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Accommodation: Theories and Mechanism
- 1. Accommodation: Mechanism, theories, ocularchanges, ranges and amplitudes. Moderator: Presenters Dr Sanjeev Bhattarai Aaratee Jha Garima Paudyal
- 2. ACCOMMODATION • Accommodation isthe mechanism by which the eye changes refractive power by altering the shape of the lens in order to focus objects at variable distances.
- 3. • In anemmetropic eye -Parallel rays of light coming from infinity are brought to focus on retina being accommodation at rest. -Eyes can also focus diverging rays coming from near object on retina to see clearly due to accommodation.
- 5. Mechanism of Accommodation Ciliarymuscle contracts Ciliary ring shortens Equator of lens move forward As a result Zonules are relaxed Tension in capsule is relieved Lens becomes spherical i.e. convexity increases Increase in Dioptric power Allowing near object to be focused clearly on retina
- 7. Anatomy of Accommodation •The lens fibres of the young crystalline lens forms an elastic substance. • Fibres are surrounded by elastic capsule with its maximum thickness at the equator. • Age changes in lens on accommodation is explained by changes in the relative sizes of the lens cortex and nucleus and their relative softness. • Eg: In child’s eye, the whole lens is soft and adopts a more convex spherical shape on accommodation.
- 8. • In anadult upto 30 years, the cortex has significance thickness, and hence the softer nucleus forces the central zone to become more spheric on accommodation.
- 9. After 30-40 yearsof age both nucleus and cortex soften Asphericity of the lens and amplitude of accommodation decreases with age Effort required to accommodate decreases Giving rise to presbyopia
- 10. With Age: (Insummary) Lens fibers & lens capsule lose elasticity The size & shape of the lens increase Reduction of accommodative amplitude Onset of presbyopia
- 11. Theories of Accommodation •The exact mechanism of accommodation is not known but the principal fact is that Accommodation is a feature of increase in curvature of the lens which affects anterior surface mainly.
- 12. Relaxation theory ofHELMHOLTZ “Capsular theory” • He considered that the lens was elastic and in normal state it is stretched and flattened by tension of suspensory ligaments. • During accommodation, contraction of the ciliary muscle shortens ciliary ring and moves towards the equator of the lens. • It relaxes the suspensory ligaments, relieving strain. • Lens assumes more spherical form, increasing thickness and decreasing diameter.
- 15. Points in favourof the relaxation theory • Imaging technique showed that ciliary muscle move anteriorly and the equatorial edge of lens move away from sclera during accommodation. • Gonio-videography show zonular fibers extending from ciliary processes to lens equator, are relaxed during accommodation.
- 16. GULLSTRAND mechanical modelof 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 i.e. ciliary muscles contract to overcome elasticity of choroid. • It helps lens to take accommodated shape.
- 18. Points against thetheory • It is not clear how lens alter its shape when tension in suspensory ligaments is relaxed. • It is unable to explain what is responsible for decline in power of accommodation with age.
- 19. TSCHERNING’STHEORY OF ACCOMMODATION Increased curvatureof capsule to increasing tension of the zonules. Contraction of ciliary muscles pulls zonules directly Increases tension of capsule at equator of lens. Bulging of poles.
- 20. Schachar’s Theory of Accommodation Ciliarymuscles contracts. Equatorial zonular tension is increased. Anterior and posterior zonules are simultaneously relaxed. Central surface of the lens steepen. Peripheral surfaces of the lens flatten.
- 22. COTENARYTHEORY As ciliary musclecontracts It forms a pressure gradient, causing anterior movement of lens zonules diaphragm Increasing anterior central curvature Presbyopia is due to increase in lens volume, results in reduced response to pressure gradient created by ciliary body contraction.
- 23. Accommodation reflex • Wheneyes are focused from a distant to near object, three reactions takes place. Constriction of pupil Thickening of the lens due to contraction of ciliary muscles. Convergence of both eyes. These three reactions together constitute near reflex triad.
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- 27. Ocular changes inAccommodation Changes in zonules • Zonules are normally under tension and keep the lens flat • During accommodation, ciliary muscle contracts • As a result, tension in zonules is released
- 28. Contd… Changes in lens •Mainly curvature of the lens changes • Principal change occurs in the anterior surface of the lens • In unaccommodated state, radius of curvature of anterior surface of the lens is 11mm and that of posterior surface is 6mm
- 29. Contd… • In accommodatedstate, the radius of curvature of the anterior surface of the lens decreases i.e becomes 6mm from 11mm • Almost no changes occur in posterior radius of curvature of the lens • Anterior pole of the lens moves forward as the radius of curvature of anterior surface of the lens is decreased • As a result, Axial thickness of the lens increases
- 30. Contd… • Lens sinksdown within the globe due to gravity as in accommodated state lens is held less firmly by zonular attachment • In addition to the changes in lens, the pupil constricts and there occurs simultaneous convergence of the visual axis of both the eyes • Choroid is stretched forward due to ciliary muscle contraction and ora serrata moves forward
- 31. Contraction of ciliarymuscles Decrease in zonular tension More spherical shape of the lens Increased axial thickness Steepening of the lens anterior curvature Increased lens dioptric power
- 32. Far and NearPoint of accommodation Punctum Remotum (Far Point ofAccommodation)/ FPA The object point that is conjugate with the retina when accommodation is fully relaxed In this case, an object placed at the far point will be imaged on the Retina
- 34. Far point ofaccommodation in different refractive state of eye Emmetropia The Far point of accommodation for an emmetropic eye is at infinity
- 35. Contd… Myopia Far point ofaccommodation for an uncorrected myopic eye is a real object point lying in front of the eye
- 36. Contd… Hyperopia Far point ofaccommodation for an uncorrected hyperopic eye is a virtual object point located behind the retina
- 37. Near point ofaccommodation (Punctum proximum) / NPA The object point that is conjugate to the retina with accommodation fully at play The nearest point at which the object can be seen clearly
- 38. Near point ofaccommodation in different refractive state of the eye Emmetropia NPA is a real object point located in front of the eye
- 39. Contd… Myopia NPA is areal object point located in front of the eye similar to that of emmetropia
- 40. Contd… Hyperopia May be eithera real object point located in front of the eye or a virtual object point located behind the eye depending upon the amount of hyperopia and amplitude of accommodation
- 41. Range of Accommodation Definedas the linear distance traversed by the point of conjugacy in moving from the far point to the NPA It is the distance over which accommodation is available for a subject and accommodation is effective An object can be seen clearly only if located at the far point or near point or at any point between the FPA and NPA The range of accommodation in either state (Corrected or uncorrected )is equal to the distance from the FPA to the NPA
- 42. Range of accommodationin different refractive state of the eye Emmetropia An emmetropic eye that has a finite (real) NPA and FPA at infinity has an infinite range of accommodation extending from infinity to NPA
- 43. Contd… Myopia For a myopiceye , the FPA and NPA are always real object points located at finite distances in front of the eye Hence the range of accommodation of a myopic eye is always less than infinity
- 45. Contd… Hyperopia For a hyperopiceye , having the FPA behind the eye , has the NPA located at a finite distance in front of the eye, the range of accommodation is infinite
- 46. Amplitude of accommodation Differencein the focus power of the eye while fixating for near distance and fixating for far distance Or The difference between the dioptric power needed to focus ta near point (P) and to focus at far point (F) i.e. AA = P – R
- 47. Q.Why measure Amplitudeof Accommodation ? • Recommended component of the routine clinical eye examination • The detection and management of common refractive conditions including Presbyopia and Latent hyperopia are frequently assisted by measuringAA
- 48. Assessment of Amplitudeof Accommodation Accommodation can be stimulated either by moving a test object closer to the eyes or by placing minus lenses Following procedures are used to determine the AA -AA using RAF rule -Push up method / Donders method -Minus lens method / Sheard’s method
- 49. Push up method/ Donders method • Ubiquitous • The commonest and simplest clinical technique to measure AA • Determines maximum amount of accommodation that eyes are capable of producing individually or together • Well established in clinical practice and research and is often assessed using RAF (Royal Airforce Rule) or Prince rule
- 51. RAF Rule • Speciallydesigned for measuring NPC,NPA and hence AA • Consists of bar or rule made from plastic, metal or wood on which a rider with the test chart can be moved back and forth(fixation target) • At one end of the bar is the winglike support that fits over the nose and rests against the lower orbital margins during the measurement
- 52. Procedure • Near visualacuity chart placed on near point card • Direct patient’s attention to 20/20 line of letters on near point card • Patient left eye is occluded first
- 53. Contd… • Near pointcard is brought closer to the patient (2-3 inches per sec) • Patient is instructed to keep the letters as clear as possible and report when it blurs • Prompt the patient to clear the target
- 54. Contd… • Stop whenthe patient can no longer clear the target within 2-3 secs of viewing • Record the dioptric point on the near point rod that corresponds with the first sustained blur • Procedure is repeated for left eye with right eye occluded
- 55. Minus lens method/ Sheard’s method • Amplitude of accommodation is determined interposing minus lenses • Each eye is tested monocularly first • Then tested binocularly • Full refractive correction is worn by the patient
- 56. Procedure • Near pointcard is placed at distance of 40cm • Patient is instructed to watch the 20/20 line of letters first with right eye and then with left eye ,the opposite being occluded • Patient is then asked to report when the letters begin to blur as minus power is gradually added to his/her subjective correction
- 57. Interpretation • Amplitude ofaccommodation is then calculated by adding dioptric power of the minus lens to produce first sustained blur with the dioptric power of the working distance • For e.g. If working distance (WD)= 40cm then, dioptric power ofWD = 2.50D Power of minus lens to make first sustained blur = - 4.00D Then, AA = (4.00 + 2.50)D = 6.00 D
- 58. Contd… • In somecases , it is necessary to add plus lenses to the subjective lenses to clear up the letters at 40cm • The amount of plus power necessary to clear up the letter is subtracted from the considered working distance to determine the Amplitude of Accommodation
- 59. Comparison between pushup method and minus lens method • The angular size of the retinal image of 20/20 letters increases in direct proportion to the decrease in distance between the near point card and the eyes • In minus lens method the angular size of the retinal image almost remains the same • For this reason, AA measured by Push-up method is overestimated compared to that of Minus lens method
- 60. Amplitude of Accommodationand age The amplitude of accommodation declines throughout the life until at about 50 or 60 years of age when it becomes zero
- 61. Formula to determinethe Amplitude of Accommodation Hofstetter formulae for expected amplitude as a function of age (using the data of Donders, Duane and Kaufman) • Maximum amplitude = 25 – 0.4 (age) • Probable amplitude = 18.5 – 0.3 (age) • Minimum amplitude = 15 – 0.25 (age)
- 62. Example • For 20years old patient, Maximum AA = 25 – 0.4*20 = 17D Average AA = 18.5 – 0.3*20 = 12.5 Minimum AA = 15 – 0.25*20 = 10D NPA = 1/10 = 0.1m = 10cm
- 63. Rule of 4’sto determine AA with Age Given by, Amplitude = 4x4 – (Age/4) Example Age of 20 AA = 16 – 20/4 = 11 D
- 64. Anomalies related toAmplitude of Accommodation • Deficient or decreased Amplitude of Accommodation results in: - Accommodation Insufficiency - Fatigue of accommodation -Presbyopia
- 65. Accommodation Insufficiency • Theaccommodative power is significantly less than the normal physiological limits for the patient’s age • Caused due to premature sclerosis of lens and weakness of ciliary muscles • Blurring of near vision along with prominent asthenopic symptoms are present • Near vision spectacles along with the accommodative exercise can be used for the treatment
- 66. Fatigue of Accommodation •Also called as ill-sustained accommodation • Measures ability of the eyes to adequately sustain sufficient accommodation over an extended period of time • Can be tested using RAF rule • With RAF rule, AA is measured repeatedly
- 67. Contd… • AA initiallysustained with considerable effort, overtime , it cannot be maintained • Hence , decrease ofAA on repeated testing suggests Fatigue ofAccommodation • Patient complains of letters becoming blurred after reading for sometime • Patient is treated with Push-up therapy or flipper lens therapy
- 68. Presbyopia (eyesight ofold age) • Not an error of refraction but a condition of physiological insufficiency of accommodation due to reduced AA , leading to progressive fall in near vision or increase in NPA • Causes are age related changes in lens and age related decline in ciliary muscle power • Symptoms are blurring of near vision and sometimes asthenopic symptoms on prolonged near work • Optical correction for near can be given for presbyopia
- 70. Relative Accommodation • Theamount of accommodation to be exerted under fixed convergence is called relative accommodation • Two types Negative relative accommodation Positive relative accommodation Negative relative accommodation • To test patient’s ability to decrease accommodation while maintaining convergence at 40 cm
- 71. Contd… • A reducedsnellen chart with 20/20 letters at 40cm • Place patient’s distance correction in trial frame • Add plus power in 0.25 steps every 2 secs until patient reports first blur Value of negative relative accommodation (NRA) • Normal values = +1.75 to +2.00DS
- 72. Positive RelativeAccommodation • To test the patient’s ability to increase the accommodation while maintaining convergence at 40 cm • Add minus power in 0.25 steps every 2 secs until patient reports first blur Value of Positive relative accommodation (PRA) • Normal values : -2.25 to -2.50 DS
- 73. Interpretation • NRA andPRA is calculated • Range of accommodation = NRA + PRA • Similarly, ADD for presbyopia can also be calculated using NRA and PRA as ADD = ½(NRA + PRA)
- 74. Example • Working distance= 2.00D • NRA = +3.00D and PRA = -1.5 D • Hence ,Range of Accommodation = NRA + PRA = +1.50D • ADD = ½(+1.50D) = 0.75D
- 75. References • Anatomy andPhysiology of eye by A.K Khurana • Optics and Refraction by A.K Khurana • Clinical optics • Primary care Optometry
Editor's Notes
- #8 ii helps to drive the lens into a more convex shape when the zonular tension is relieved.