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Grupo de Óptica y Percepción Visual (GOPV), University of AlicanteDepartment of Ophthalmology (OFTALMAR), Vithas Medimar International HospitalFoundation for the Visual Quality (Fundación para la Calidad Visual, FUNCAVIS), Alicante, Spain
Age-related neurodegenerative diseases, such as Alzheimer’s disease (AD), are conditions in which a degenerative process affects the central nervous system, as well as visual functioning.
is a relatively new clinical tool that allows the clinician to analyze the visual function for the magno-, parvo-, and koniocellular pathways, with the possibility of detecting incipient alterations in each of these visual pathways that could not be detected in a conventional perimetric exam.
A 78-year-old female with AD and mild cognitive impairment had been treated for 6 months before our visual examination. She had a manifest refraction of plano, uncorrected distance visual acuity of 20/40, and near addition of +2.50 D OD. She had a manifest refraction of plano, corrected distance visual acuity of 20/25, and near addition of +2.50 D OS. Intraocular pressure was 13.5 and 14.5 mm Hg OD and OS, respectively. No abnormalities were detected in the fundoscopic examination. Both eyes were pseudophakic with clear posterior capsule. No alterations were detected in the examination of the macular structure by optical coherence tomography (Fig. 1).
Fig. 1Retinographies (top) and macular scans obtained by optical coherence tomography (bottom) for both eyes.
The results of the color vision examination (FM 100 Hue test) showed a total error score out of the range of normality and worse colour discrimination OD. Specifically, the red-green (R-G) and blue-yellow (B-Y) partial errors were higher than those established as the mean values for the age-matched normal population. The error value in the R-G axis was worse OS, whereas the error value in the B-Y axis was worse OD. Regarding achromatic contrast sensitivity evaluation, a decrease in the contrast sensitivity functions (CSFs) was observed OS for intermediate spatial frequencies, whereas the CSF OD was within the range of normality defined by the manufacturer (Fig. 2).
Fig. 2Achromatic contrast sensitivity function obtained by means of the CSV-1000E test for both eyes. cpd, cycles per degree.
Achromatic channel of magnocellular origin (A): 0.5 cycles per degree (cpd) and 24 Hz
•
Parvocellular R-G channel 0.5 cpd and 0 Hz
•
Koniocellular B-Y channel: 0.5 cpd and 0 Hz
The results of this test are displayed in Figure 3 and are summarized in Table 1.
Fig. 3Results obtained with the multichannel perimetry. Top: achromatic channel; centre: R-G channel; bottom: B-Y channel. Left: right eye; right: left eye. In each map of each eye. the sensitivity (left), total deviation (centre), and pattern deviation (right). R-G, red-green; B-Y, blue-yellow.
For each mechanism evaluated, the values of the MD and PSD are shown, as well as the statistical significance (p) of the comparison with the normal healthy pattern.
Achromatic
R-G
B-Y
RE
LE
RE
LE
RE
LE
MD
–1.00
–0.30
–4.49
–2.80
–8.06
–5.35
p-value MD
0.04
0.42
0
0
0
0
PSD
2.87
3.05
3.41
3.07
2.26
1.86
p-value PSD
0
0
0
0
0.11
0.20
R-G, red-green; B-Y, bue-yellow; RE, right eye; LE, left eye; MD, médium defect; PSD, pattern standard deviation
* For each mechanism evaluated, the values of the MD and PSD are shown, as well as the statistical significance (p) of the comparison with the normal healthy pattern.
Multichannel perimetry measures the detection thresholds in a horizontal field of 60 × 40 degrees, centred at the fovea, and at a viewing distance of 25 cm. It allows the clinician to detect incipient alterations in magno-, parvo-, and koniocellular pathways that cannot be detected in a conventional perimetric exam. To date, this technology has been reported to detect impairment in the achromatic and R-G and B-Y chromatic mechanisms in a case of essential tremor—a neurologic disorder of unknown cause—characterized by the presence of an 8–12 Hz kinetic tremor of the arms, often accompanied by head and voice tremors.
In the present case, the medium defect was negative for all channels in both eyes (Table 1). The defect was small and very localized in the achromatic channel, whereas the losses were significant and covered a wider area in the chromatic channels, especiallly in the B-Y channel. Blue hue-specific colour discrimination deficits have been previously reported in AD,
but using tests only evaluating the central visual field. In contrast, other authors have described nonspecific colour vision deficiencies independently of the severity of the disease.
Likewise, high values of pattern standard deviation were obtained in the present case that indicated the presence of localized losses that were statistically significant compared with the normal healthy pattern
in the achromatic and R-G channels. In other studies, the reduction in colour discrimination capacity in AD has been found to depend on cognitive impairment and the involvement of the primary visual cortex and/or retinal ganglion cells.
The results of multichannel perimetry were consistent with those obtained with the FM 100 Hue test for the B-Y channel. In contrast, this did not happen with alterations detected in the R-G channel. Therefore, multichannel perimetry was more sensitive than the FM 100 Hue test in the present case for the detection of colour vision alterations in the central visual field. These results suggest that some alterations in the R-G and B-Y visual pathways may be present in cases of AD compared with the healthy population that cannot be detected with conventional colour vision tests.
The results obtained in the multichannel perimetry for the achromatic channel were relatively consistent with the degradation of the achromatic contrast sensitivity detected for medium spatial frequencies with the Contrast Sensitivity Vision 1000E (CSV1000E) test in the left eye. Specifically, the spatial frequency tested (0.5 cpd) with the multichannel perimeter did not show exactly the same values than the CSV1000E test, but it should be taken into account that the multichannel perimeter evaluates all the retina and the stimulus flickers at 24 Hz, whereas with the CSV1000E test the stimulus had temporal frequency 0 and was localized at the center of the visual field. In the right eye, the degradation of the achromatic channel detected with multichannel perimetry was not observed in the contrast sensitivity evaluation. This is to be expected considering most of losses in the achromatic channel OD were peripheral. Some studies have also confirmed the presence of deficits in contrast sensitivity in patients with AD, even in cases of mild cognitive impairment.
Some achromatic and colour vision alterations may be present in AD that cannot be detected with conventional colour and contrast sensitivity tests, such as the FM 100 Hue or the CSV1000E. It is possible that currently available tests for evaluating colour vision do not accurately isolate the different chromatic mechanisms and are based on stimulation of the central visual field, without taking paracentral chromatic sensitivity into account. Future studies should investigate whether these colour vision alterations detected by means of a multichannel perimetry may be used for the detection of mild impairment or earlier stages of this neurodegenerative disorder.
References
Tzekov R.
Mullan M.
Vision function abnormalities in Alzheimer disease.
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