What is the difference between Ishihara and HRR color vision plates?

Ishihara plates screen specifically for red-green color deficiency using numbers embedded in dot patterns. They are quick and widely used but cannot detect blue-yellow (tritan) deficiencies. HRR (Hardy Rand Rittler) plates use symbol patterns (circles, crosses, triangles) and can detect red-green AND blue-yellow deficiencies, classify the type, and grade severity. HRR is more comprehensive and can be used with children or illiterate patients since it uses shapes rather than numbers.

Why are color deficiencies more common in males?

The most common color deficiencies (protanopia and deuteranopia, both red-green types) are caused by X-linked recessive gene variants. Males have only one X chromosome (XY), so a single abnormal gene causes the deficiency. Females have two X chromosomes (XX) and need both copies to carry the variant, making them far less commonly affected. This is why about 8% of males have red-green deficiency compared to only 0.5% of females.

Can acquired disease cause color vision loss?

Yes. Acquired color vision deficiency can result from optic neuritis, glaucoma, macular disease, retinal conditions, and certain medications or toxins. Key features that distinguish acquired from congenital deficiency include unilateral or asymmetric loss, blue-yellow deficiency pattern (rather than red-green), and progression over time. Sudden or asymmetric color vision loss, especially in one eye, warrants physician evaluation for optic nerve or retinal pathology.

Color Vision Testing: CPO Exam Guide

Why Color Vision Testing Matters

Color vision testing identifies deficiencies in the ability to distinguish colors, which affects approximately 8% of males and 0.5% of females. In the clinical setting, color vision testing serves two purposes: screening for congenital color deficiencies and detecting acquired color vision changes that may signal optic nerve or retinal pathology.

How Color Vision Works

Normal color vision depends on three types of cone photoreceptors in the retina, each sensitive to different wavelengths of light:

L cones (long wavelength): primarily sensitive to red light
M cones (medium wavelength): primarily sensitive to green light
S cones (short wavelength): primarily sensitive to blue light

Color deficiency occurs when one or more cone types is absent or functions abnormally, typically due to a genetic variant on the X chromosome.

Types of Color Deficiency

Type	Defect	Affected Colors	Prevalence
Protanomaly	Reduced red sensitivity	Red-green confusion	1% males
Protanopia	Absent L cones (red)	Red-green confusion, severe	1% males
Deuteranomaly	Reduced green sensitivity	Red-green confusion (most common)	5% males
Deuteranopia	Absent M cones (green)	Red-green confusion, severe	1% males
Tritanomaly/Tritanopia	Reduced/absent S cones (blue)	Blue-yellow confusion	Very rare, acquired form more common

Red-green deficiencies (protanopia and deuteranopia) are the most common color deficiencies and are X-linked recessive, which is why they are far more common in males (who have only one X chromosome) than females.

Ishihara Plates

The Ishihara pseudoisochromatic plates are the most common screening tool for color deficiency. Each plate contains a pattern of colored dots with a number embedded in the pattern. Normal color vision reads the number; color-deficient vision either sees a different number, no number, or an incorrect path on tracing plates.

Key points about Ishihara testing:

Screen primarily for red-green deficiencies; not designed to detect blue-yellow deficiency
Test monocularly under standard daylight-equivalent illumination
Patient should respond within 3-5 seconds per plate
A standard adult set has 38 plates; screening sets may have 14 or 24 plates
Passing criterion: typically reading 13 of 17 color vision plates correctly in standard sets

Ishihara testing must be done under appropriate lighting. Dim or incandescent lighting can cause errors that do not reflect true color vision ability. Use a daylight-equivalent light source (5000-6500K color temperature) or natural daylight whenever possible.

Hardy Rand Rittler (HRR) Plates

The HRR pseudoisochromatic plates are a more comprehensive diagnostic tool. Unlike Ishihara, HRR can:

Detect red-green deficiencies
Detect blue-yellow (tritan) deficiencies
Classify the type and severity of deficiency

HRR plates use symbols (circle, cross, triangle) rather than numbers, making them useful for children or patients with reading difficulties. The first eight plates are screening plates; subsequent plates classify type and grade severity.

Congenital vs. Acquired Color Vision Loss

Congenital deficiencies are typically:

Bilateral and symmetric
Stable over time
Red-green type most common

Acquired color vision changes suggest pathology:

Unilateral or asymmetric loss suggests optic nerve disease
Blue-yellow loss is more common in acquired disease
Optic neuritis, glaucoma, and macular disease all cause acquired color deficiency

Testing both eyes simultaneously when screening for acquired color deficiency. Each eye must be tested separately. Acquired unilateral color deficiency is a red flag for optic nerve pathology that would be missed if both eyes are tested together.

Key Takeaways

Color vision depends on three cone types (L, M, S) sensitive to red, green, and blue light
Red-green deficiencies are the most common and are X-linked recessive
Ishihara plates screen for red-green deficiency; HRR plates detect all types and grade severity
Test under daylight-equivalent illumination
Acquired color deficiency (especially unilateral) suggests optic nerve or macular pathology
Test each eye separately to detect asymmetric acquired loss