Comprehensive Study Guide to Congenital and Acquired Colour Vision Defects

Normal Trichromacy and Photoreceptor Physiology

• Normal trichromacy is the standard state of human colour vision, predicated on the existence of three distinct types of photoreceptors in the retina.
• These photoreceptors exhibit peak sensitivities at the following wavelengths:
  • $\\approx 435\\,\\text{nm}$
  • $\\approx 535\\,\\text{nm}$
  • $\\approx 570\\,\\text{nm}$
• Variability in the peak sensitivity of each photoreceptor type is created by different amino acids within the pigment molecule.
• Conventional naming vs. technical naming:
  • Conventionally, these are known as the blue, green, and red cones.
  • Technically, the term "red" is a misnomer for the cone with a peak at $570\\,\\text{nm}$, as this wavelength does not correspond precisely to red.
  • More accurate terminology refers to them by wavelength range: short-wavelength (S), medium-wavelength (M), and long-wavelength (L) cones.
• The perception of different "colours" is the result of differing patterns of stimulation across these three cone types. If stimulation across all receptors is identical, no change in colour is perceived.

Classification of Congenital Colour Vision Defects

Congenital colour deficiencies are classified based on the number of variables required in wavelength matching, the number of cone photopigments present, and the resulting hue discrimination capability.

• Monochromat (Achromat) - Typical or Rod Monochromat:

  • Number of variables in wavelength matching: $1$
  • Number of cone photopigments: None
  • Hue discrimination: Absent

• Monochromat (Achromat) - Atypical, Incomplete, or Cone Monochromat:

  • Number of variables in wavelength matching: $1$
  • Number of cone photopigments: One
  • Hue discrimination: Absent

• Dichromat:

  • Number of variables in wavelength matching: $2$
  • Number of cone photopigments: Two
  • Hue discrimination: Severely impaired
  • Sub-types:
  • Protanope
  • Deuteranope
  • Tritanope

• Anomalous Trichromat:

  • Number of variables in wavelength matching: $3$
  • Number of cone photopigments: Three (one of which is abnormal)
  • Hue discrimination: Continuous range of severity from severe to mildly impaired
  • Sub-types:
  • Protanomalous
  • Deuteranomalous
  • Tritanomalous

• Normal Trichromat:

  • Number of variables in wavelength matching: $3$
  • Number of cone photopigments: Three
  • Hue discrimination: Optimum

Spectral Sensitivity and Human Cone Pigments

• The peak sensitivity (absorption) functions for human cones are calculated at the retinal level.
• Data provided by Estévez (PhD thesis, 1979, unpublished) illustrates the log sensitivity across the spectrum for the three cone types:
  • Blue cones (S-cones): Represented by filled circles.
  • Green cones (M-cones): Represented by filled squares.
  • Red cones (L-cones): Represented by filled triangles.
• Comparative scientific data indicates that measurements of Rhesus pigments (identified as data from measurements 13) are comparable to human green and red pigments (represented by open squares and open triangles respectively).
• Protanopes view scenes differently than normal trichromats due to the absence of functioning long-wavelength (red) cone photopigments.

Comparison of Congenital vs. Acquired Colour Vision Defects

There are considerable differences between congenital and acquired defects, necessitating different testing approaches depending on the purpose.

• Congenital Defects:

  • Onset: Present at birth.
  • Progression: Stationary throughout life.
  • Classification: Easier to classify into specific types.
  • Laterality: Binocular (affects both eyes identically).
  • Vision: Visual Acuity (VA) is typically normal.
  • Inheritance: Usually X-linked; predominantly affecting males.
  • Defect Type: Usually Red-Green.

• Acquired Defects:

  • Onset: Appears later in life.
  • Progression: Often progressive over time.
  • Classification: Often consists of combined defects, making them harder to categorize.
  • Laterality: Often monocular (may affect one eye differently than the other).
  • Vision: Visual Acuity (VA) and visual fields are often abnormal.
  • Demographics: Affects males or females equally.
  • Defect Type: Often Blue-Yellow (Tritan).

Occupational Significance and Incidence

Congenital colour vision defects are of significant concern in occupational health and safety.

• Incidence of Red-Green (R-G) defects:
  • Occurs in up to $8\\%$ of the male population.
  • Occurs in up to $0.5\\%$ of the female population.
  • Marked racial variations exist in these statistics.
• Incidence of Blue-Yellow (B-Y or Tritan) defects:
  • Suggested prevalence is between $1 : 15,000$ and $1 : 50,000$.
• Safety Implications:
  • Safety often depends on the ability to distinguish or see red and green lights/signals.
  • Critical in maritime and aviation traffic signals.
  • Historical Context: The first "occupational vision screening" for colour vision occurred around the 1850s, specifically targeting sailors and railwaymen.
  • Road signals represent a specific case where discrimination can partially be judged by the vertical or horizontal position of the light rather than colour alone.

Occupational Hazards and Health Monitoring

• Certain occupations expose workers to risks that can manifest as acquired colour vision loss.
• Exposure to neurotoxic chemicals, organic solvents, and metals can degrade colour vision even if levels are below specified limits and safe working practices are followed.
• Monitoring strategies:
  • Colour vision can be used as a biological monitor for neurotoxicity.
  • Measurements should be taken at a baseline and then at regular intervals.
• Nature of loss: Acquired loss is typically blue-yellow, though red-green loss can occur.
• Legal implications: Employers may document pre-employment colour vision status to avoid future litigation regarding occupational exposure.

Normal Colour Discrimination and McAdam Ellipses

• Normal colour discrimination is often represented using McAdam ellipses.
• McAdam ellipses represent the "jnd" (just noticeable difference) in colour perception.
• In graphical representations, these ellipses are often Shown at $10 \\times$ their normal size for visibility.
• They represent the mean $\\pm 2SD$ ($95\\%$ of the population).
• Physically, these ellipses represent the minimum change in the excitation of receptors required for a human to perceive a change in hue.

Colour Confusion in Congenital Defects

• In individuals with congenital defects, the short, subtle ellipses of normal vision are replaced by very long, thin "confusion ellipses."
• These ellipses indicate the directions in colour space where an observer cannot distinguish between different tints.
• Specific orientations are identified for:
  • Deutan
  • Protan
  • Tritan
• For individuals with severe red-green deficiency (such as Protanopes or Deuteranopes), specific sets of colours—often arranged in vertical columns on screening plates—look identical to one another.