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Accommodation and Presbyopia Prof. Roger S. Anderson

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Presentation on theme: "Accommodation and Presbyopia Prof. Roger S. Anderson"— Presentation transcript:

1 Accommodation and Presbyopia Prof. Roger S. Anderson
University of Ulster, Coleraine UCL Institute of Ophthalmology, London

2 Makes up remaining 1/3 refractive power of eye Accommodation
Role of the lens: Makes up remaining 1/3 refractive power of eye Accommodation Compensation for aberrations of cornea Accommodated o Unaccommodated Ciliary muscle

3 Control of Accommodation of utmost importance in refraction.
A hypermetrope can bring the focal point of the eye forward by accommodation. Unaccommodated Accommodated f Therefore, young hyperope may still read 6/5! Myope can not put focal point backwards since ciliary muscle already relaxed. Type of optical defect (e.g. myopia, hyperopia, etc) refers to position of focal point of eye in unaccommodated state.

4 AMPLITUDE OF ACCOMMODATION. Accommodation occurs at ~1 mth old.
More regular by 2-3 mths. Almost adult-like by 6 mths. Falls from maximum of 18D at 10yrs to zero by 70 yrs. => PRESBYOPIA (“old eye”) Binocular Amplitude of Accommodation is usually slightly greater than monocular. Also dependent on pupil size. 24 22 20 18 16 14 12 10 8 6 4 2 Age (yrs) After Donders

5 Anatomy of Accommodation: the ciliary muscle
Oblique fibres Circular fibres Longitudinal fibres

6 Accommodation: according to Helmholtz
Contraction of circular and longitudinal muscle fibres results in decreased zonular tension. Lens takes up more relaxed, less elongated state. Increased central thickness and reduced radius of curvature

7 Accommodation: according to Schachar
Central thinning Decreased curvature Aspect ratio > 0.6 Peripheral thinning but central thickening. Increased central curvature. Decreased spherical aberration. Aspect ratio ≤ 0.6 (“long oval”)

8 Presbyopia: the common view
Increased sclerosis of lens leads to loss of flexibility and inability to change shape Young Old

9 But…. embryologically the lens is derived from surface ectoderm…..
…and keeps growing throughout life!

10

11 Presbyopia: according to Schachar
As lens grows, tension on zonules slackens and ciliary muscle must contract more to take up slack before accommodation can occur. Young Old

12 MEASUREMENT OF ACCOMMODATION ‘Push-up’ method (Donders 1864)
Requires the use of a small target so that small errors of focus are detectable. e.g. Using RAF rule: 1. Place on face with one eye occluded 2. Type moved closer until print “just goes blurred” 3. Move closer still to ensure that print goes even more blurred 4. Move back until print “just becomes clear” (take average) 5. Repeat for other eye and binocularly Amp. of Accommodation = _______ Near pt. of Acc.

13 ‘Minus Lens Method’ (Sheard 1917)
1. Near point card placed at 40cm 2. Minus lenses added in -1.00D steps until blur first noticed, which can not be cleared by straining. A-of-A = Negative power added D NB. In many presbyopes, A-of-A often less than 2.50D. Must add positive lenses first to clear target. Then add negative. Push-up method usually gives higher A-of-A than minus lens method. Effects of: 1. Pupil size 2. Object distance (accommodative lag) 3. Fatigue 4. Accommodative Convergence (binocular)

14 Accommodation should be measured
on every patient because it may be reduced by….. Latent hypermetropia or accommodative spasm Poor general health Ocular disease Side effect of medical treatment

15 Spectacle and Ocular Accommodation
ls d d = 14mm, object at 350mm from PP Ocular Acc. = = 2.86D ls + d Spectacle Acc. = = 2.98D ls So, for an emmetrope, Spectacle Acc. > Ocular Acc.

16 Myope. e.g. -4.00DS (spec ref), d = 14mm
object at 350mm from PP Distance Vergence D mm 0 <-- ¥ Fsp > f’sp -d -14 (ocular ref) Near Vergence D mm Ls < ls +Fsp > f’sp (Verg. at PP < for near) Ocular Accommodation = (-6.36) = 2.57D Therefore, myope accommodates less wearing spectacles.

17 For Hyperope: +4.00DS (spec ref), d = 14mm
object at 350mm from PP Distance Vergence D mm 0 <-- ¥ Fsp > f’sp -d -14 (ocular ref) Near Vergence D mm Ls < ls +Fsp > f’sp (Verg. at PP < for near) Ocular accommodation = = 3.21D Therefore hyperope accommodates more wearing spectacles.

18 Calculating the Near Addition
The power of the Add depends on: Myope or hyperope Habitual working distance Amplitude of Accommodation Nature of the task Pupil size Not possible to employ full accommodative power for long periods, But about 1/2 - 2/3 can be sustained. Thus Add = (-Ls) - Amp/2 where Ls = dioptric working distance

19 Thus, if working distance = -400mm from spectacle plane
and amplitude of accommodation = 3 dioptres Add = (2.50) = D Need to check reading range as well. Patients with poorer VA can obtain greater magnification by prescribing higher Adds and making them read closer.

20 Correction Methods Single Vision Readers Bifocals Varifocals Bifocal IOL

21 Thank You


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