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Accommodation

Briefly about Accommodation , Assessment and Anomalies

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Accommodation

  1. 1. ACCOMMODATION PRESENTED BY: REEMA DANDAVATE T.Y.B.OPTOMETRY
  2. 2. WHAT IS ACCOMMODATION ? • KNOWN FACT : in an emmetropic eye, parallel rays of light coming from infinity are brought to focus on retina, with accommodation at rest. • WHAT ABOUT THE DIVERGING RAYS COMING FROM NEAR OBJECT ? • Our eyes has been provided with a unique mechanism by which we can even focus the diverging rays coming from near object on retina in bid to see clearly.  ACCOMMODATION. • IN IT THERE IS INCREASE IN LENS POWER.
  3. 3. SOME TERMINOLOGIES... • NEAR POINT OF ACCOMMODATION : • The nearest point at which small objects can be seen clearly is called near point of accommodation or punctum proximum. • the distant point is called far point off accommodation or punctum remotum.
  4. 4. • RANGE OF ACCOMMODATION: • The distance between near point and far point is called range of accommodation. • AMPLITUDE OF ACCOMMODATION: • The difference between DIOPTRIC power, needed to focus at near point (P) and to focus at far point (R), is called amplitude of accommodation (A). Thus, A=P-R
  5. 5. POINTS TO BE KNOWN... • In HYPROPIC eye, far point is virtual and lies behind the eye. • In MYOPIC eye, far point is real and lies in front of the eye. • In an EMMETROPIC eye, far point is at infinity and near point varies with age...
  6. 6. Near point in centimeters Age in years 7 10 25 40 33 45
  7. 7. DEPTH OF FIELD... • The range of distance from the eye in which an object appears clear without change in accommodation is termed depth of field. • It reduces the necessity for precise accommodation.
  8. 8. DEPTH OF FOCUS... • The range at the retina in which an optical image may move without impairment of clarity is termed as depth of focus.
  9. 9. THEORIES OF ACCOMMODATION… • Numerous theories have been proposed on accommodation. • Few of these are as follows.
  10. 10. PROOF OF EXISTENCE OF ACCOMMODATION • Till 17th century  unknown that it is necessry for eye to change its power in order to focus • 1619  Christopher Scheiner gave proof of existence of accommodation. • 1801 Thomas Young demonstrated lens is responsible for accommodation
  11. 11. TSHERNING’S theory • This theory attributed increased curvature of capsule to increasing tension of the zonules. • It states that contraction of ciliary muscle pulls zonules directly and increases tension of capsule at equator of lens, which leads to bulging of poles.
  12. 12. Relaxation theory of HELMHOLTZ • Also known as the “Capsular Theory”. • He considered that lens was elastic and in normal state it is stretched and flattened by tension of the suspensory ligaments. • During accommodation, contraction of ciliary muscle shortens ciliary ring and moves towards the equator of the lens. • Relax the suspensory ligaments, relieving strain. • Lens assumes more spherical form, increasing thickness and decreasing diameter.
  13. 13. GULLSTRAND mechanical model of accommodation • It is based on HELMHOLTZ hypothesis • GULLSTRAND devised a mechanical model to explain accommodation. • It shows in unaccommodated state elasticity of choroid is stronger than lens. When accommodation comes into play weight i.e ciliary muscles contract to overcome elasticity of choroid. • It helps lens to take accommodated shape.
  14. 14. MECHANISM OF ACCOMMODATION
  15. 15. Changes in eye due to accommodation...  Slackening of the zonules  Change in the curvature of lens surface  Anterior pole  Axial thickness  Changes in the tension of lens capsule  The lens sinks down
  16. 16. Change within the lens substance Pupillary constriction and convergence of eyes The choroid The ora serrata
  17. 17. AC/A ratio • The AC/A ratio is the relationship between accommodative convergence (AC) expressed in prism diopetrs, and accommodation (A) expressed in lens diopters. • This relationship is linear one and is thought to be relatively stable throughout life. • Normal AC/A ratio- 3-5 prism D for 1D of accommodation.
  18. 18. NEGATIVE RELATIVE ACCOMMODATION (NRA) • NRA is a measure of its maximum ability to relax accommodation while maintaining clear, single binocular vision.
  19. 19. POSITIVE RELATIVE ACCOMMODATION (PRA) • PRA is ameasure of maximum ability to stimulate accommodation while maintaining clear, single binocular vision.
  20. 20. REACTION TIME It refers to a time lapse between the presentation of an accommodative stimulus and occurrence of accommodative response. •Average reaction time for far-to-near accommodation is 0.64 sec and for near-to-far is 0.56 sec
  21. 21. Types of Accommodation • Tonic accommodation – It is due to tonus of ciliary muscle and is active in absence of a stimulus. The resting state of accommodation is not at infinity but rather at an intermediate distance. • Proximal accommodation – Is induced by the awareness of the nearness of a target. This is independent of the actual dioptric stimulus.
  22. 22. • Reflex accommodation – Is an automatic adjustment response to blur which is made to maintain a clear and sharp retinal image. • Convergence-accommodation – Amount of accommodation stimulated or relaxed associated with convergence. – The link between accommodation and convergence is known as accommodative convergence and is expressed clinically as AC/A ratio.
  23. 23. ASSESSMENT FOR ACCOMMODATION.
  24. 24. ANNOMALIES OF ACCOMMODATON…
  25. 25. ANOMALIES OF ACCOMMODATION • DIMINISHED ACCOMMODATION • PRESBYOPIA
  26. 26. Presbyopia Presbyopia is a condition of physiological insufficiency of accommodation leading to a progressive fall in near vision.
  27. 27. Pathophysiology • In emmetropic eye far point is infinity and near point varies with age (being about 7 cm at 10 years, 25 cm at 40 years and 33 cm at 45 years). • We read from 25 cm. After 40 years, the near point recedes beyond normal reading or working range. • Failing near vision due to age-related decrease in amplitude of accommodation is called presbyopia.
  28. 28. Causes • Decrease in accommodative power of lens with increasing age, leads to presbyopia, occurs due to: – Age-related changes in lens: oDecrease in elasticity of lens capsule, and oProgressive, increase in size and hardness (sclerosis) of lens substance which is not easily moulded. – Age related decline in ciliary muscle power.
  29. 29. Premature presbyopia: • Uncorrected hypermetropia. • Premature sclerosis of the crystalline lens. • General debility causing pre-senile weakness of ciliary muscle. • Chronic simple glaucoma.
  30. 30. Symptoms • Difficulty in near vision. • Patients complaint of difficulty in reading small prints • Asthenopic symptoms due to fatigue of the ciliary muscle are also complained after reading or doing any near work.
  31. 31. Optical treatment • Prescription of appropriate convex glasses for near work. • A rough guide for providing presbyopic glasses in an emmetrope can be made from patient’s age. – About +1 DS is required at the age of 40-45 years, – +1.5 DS at 45-50 years, – + 2 DS at 50-55 years, – +2.5 DS at 55-60 years.
  32. 32. Basic principles of presbyopic correction • Refractive error for distance is corrected first. • Correction needed in each eye should be tested separately and add it to distant correction. • Near point should be fixed according to the profession of patient. • Weakest convex lens with which one can see clearly at near point should be prescribed, overcorrection will also result in asthenopic symptoms. • Presbyopic spectacles may be unifocal, bifocal or varifocal.
  33. 33. Surgical Treatment • Corneal procedures – Non ablative corneal procedure • Monovision CK – Laser based corneal procedure • Laser thermal keratoplasty (LTK) • Monovision LASIK. • Presbyopic bifocal LASIK • Presbyopic multifocal LASIK C Near Vision Distant Vision
  34. 34. • Intraocular refractive procedure – Refractive lens exchange – Phakic refractive lens – Monovision with IOLs • Scleral based procedures – Anterior sclerotomy with tissue barriers – Scleral spacing procedure – Scleral ablation with erbium : yag laser
  35. 35. Insufficiency of accommodation • Condition in which accommodative power is constantly less than lower limit of normal range according to patient’s age.
  36. 36. Etiology • Premature sclerosis of lens • Weakness of ciliary muscle due to systemic causes: Debilitating illness, anemia, toxemia, malnutrition, diabetes mellitus, pregnancy, stress etc. • Weakness of ciliary muscle due to local causes: PAOG, mild cyclitis as during onset of sympathetic ophthalmia.
  37. 37. Clinical features • Features of eye strain and asthenopia. • Head ach, fatigue & irritability of the eyes, while attempting near work. • Near work is blurred & becomes difficult or impossible. • Disturbance of convergence : intermittent diplopia. • It is stable condition, if due to sclerosis of lens. • But is not stable in association with ciliary muscle weakness.
  38. 38. Treatment • Identification & treatment of any systemic cause. • Any refractive error should be corrected & if vision for near work is seriously blurred then additional near correction has to be prescribed same as presbyopia. • If associated with convergence excess then full spherical correction.
  39. 39. • Convergence insufficiency is there, then base in prisms can be added. • Prismatic correction added should bring near point of convergence to same distance as near point of accommodation. • Weakest convex lenses should be prescribed, so as to exercise and stimulate accommodation. • After recovery additional correction should be made weaker and weaker from time to time.
  40. 40. • Accommodative exercises. – While do exercises patient should wear correction for distance. – Should be done simultaneously in both eyes, even if associated with convergence insufficiency. – But with convergence excess then the exercise should done with one eye alternately. – Accommodation test card exercise. – Useless in generalized debility and sclerosis of lens.
  41. 41. Ill-Sustained accommodation • Accommodation fatigue. • It is a situation in which though range of accommodation is in normal range but it cannot sustain it for a sufficient period of time. • Initial stage of insufficiency of accommodation. • It occurs due to – Stage of convalescence from debilitating illness – Stage of generalized tiredness – When the patient is relaxed in the bed
  42. 42. Clinical features • These symptoms are most commonly reported at the end of the day • Blurred vision after prolonged near work. • Headaches • Eyestrain • Fatigue, sleepiness and a loss of comprehension with continued reading • A dull 'pulling' sensation around the eye.
  43. 43. Treatment • Near work should be curtailed during debilitating illness. • General tonic measures should be taken. • The condition of illumination and posture while doing near work, should be improved.
  44. 44. Inertia of accommodation • It is a condition in which patient faces difficulty in altering the range of accommodation. • Amplitude of accommodation is normal. • Ability to make use of this amplitude quickly and for long periods of time is inadequate.
  45. 45. Clinical features • Difficulty changing focus from one distance to another • Headaches • Eyestrain • Fatigue • Difficulty sustaining near tasks • Blurred vision Treatment: correcting any refractive error and accommodative exercises.
  46. 46. Paralysis of accommodation • Cycloplegia, refers to complete absence of accommodation. • Causes – Atropine, homatropine or other parasympatholytic drugs. – Internal ophthalmoplegia (paralysis of ciliary muscle and sphincter pupillae)due to neuritis associated with diphtheria, syphilis, diabetes, alcoholism, cerebral or meningeal diseases.
  47. 47. – Complete third nerve paralysis due to intracranial or orbital causes. – Systemic medications such as anti-hypertensive, antidepressants.
  48. 48. Clinical features • Blurred vision at near • Photophobia or a 'dazzling' effect • Diplopia • Micropsia: objects may appear smaller than they are due to a false sense of distance • Enlarged pupil.
  49. 49. Treatment • An effort should be made to find out the cause and try to eliminate it. • Self-recovery occurs in drug-induced paralysis and in diphtheric cases (once systemic disease is treated). • Dark-glasses effective in reducing glare. • Convex lenses for near vision, if the paralysis is permanent.
  50. 50. Excessive accommodation • Accommodative response is greater than the accommodative stimulus. • There is functional increase in tonus of ciliary muscle, results in a constant accommodative effect.
  51. 51. Causes • Young hypermetropes frequently uses excessive accommodation as a physiological adaptation • Young myopes performing excessive near work, associated with excessive convergence. • Astigmatic error in young patients • Presbyopes in the beginning • Use of improper and ill fitting spectacles
  52. 52. Precipitating factors • Excessive near work done, especially in dim or excessive illumination. • General debility, physical or mental ill health
  53. 53. Symptoms • Blurred vision at near is uncommon • Blurred vision at distance • Headaches • Eyestrain • Photophobia • Difficulty changing focus from distance to near • Diplopia
  54. 54. Treatment • It has a good prognosis. • Refractive error should be corrected after carefully performed cycloplegic refraction. • Near work should be stopped for some time, after that it should be done with proper illumination conditions.
  55. 55. Spasm of accommodation • Spasm of accommodation refers to exertion of abnormally excessive accommodation.
  56. 56. Causes • Drug induced spasm of accommodation is known to occur after use of strong miotics. • Spontaneous spasm of accommodation: attempt to compensate for a refractive anomaly. • Occurs when excessive near work is done with bad illumination, bad reading position, state of neurosis, mental stress or anxiety.
  57. 57. Clinical features • Defective vision: due to induced myopia. • Asthenopic symptoms • Precipitating factors like marked degree of muscular imbalance, trigeminal neuralgia, a dental lesion, general intoxication.
  58. 58. Treatment • Relaxation of ciliary muscle by atropine for 4 weeks or more and • Prohibition of near work allow prompt recovery from spasm of accommodation. • Elimination of the associated causative factors to prevent the recurrence.
  59. 59. Reference… • BOOKS… • Primary care optometry • A.K.Khurana (optics and refraction) • A.K.Khurana (squint and orthoptics) • REFERENCE PPT… • www.slideshare.net/RohitRao2/accommodation-of-eye • www.slideshare.net/laxmieyeinstitute/accommodation- 35905316 • IMAGES… • Google

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