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Visual acuity and its disturbances (asik)
- 1. Asik Pradhan Consultant Optometrist Tilganga Institute of Ophthalmology VISUAL ACUITY AND ITS DISTURBANCES
- 2. Vision •Mental perception of the outside world with light as stimulus.
- 3. Vision • Purpose of the visual system • Transform light energy into an electro-chemical neural response • Represent characteristics of objects in our environment such as • Size, • Shape • Color, • Location, and • Movement
- 4. Light - The Visual Stimulus Gamma rays X-rays Ultra- violet rays Infrared rays Radar Broadcast bands AC circuits Visible light Prism White light 400 500 600 700 10-5 10-3 10-1 10 1 10 3 10 5 10 7 10 9 10 11 10 13 10 15 10 17 Wavelength in nanometers (billionths of a meter)
- 5. Light - The Visual Stimulus •Visible light has wavelengths from about 400nm to 700nm •Wavelength of light is related to its perceived color
- 6. Structure of the Eye The eye works like a camera, using a lens to focus light onto a photo- sensitive surface at the back of a sealed structure. Light rays Cornea Pupil Blind spot Optic nerve Retina Fovea (point of central focus) Lens Iris
- 7. Organization of Retina To optic nerve Ganglion cell Amacrine cell Bipolar cell Horizontal cell Cone Rod Light Light Cross section of retina shown vastly magnified in the diagram to the right Photochemical is located here Back of the eye
- 8. Visual Pathway RetinaOptic tract Optic chiasm Optic nerve Visual area of the thalamus Visual cortex
- 9. Visual Acuity • Refers to an angular measurement relating testing distance to the minimal object size resolvable at that distance. • Simply it is the measure of the eye’s ability to distinguish object details at a given distance.
- 10. Visual Acuity • Visual Acuity is the resolving power of eye. It is the ability of eye to perceive two points separately. • Eye can resolve two points separated at 1 min of Arc • Minimum Angle of Resolution (MAR)
- 11. Visual Acuity • Evaluated both uncorrected and corrected, at distance and near, with each eye being tested independently. • Distance Visual Acuity (DVA) • @ 6meters (20 feet) equivalent to optical infinity Mirrors and reflective systems (if testing lane is shorter) Chart re-calibration for shorter testing distances • Near visual acuity (NVA) • 40cm
- 12. Why to check Visual Acuity??? • To monitor a change in vision due to a disorder (progression and/or treatment plan) • Visual acuity must be measured on every patient at every visit!
- 13. Procedure • Position the patient at the appropriate testing distance. • Cover the eye not being tested.
- 14. Procedure • Ask the patient to read the optotype (letters in the acuity chart). • Continually decrease the size of the optotype until the patient is unable to read or will miss more than half of the characters on anyone line • Note the line and record the result. • Repeat the procedure for the fellow eye.
- 17. Linear Kay Pictures for children
- 18. SHERIDAN GARDNER for Children “One thing like other thing”
- 19. PIN HOLE •A pinhole converts the eye into a pinhole camera and neutralises refractive error. •A pinhole is used to create a shorthand refraction. •The pinhole test allow us to assess the visual potential of the eye.
- 20. Pinhole Visual Acuity •Purpose • To determine if a decrease in vision is correctable by lenses •Pinhole visual acuities are taken when the patient’s entering VA is worse than 20/30 at distance through the habitual correction
- 21. Expected Findings • If the cause of the patient’s decreased visual acuity is due to an uncorrected refractive error, the VA is expected to improve through the pinhole • If the cause of decreased visual acuity is not optically based, no improvement, and possibly a decrease, will occur through the pinhole
- 23. Causes of Subnormal Acuity • Refractive Errors • Non-refractive/Pathological Errors • Media opacity, Retinal Pathology, Optic Nerve and Visual Pathway disorders • Amblyopia
- 24. Refractive Error • The result of a mismatch between optics and the growth of the eye • It is due to a combination of genetic and environmental influences • It is NOT considered an eye disease • Treatment includes spectacles, contact lenses, and refractive surgery (RK, PRK, LASIK, LASEK)
- 25. Refractive Errors • Emmetropia • Ametropia (Refractive Error) • Myopia • Hypermetropia/ Hyperopia • Astigmatism
- 26. Emmetropia • The average emmetrope has a VA of 20/20 or better
- 27. Myopia • When parallel rays of light enter the eye (with accommodation relaxed) and come to a single point focus in front of the retina
- 28. Mechanism • Increased anterio-posterior length • Axial myopia • Increased curvature of cornea • Curvatural myopia • Anterior displacement of the lens • Positional myopia • Change in refractive index of lens • Index myopia • Myopia due to spasm accommodation • Excessive accommodation
- 29. Myopia • Blurry vision at distance • Clear vision at near • Asthenopic symptoms • Headache • Watering • Ocular pain. • Poor school performance
- 30. Myopia • It is corrected by divergent or minus lenses
- 31. Hyperopia • When parallel rays of light enter the eye (with accommodation relaxed) and come to a single point focus behind the retina
- 32. Mechanism • Decreased anterior-posterior length •Axial • Decreased curvature •Curvatural • Posterior displacement of the lens •Positional • Change in refractive index of the lens •Index hyperopia • Absence of the crystalline lens •aphakia
- 33. Hyperopia • Blurry vision at near • Asthenopic symptoms • Headache • Eye strain • Poor school performance
- 34. Hyperopia • It is corrected by convergent or plus lenses
- 35. Astigmatism • When parallel rays of light enter the eye (with accommodation relaxed) and do not come to a single point focus on or near the retina
- 36. Astigmatism • It is due to a distortion of the cornea and/or lens • The refracting power is not uniform in all meridians • The amount of astigmatism is equal to the difference in refracting power of the two principal meridians
- 37. Astigmatism • Blurry vision @ near and distance • Asthenopic symptoms (Headache, Eye Strain) • Tilting of head • Letter confusion • P versus F • A versus R • H versus N • T versus Y • C versus G • V versus Y • It is corrected by cylindrical or spherocylindrical lenses
- 38. Presbyopia • Presbyopia = “old man’s eye” (Latin) • Decrease in the amplitude of accommodation or loss of accommodative ability with age • It is a natural part of the ageing process • The onset of presbyopia is at approximately 40 years of age and over though it may be earlier in low hyperopes
- 39. Presbyopia • Blurry vision at near • Difficult or impossible to accommodate sufficiently for near work • It is corrected by convergent or plus lenses for near work only (near Add)
- 40. Amblyopia • “Lazy Eye” • An ocular condition in an otherwise healthy eye, in which there is an abnormality of cortical response in the occipital lobe of the brain due to insufficient or inadequate stimulation of the fovea, neural pathway, and cortex that may result in unilateral vision loss if untreated.
- 41. Amblyopia Good eye Amblyopic eye
- 42. Amblyopia • Treatment • Optical Correction • Glasses & contact lenses • Patching Therapy • Active Vision Therapy