Week 9: Objective Refraction Flashcards
Describe what are autorefractors?
- Autorefractors automatically & objectively measures refractive error on patients
- Starting point for subjective refraction
- Fast with reasonable accuracy & repeatability (may be slightly over minus)
- Latest models provide keratometry measurements, displaying corneal mapping, detect corneal irregularities & measure IOPs
What different principles are based on the autorefractors?
- Badal Optometer
- Analysis of image quality
- Retinoscopic Scanning
- Scheiner Disc
- Photorefraction
Advantages & disadvantages of autorefractors?
Advantages:
- Quick, reasonable accuracy & repeatability
- Can be used by non-expert staff
- Works best in an unaccommodated eye
- Objective
Disadvantages:
- Doesn’t work well in media opacities, irregular astigmatism, latent hyperopia, people accommodating
- Cost
- Lack of portability
- Children & young hyperopes need cycloplegia (they accommodate)
- Calibration needed & can break down
What are the basic designs for the autorefractor?
- Infrared light source – 780 to 950 nm excellent transmittance through clear media
- Fixation target
- Based on the Badal Optometer and/or Scheiner Disc Principle
Explain the Badal Principle
Principle states that if the eye is placed at the back focal point of the (Badal) lens then;
a) Target image vergence is proportional to the distance of the target from the front focal point of the lens
b) Angular size of the image is independent of target vergence
What is the Badal optometer?
- Single positive lens (Badal lens), power F
- Target moves towards or away from the lens
- There is a change in vergence of the light hitting the lens
- No change in the magnification of the retinal image
- Linear relationship between target position & refractive error
Describe Analysis of Image Quality
- Infra-red radiation illuminates a test graticule
- If the image is correctly focused, it will pass through the mask, with a maximal proportion of the radiation reaching the detector
- Z’ > 0 for myopes, z’ < 0 for hyperopes
Describe Retinoscoping Scanning
- A chopper drum (C) directs a beam of infra-red radiation across the central area of the patient’s pupil in a manner similar to streak retinoscopy
- The retinoscopist’s eye is replaced by a detector system of two photoelectric cells (H, J)
- Like retinoscopy, speed & direction of the reflex are used to determine refractive error with lenses to neutralise the refractive error
Describe Scheiner Disc
- First proposed by Thomas Scheiner in 1619
- Based on a double pinhole being placed in front of the patient’s eye
- Most autorefractors use the Scheiner disc principle in a modified fashion
How does Scheiner Disc operate?
- Two infrared light sources are imaged in the plane of the pupil to simulate the Scheiner pinhole apertures
- Photodetector observes the degree of coincidence between the two images on the fundus
- The Badal system then moves the target to & from the Badal lens to focus the target on retina
- Final Rx is computed
- For emmetropic eye, a sharp spot image if formed
- For ametropic eye, two blurred spots are formed
Describe Photorefraction: Vision Screening
- A method to estimate the refractive state of patient’s eye
- Instantaneous test, it is an ideal test for children
- Quick & remote: useful for measuring refractive error on difficult children
What are the principles for Photorefraction
- A small point source of light is placed at the centre of the front of a suitable camera
- If the eye is correctly focused, the reflected light returns to the source & pupil appears dark in the photograph
- If not focused correctly, there will be an illuminated reflex visible around the source
- To determine the degree of ametropia, the camera must be defocused by a known amount
Describe Eccentric Photorefraction
- The light source is moved to the edge, or beyond the edge of the camera lens
- Like retinoscopy, a crescent of light may now appear in the pupil
What do you need to consider when measuring the rx with the use of Eccentric Photorefraction?
- Position of the crescent : if it appears on the same/opposite side of the flash, then the script is myopic/hyperopic respectively
- Size of the crescent: measurement of the size of the crescent can be used to calculate the degree of ametropia
Describe accommodation and fixation control in autorefractors
- Most autorefractors measure refractive error monocularly
- Proximal accommodation
- Photographic target
- Fogging lens used to relax accommodation
- Provides fixation target so refraction measured on optical axis
Describe wavefront aberrometry
- Commonly used in refractive surgery
- It measures;
1. Lower order ocular aberrations such as sphere + cylinder (defocus)
2. Higher order aberrations: degrade quality of the image received by retina
List the difference between traditional vs wavefront aberrometry
Traditional:
- Limited to measuring lower order aberrations (spherical & astigmatic)
- Measure over small central region of pupil
- Cheaper
- Less likely to over-stimulate accommodation
Wavefront:
- Measure lower & higher order aberration
- Measure over larger pupil area
- Account for local variations in refractive power
- More expensive
- Tend to stimulate accommodation resulting in over-minused prescriptions
Can you replace autorefraction with retinoscopy?
- Autorefraction tends to provide prescriptions that are more negative/less positive
- Does not pick up irregular astigmatism e.g. keratoconus or media opacities
- Overestimation of hyperopia with autorefraction compared with retinoscopy under cycloplegia in school-age children
- Need to use retinoscopy in mixed astigmatism
Describe ketatometry
Assessment of corneal shape by measuring anterior corneal radius of curvature & regularity of corneal surface
What are the main applications of keratometry measurements?
- Contact lens practice
- Corneal disease
- Keratoconus, pterygium, dry eye
- Investigation of patients with best corrected visual acuity (BCVA) less than expected
- IOL design (phakic & aphakic)
- Pre-LASIK corneal evaluation
What are the disadvantages of ketatometry?
- Only measures curvature of anterior central cornea (radius 2 – 4 mm)
- Expressed in either radius of curvature (mm) or dioptric power
- Optical principle
What is the Paraxial Theory of Keratometry?
- Principles of keratometry & corneal topography are identical
- % of light incident at corneal surface reflected produces a Purkinje Image 1
- The Purkinje image is virtual
Describe Doubling Principle Keratometry
- Due to involuntary eye movements, image formed on cornea would be constantly moving
- Measurement of corneal radius is made using an optical doubling system
- Prism introduced so two images are formed
- Prism moved until images touch each other
What is the estimation of total power?
- Keratometer measures only anterior corneal radius – it cannot measure posterior radius
- Total corneal power reading is an estimate
What are the two types of keratometer?
- Bausch& Lomb
- Fixed object size with variable image size (variable doubling) - Javal Schiotz type
- Fixed image size with variable object size (fixed doubling)
Describe Javal Schiotz type?
- Two position instrument which uses a fixed image & doubling size, and adjustable object size to determine the radius of curvature of the reflective surface
- Uses two self illuminated mires (the object)
- one a red square, other a green staircase
Describe Burton (B&L Clone) Keratometer
- One position keratometer: measures both meridians simultaneously
- Object size is kept constant, the amount of doubling is varied to produce an image of fixed size
- Amount of doubling related to corneal curvature
What are some common errors in keratometry?
- Inadequate instrument calibration
- Incorrect examination technique
- Corneal abnormalities
- Inadequate/unstable patient fixation
What are some common errors in keratometry?
- Inadequate instrument calibration
- Incorrect examination technique
- Corneal abnormalities
- Inadequate/unstable patient fixation
What is Javal’s Rules?
- Estimates total refractive astigmatism (TRA) from keratometry readings
- Tells us if there is an internal contribution of against the rule astigmatism
What is Placido Disc?
- The original corneal topographer
- Placido Disc: observer views the pattern of concentric white rings (mires) reflected from the px’s cornea through a central +2.00 D lens
- Very subjective/qualitative
Describe corneal topography
- Some topographers can measure both anterior and posterior corneal surface
- Aids in diagnosis of various corneal diseases/degenerations
- Tear Film Surface Quality analysis
- Used in refractive surgery, including laser refractive surgery & IOL power calculations
- Specialty contact lens fittings
Explain Axial map
- Used most often
- As axial curvature related to corneal power, correlates anterior surface shape to refractive status & provides a quick overview
- Averages data = less accurate than tangential, central data is more accurate
Explain Tangential map
- More sensitive
- Defines small curvature changes, calculates each measured point at a 90° tangent to the corneal surface
Explain Elevation display map:
- Conveys true shape of cornea
Advantages vs Disadvantages in Corneal Topographers
Advantages:
1. Map of almost entire corneal surface
2. All meridians, not just two
3. Colour coding
4. Measures posterior surface curvature
5. Measures corneal thickness
6. Objective
Disadvantages:
1. Much more expensive
2. Interpretation of results/indices
3. Different topography instruments may measure slightly different
