acquired colour vision deficiency 1 and 2 Flashcards
what do human with normal trichromatic colour vision possess?
- humans with normal trichromatic colour vision possess three distinct classes of cone photoreceptors in the eye which contain three different pigments that absorb in different parts of the visible spectrum
where do wave length sensitive pigment peak ?
- peak at 440 nm , 540 and 570 nm
what do genetic mutations in L and M cones do ?
- genetic mutations or expression in the L and M cones can cause a shift in peak sensitivity that results in difference in colour vision
what do pre-receptor filters do ?
- pre-receptoral filters like the macular pigment can explain some differences in colour matching experiment
what are other factors that may or may not be genetically related that can causes changes in chromatic sensitivity ?
- L:M cone ratio among normal people
- variation in optical density of cones
- variation of post-receptoral amplification of cone signals
what do the different classes of cone receptors have ?
- the different classes of cones have overlapping but distinct spectral sensitivities
- the cone photoreceptors in the retina contain three different pigments (opsins) that absorb in different parts of the visible spectrum
what is the similarity between L and M cones ?
- the L and M cones have many similarities in terms of their known histology , physiology and molecular genetics
- the M and L cones pigments are coded in an array of the X chromosome and share 96% similarity with each other , differing only in 15 amino acids
where is the largest shift in peal sensitivity ?
the largest shift in peak sensitivity are produced by substitutions at key sites on sequence of amino acid that make up the heptical opsin molecule
what is the similarity of the S cone with the M and L cone ?
- the S cone pigment is coded on chromosome 7 and shares 43% identity with the M and L cone
what complex objects produce ?
- complex objects produce two-dimensional spatial modulations of intensity and spectral content on the retina
who’s function is it to respond to light and produce local signals ?
- it is a function of retinal photoreceptors to respond to light and to produce local signals that reflect spatial changes in the amount of light as well as its spectral composition
- this information is extracted, and channelled to cortex in 4 principle channels
what do the two achromatic channels rely on ?
- the two achromatic channels rely on cones ( photopic) and rods (scotopic) signals to extract spatial modulations of light intensity
what does the comparison of L and M cone signals form ?
the comparison of L and M cone signals forms a chromatic channel that mediates red-green discrimination
how is the second yellow-blue channel achieved ?
- the second yellow-blue channel is achieved from the comparison of s cone signals against the combined L and M cone signals
what do the two chromatic channels mediate ?
- these two chromatic channels mediate detection of colour signals selectively in red green and yellow blue directions
what is the classification of anomalies ?
- congenital deficiency
- acquired deficiency
what is congenital deficiency ?
- present at birth, stable, bilaterally symmetrical , and is thought to affect the entire filed of vision
what is acquired deficiency ?
- may exhibit progression and regression, may affect one eye or both eyes asymmetrically, and may affect only a portion of the visual field
how does congenital deficiency arise?
- those that arise from disorders in the genes coding for the cone pigments in genes controlling the expression of proteins and molecules of cones photopigment
- these colour anomalies are the most common
how is congenital deficiency sub classified ?
- congenital deficiency is sub classified by the severity of the defect and the class of cones affected
what happens in normal trichromacy ?
- in normal trichromat with three classes of cones
- there is a peak separation between the M and L cones of about 28nm
what happens in anomalous trichromacy ?
- trichromatic colour vision is displayed
- there is a shift in the peak sensitivity of the normal photoreceptors
what is deuteranomaly?
- three classes of cones
- instead of having an M cone , there is anomalous pigment L’ which reduces separation to approx 3-11 nm in comparison to L cone
- (S,L’,L)
- the mildest deficiency arises with the largest separation of 11 nm
what is protanomaly?
- there are three classes of cones
- instead of having an L cone there is an M’ and normal M with a peak separation of 3-6nm
- larger separation = milder separation
- (S,M,M’)
what is dichromacy ?
- dichromats have only two types of cones
- most severe type
what is protanope ?
- L cone is missing
S,M
what is deuteranope ?
- M cone is missing
S,L
what is the prevalence of red/green deficiency ?
- red/green are the most common, affecting 8% of men and 0.4% of women
what is the prevalence of tritan-type deficiency ?
- involves the S-cone
- prevalence in 1 in 500
what is the prevalence of rod monochromacy ( only rods )
- 1 in 33,000 to 50,000
what are the characteristics of acquired anomalies ?
- frequently associated with other visual function loss ( i.e. acuity, visual fields )
- onset after birth ( colour vision previously normal )
- type and severity changes with time
- monocular difference in severity frequently occur
- monocular testing required
- equal prevalence in males and females
- may affect both red/green and yellow/blue colour channels
what are the most common colour vision test used ?
. pseudoisochromatic plates
1- ishihara
2- American optical - hardy-rand-rittler
3- lanterns
4- hue discrimination
. Farnsworth D15
.Farnswoth-Munsell 100 Hue
. city university
5- colour matching ( anomaloscopes )
- computerised diagnostic test
. colour assessment and diagnosis (CAD)
what is important for colour vision test to employ?
- visual stimuli tend to produce signals in several vision information channels , it is important for colour vision test to use stimulus conditions that isolate the use of colour signals
- most tests employ some form of luminance contrast masking to reduce the detection of luminance cues in the coloured stimulus
what are the different functions colour vision test perform ?
- screening test; which identify people with normal RG and YB colour vision
- classify type pf colour vision defect;protan, deutan or tritaan type
- quantify or grade the severity of the colour vision loss
- diagnose dichromacy/anomalous trichromacy
- occupational suitability
what is important for an efficient colour vision asesssment ?
- true isolation of colour signals is obtained
what does the CAD test employ ?
- CAD test employs dynamic luminance contrast noise to mask the detection of luminance signals and thus isolate the red/green and yellow/blue signals
how is acquired colour vision deficiency classified ?
- acquired colour vision deficiency may be classified by its mechanism or the site of pathology or by the type of colour deficiency present
what is the prevalence of acquired deficiency ?
- it is difficult to determine with accuracy
- several epidemiology studies have suggested that acquired forms may affect between 5-15% of the population
- however the prevalence within the populations would be anticipated to be influenced by aging
explain the study that examines how colour vision threshold vary as one ages ?
- participants of ( 4-90 years)
- detailed medical hx and clinical assessment was recorded for each subject
- chromatic sensitivity was examined monocularly using the CAD test
- a number of filtering steps were implemented to exclude abnormalities
- subject with congenital deficiency exhibit elevated red/green thresholds and normal yellow blue
four filters used
- exclusion of congenital colour deficiency
- exclusion of medical conditions ( hypertension, diabetes and ocular diseases )
- exclusion of fundus abnormalities and drusen
- asymmetry criterion - statistically significant difference in RG and YB thresholds between the two eyes were excluded - asymmetry criterion
what happen to RG and YB threshold with age ?
- RG threshold increases with age by 0.1% every year for RG
- YB threshold increases by 1.6% per year
what are the thresholds for normal trichromat ?
- RG- 1 CAD unit
- YB - 1 CAD unit
explain colour vision and diabetes study result ?
- inclusion criteria : best corrected visual acuity of 6/18 or better, no more than moderate non-proliferative retinopathy or maculopathy and no co-existing glaucoma
- over 70% of these patients had a significant loss of both YB and RG colour vision
- the severity of colour vision loss is not correlated with age or gender of the patients, duration or severity of diabetes, or current diabetic control
what is the relationship between colour vision loss and AMD ?
- patients with AMD can exhibit large loss of both RG and YB chromatic sensitivity
- have elevated RG and YB threshold
- AMD px are mainly over 60
- px with AMD tend to lose more the YB colour vision compared to RG- as most YB thresholds are above the dotted grey line of unit gradient.
