Glaucoma & Colour Vision Defect

Aishwarya Gupta, B. Optom

Fellow Optometrist, Dr. Shroff’s Charity Eye Hospital, New Delhi

Introduction

Glaucoma is defined as a group of optic neuropathies with structural changes at the optic nerve head that may lead to visual field loss. ⁽¹⁾ The types of glaucoma can be classified as congenital and acquired where it is sub-classified as open angle and angle closure glaucoma. The mechanism of aqueous outflow to the anterior chamber configuration is distinct from the primary and secondary glaucoma.   People with glaucoma may not lead immediately to blindness but have limited functional visual activities such as colour vision deficiency, visual field defect, contrast sensitivity defects etc. This blog talks about the colour vision deficiency in glaucoma.

Colour vision is defined as the ability to discriminate between a light stimulus as a function of its wavelength. The sense of colour has many light stimulus absorptions resulting in the process of transmission of the sensory signal to the occipital lobe. ⁽²⁾

Pathophysiology

Abnormal colour vision defects are typically categorised as congenital and/or acquired forms. The occurrence of congenital colour vision deficiency is due to a gene defect that is encoding cone opsins, and the gene protein involved in photo-transduction. The acquired colour vision defect or dyschromatopsia is caused due to any kind of injury or disruption in the visual pathway. ⁽³⁾ 

The compression of the optic nerve in glaucoma sometimes leads to colour vision defects. Here colour perception may suffer alteration in a change of functional or structural that is detected and appears to be primary blue-yellow deficiency (Tritan) and in advance glaucoma red-green deficiency (deutan). ⁽²⁾ Tritan defects may lead to poor vision and photophobia.

Diagnosis

Photo-pigments are combined apoprotein and an 11-cis retinal chromophore. Blue-yellow colour vision defect results from the mutation in the OPN1SW gene. This is due to premature destruction of the S cone resulting in difficulties in differentiating between these colours. Red-green colour defects involve mutation of OPN1LW and OPN1MW. ⁽⁴⁾

The standard colour vision test can be performed with pseudo-isochromatic plates or Farnsworth D-15 hue discrimination colour plates frequently as correlated to colour vision deficiency. However, it is difficult to specify the colour vision defects in ocular conditions such as glaucoma.

To detect the acquired colour vision defects, the Rabin cone contrast test (RCCT) plays a significant role. ⁽⁵⁾    

Rabin Cone Contrast Test: The Rabin Cone Contrast Test (RCCT) is an arbitrary series of red (protan), green (deutan), and blue (tritan) letters which is visible to single cone type and in decreasing contrast threshold of letters. The letter’s contrast and the luminance background have only differed in chromaticity. The projection function of the cone is to maintain the limit of stimulus on each cone type while maintaining a similar background stimulus. The subject must identify the threshold of letters in the form of reducing contrast. It is measured in numeric scores and the recorded duration is a 6 minutes test, which is vital and quick in the objective examination. The result has been given in the robust index of colour vision that precisely detects the colour vision deficiency.

The RCCT measures the chromatic contrast threshold faster and more effortlessly which in turn helps in monitoring glaucoma progression. ⁽⁵⁾    

References

1. Gupta, D., & Asrani, S. (2016). Macular thickness analysis for glaucoma diagnosis and management. Taiwan Journal of Ophthalmology, 6(1), 3-7.
2. Papaconstantinou, D., Georgalas, I., Kalantzis, G., Karmiris, E., Koutsandrea, C., Diagourtas, A., … & Georgopoulos, G. (2009). Acquired color vision and visual field defects in patients with ocular hypertension and early glaucoma. Clinical ophthalmology (Auckland, NZ), 3, 251.
3. Dahal, M., Adhikari, P. G., Bhattarai, S., & Mishra, S. (2021). Pattern of Colour Vision Anomalies Among Patients Presenting to a Tertiary Eye Center of Nepal. Journal of Institute of Medicine Nepal (JIOMN), 43(3).
4. Deeb, S. S. (2004). Molecular genetics of colour vision deficiencies. Clinical and Experimental Optometry, 87(4‐5), 224-229.
5. Fujikawa, M., Muraki, S., Niwa, Y., & Ohji, M. (2018). Evaluation of clinical validity of the Rabin cone contrast test in normal phakic or pseudophakic eyes and severely dichromatic eyes. Acta Ophthalmologica, 96(2), e164-e167.