H53.55 Tritan Color Vision Defect

People with an inherited tritan color vision have an abnormal or functionally absent blue cone photopigment. 

Color is perceived because objects selectively absorb certain wavelengths of light while transmitting the other wavelength; and, the object will take on the color of the wavelength of light it transmits.  

The perception of color is defined by three variables:

• Hue — the dominant wavelength of the light the object transmits
• Saturation — the absence of white
• Brightness — the intensity of the color

The retina contains two types of photoreceptors: 

• Rods — used for vision in low light
• Cones — used for vision in bright light and for color vision

Color vision is a function of the retinal cones and each photoreceptor cell contains a specific type of photopigment.  Normal color vision is trichromatic and is based on three types of cones that are maximally sensitive to light at certain wavelengths. 

• Blue cones — sensitive to light rays of short wavelength (approx. 420 nm)
• Green cones — sensitive to light rays of middle wavelength (approx. 530 nm)
• Red cones — sensitive to light rays of long wavelength (approx. 560 nm)

Genetic tritan color vision defects appear in early infancy and once fully expressed, the defect remains stable throughout life.  The defect is caused by a mutation of the gene encoding the blue retinal cone pigment.  Blue-yellow color vision defects are not accompanied by any other ocular abnormalities and the retina appears normal.  The defects are divided into subclasses based on severity and the type of missing or anomalous color cone receptor.  

Structural Damage to the Eye

• Tritanomaly — normal blue and green cones plus anomalous red-like cones 
• Tritanopia — blue and green cones only; no functional red cones 

Tritanomaly is the milder form of the disease while protanopia reflects the severe form.   

Functional Damage to the Eye

• Decreased color vision
• Most people with tritanomalous defects have no problem naming colors

People with tritan defects may be excluded from a variety of industrial, marine, air, rail, and military occupations that require the ability to distinguish blue and yellow colors.  Although people with mild defects may be able to discriminate color, they may fail the strict requirements of color discrimination on extended color vision examinations.

The goal of the diagnostic evaluation in a patient with an tritan color vision deficiency is to accomplish the following:

• Differentiate whether the color vision defect is genetic or acquired
• Determine if the color vision defect is unilateral, asymmetric, or transient
• Prescribe a treatment program 

Patient History

Tritanopia

• Most people with tritanopia perceive the visible spectrum as lacking blue, green, and yellow
• There is marked difficulty in selecting colored articles, materials, and foods
• Colors of the blue family may appear green 

Tritanomaly

• Most people with tritanomaly can perceive colors but color saturation is weakened
• Color discrimination deficits vary widely in severity

Clinical Appearance of the Retina

There are no retinal changes associated with a genetic tritan color vision deficiency.

DIAGNOSTIC TESTS

Color Vision Examination

Extended color vision testing divides people into two groups:

• The first group consist of people with normal color vision and slight color deficiency
• The second group consists of people with moderate or severe color deficiency

Farnsworth D-15

The most common test performed in clinical practice is the Farnsworth D-15 and the procedure can be accomplished using Konan’s ColorDx software.  The test consists of fifteen colored bars and one fixed bar.  The hue of each bar has been chosen so that adjacent bars have approximately equal hue differences.  When the bars are arranged in order they form a hue circle. As a result, errors in hue discrimination can be made across the hue circle.

		ColorDx — Color Vision Diagnostics Self-administering Self-scoring Self-reporting Qualitative and quantitative results Comparable to anomaloscope Validated by Naval Aerospace Medical Institute
ColorDx — Pediatric Testing Examine children as young as 3 years old Used by the U.S. Special Olympics Novel testing method with easy shape and easy answer buttons Features success-reinforcement using shapes seen by all children Color deficient children feel encourage to “play the game” Excellent examination technique to help identify malingering
		Patients with normal color vision usually make no error but may make one or two minor transposition errors, as do those with mild color vision deficiency.  Patients with more advanced color deficiency make some or more of the following errors: They place colors that lie on the opposite side of the hue circle next to colors that lie on their confusion locus (e.g., diametrical errors) Two or more diametrical crossing errors indicates a “failed” test If diametrical crossing errors are made, the orientation of the crossing produces a diagnosis of protan, deutan, or tritan color vision deficiency

Acquired Color Vision Deficiency

Several eye diseases and clinical conditions affect a person’s color vision.  These acquired color vision deficiencies are characterized by a reduction in a person’s ability to discriminate between different wavelengths of visible light.  In contrast to genetic color vision defects, which are always bilateral, acquired color vision deficiencies can be monocular. 

Common conditions that can affect a person’s color vision include the following:

• Normal age-related deterioration in chromatic discrimination ability
• Yellowing caused by cataract results in a loss of hue discrimination
• Diseases that result in a loss of foveal function
• Optic nerve disease
• Retinal dystrophies
• Neurologic disease
• Neurologic injury
• Visual field defects
• Common drugs and substances

The goals of treating a patient with a tritan color vision deficiency include the following:

• Differentiate between congenital and acquired color vision deficiency
• Differentiate between tritanomaly and tritanopia  
• Prescribe a program to treat the manifestations associated with tritan color vision defects

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