COLOR VISION CHANGES IN GLAUCOMA

 

Color vision defects in glaucoma have been known since 1883.

Köllner in 1912 divided color vision defects into “blue-yellow” and “progressive red-green blindness”. This classification is known as Köllner’s rule.

The human visual system is capable of processing chromatic (i.e., color) information, apart from achromatic parameters, such as visual acuity (VA) and contrast sensitivity.

Reduced sensitivity to colors has been described in patients with ocular hypertension, tilted discs, and various forms of glaucoma, and may precede any detectable loss of peripheral or central vision by standard acuity or visual field testing.

Retinal Cone cells are classified by their pigment into three types: L-, M-, and S-cones. 

The L-cones are the most common type (more than 50%), followed by the M-cones, with the S-cones comprising only a very minor proportion (less than 10%) of the cone cells.

The S-cones are highly susceptible to damage, which might lead to acquired blue/yellow color vision deficiency.

However, in experimental glaucoma there is preferential loss of L- and M-cones reported, leading to acquired color vision loss. This suggests that the ability of the eye to sharply resolve objects of different colors, known as color visual acuity (CVA), might change with the progression of glaucoma. In early, mild-to-moderate glaucoma blue-yellow and in advanced disease red color defects are found to develop.

Blue signals are detected by the short-wavelength cones, and then processed by the blue-yellow bistratified ganglion cells. These cells project their axons to the interlaminar koniocellular layers of the lateral geniculate nucleus.

Blue cones contribute little to the sensation of brightness or to visual acuity, which may account for why standard visual acuity tests, perimetry, or contrast sensitivity studies might miss an associated visual deficit.

The color visual dysfunction is strongly related to elevated intraocular pressure (IOP) levels suggesting that the damage is pressure induced.

It is unclear whether the loss of color vision and the visual field changes associated with nerve fiber bundle loss share the same mechanism.

Ocular hypertensive eyes with yellow-blue and blue-green defects were found to have diffuse early changes in visual field sensitivity and an increased risk of glaucomatous visual field loss, compared with similar eyes that did not have these color vision disturbances.

The same color abnormalities in patients with early glaucoma correlated significantly with diffuse retinal nerve fiber loss. However, some studies did not find significant correlation between color vision scores and visual field performance among patients with ocular hypertension when age correction was applied to the color variable, and another study revealed no clear association between early glaucomatous cupping and color vision anomalies.

Specificity of color vision loss in glaucoma is limited by the fact that the tritan deficit is also the one most frequently seen with age-related changes. When study populations were matched for age and lens density, however, color vision loss in glaucoma was still attributable in part to the disease process.

 

Optic nerve disease often produces a type II (red-green) deficiency, but if visual acuity is preserved then the predominant colour deficiency is type III (blue-yellow). In early glaucomatous optic neuropathy, paracentral scotomas and a reduction of sensitivity in the arcuate regions are common visual field defects, while visual acuity is spared; hence the most frequent chromatic anomaly associated with POAG is a type III defect.

Prevalence estimates for the different types of colour vision defect in POAG have been obtained using a variety of noncomputerised tests. Based on these reports typical prevalences are 20–40% for normal colour discrimination, 30–50% blue-yellow defects, 5% for red-green defects, and 20–30% for a general loss of chromatic discrimination.

Several possible explanations have been suggested for this predominance of blue-yellow (tritan-like) defects in POAG, including: + short wavelength cones or their neuronal connections are less able to resist the effects of raised IOP+ there is selective damage to blue-yellow sensitive ganglion cells or their axons.

Blue-yellow ganglion cells have larger receptive fields, are larger than red-green cells, and have a unique morphology and connectivity to second order neurons, which may make blue-yellow ganglion cells more susceptible to IOP related damage + the relative scarcity of ganglion cells which code blue-yellow signals, and the relatively little overlap between adjacent receptive fields of these ganglion cells.

Consequently, although only a few ganglion cells may cease to function, there is preferential impairment of the blue-yellow discrimination threshold compared with red-green, even if the proportion of damaged fibres is the same for both types.

Specific losses of the red-green chromatic mechanism are usually associated with advanced POAG.

In many patients with POAG, colour vision defects precede the development of standard white on white visual field loss.

However, some patients with POAG never develop chromatic defects or only develop them in advanced disease. Several studies have found greater colour vision losses in high tension POAG compared with normal tension glaucoma, suggesting that there may be two separate mechanisms for damage to visual function in glaucoma. One mechanism operates as a result of elevated IOP and is responsible for central and paracentral visual function loss, including chromatic discrimination loss, and the second mechanism is independent of the level of IOP.

Loss of foveal sensitivity in both mechanisms in glaucoma patients and in some glaucoma suspects indicating that the foveal sensitivities are also affected relatively early in the disease process.

Both diffuse and localized nerve fibre damage occurs in POAG. Flammer has suggested that a diffuse loss may be as a result of a direct mechanical damage related to an increased IOP, whereas a localised nerve fibre loss may be primarily caused by a vascular disorder. Both diffuse and localised losses may result in functional deficits.

Short wavelength sensitive cones and ganglion cells are relatively sparsely distributed throughout the retina.

In eyes with early to moderate glaucomatous loss, scotomas detected using SWAP are more extensive and deeper than conventional perimetry, and blue on yellow defects may precede the development of white on white defects by several years.

However, short wavelength transmission losses resulting from absorption and forward light scatter by the ocular media may be indistinguishable from early glaucomatous loss.

Data from SWAP exhibit increased test variability compared with conventional perimetry which may result in less sensitive detection of visual field progression.

There was also an increase in short term fluctuation with SWAP.

Increased interindividual variability is another clinical limitation of current methods of SWAP.

A small, statistically significant loss of short-wave cone sensitivity with aging remains.

Clinical reports confirm that patients with acquired tritan type defects observe that colours in general appear desaturated or “washed out”. Perception of specific colours can be especially impaired, with yellow appearing white and blues appearing black.

Ouchi et al. found that Red Visual Acuity and Blue-Green VA were detectably impaired in eyes with glaucoma, in close association with the degree of functional loss.

In conclusion, glaucoma in early phase is associated with loss of blue-green sensitivity due to preferential loss of short-wave cones and lack of their redundancy. However, late stage disease shows red-green loss. These changes may occur prior to development of conventional visual field defects. Therefore, they can be utilized for glaucoma diagnosis and screening.