VORBV Revision PDF

Vision- the smallest detail that can be seen without correction in the form of glasses or contact lenses Visual acuity- the smallest detail that can be seen when wearing glasses or contact lenses Levels of Visual Function: 1. Light Perception 2. Detection 3. Resolution 4. Recognition 5....

Vision- the smallest detail that can be seen without correction in the form of glasses or contact lenses Visual acuity- the smallest detail that can be seen when wearing glasses or contact lenses Levels of Visual Function: 1. Light Perception 2. Detection 3. Resolution 4. Recognition 5. Relative Position 6. High Order Functions Simple to most complex. **Light Perception** Ability to tell whether something is bright or dark, examiner shines a light and patient asked if they could identify it. Recorded as PL or NPL. **Detection** Ability of the eye to detect a difference in retinal contrast, for example two shades of grey with different intensities. How much the intensity has to change for the patient to notice it. Teller acuity cards- black and white grating detection. **Resolution** Ability of the eye to see detail. For example, are there one or two stars in the sky. Resolution threshold is the smallest separation of detail that can be seen. **Recognition** Ability to recognise shapes. Kay picture cards- children asked to identify simple shapes. **Relative Position** Ability of the eye to tell whether an object is to the left or to the right of another object. Vernier Acuity- is a line to the left or right of another. **High Order Functions** Perception of colour, flicker, motion and memory. Relates to tasks where the visual system stimulates a response in another part of the body. 1. Light perception- ability to recognise light and dark, PL or NPL 2. Detection- ability to distinguish between intensities, teller acuity- b&w grating 3. Resolution- ability to distinguish detail for example 1 or 2 stars 4. Recognition- ability to recognise shapes, Kay Picture Cards 5. Relative position- vernier- is a line to left or right of another 6. High order functions- colour, flicker, motion, memory Minutes of arc- sixtieth or a degree Degrees to minutes of arc- multiply by 60 Minutes of arc to seconds of arc- multiply by 60 All eyes tested at 6m Donders- an angular separation of 1 minute of arc is needed for letter charts **Snellen** Each detail of the letter is 1 minute of arc with each letter being 5 minutes of arc vertically and horizontally. VA is a fraction- numerator is standard MAR of letter, denominator is MAR of the patient 6/18- standard observer can resolve at 18m, patient can only see it at 6m. MAR in minutes of arc- divide Snellen denominator by numerator Snellen Fraction- 6/12 is 0.5 (divide num by denom) Imperial Snellen- numerator becomes 20, multiply denominator by 3.333 Letter size- 6/6 letter is 8.73mm. Flip Snellen fraction and multiply by 8.7. or use TOA. Converting to 6/x- flip fraction and x by 6, this will give new denominator, numerator will be 6 LETTER SIZE DOUBLES APPROX EVERY SECOND LINE. **Snellen** Each limb of a letter is one minute of arc. Each letter is 5 minutes of arc both horizontally and vertically. Letter size doubles approximately every second line. Fraction with numerator 6m, denominator is distance normal person sees at. Height of 6/6 letter is 8.73m. Snellen to MAR- flip the fraction Snellen to imperial Snellen- change numerator to 20, multiply denominator by 3.33 Height of letter- flip fraction and multiply by 8.73 Converting to 6m equivalent- flip fraction and multiply by 6 to give new denominator. **LogMAR** Log mar is a more accurate way to measure VA with more repeatable results. Possible to determine if there has been a real change in VA over time. VA measured by taking the log of the mar: Snellen 6/24 mar is 4'. Log4 = 0.602 Letter size increases by a factor of 1.2589. Letter size doubles every 3 lines. 5 letters on each line. Each letter valued at 0.02. Negative log mar means VA is better than 6/6 Snellen. Lower log mar number = better VA Log MAR Snellen ------------------------------------ ---------------------------------------- Regular progression in letter size Irregular progression in letter size Letters of equal visibility Some letters easier to see than others Sans serif letters Serif letters All lines have 5 letters Lines have different number of letters Equal line spacing Irregular line spacing **Poor Vision** If patient cannot read 6/60. Move them closer to the chart, for example 3/60 instead of 6/60. Record light perception HM = hand movements CF @ 25- counting fingers. **Physiological Factors** VA decreases further from the fovea Infants are born with VA of approximately 6/200. This value goes down in the months following birth. VA declines after the age of 50, more rapidly after 70. **Near VA** Jaeger System- non-linear scale. Smallest line is smaller than most available print Faculty of Ophthalmologists- commonly used, N48-N5, used at a distance of 35cm. Times new roman Meter notation- represents distance in metres, used at 40cm. **Other tests for VA** Objective- does not require a verbal response form the patient Forced choice preferential looking- child fixates on grating rather than plain background Cardiff Acuity- child focuses on picture rather than plain background Subjective- requires a verbal response form the patient Kay picture cards- child recognises a series of shapes, log mar progression **Introduction to Refractive Error** Refraction aims- determine the size of refractive error and hat is needed to correct it. Light from close up objects will travel in divergent rays. Light from far away objects will travel in parallel rays. Emmetropia Image is formed directly on the retinal and a clear image can be seen. There is a perfect balance between the power and axial length of the eye. Emmetropic far point is at infinity. Accommodation needed to focus near objects Myopia Short sightedness. Image is formed in front of the retina. Eye is too powerful, or the axial length is too long. Minus lenses are used to correct, these diverge light and allow an image to form on the retina. Near objects can be focused without accommodation Hypermetropia Long sightedness. Image is formed theoretically behind the retina. Eye is not powerful enough or axial length is too short. Convex plus lenses are used to correct, these converge light and allow image to form on the retina. Eye can accommodate to increase power and focus distance objects. Near objects cannot focus even with accommodation. Presbyopia Crystalline lens hardens with age. Unable to accommodate to view objects which are near or distant. Progressive lenses used to correct. Astigmatism Image formed is not a single point, but two perpendicular focal lines separated by a circle of least confusion- area of least blurring. Interval of Sturm has to be closed to correct astigmatism. Steepest and flattest meridians. Cornea or lens is more curved in one direction than another. **Retinoscopy** Objective method of determining prescription. Streak directed across the eye and fundal reflex observed. With- streak and reflex in same direction, HYPEROPIA, EMMETROPIA OR LOW MYOPIA Against- streak and reflex move in opposite directions MYOPIA Neutral- no movement of reflex. Refractive error has been neutralised. Overcorrecting- with should become against, against should become with. Changing distance- moving closer shows with movement, moving back shows against Speed of reflex increases as neutral point is approached. **Structure of the eye** Cornea- first refracting surface, 2/3 of eye's overall power, transparent surface that allows light to enter the eye Iris- band of pigmented tissue which sits around the pupil, contains muscles which dilate and constrict the pupil depending on light levels. Lens- biconvex tissue which refracts light. 1/3 of eye's total power. Can change shape and accommodate to view distant or near objects Pupil- circular aperture in the centre of the eye which allows light to enter Ciliary body- contains zonular fibres which suspend the lens and can expand and contract to accommodate it Choroid- layer of blood vessels which surround the retina and provide it with nourishment. Aqueous- clear watery liquid in anterior chamber, provides nutrients to the cornea and lens Vitreous- gelatinous liquid found in vitreous chamber, keeps the retina attached to the choroid Retina- area at the back of the eye where images are formed, fovea is region of high cone density, sends signals to the brain along the optic nerve to form images Sclera- thick opaque white tissue surrounding the eyeball, posterior 5/6, provides shape and protection. 120 million rods- low light levels 8 million cones- bright light levels **Cornea** Transparent tissue at the front of the eye which refracts light. 2/3 of the eye's overall power. Cornea is avascular but has many nerves. Thickness: 0.5mm Diameter: 12mm RI: 1.376 **Anterior Chamber** Anterior chamber is filled with aqueous humour. RI: 1.33 **Pupil** Small aperture in the centre of the eye which allows light to enter. Constricts and dilates depending on light conditions. Diameter varies, between 1-8mm Dark: 5-8mm Bright: 2-5mm Pupil size decreases as age increases Consensual pupil response- when light is shone into one eye the other should also respond Pupil size reduces when viewing a near object Mydriatic drugs dilate Miotic drugs constrict **Lens** Supplies 1/3 of the eye's refractive power and can accommodate to make the eye more powerful to view near objects. Stiffer with age, presbyopia occurs when lens can no longer accommodate. Opacity of the lens= cataract. Thickness: 3.6mm Diameter: 9mm RI: 1.46 Accommodation: increase in power is positive accommodation, decrease in power is negative accommodation Ciliary muscle relaxed, lens can focus in the distance. When ciliary muscle contracts the lens surface steepens to view near objects. **Retina** Sensitive neural tissue at the back of the eye, has 10 layers. Fovea is region of high cone density, no rods present. Cones sensitive to blue green and red light Rods sensitive to blue green light at 500nm **Dark Adaptation** Coming from bright to dark environment, eyes have to adapt Cones take 7-10 minutes to adjust Rods take 20-30 minutes Rods contain a photopigment called rhodopsin, this chemical is purple, and absorption of light causes it to become bleached. Colour change- purple- orange- yellow- bleached. Reversible reaction- bleached molecules can be regenerated. Blind spot- no receptors at the trunk of the optic disc or the optic nerve head, blind spot in vision.