Abnormal Colour Vision

Abnormal Colour Vision: Comprehensive Notes

• Colour Vision (CV) and Abnormal Colour Vision (Abnormal CV)

  • Abnormal CV characterized by:

  • Abnormal colour matching

  • Colour confusions

  • Reduced number of colours that can be distinguished compared to those with normal CV

Causes and Types of Abnormal Colour Vision

• Abnormal CV can be due to:

  • Congenital conditions: inherited genetic defects affecting photoreceptor pigments

  • Red/Green (R/G) defects: X-linked recessive

  • Blue/Yellow (B/Y) defects: Autosomal dominant

  • Monochromats: X-linked or autosomal recessive

• Acquired conditions: altered CV perception secondary to eye disease

  • Retinal, optic nerve, or visual pathway conditions

  • Often with other visual function changes

  • Less predictable pattern of loss

Cone Photo-pigments and Colour Differentiation

• 3 cone photo-pigments enable differentiation of all wavelengths in the visible spectrum

  • If normal function, can discriminate around 2-5{ nm} variation in wavelength

• 2 cone photo-pigments differentiate some but not all wavelengths

  • Results in limited colour vision; some colours perceived as identical

• 1 cone photo-pigment leads to no colour differentiation; brightness differences used for discrimination; effectively colour blind

  

Terminology: Cone Function and Colour Vision Defects

• Status of cone functioning:

  • Normal trichromat: three cones function normally

  • Anomalous trichromacy / anomalous trichromat: abnormal sensitivity in one cone

  • Dichromacy / dichromat: absence of one cone, leaving two functioning cones

  • Typically avoid the term “colour blind”

• Affected cone pigments:

  • Protan: L-cone affected

  • Deutan: M-cone affected

  • Tritan: S-cone affected

Congenital Colour Vision Defects (R/G and B/Y)

• Congenital red/green defects:

  • Deutan (M cone affected)

  • Deuteranomaly

  • Deuteranopia

  • Protan (L cone affected)

  • Protanomaly

  • Protanopia

• Congenital blue/yellow defects:

  • Tritan (S cone affected)

  • Tritanomaly (rare)

  • Tritanopia

Colour Matching and Judgement

• Normal trichromats (CV normal): can match any reference wavelength with 3 appropriate spectral wavelengths

• Anomalous trichromats: can match any reference wavelength with 3 spectral wavelengths but in different proportions from normals

• Dichromats: can match any reference wavelength using two other appropriately chosen wavelengths

• Monochromats: can match any reference wavelength using any one other selected wavelength; matched on brightness

Anomalous Trichromacy: Abnormal Absorption and Shifts

• Abnormal absorption spectrum for the affected cone photopigment

• Shifting of the relative luminous efficiency curves

• Specific shifts:

  • Deuteranomaly: peak shift toward the long-wave end

  • Tritanomaly: peak shift toward the long-wave end

  • Protanomaly: peak shift toward the short-wave end

Anomalous Trichromacy: Range and Performance

• Three primaries can match many colours; but the proportions differ from normal trichromats

• Range of severity:

  • Mild: close to normal functioning

  • Moderate: intermediate functioning but may be colour unsafe for various tasks

  • Severe: close to dichromatic functioning

• Performance tends to be worse with:

  • Desaturated colours

  • Low luminance

  • Small targets

  • Fatigue

Dichromacy (Absence of One Cone Type)

• Types:

  • Deuteranopia: M-cone absent

  • Protanopia: L-cone absent

  • Tritanopia (rare): S-cone absent

• Can match all colours with 2 primaries

• Severity terminology is not appropriate; all types perform similarly to each other

Characteristics of Red/Green Defects

• Relative luminous efficiency: normal peak around ~555nm

  • Protan: peak shifts to shorter wavelengths ~530nm, reduced long-wavelength sensitivity

  • Deutan: peak shifts to longer wavelengths~565 nm

  • Anomalous trichromats have peaks between normal and dichromatic values.

    • Protan – peak shifts towards short wavelengths

    • Deutan – peak shifts towards long wavelengths

• Hue discrimination: optimum around ~495 nm; similar for protanopes and deuteranopes

  • above 540nm, colour distinction reduces

  • anomalous trichromats vary

• Saturation discrimination: dichromats have a neutral point where a spectrum is indistinguishable from white

  • Protanopes ~490nm

  • Deuteranopes ~500nm

  • poorest saturation in the blue-green end of the spectrum– Need to add much more colour to white before it looks different from white

  • Anomalous trichromats have similar saturation discrimination to dichromats but no neutral point

• Neutral points appear grey/achromatic at specific wavelengths (spectrum points)

• Confusion lines on a chromaticity diagram: colours along a confusion line appear the same

  • A line through white intersecting the spectral locus defines a point that appears achromatic

  • Neutral points:

    • ~490nm Protanopes

    • ~ 500 nm Deuteranope

• 

Anomalous Trichromats: Confusion Zones

• Confusion zones vary in length depending on how far the abnormal pigment shifts the spectral sensitivity

• Confusion zones may be almost as long as dichromats (severe anomalous trichromats) or not far off normal colour vision (mild anomalous trichromat)

Specific Defects: Deuteranomaly and Deuteranopia

• Deuteranomaly:

  • Production of abnormal opsin pigments for the M-cone; genetic abnormality on the X chromosome

  • Prevalence: 5\% males, 0.35\% females

  • Practical implications depend on degree affected:

    • M-cone absorption curve shifted; diminished discrimination above 540\text{ nm}

    • Best discrimination around 495\text{ nm} (blue/green)

  • May confuse reds, yellows, and greens; blue/green with grey and red/purple

  • Mild: may be colour safe and pass some vocational tests

  • Moderate/severe: likely colour unsafe (closer to deuteranopia levels)

• Deuteranopia:

  • Absence of M-cones; genetic abnormality on X chromosome

  • Prevalence:1\% males, 0.01\% females

  • Practical implications invariant: M-cone absent; no long-wavelength discrimination above 540\text{ nm}; best around 495\text{ nm}

  • Will confuse reds, yellows, greens; blue/green with grey and red/purple; brightness cues also matter

Protanomaly and Protanopia

• Protanomaly:

  • Abnormal opsin pigments for the L-cone; X-linked

  • Prevalence: 1\% males, 0.03\% females

  • L-cone absorption curve shifted; variable loss above 540\text{ nm}; best discrimination around 495\text{ nm}

  • May confuse reds, yellows, greens; red & blue/green with grey; blue with purple

  • May have reduced sensitivity to red light

  • Mild: less likely to pass vocational tests than mild DA

  • Moderate/severe: colour unsafe; may be close to protanopic function

• Protanopia:

  • Absence of L-cones; X-linked

  • Prevalence: 1\% males, 0.01\% females

  • Invariant practical implications: no L-cone function; no long-wavelength discrimination above 540\text{ nm}; best around 495\text{ nm}

  • Will confuse reds, yellows, greens; red & blue/green with grey; blue with purple; reduced sensitivity to red light

Inheritance of Red/Green Defects

• L and M cones are encoded on the X-chromosome; defects are recessive

• Expression tends to be in males with a single defective X chromosome; females may be carriers if only one defective X is present

• Carrier concepts and cross outcomes (simplified representations):

  • (a) Mother carrier (X'X), Father normal (X Y): 50% daughters carriers; 50% sons abnormal

  • (b) Mother normal (XX), Father abnormal (X'Y): all daughters carriers; sons normal

  • (c) Mother carrier, Father abnormal: 50% daughters abnormal or carriers; 50% sons abnormal

Tritan Defects: Blue/Yellow Defects

• Tritanopia (S-cone absent): Prevalence ~1 in 10,000; Autosomal dominant

  • Reduced ability to see colours; confuses blue with green and purple; yellow with grey

  • Not colour safe; may fail some occupational tests

• Tritanomaly (incomplete Tritan): Prevalence unclear (~1 in 500?); S-cone shifted or partially absent

  • May be colour safe; may pass more occupational tests; R/G connotative codes more common than B/Y

• Characteristics:

  • Relative luminous efficiency: no peak shift, but reduced sensitivity at short wavelength end

  • Hue discrimination: Tritanopes cannot detect wavelength differences between 450-480\text{ nm}

  • Tritanomalous: reduced colour discrimination in this range

• Confusion lines and neutral points:

  • Confusion lines on a chromaticity diagram: colours on a line appear the same (perceived as grey / achromatic)

  • Neutral point occurs at approximately 570\text{ nm} for Tritanopes

• Inheritance: Autosomal dominant; one defective gene expresses deficiency

  • a parent with the defect passes the gene to 50% of offspring regardless of gender

Monochromatism (Inherited Achromatopsia)

• Monochromats cannot distinguish wavelength differences in photopic illumination

• They can match all spectral hues using a single spectral wavelength

• Rod monochromatism (typical, complete):

  • Normal rod function only; absence of cone photoreceptors

  • Scotopic function only

  • Autosomal recessive inheritance

    • Both parents with same genetic abnormality, 25% of children are affected

    • Predisposing factor: consanguinity (inter-related marriage)

  • Prevalence ~ 1 in 35,000000

  • Features: poor visual acuity (~6/60); severe photophobia; central scotoma; nystagmus; fundus and optic nerve head abnormalities; high refractive error; strabismus

• Blue cone monochromatism (BCM, “atypical”, “incomplete”):

  • X-linked inheritance

  • Prevalence ~1 in 100,000

  • Absence of L and M cones, but has S-cones and rod function

  • Reduced visual acuity (~6/18$$); moderate photophobia; low-grade nystagmus

  • ERG may be abnormal

  • Some colour perception possible in mesopic conditions

  • Both S-cones and rods active, but no colour perception in photopic or scotopic conditions

Summary of Inheritance and Incidence

Treating Congenital Colour Vision Defects

• short answer is “no” - alex black

• Selective wavelength transmission filters may help some individuals

  • May assist passing a CV test but likely shifts rather than removes colour confusions

  • May reduce luminance; avoid for critical tasks (e.g., night driving)

• X-chrom, Chromagen and ColourMax lenses: marketed for selective wavelength filtering

  • Evidence does NOT support their efficacy

  • Beware of stereo disadvantage and Pulfrich effect during use

• Ensure coloured contact lenses or other wavelength filters are NOT used during a CV assessment

• Potential gene therapy may exist in the future

Acquired Colour Vision Deficiencies

• Abnormal CV can be secondary to disease or injury along the visual pathway from retina to cortex

• May be associated with systemic or CNS diseases or medication use

• Variable severity and progression; may outlast the underlying condition after treatment

• Presentation is less specific; classification not straightforward

• May present with monocular differences (asymmetric CV defect)

Clinical Associations and Rules

• May be accompanied by reduced visual acuity (VA), visual field (VF) defects, and/or relative afferent pupillary defect (RAPD)

• Impaired dark adaptation, abnormal ERG responses, flicker sensitivity changes

• Kollner’s rule (general associations):

  • Blue/Yellow (B/Y) defects: outer retinal and media changes

  • Red/Green (R/G) defects: inner retina, optic nerve, visual pathway and cortex involvement

  • Exceptions exist (e.g., glaucoma—early B/Y defects possible)

Classification of Acquired CV Deficiencies

• Type 1: Red-green Protan type defects

  • Symptoms: displaced relative luminous efficiency toward shorter wavelengths; reduced long-wavelength sensitivity

  • Associated with cone and RPE dystrophies (e.g., Stargardt disease, chloroquine toxicity)

• Type 2: Red-green Deutan type defects

  • Symptoms: reduced sensitivity to short wavelengths

  • Associated with optic neuropathy and retinal ganglion cell disease (e.g., ethambutol toxicity)

• Type 3: Blue-yellow defects

  • A. Tritan type defects: peripheral field defects

  • B. Tritan type defects with reduced sensitivity to long wavelengths: central field defects

  • Associations: glaucoma (SWAP defects); vascular disorders (e.g., proliferative diabetic retinopathy); peripheral retinal lesions (retinal detachment); rod dystrophies (RP); macular edema (AMD, diabetic macular edema, central serous retinopathy)

• RLE = relative luminous efficiency; λ = wavelengths

Medications Known to Affect Colour Vision

• Not exhaustive list; examples include:

  • Chloroquine, amiodarone, digitalis: Blue-yellow defects

  • Ethambutol (TB) and others: Red-green defects

  • Indomethacin, butazolidin (NSAIDs); tamoxifen; oral contraceptives and estrogens; antihistamines: various effects

  • Tri- and bicyclic antidepressants: mixed defect types

Summary: Congenital vs Acquired Colour Vision Defects

• Congenital defects:

  • Typically R/G defects

  • Higher incidence in males

  • Present at birth

  • Symmetric between eyes

  • Colour naming errors are rare

  • Defect generally stable over time

• Acquired defects:

  • Equal incidence in males and females

  • Onset after birth

  • Often asymmetric between eyes

  • Colour naming errors may be present

  • Defect may be unstable and change over time