clinical assessment of contrast sensitivity Flashcards
(24 cards)
What is Contrast Sensitivity (CS)?
Contrast Sensitivity measures the eye’s ability to detect differences in luminance between an object and its background. It helps assess vision in low-contrast situations, such as fog, glare, or low light conditions.
Why is Contrast Sensitivity (CS) testing important?
CS testing is important for:
Assessing and managing ocular diseases like cataracts, age-related macular degeneration (ARMD), and glaucoma.
Evaluating the vision of bifocal and multifocal contact lens/IOL patients.
Vocational assessment and safety (e.g., driving, electrical work).
What is the Contrast Sensitivity Function (CSF)?
The CSF describes how contrast sensitivity varies across different spatial frequencies. It provides information about a patient’s vision at various levels of contrast and spatial frequency, indicating how well they can see patterns and objects under low contrast conditions.
How is Contrast Sensitivity measured clinically?
CS is measured using charts like the Pelli-Robson, VISTECH, and Farnsworth D-15 tests, which involve presenting gratings or letters at varying spatial frequencies and contrast levels until the patient can just distinguish them.
What are the four main parameters of contrast sensitivity?
Spatial Frequency (cpd): The number of cycles per degree of visual field, which represents the level of detail visible.
Contrast (modulation): The difference in brightness between the object and its background.
Phase: The positioning of the light and dark bars in a grating pattern.
Orientation: The angle at which the bars or gratings are presented.
What is the VISTECH test used for?
The VISTECH test measures contrast sensitivity using circular gratings of different spatial frequencies and contrasts, and the patient must identify the bars until they can no longer see them. It is used to assess CS across varying orientations (vertical, left, right).
What does a low contrast letter chart measure?
: It measures the ability to recognize letters at different contrast levels (e.g., 96% contrast vs. 10% contrast), helping assess how well a person sees under reduced contrast conditions.
Pelli-Robson Chart?
: The Pelli-Robson Chart is a standard test for assessing contrast sensitivity. It presents triplets of letters at varying contrast levels. The score is based on the last triplet where at least two letters are identified correctly.
What is the significance of log CS units in CS testing?
Log CS units are used to quantify contrast sensitivity. Higher log CS values indicate better sensitivity (better vision), while lower values indicate poorer contrast sensitivity.
What is disability glare?
Disability glare refers to the loss of contrast in the retinal image caused by scattered light within the eye. It reduces contrast sensitivity, particularly in low-light conditions, without necessarily causing discomfort.
What is discomfort glare?
Discomfort glare is the psychological sensation of glare, which may or may not affect vision. It is caused by excessive brightness or light contrast in the visual field.
How does disability glare affect contrast sensitivity?
Disability glare reduces the apparent contrast in the retinal image by introducing veiling luminance, which lowers contrast sensitivity and makes it harder to detect details, particularly in patients with cataracts or certain retinal diseases.
What is the Inverse Square Law in lighting?
The Inverse Square Law states that illumination (brightness) is inversely proportional to the square of the distance from the light source. It means that as the distance from the light source increases, the brightness decreases significantly.
How does age affect contrast sensitivity?
As people age, contrast sensitivity generally decreases, particularly for blue light due to the absorption of shorter wavelengths by the crystalline lens. Older individuals also experience reduced CS under glare conditions.
What effect does cataract have on contrast sensitivity?
Cataracts cause a reduction in contrast sensitivity, particularly for low spatial frequencies. They create a cloudy effect in the lens, leading to disability glare and a general decline in the ability to detect low-contrast objects.
How does refractive surgery (e.g., LASIK) affect contrast sensitivity?
After refractive surgery, there is typically a reduction in contrast sensitivity, especially at low spatial frequencies (1.5 cpd and 3.4 cpd). This can lead to difficulties in vision under low-contrast conditions, such as night driving.
What are some diseases that affect contrast sensitivity?
Keratoconus and corneal dystrophies: Can reduce contrast sensitivity due to changes in the cornea’s shape and transparency.
Cataracts: Cause a general depression in the contrast sensitivity function.
Retinal diseases (e.g., macular degeneration): Affect the ability to distinguish contrasts, especially in the central visual field.
What does a reduced contrast sensitivity in a patient indicate?
A reduced contrast sensitivity can indicate the presence of ocular diseases such as cataracts, macular degeneration, or glaucoma, or it could be a sign of visual disturbances following refractive surgery.
What are normal values for contrast sensitivity?
Young adults: 1.86 log CS units
Older adults: 1.80 log CS units
With glare: Reduced CS (e.g., older with glare: 1.65 log CS units)
How can contrast sensitivity help in managing cataracts?
CS testing helps to assess the severity of cataracts and the impact on vision. It also aids in determining the best time for surgical intervention and monitoring post-surgical recovery.
What does Pelli-Robson scoring reveal about a patient’s CS?
The Pelli-Robson test reveals the patient’s threshold for contrast sensitivity by measuring the last triplet of letters correctly identified, giving insight into how well the patient can see low-contrast objects.
Trichromatic Theory (Young-Helmholtz Theory):
Colour vision is based on three types of cone receptors:
Long-wavelength (Red)
Medium-wavelength (Green)
Short-wavelength (Blue)
These cones have overlapping spectral sensitivities, which allow colour discrimination.
Opponent Colour Theory (Hering):
Proposed that there are four primary colours (Red, Green, Yellow, Blue) arranged in opponent pairs.
These colours work against each other (e.g., Red vs. Green, Yellow vs. Blue), with a luminance mechanism involving White vs. Black.
Zone Theory (Donders):
The visual system processes colour in a series of zones:
Receptor Level: Trichromatic, mediated by cones.
Ganglion Cell Level: Colour information is coded into opponent channels
