Alzheimer disease (AD) is the most common degenerative dementia and causes a progressive decline in cognitive function. In most cases, an episodic memory deficit is the predominant initial complaint. As time passes, changes in daily living activities, cognitive functions, and visual disturbances supervene.
Visual disturbances consist of impairment in spatial contrast sensitivity (1), motion perception (2,3), color discrimination (4), and blurred vision (5). Several recent articles have attributed the visual dysfunction in AD to damage in primary visual cortex and to degeneration of higher cortical areas (6-8). Other studies have shown evidence of pre-cortical involvement on the basis of reduction in the number of retinal ganglion cells and axons of the optic nerve (9,10). In contrast to these reports, some histopathologic studies have shown no retinal nerve damage in AD (11,12). Although recent studies have used sophisticated imaging techniques, such as optical coherence tomography (OCT), scanning laser polarimetry, and pattern electroretinography (PERG), to assess the morphologic and functional changes of the retina in AD, disagreement about retinal involvement persists (13-15).
The macula is defined anatomically as that region of the retina where the ganglion cell layer has a thickness of more than one cell. The ganglion cells and retinal nerve fiber layer (RNFL) contribute 30% to 35% of retinal thickness in the macula, where the ganglion cells are known to be most concentrated (16). Blanks et al (17) histologically observed a total decrease of 25% of neurons in the ganglion cell layer at the level of the fovea/parafoveal retina in AD. The greatest decrease was in the foveal region. To our knowledge, no previous studies have investigated macular thickness and volume in living AD patients.
OCT is a relatively new non-invasive, non-contact, transpupillary imaging technology that provides high-resolution cross-sectional images of the retina. OCT has been reported to be useful in assessing glaucoma, diabetic neuropathy, and macular edema (18-22).
The aim of this study was to investigate whether a correlation exists between structural (RNFL thickness, macular thickness and volume) and functional (visual evoked potential [VEP]) measures and cognitive impairment in AD.
We compared 28 eyes of 14 patients with AD to 30 eyes from 15 age-matched control subjects. The AD patients were obtained from the Kocaeli University Neurology Department Dementia Clinic; the control subjects were obtained from the Kocaeli University Eye Diseases Department General Clinic, Kocaeli, Turkey. The patients met criteria for probable AD set by the National Institute of Neurological and Communicative Disorders and Stroke-Alzheimer's Disease and Related Disorders Association (NINCDS-ADRDA) (23) and DSM-IV. AD patients had mild and moderate cognitive impairment according to the Clinical Dementia Rating (CDR) scale (24). All participants were free of ocular disease and systemic disorders affecting vision. Informed consent of AD patients was obtained from their first-degree relatives. The research followed the tenets of the Declaration of Helsinki, and the protocol was approved by the local ethics committee.
Each AD patient underwent a detailed neurological examination including laboratory, neuro-imaging evaluations, and psychometric testing. The control subjects also underwent a detailed neurological examination to rule out the presence of cognitive impairment. We excluded patients with dysmetabolic diseases, a history of alcohol abuse, psychiatric disorders, or other neurological diseases. Mini-Mental State Examination (MMSE) (25), Blessed short orientation, and Clock drawing tests for clinical evaluation were used to assess cognition in AD and control subjects.
All AD patients and control subjects underwent a complete ophthalmologic examination, including assessment of visual acuity, refraction, ocular motility, pupillary reflexes, anterior and posterior segment biomicroscopy, applanation tonometry, dilated fundus examination, and Octopus 101 perimeter program G2 visual field testing. The visual field results were uniformly normal in AD and control subjects. All participants had a corrected visual acuity of 5/10 or better with a refractive error between ± 3 spheric diopters and intraocular pressures less than 22 mmHg. Eyes with posterior pole pathology such as macular degeneration, glaucoma suspect, or glaucoma, or patients with media opacification such as cataract that prevented ocular and OCT examination were excluded.
Visual Evoked Potential Examination
All AD patients and control subjects underwent VEP examination. The VEP was generated using a black-and-white checkerboard pattern on a television monitor with a dimension of 5 × 5 cm for every check under the following conditions: contrast 95%, check size subtense 50', reversal rate 2 sec−1, mean luminance 12 Cd.m−2 field size 17° × 14°. One hundred stimulus epochs of 200 msn were obtained in each average and the amplifier bandwidth was 1-100 Hz.
Patients sat one meter away from the monitor. Ag/AgCl cup-shaped electrodes fixed with collodion were placed over the left and right occiputs at O1 and O2 with a common reference at Fpz and a ground on the left arm. The patients' gaze was fixed on a point at the center of the television monitor monocularly. The bioelectric signal was filtered (bandpass 0.5-200 Hz). Two hundred responses were averaged for every trial (Neuropack Nihon Kohden, MEB-5504 K, Tokyo, Japan). The analysis time was 250 milliseconds. Sweep length was 300 milliseconds (30 msec/div) and stimulus rate was 1 Hz. The normal range of P100 peak latency is 100.26 ± 7.04 milliseconds based on testing of normal controls in our laboratory. Peak latencies of the P100 component were measured.
Optical Coherence Tomography Examination
All AD patients and control subjects underwent OCT examination. The RNFL thickness, and macular thickness and volume were measured by OCT Model 3000 unit (Model 3000, software version A1.1, Carl Zeiss Meditec, Inc., Dublin, California, USA) after pupillary dilatation. Tomography images were constructed from a series of axial reflectance profiles (A-scans) over 2 mm of depth in less than 1 second. Retinal thickness and RNFL thickness were calculated by processing the cross-sectional images using computer algorithms to detect boundaries by searching each A-scan for the highest rates of changes in reflectivity. The software allows the mapping of the thickness data according to quadrant-by-quadrant and clock-hour analyses. Retinal thickness was determined by computer as the distance between the first reflection at the vitreo-retinal interface and the anterior boundary of the second reflective layer, corresponding to the retinal pigment epithelium and the choriocapillaris. RNFL thickness was automatically assessed by computer assuming the correlation with the red highly reflective layer at the vitreo-retinal interface. The posterior margin of the RNFL was automatically located by starting within the photoreceptor layer (posterior) and searching forward in the image (19,26,27).
Throughout scanning, the patient kept both eyes constantly fixed on an internal target provided by the equipment. Each subject eye underwent fast RNFL and macula scan protocols. Mean OCT values were calculated from the values of the three scans. One of the authors (OA) performed the image acquisition and judged its quality. Scans with poor image quality were defined as scans with signal-to-noise ratio of less than 45dB or excessive eye movement during measurement. We excluded three patients who had difficulty in cooperating with testing.
The fast RNFL scan protocol consisted of three consecutive 360° circular scans with a diameter of 3.4 mm centered on the optic disc, each containing 256 A-scans taken in a single session of 1.92 seconds. The parapapillary RNFL thickness parameters evaluated in this study were average thickness (360° measurement), temporal quadrant thickness (316° to 45°), superior quadrant thickness (46° to 135°), nasal quadrant thickness (136° to 225°), inferior quadrant thickness (226° to 315°), and thickness for each 12:00 position with the 3:00 as nasal, 6:00 position as inferior, 9:00 as temporal, and 12:00 position as superior.
The fast macula scan protocol consisted of six consecutive 6 mm radial line scans centered on the macula, each containing 128 A-scans taken in a single session of 1.92 seconds. Six sets of intersecting and equally spaced scans were obtained, each crossing the central fovea. The retinal thickness/volume tubular analysis program was used to evaluate macular scans. This analysis program presents mean foveal thickness and total macular volume in 3.5 and 6.00 mm macular maps. Macular retinal thickness data were displayed in three concentric circles. The central disk was the foveal region measuring 1.00 mm in diameter. The inner and outer rings were each divided into four quadrants; the rings had diameters of 3 mm and 6 mm, respectively, in 6.00 mm macular maps. An average retinal thickness and volume were reported for each of the nine regions.
The data are reported as mean values ± standard deviation (SD). The differences between AD and control eyes were statistically evaluated with the Student t test. To assess whether a correlation existed between OCT, VEP, and clinical severity of disease, Pearson's correlation test was used. P < 0.05 was considered significant.
The mean ages of AD and control groups were 70.1 ± 9.7 years and 65.1 ± 9.8 years, respectively. There was no statistically significant difference in age and gender between groups. Demographic and clinical data of AD and control groups are shown in Table 1. Examples of OCT and VEP recordings from control and AD eyes are shown in Figure 1.
Optical Coherence Tomography
The mean RNFL average thickness was significantly reduced in AD patients (87.46 ± 23.78 microns) when compared with control subjects (113.16 ± 6.72 μm) (P < 0.05) (Table 2). The RNFL thickness in all quadrants and positions of AD patients was thinner than in control subjects. RNFL differences were statistically significant (P < 0.05) except in the temporal quadrant, 8:00 and 9:00 positions (Table 1).
The retinal thickness in all quadrants of the macula of AD patients was less than in control subjects. This thinning was prominent in the nasal, temporal, and inferior quadrants (P < 0.05) (Table 2). The macular volume of the AD patients was less than that of control subjects in all macular regions except the foveal minimum. The reduction in macular volume was statistically significant except in the nasal and inferior inner quadrants and the temporal and superior outer quadrants of the macula. The mean total macular volume of AD patients (6.80 ± 0.41 mm3) was significantly reduced when compared with that of control subjects (7.10 ± 0.23 mm3) (P < 0.05) (Table 1).
Visual Evoked Potentials
No significant difference was found in the latencies and amplitudes of the VEP P100 of AD patients (107.96 ± 9.93 msec) (16.35 ± 2.69 μv) and control subjects (107.80 ± 10.40 msec) (18 ± 3.59 μv) (P > 0.05) (Table 1).
Correlation between Optical Coherence Tomography, Visual Evoked Potential, and Mini-Mental State Examination
A highly significant correlation was found between total macular volume and MMSE scores in AD patients (r = 0.696; P = 0.006) (Fig. 2). There was no correlation between OCT and VEP changes (Table 3).
The results of our study suggest a significant reduction in parapapillary RNFL, macular thickness and volume in patients with AD. We have also demonstrated a highly significant correlation between total macular volume and MMSE scores. Structural changes were not significantly correlated to the VEP changes.
Our data are consistent with histologic studies (9,10) and other methods of evaluating the RNFL and optic nerve in vivo (15,28), which demonstrate substantial decline in the quantity of optic nerve fibers and a degeneration of retinal ganglion cells in AD. Optic disc pallor, pathologic disc cupping, and thinning of the neuroretinal rim and the RNFL have been reported in two clinical studies based on the subjective evaluation of fundus photographs (28,29). Tsai et al (28) have observed an increased cup-to-disc ratio, cup volume, and decreased disc rim area in AD patients by optic nerve analyzer. Parisi et al (15) demonstrated a reduction in RNFL thickness by using OCT and suggested that this morphologic abnormality is related to retinal dysfunction as revealed by abnormal PERG responses.
Our data indicate not only a significant decline in parapapillary RNFL but also in macular thickness and volume in AD eyes. These findings suggest a loss of retinal ganglion cells in AD patients. To our knowledge, no report has previously documented the macular volume and thickness in vivo in AD patients. These findings are in agreement with the postmortem study of Blanks et al (17) demonstrating a total decrease of 25% of neurons in the ganglion cell layer at the level of the foveal and parafoveal retina.
In the pathophysiology of AD, beta-amyloid peptides that are cleaved from the amyloid precursor protein (APP) play a critical role. In the later stages of the disease, beta-amyloid peptides compose the characteristic pathologic findings of AD, including neurofibrillary tangles and neuritic plaques. The cortical degeneration characteristic of AD is present especially in visual association areas. Histopathologic studies have disclosed the pathologic hallmarks of AD (B-amyloid, tau, and APP neurofibrillary tangles and neuritic plaques) in subcortical visual centers, including the lateral geniculate nucleus and superior colliculus (31) but not in the retina (9,30). On the other hand, degeneration of retinal ganglion cells and their axons in the nerve fiber layer has been reported (10). Morphometric analysis has shown that in AD, the optic nerve has predominant loss of the largest class of retinal ganglion cells (M-cells) (10).
The loss of retinal ganglion cells may be a primary process or a consequence of retrograde neurodegeration occurring in the cortical regions.
We found no abnormalities in VEP in our AD patients. Normal pattern VEP responses have been described before in AD, although there is evidence that the P2 component of pattern VEP is delayed (32,33). In an earlier report, progressive increase in the latency of the flash VEP was related to the severity of dementia (33). These findings stem largely from the fact that flash VEP reflects the abnormality of visual association regions of the brain while pattern VEP shows the function of primary visual cortex and visual pathways (34). Sparing of the primary visual cortex with extensive cortical disease has been shown in AD (33) with positron emission tomography and histopathologic studies (35,36). The normal VEP responses in our study suggest that primary cortical region and optic nerve function is normal despite considerable RNFL loss.
The significant correlation we found between MMSE scores and macular volume as measured by OCT may be useful in providing a basis for further studies evaluating the effect of disease severity on RNFL.
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