techniques and keratometry

• Keratometers and topographers measure corneal curvature.
• Children may not be able to respond to subjective techniques so objective techniques are needed to measure the refractive error.
• Keratometers measure corneal curvature by measuring the size of an image formed by a convex reflecting surface.
• In Karetometry 0.05mm difference between reading is equivalent to 0.25DC.
• Keratometers are used to: Estimate corneal astigmatism, Differentially diagnose axial vs refractive anisometropia - see which meridian is more powerful.
• In patients with large strabismus, the trial frame allows retinoscopy on the line of sight without occluding the fellow eye.
• The trial frame is better for children as it stimulates less proximal accommodation and is less intimidating.
• Some children love phoropters.
• The advantages of a trial frame are that it is beneficial for patients with low vision as it allows larger power spheres and cross-cyls to be used easily, and it is advantageous for patients with high refractive error as it allows BVP to be measured.
• The trial frame is useful in patients with hearing problems.
• The trial frame is adaptable for the patient's head and any head tilt.
• With cataracts and astigmatism cutting in that meridian can relax the astigmatism, Contact lens base curve selection (gas permeable lens), Detect gas permeable lens flexure - if the lens bends or there is distortion when blinking, Detect and monitor corneal surface distortion, Verify contact lens base curves, Diagnose and monitor corneal ectasia, Calculate intraocular lens power - if a px has cataracts need to find if the lens is the right power, Monitor intrasurgical and postsurgical astigmatism.
• Karetometers are disadvantageous because they assume uniformity over the area, eyepeice needs to be focussed to ensure an accurate measurement and they only measure 4 points within the central cornea.
• Objective optometers are instruments that measure refractive error, there are visual optometers and automated electronic optometer.
• In a simple optometer as soon as the target is seen clearly the refractive error can be deduced.
• Problems with optometers are: large depth of focus, non - linear scale, apparent size of target varies, proximal accommodation, target can stimulate through accommodation.
• Fsp = L + F0 L = Dioptric distance of text from lens F0 = optometer lens power.
• Badal optometers consist of a target and a positive lens placed at its focal distance from the eye.
• Phoropters are good because they are quicker, more comfortable for patient and practitioner, Jackson cross-cylinder aligned automatically with cyl axis, no lens smear, Risley prisms for measuring heterophorias and fusional reserves, computerised phoropters linked to autorefractors and focimeters, high-tech which some patients prefer.
• Photorefraction is usually used for the screening of infants or young children and requires a dim room.
• If the patient sees only one image, they are an emmetrope as the image is formed on the retina but if there are two images seen, the patient has an ametropia and the direction of the movement of the target will give the ametropia.
• In an emmetropic eye, the images will be coincident but in a myope, the images will be crossed in the eye but will be uncrossed in a diopliopia. photorefractor
• Fogging the target to relax accommodation a bit during refraction is a common practice in difficult patients.
• If the eye is in focus, the light returning to the source is occluded from the camera and the pupil is black. photorefraction
• The angular retinal image size of an annular target depends on the ametropia in the Scheiner principle.
• The images are uncrossed but the brain makes them appear as though they are crossed in hyperope. photorefraction
• If the eye is out of focus, there is a blur circle or ellipse where the illuminated area around the source. photorefraction
• Looking through an instrument will stimulate the eye to accommodate more than would be necessary for the naked eye, this is called instrument myopia.
• Photorefraction assesses the refractive state of the patient's eyes and is a binocular technique where there is a small light source mounted in front of a camera.
• In hyperope, the images will be uncrossed in the eye but will appear crossed in a dipliopia. (scheiner Principle)
• The Pulsoptix provides measurements for the values of refraction, IPD and pupil distance.
• The rate at which the images separate in the Scheiner principle depends on the pinhole separation.
• The badal principle is the angular subtense of image of a target in the lens is unaffected by target position.
• The badal system produces a linear relationship between the subjects ametropia and the target movement.
• Optometer lens is placed with the second focal point at the eyes focal point. (Badal System)
• The badal system is good because there is a better assessment of focus, a linear scale is used and the target point is constant.
• The badal optometer is bad because of proximal accommodation and the target can also stimulate accommodation.
• Autorefractors relax more accommodation as you are looking at a distance.
• There are both open and closed autorefractors - closed autorefractors stimulate accommodation because the image is close.
• Retinoscopic principle - time difference for peak intensity.