ALTERATIONS OF THE COLOR VISION IN ELDERS
Received for publication: 12th of November, 2004
Revised: 5th of April, 2005
SUMMARY: (Hide the summary)
The ageing process affects the eye as part of the whole human body. The troubles are initially placed at the borderline between physiology and pathology, hence the need of a very sensitive instrument to study them, instrument able to detect minimal changes. The color vision, as the phylogenetic most recent developed function of the visual analyzer is such an instrument. The color vision defects can be caused by the alterations of the clear medias (troubles of light absorption), of the retina (the receptor level) or of the central nervous system (post receptor level). For the transparent media ageing process pleads an acquired green-red dyscromatopsia whit early alteration of the red component, which progress as the refraction index of the lens increases, but does not realize a true tricromazia. For the receptor cells of the macula (cones) ageing process pleads a yellow-blue disorder and when it associates visual acuity impairment also appears a deviation to red of the Rayleigh equation. On central level, occipital lobe disorders manifest themselves by color amnesia or agnosia (disturbance of the verbal and visual association between objects and color) or color aphasia (disturbance of the vocabulary for color description).
During lifetime, the human organism is subject to wear, and the first affected are the most delicate mechanisms. The troubles are initially placed at the borderline between physiology and pathology, hence the need of a very sensitive instrument to study them, instrument able to detect minimal changes. The color vision, as the phylogenetic most recent developed function of the visual analyzer is such an instrument applicable on the human eye as part of the ageing organism. The color vision is the most delicate and sensible, the most „human” sense, that is why it is the first to modify during local or general pathological states. Alas, this signal-value is little to benefit for now just for the multitude of factors that can impact on, and also because of the subjectiveness of the tests we dispose to evaluate it.
In elders, chromatic perception can be altered at different levels:
- the clear medias (alteration of prereceptoral filters)
- the receptors (reduced cone photo pigment optical density, greater loss of one cone type than the others)
- the central analyzer
Often, those mechanisms overlap, resulting in atypical, nonspecific, noninterpretable dyschromatopsies.
Material and method
To illustrate the different types of color defects we selected two groups of patients with isolated pathology of the eye; we eliminated from the study those patients with systemic diseases that can potentially lead to color alterations – diabetes mellitus, chronic liver diseases, alcoholism, and heavy smokers.
The cortical pathology (occipital lobe stroke, multiple disseminated micro strokes) did not represent a subject to this study because of the specific symptoms – color aphasia, amnesia, agnosia, which necessitates neurological instruments to highlight.
- Group A – cataract (clear media pathology)
- Nuclear cataract
- Cortical cataract
- Group B – central senile degenerative chorioretinal lesions (receptor pathology)
To assess the color vision troubles we used the following tests:
- plates from Velhagen and Rabkin test types – for screening; they are portable, easy to administrate;
- special plates – „P” plates from Rabkin – to asses the saturation threshold;
- Panel D-15 test – to identify the axis of the color defect;
- AN 59 anomaloscope – for the quantitative assessment of the chromatic defect and for quantification of the chromatic discriminative threshold by tone.
Group A – included 67 patients by the age between 65 and 82 years (mean age 73, 5 years) subdivided in two subgroups:
- Nuclear cataract – 31 patients – presenting a more refringent nucleus, reddish-brown, associated with a index myopia;
- Cortical cataract without volume changes of the lens – 36 patients.
All those patients presented a visual acuity with the best correction over 5/50 (over 10%).
We determined that:
- the color vision defects appear within group 1 by a visual acuity of 5/7-5/10, and within group 2 they are absent by the visual acuity of 5/50 considered in this study. In fact, in the second group, the visual acuity drops firs, early before the chromatic test can yield significant results. For this reason, all comments are referring to the first group patients.
- the first affected is the discriminative threshold for red; the thresholds for blue and green increase very little even in advance states of nuclear densification;
- along with the deterioration of the visual acuity (V.A. under ½) difficulties appear on pseudoisochromatic plates; first the patients cannot read/make reading errors on the green plates (Velhagen plates no. 8 and 19, Rabkin plates no. V, VI, X and XIV), then also on the red plates (part of the Velhagen plates no.3-23, Rabkin plates no. VII, XIX, XX); by now we can affirm a red-green color defect.
- the last test affected is the arrangement test Panel D-15. Usually, the errors are minor, the most we can observe 1 major error in the green axis, maximum two major errors in the red-green axis.
- as for the colorimetric equation, the one that evaluates the most accurate in quantitative manner the value of the color deficit never attends a true pseudodeuteranomalia (Q = 1,8-17), but overpasses to green the strictly normal limits (N=0,8-1,18), by registrating values between 1,1 and 1,3 (table 1).
In conclusion, in nuclear cataract the identifiable dischromatopsia is a minimal form of pseudodeuteranomalia (red-green chromatic deficit type II), similar to the one encountered in medium myopia.
The demonstration is made during the surgical removal of the lens – 27 patients in a third group (C) underwent ECE (extra capsular extraction) of the lens followed by intraocular artificial lens implantation in the capsular bag. In all those patients the chromatic tests resulted in normal responses (Table 2).
In literature we encounter studies made on aphakic patients (after surgical removal of the lens). Cernea demonstrates that the thresholds for red and green are directly opposed to the ones in cataract: the red regains the normal discriminative threshold and the threshold for green becomes altered reaching the values of the red after the surgery.2,4 The settings (matchings) are made at 29-30 lines on the D scale (pseudoprotanomalia). If repeated with adequate spectacles the color deficit disappears. The patients cannot read the pseudoisochromatic plates without proper spectacles, but they promptly recognize the plates by wearing the optical correction.
Group B included 42 patients with ophthalmoscopic changes on the posterior pole suggestive for the „dry” form of age-related macular degeneration (ARMD), ranging from lack of foveolar reflex, macular microdrusen, irregular aspect of the macular area with pigment scattering to atrophic areas of the retinal pigment epithelium.
The hemorrhagic form of the ARMD and the form with choroidal neovascularization (CNV) were not taken in to the study because of the important loss of visual acuity (below 5/50). The tests were performed using the near correction. If associated, the lens opacities were strictly located at the cortical periphery of the lens (23 patients).
We determined that:
- the color troubles appear first at a visual acuity of 5/7;
- the discriminative thresholds by tone were higher for all the three fundamental colors, earlier and more prominent for blue, followed by green and finally by red;
- the abnormal quotient Q was normal in 23 patients (54,7%) and slightly deviated to red (pseudodeuteranomalia) in 29 patients (45,3%).
For the last ones, the abnormal quotient Q ranged between 1,4 and 1,84. For the entire patients the setting range was significantly enlarged, indifferent to the calculated mid point (normal or shifted); 9 patients realized full-scale dichromate- type equalization (table 3).
- On the arrangement Panel D-15 test the patients made no major errors by an visual acuity higher of 5/10, but if the visual accuity was worse, we recorded 1 to 3 errors on the blue-yellow axis.
- the pseudoisochromatic plates of Rabkin and Velhagen were readed fairy correct, if we take into consideration that we have only a few specific plates for the blue axis defect (3 plates in the Velhagen test type and none in the Rabkin test type). We recorded 2-4 inconstant, unorganized errors on Rabkin plates and on Velhagen plates 24, 25 and F1 (see table 4.)
- the most interesting aspect emerged from the special Rabkin P-plates: the saturation levels for red and yellow were not altered. On the contrary, especially the blue but also the green are termed as “grey” by low and medium saturation levels and they become distinct as color by the 12-14 spot. 4 patients (9,5%) were unable to distinguish the color, calling it “black” even on very high saturation level.
Hence, the dischromatopsia in ARMD is more prominent as the one in cataract, but we had fewer instruments to diagnose it. It is a tritanopia (blue-yellow defect), evolving from trichromasia to dichromasia. In literature we encountered the end stage – the absence of color perception (achromasia).
Testing color vision troubles on lenses with transparency alteration are based on the premises that the biconvex lens serves for two purposes into the eye: to focalize the image on the retina and to absorb the light with wavelength under 300 nm.
Experimental observations on senescent lenses pointed out that the inferior limit of the absorbed wavelength rises at 450 nm. The modified absorption of the light through a lens with higher refractive index causes part of the chromatic troubles in cataract - the denser nucleus absorbs the radiations with lower wavelengths near green and allows prevailingly the passing trough of the red wavelengths, which are desaturated. On the other part, the augmentation of the refractive index of the lens determines the shift of its focal point in front of the retina. While the different wavelengths focalize different along the optical axis results the displacement of the focal point of the lens in the lower wavelength (green) area. This is the motive for the patient needs more red to obtain chromatic equalization.
In age-related macular degeneration the altered chromatic perception is caused obviously by color receptors destruction (the cones reside only in the macular area). The axis of the defect can be explained by both theories of color perception. The trichromatic theory of color vision is based on the premise that there are three classes of cone receptors subserving color vision – one for blue (S - cones “short wavelength”), one for green (M - cones “medium wavelength”) and the third for red (L – cones “long wavelength”). According to this, the blue axis defect results from the undifferentiated destruction during the aging process of all the three types of cones; but the number of the S – cones is significantly less that the others – hence the resulting quantifiable defect will be on the blue axis. The Hering’s Theory of opponent color states that there are 3 channels transmitting color information in the visual system, which perform in couples: red-green, blue-yellow and black-white. Stimulation of one member of a couple produces the sensation of the corresponding color. According to this theory, the blue defect results from preponderant destruction during aging process of the blue-yellow channels, which are thinner, more fragile and more susceptible to hypoxia.
- In nuclear cataracts we can identify a minimal form of pseudodeuteranomalia (red-green type II defect) similar to the one in medium myopia. Cortical cataract does not induct a color deficit.
- An identifiable dischromatopsia in a cortical anterior/posterior or white cataract means an associated lesion on receptor/post receptor level;
- In all types of cataract, a blue-yellow axis defect means an associated macular pathology;
- In all types of cataract a significant dischromatopsia (an abnormal quotient higher than 1,4) means an associated lesion of the optic nerve/optic pathways.
- In ARMD we can identify a more important dyschromatopsia that the one encountered in cataract. It is a blue-yellow axis defect that evolves from dichromatic to trichromatic stage. In the end-stage we encountered in literature mentioned the absence of color perception (achromatic stage).
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2. Cernea P. , Constantin Florica – Vederea culorilor , Ed. Scrisul Romanesc, Craiova 1977
3. Grutzner P – Aquired color vision defects – in Hurvich LM Handbook of sensory phisiology vol III/4 – visual psychophisics
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