that reflect the nature and severity of neuropathological processes in the brains of AD patients [15, 16].

Amyloid precursor protein (APP) is a single-pass trans-membrane protein. It exists in high concentration in neuronal brain tissue. Its metabolism carried out through rapidly acting and complex series of sequential proteases. To date, the precise physiological function of APP is still one of the vexing issues in the molecular biology field. After a cascade of breakdown process, the cleavage of APP forms two Aβ peptides that are 40 and 42 amino acid residues long and that may oligomerize into unique aggregations named neuritic plaques that concentrate mainly in the extracellular space [17].

Measurement of plasma concentration of Aβ1–40 and Aβ1–42 peptides derived from APP could potentially be used as markers of disease risk in early stage of AD [18]. Several studies have demonstrated a head-to-head correspondence between high precision plasma Aβ42/Aβ40 assays and amyloid PET imaging [19‐22].

Magnetic resonance imaging (MRI) is a major imaging modality used for imaging of AD. MRI has a good tissue contrast and accurate three-dimensional (3D) measurement of the brain parts volume and allows accurate measurement of the volume of the hippocampus and related parts [23]. Changes of hippocampal volume were used as biomarkers for diagnosis of AD [24]. Measurement of hippocampal atrophy by magnetic resonance imaging considers as a valuable biomarkers for follow-up of Alzheimer’s disease progression [25].

Embryonically, the retina is a developmental outgrowth of brain with common similar structural features in between. Many anatomist consider retina as a peripheral part of central nervous system [26‐28]. For that, several clinical and histologic studies showed that nearly the same neurodegenerative processes that occur in the brain usually reflect on the retina. Recently, pathologists found marked loss of retinal ganglion cell (RGC) and their axons in the optic nerve in post-mortem studies of human AD patients` eyes [28].

Accordingly, as both brain and retina are like a double-faced coin, it is now accepted to relay upon the degenerative retinal changes in evaluation of many neurodegenerative diseases. Optical coherence tomography (OCT) is a noninvasive method for evaluating retinal changes that occur in neurodegenerative disease. The potentiality of its use as a biomarker in AD is progressing [29].

Optical coherence tomography is a uniquely safe method serves high resolution two-dimensional cross-sectional imaging and three-dimensional volumetric measurements which provides multiple parameters that allow to measure the volumetric changes in specified retinal layers [30, 31].

Our aim was to investigate plasma levels of tau and Aβ alongside with measuring the change of hippocampal volume and thickness of retinal fiber layer in patients who clinically diagnosed with AD spectrum. Consequently, we correlated those different biomarkers with the severity of illness.

This was a cross-sectional study that performed in outpatient clinics of neurology department in period from July 2020 to July 2021 after assent from Research Ethics Committee (REC) for Human Subject and animal Research at the Faculty of Medicine. The study included two groups: the first group enrolled 60 patients with AD diagnosed on clinical basis using mini mental state examination (MMSE) with exclusion of structural brain insult by performing MRI brain. The second group consisted of 60 healthy age- and sex-matched control volunteers.

Inclusion criteria were applied to individuals with cognitive impairment who meet the DSM-V criteria for AD aged 60 years or more and a written informed agreement either from the patient or his relatives was obtained.

While exclusion from this study included patients under age of 60 years old, given history of vascular or other neurological/medical disorders interfere with AD diagnosis (based on either clinical history, neurologic examination or radiologic findings), regular administration of medications known to cause cognitive impairment, known contraindication for MRI scanning or refusal of giving a written informed consent.

All patients and control individuals were subjected to detailed history taking, complete general and neurological examination focusing on the following: age, sex. MMSE and disease duration. Routine laboratory investigations; complete blood pictures (CBCs), liver function tests, renal function tests, hemoglobin A1c (HbA1c), vitamin B12 assay and thyroid function tests were performed for all patients.

Brain MRI scanning for hippocampal volumetric measurement, measuring serum tau, Aβ 1–40 and Aβ 1–42 were performed for both patients and control subjects. OCT for both right and left eyes was done for all patients. MMSEs is a brief neuropsychiatric rapid test that considered one of the most-known and the best often tool used as a short screening score for providing accurate measure of cognitive dysfunction in clinical, community and research settings [32]. The MMSEs Consists of eleven questions scored from 0 to 30 that assess problems with orientation, memory, registration, concentration, language and nonstructural praxis. The scores grouped into normal cognition for home achieved 27–30 points, mild dementia for home achieved 20–26 points, moderate dementia for home achieved 10–19 points, while score 0–9 points represent severe dementia [33, 34].

As for tau, amyloid beta peptide 40 and 42 plasma assessment: blood samples were drawn into ethylenediaminetetraacetic acid (EDTA) tubes for tau, amyloid beta peptide 40 and 42 plasma assessment, after centrifugation (2500 × g, +4 °C for 20 min), plasma samples stored at − 80 °C within 60 min of collection. Plasma levels of Aβ1–40, and A β1–42 quantified by EUROIMMUN β-Amyloid 1–42, and 1–40 plasma enzyme-linked immunosorbent assays [35].

Coming to MRI imaging: brain MRI scanning with hippocampal volumetric measurement on right and left sided for all participants were done using a 1.5 T magnetic resonance imaging machine (Magnetom Aera, Siemens healthcare, Germany) with a head coil. Conventional MRI sequences were including; axial T1-axial-weighted image, axial and coronal T2-weighted image, axial FLAIR images and diffusion-sensitizing gradients images.

MRI protocol and segmentation method was done for all participants of the study. The volumetric measurement of both right and left hippocampi were done using coronal oblique three-dimensional T1 magnetization-prepared rapid gradient echo (MPRAGE) sequence that was done perpendicular to the hippocampal longitudinal axis with the following MR parameters: repetition time (TR) equal 2400 ms, echo time (TE) equal 3.54 ms, field of view (FOV) equal 240 mm and slice thickness equal 1mm. In coronal oblique 3-D T1 MPRAGE right and left hippocampi were measured manually by tracing hippocampal boundaries starting from head to tail. The cerebrospinal fluid (CSF) seen in temporal horn uncal recess (between the amygdala and head of hippocampus) was used to detect the hippocampal head area. Hippocampal posterior boundary was defined by the crus of fornix. Hippocampal medial boundary was defined by CSF in uncal/ambient cisterns. Hippocampal lateral boundary was defined by CSF in temporal horn of lateral ventricle. Hippocampal inferior boundary was defined by the gray and white matter junction between para-hippocampal gyrus white matter and subiculum (Figs. 1, 2, 3, 4).

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As regard OCT on right and left eyes: Cirrus HD-OCT software version 6.0.0.599 (Carl Zeiss) was used to assess retinal thickness. Images were rejected if there were movement artifacts, segmentation errors or poor concentering on the fovea for the macular cube protocols. The Macular Cube 512 × 128 protocol was used for the mean macular and GCL plus IPL (GC-IPL) thickness measurements [36].

Statistical analysis: Data were entered and analyzed using IBM–SPSS software (Version 26.0). Qualitative data were expressed as N (%) and compared using chi-square test. Quantitative data were initially tested for normality using Shapiro–Wilk’s test, and z-scores of skewness and kurtosis with data being normally distributed if two of the three supports normality (Shapiro p > 0.050, zscores ± 2.58). The presence of significant outliers was tested for by inspecting boxplots. Quantitative data were expressed as mean ± SD and compared between two groups by Student’s t test. Point biserial correlation test was used to assess the association between a dichotomous and a continuous variable, while Spearman’s correlation test was used to assess the association between two continuous variables. The measures for assessing diagnostic test performance were calculated by confusion matrix online calculator (https://onlineconfusionmatrix.com, accessed February 24, 2023) and MedCalc Software Ltd. Diagnostic test evaluation calculator, Version 20.018 (https://www.medcalc.org/calc/diagnostic_test.php, accessed February 24, 2023). AUCs comparisons were run using MedCalc Statistical Software (version 18.9.1). For any of the used tests, results were considered as statistically significant if p value ≤ 0.050.

This study included 120 participants divided into two groups: patients group includes 60 patients with diagnosis of AD, while the control group included 60 age- and sex-matched healthy participants. After analysis of our results, we found that proteins tau, Aβ1–40, and Aβ1–40/Aβ1–42 ratio were significant discriminators of AD at cutoff values of >23.45, > 84.4, and > 1.95, respectively. MRI hippocampal volume in both right and left sides are also could be considered good discriminators of AD at cutoff values of ≤2.997, and ≤2.994, respectively. A significant correlations were reported between tau with Aβ1–40, Aβ1–42, MMSE and MRI right and left hippocampi volumes. On comparing moderate versus mild AD, there was a high significant levels of tau, Aβ1–42 and Aβ1–40/Aβ1–42 ratio.

Table 1 shows no statistically significant difference in age and sex between Alzheimer’s disease cases versus control subjects.

Table 2 shows that Tau, Aβ1–40, and Aβ1–40/Aβ1–42 ratio are statistically significant discriminators of AD versus control at cutoff values of >23.45, > 84.4, and > 1.95, respectively. Comparisons of the three AUCs revealed no statistically significant difference between the three AUCs (TAU versus Aβ1–40, difference = 0.066, p = 0.3671; TAU versus Aβ1–40/Aβ1–42 ratio, difference = 0.040, p = 0.5638, and Aβ1–40/Aβ1–42 ratio versus Aβ1–40, difference = 0.026, p = 0.7199)

Table 3 shows a statistically significant correlations between tau with Aβ1–40, Aβ1–42, MMSE and MRI right and left hippocampi volumes. There were statistically significant correlations between Aβ1–40 with tau, Aβ1–42, Aβ1–40/Aβ1–42 ratio and left temporal retinal thickness. In addition, there were statistically significant correlations between Aβ1–42 with Aβ1–40, Aβ1–40/Aβ1–42 ratio, MMSE and MRI right and left hippocampi volumes. Meanwhile, there were no statistically significant correlations between three biomarkers with sex, educational level, diabetes mellitus, hypertension, age in years and retinal thickness except between Aβ1–40 with left temporal thickness.

Table 4 shows a statistically significantly high tau, Aβ1–42, Aβ1–40/Aβ1–42 ratio, disease duration in moderate versus mild Alzheimer`s patients, while right average RNFLs, right superior, right nasal, left RNFLs, left nasal, right hippocampus, and left hippocampus showed statistically significantly low in moderate versus mild Alzheimer disease patients.

Table 5 shows that MRI hippocampus volume, both right and left are statistically significant discriminators of AD versus control at cutoff values of ≤ 2.997, and ≤ 2.994, respectively (Figs. 5, 6, 7).

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A diverse of pathological changes take place in the brain of AD patient. The cornerstone of which is accumulation of Tau and amyloid-β (Aβ) peptide going in hand with synaptic and neuronal loss particularly in hippocampus and neocortex. Consequently, a loss of functional connectivity ends in apparent cognitive deterioration hindering everyday life activity [37]. Some of those pathological changes begin to occur very early in the tempo of disease progression that correspond to the clinical phase of mild cognitive impairment (MCI) when the symptoms are not yet troublesome [38‐40]

Although many of previous studies failed to address an explanation for the accelerated rates of brain atrophy even prior to the stage of MCI, yet none argue about the hippocampus as the preferential affected brain region. Fortunately, the recent advance in using high-resolution MRI and voxel-based volumetry enabled the detection of even a minute atrophy of hippocampus in AD patients’ brains a decade before the dementia becomes overt [41‐43].

In the meanwhile, OCT is currently used in a widening manner to judge the degenerative processes occur in several neurologic disorders. The principle depends on measuring the extent of R NFLs loss and macular thickness and volume. For this reason, OCT is a biomarker with significant potentiality in monitoring AD progression and in the treatment strategies evaluation [44].

In our study, we did not found any statistically significant difference between AD patients and control subjects as regarded sex and age (P = 0.855 and 0.114, respectively).When we compared serum Tau level of patients with control group, we found that, there was statistically significant higher level of tau protein (p < 0.001) at cutoff of values >23.45 in patients, this was congruent with Nam and his colleagues 2020 as they found that serum tau was statistically significant higher in mild Alzheimer patients [45]. Mattsson and his colleagues 2016 reported that MMSE was inversely proportionate with serum tau protein levels in AD [46]. Definitely, this was accepted as the decline of MMSE scores matches with the clinical progression of the disease. This was similar to Koychev and his colleagues 2021 who reported that there is significant elevated level of blood tau protein in Alzheimer’s disease [47]. This can be explained by correlation between tau proteins with ongoing injured or degenerated axons that in turn reflect disease intensity [48]. On the other hand, some studies do not consider blood tau protein as a perfect biomarker of Alzheimer`s patient due to its short half-life and lack of analytical sensitivity for accurate measurement in blood samples. For that reason, a thorough effort is currently being done to develop an ultra-sensitive assay for tau in peripheral blood based on digital array technology making the assay sensitive to all known tau isoforms [49].

As regard Aβ1–40, we found that there was a highly statistically significant difference in patients when compared with control group (P = 0.003, 95% Cl 0.565–0.742) with cutoff value> 84.4 that was in agreement with Manafikhi and his colleagues 2021 they found that Aβ1–40 plasma level in AD showed statistically significant higher value when compared with cognitively normal individuals with sex and educational matching (P<0.001, 95% CI 1.012–1.051) [50]. Nonetheless, other researchers documented reduction in plasma concentration [51]. These contradictory results may be attributed to analytical interferences like epitoic masking that caused by hydrophobic amyloid beta binding to plasma protein. In addition, many undocumented factors concerning the origin of plasma amyloid beta or its production by proteolytic cleavage of APP that located in peripheral tissues could explain this conflict [52].

As regard our results on Aβ1–40, we did not noticed any statistically significant differences when compared with normal individual this was consistent with Nam and his colleagues 2020 they found no correlation or changes in serum amyloid beta with neurocognitive test assessment [45].

Our results also consistent with Manafikhi and his colleagues 2021 also with Soni and his colleagues 2021 they documented that Amyloid beta1–42 was significantly higher in Alzheimer disease than normally cognitive participants [50, 53].

Moreover, we found that Aβ1–40/Aβ1–42 ratio is also informative and reliable as we found highly statistically significant differences between patients and controlled subjects (P<0.001, 95% Cl 0.593–0.766) with cutoff value > 1.95. This was consistent with Nakamura and his colleagues 2018 they found Aβ40/Aβ42 ratio predict brain amyloid beta burden [20].

Comparisons of the three area under the ROC curves (AUCs) revealed no statistically significant difference in between the three AUCs (TAU versus Aβ1–40, difference = 0.066, P = 0.3671; TAU versus Aβ1–40/Aβ1–42 ratio, difference = 0.040, P = 0.5638, and Aβ1–40/Aβ1–42 ratio versus Aβ1–40, difference = 0.026, P = 0.7199). The discrepancy between the previous results and ours mostly due to differences in sample size, method of collecting samples or socioeconomic state.

When we did correlations of the three biomarkers (tau, Aβ1–40, Aβ1–42 and Aβ1–40/Aβ1–42) in relation to different characteristic parameters in our study we found that there were no statistically significant differences between the previous parameters as regard: sex, educational levels, diabetes mellitus, hypertension, age in years, right and left retinal thickness measurements in Alzheimer patients.

As regard tau protein, tau has negative statistically significant correlation with MMSE (P < 0.001). This was consistent with Tsai and his colleagues 2019, Nam with his colleagues 2020 and Mattsson and his colleagues 2016 they found close association of plasma tau level with impairment cognition and so strong correlation with MMSE score without gender or age dependent changes which indicate other specific factors for disease progression [45, 46, 54].

When we compared plasma tau protein with Aβ1–40 and Aβ1–42, there was positive correlation with statistically significant difference (P< 0.001 and P< 0.001, respectively). This was in agreement with Roda and his colleagues 2022 they found that tau and amyloid beta work cooperatively to impair gene transcription involved in synaptic function leading to tau down regulation that reverses partially perturbation transcription. Recently, supporting data proposed adding human tau was associated with increased plaque size, this display the ability of tau in facilitation of amyloid aggregation [55, 56].

Our research exhibited that there was significant correlation between plasma tau protein and volume of right and left hippocampus as measured by MRI volumetric study (P = 0.043 and P = 0.030, respectively). This results were congruent with Chiu and his colleagues 2014 who found that there was threefold increase in plasma tau in patients with AD with marked reduction of whole brain volume when compared with healthy control. To our last knowledge, there is no studies done to create a correlation between MRI hippocampal volumes—in particular—and plasma tau protein [57].

Coming to the studying of Aβ1–40 in relation to other parameters, we did not find any significant relation with sex, educational level, diabetes mellitus, hypertension, age, MMSE, right retinal layer thickness or right and left hippocampal volume. The significances were in between tau, Aβ1–42, Aβ1–40/Aβ1–42 and left inferior retinal layer thickness. However, why the correlation between the severity of AD and Aβ1–40 differs from that with Aβ1–42 still lacking evidence-base explanation. Jin and his colleagues postulated that the load of soluble amyloid—which aggregate extracellularly in brain tissue—contribute substantially to the burden of the disease, whereas the measurable isoform of amyloid in plasma is rather a different form. This could explain the appearing conflicting results [58].

Our research revealed statistically significant positive correlation between Aβ1–40 and Aβ1–42 (P < 0.001). This was consistent with Roher and his colleagues 2009 who found that Aβ1–40 and Aβ1–42 plasma levels were higher in Alzheimer disease patients [59]. These results may be explained based on data suggested that amyloid beta may enter through blood brain barrier (BBB) and enhance amyloid beta accumulation in the brain [50].

Our study exhibits negative statistically significant correlation between Aβ1–40 and left inferior temporal retinal nerve fiber thickness (P = 0.049). This means that the increase of Aβ40 level was associated with decreased thickness of RNFLs in left temporal quadrant that was congruent with other several studies which demonstrated significant thinning of superior and inferior quadrants [26, 31, 60, 61].

We cannot explain fully this finding that revealed the more susceptibility of left temporal quadrant in retina to show decreased thickness in AD. We think that this finding could occur initially, after a while and with the progression of the disease, all retinal quadrants will be affected similarly. The reason why the left retinal quadrant is the preferential initial zone may need further pathophysiological studies.

On studying the relation between Aβ1–42 and other parameters, we found statistically significant correlation with tau, Aβ1–40, Aβ1–40/Aβ1–42 ratio, MMSE, and degree both right and left hippocampal zone atrophy (P < 0.001, P < 0.001, P < 0.001, P = 0.016, P =0.016 and P = 0.039, respectively). The significant correlation between Aβ1–42 and MMSE (P < 0.001) in our study was in agreement with Wu and his colleagues 2022 who found plasma linear relation between amyloid beta1–42 and severity of Alzheimer disease patients in china (P < 0.01) as increased from early AD passing through moderate to severe cases, taking in consideration many factors that influence the relationship including gender, sample size and body mass index [62]. While Buerger with his colleagues 2009 and Giedraitis with his colleagues 2007 found that the plasma level of Aβ1–42 was lower in Alzheimer disease patients than normal individuals [63, 64]. The observation may be due to the hypothesis suggesting that the blood brain barrier does not allow the free passage of amyloid beta from cerebrospinal fluid into the blood through lipoprotein receptors related protein 1 (LRRP-1) with subsequent aggregations of Aβ1–42 in the brain [65].

On comparing mild versus moderate AD, in our study we found that there were statistically significant higher levels of Tau, Aβ1–42, Aβ1–40/Aβ1–42 ratio, disease duration in moderate versus mild Alzheimer patients (P = 0.007, P = 0.018, P = 0.021 and P < 0.001, respectively). On the other hand, the right hippocampus volume and the left hippocampus volume showed statistically significant low values in moderate versus mild Alzheimer disease patients (P < 0.001 and P < 0.001, respectively). However, there were no statistically significant differences as regarded sex, age, educational level, diabetes, hypertension, Aβ1–42, right inferior quadrant retinal nerve fiber thickness, left nasal retinal nerve fiber thickness and left temporal retinal nerve fiber thickness between mild and moderate AD.

Concerning tau level we are consistent with Ou and his colleagues 2021 who found that the level of tau was significantly increased from control subjects pass through mild cognitive impairment to AD [66] and with Mattsson and his colleagues 2016 they found inversely proportionate of MMSE with serum tau protein levels in AD [46]. Our study was incongruent with Seppala and his colleagues 2010 they concluded that the plasma levels of Aβ1–42 were decreased in serial measurements parallel to deterioration of cognitive decline assessment [67]. The inverse correlation between Aβ1–42 and both right and left hippocampal volume (P = 0.016 and 0.039, respectively) was consistent with Hilal and his colleagues 2018 who found that the same negative relation between hippocampal volume and plasma level of Aβ1–42 [68].

OCT showed significantly statistical decrease in thickness in right average RNFL, right superior quadrant, right nasal quadrant, right temporal, left average RNFL, left nasal quadrant in patient with moderate Alzheimer when compared with mild Alzheimer (P < 0.001, P < 0.001, P < 0.001, P = 0.041, P = 0.025, P = 0.009, respectively). Den Haan and his colleagues 2017 found that marked reduction in RNFL thickness in inferior and superior quadrants than temporal and nasal quadrants may be explained by presence of more neurons in superior and inferior quadrants [60].

As regard MRI volumetry: this study showed that MRI hippocampus volumetry for both right and left hippocampi were statistically significant discriminators of AD with cutoff values of less than or equal 2.997, and less than or equal 2.994, respectively. These were matched with the results of study of Dhikav and his colleagues 2017. They concluded that the atrophy of hippocampal is considered a sensitive marker for AD diagnosis due to the pathological changes that occurred in the hippocampal olfactory cortex at early stages of the disease [69].

Current study results were matched with the results of Chen and his colleagues 2022 who concluded that right, left and total hippocampal volumes reduced in Alzheimer`s disease compared to healthy controls subjects [70]. In addition, our results were matched with the results of Ferrari and his colleagues 2017 who concluded that magnetic resonance imaging hippocampal volumetry is a very specific test, with relatively lower sensitivity compared to the PET–CT [61].

Our study was matched with study of Dolek and his colleagues 2012 they concluded that AD had significantly lower hippocampal volumes than control groups and patients of vascular dementia, also concluded that hippocampal volume measurement could be used as an imaging biomarker in differentiation between healthy individuals, patients with vascular dementia, and patients with mild cognitive impairment from AD patients [71].

Mild cognitive impairment diagnosis is as crucial as it may progress to AD. The studies of Dolek with his colleagues 2012 and Jhoo with his colleagues 2010 concluded that patients with mild cognitive impairment had significantly lower hippocampal volume than in controls subjects [71, 72]. The study of Dolek and his colleagues 2012 concluded that hippocampal volume was different between moderate dementia patients than patients with severe dementia and both were different from normal cognitive function subjects as there is a relationship between hippocampal atrophy and the degree of cognitive impairment. This was similar to what we found in our study as there was a negative correlation between the temporal lobe volume and the MMSE score [71].

Finally, we found in our study increased disease duration was associated with deterioration of cognitive function (P = < 0.001) which was consistent with Zhao and his colleagues 2014 who found the rate of decline in the MMSE scores were 1.52 point/year with significant deterioration of neuropsychological test over time [73].

Limitations: the limitations in our study included small sample size and lack of reliable comparative analysis between blood and CSF tau and amyloid beta. Besides, there was unavailability in long-term follow-up for clinical, laboratory and radiological assessment that permit an adequate monitoring of the supposed biomarker changes throughout the disease progression.

The astonishing expanding prevalence of Alzheimer’s dementia as the world population growing more and more old owing to the improving provided healthcare forces us to establish a valid diagnostic platform to pick up those threatened patients early. Currently, several protocols are created for screening even the susceptible normal persons allowing for very early intervention, including vaccination, to put a safe guard against developing AD. Recent advanced technology in medical field enabled usage of rapid, noninvasive, non-burdensome and less expensive methods for early diagnosis of AD. After accomplishing our study, we can conclude that assessment of plasma amyloid level, measuring plasma tau protein and precise detection of early reduction of hippocampus volume by MRI volumetric study can efficiently diagnose AD. Besides, detection of reduced retinal nerve fiber layers by OCT can add much for early diagnosis. All of the aforementioned biomarkers can lead in the end to better therapeutic and—hopefully—preventive strategies for AD.