Color vision testing identifies deficiencies in the ability to discriminate colors. These deficiencies may be congenital (inherited) or acquired (from disease). As a CPOA, you will perform color vision testing as part of the preliminary examination and document the results for the physician's review.
Types of Color Vision Deficiency
Color vision depends on three types of cone photoreceptors (short, medium, and long wavelength). Deficiency of any type creates characteristic color vision problems:
- Protan defects (protanopia/protanomaly): Deficiency or absence of red-sensitive (long wavelength) cones. Difficulty distinguishing red from green; red appears dark or black. X-linked inheritance.
- Deutan defects (deuteranopia/deuteranomaly): Deficiency or absence of green-sensitive (medium wavelength) cones. Most common congenital color deficiency. Difficulty distinguishing red from green; green appears grayed. X-linked inheritance.
- Tritan defects (tritanopia/tritanomaly): Deficiency of blue-sensitive (short wavelength) cones. Autosomal dominant. Difficulty distinguishing blue from yellow.
Because red-green deficiency is X-linked, it affects approximately 8% of males and only 0.5% of females. Blue-yellow deficiency (tritan) is much rarer congenitally but more commonly acquired from disease.
Ishihara Plates
The Ishihara pseudoisochromatic plates are the most widely used screening tool for congenital red-green color deficiency. Each plate consists of colored dots arranged to show a number or path visible to those with normal color vision but not to those with red-green deficiency, or vice versa.
Testing technique:
- Use in bright, natural-like illumination (or a daylight-balanced lamp). Do not test under fluorescent or incandescent light.
- Hold the plate approximately 30 inches (75 cm) from the patient.
- Allow 3 to 5 seconds per plate for a response.
- Test each eye separately for a complete assessment.
- Record the number of plates passed out of the total (e.g., "12/14 plates correct" or "fails red-green screening").
Hardy-Rand-Rittler (HRR) Plates
The Hardy-Rand-Rittler (HRR) plates are a more comprehensive color vision test that screens for both red-green and blue-yellow deficiencies. Unlike Ishihara, HRR uses symbols (circles, triangles, crosses) rather than numbers, making them useful for patients who cannot read. HRR plates can also classify deficiency severity (mild, medium, strong).
Congenital vs. Acquired Color Deficiency
| Feature | Congenital | Acquired |
|---|---|---|
| Type | Usually red-green | Often blue-yellow |
| Symmetry | Symmetric (both eyes equally) | May be asymmetric |
| Progression | Stable lifelong | May progress with underlying disease |
| Association | Family history, males predominant | Optic nerve disease, macular disease, medications |
Acquired color deficiency is an important finding because it may indicate optic neuritis, glaucoma progression, toxic optic neuropathy, or macular disease. A patient who previously had normal color vision and now fails is more clinically significant than one with a lifelong congenital deficiency.
Key Takeaways
- Congenital red-green color deficiency is X-linked, affecting 8% of males and 0.5% of females.
- Ishihara plates screen for red-green deficiency; HRR plates screen for both red-green and blue-yellow.
- Test in bright, natural-like illumination; allow 3 to 5 seconds per plate; test each eye separately.
- Acquired color deficiency affecting blue-yellow discrimination suggests optic nerve or macular disease.
- Document the number of plates passed, which eye was tested, and any relevant comparison to prior results.