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

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

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.

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 0 10 20 30 40 50 60 70 80 90 Age (yrs) After Donders

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

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

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”)

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

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

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

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 = 1_______ Near pt. of Acc.

‘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 + 2.50D 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)

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

Spectacle and Ocular Accommodation ls d d = 14mm, object at 350mm from PP Ocular Acc. = 1 = 2.86D ls + d Spectacle Acc. = 1 = 2.98D ls So, for an emmetrope, Spectacle Acc. > Ocular Acc.

Myope. e.g. -4.00DS (spec ref), d = 14mm object at 350mm from PP Distance Vergence D mm 0 <-- ¥ Fsp -4.00 --> f’sp -250 -d -14 (ocular ref) -3.79 -264 Near Vergence D mm Ls -2.98 <-- ls -336 +Fsp -4.00 -6.98 --> f’sp -143.27 (Verg. at PP -6.36 <-- -157.27 for near) Ocular Accommodation = -3.79 - (-6.36) = 2.57D Therefore, myope accommodates less wearing spectacles.

For Hyperope: +4.00DS (spec ref), d = 14mm object at 350mm from PP Distance Vergence D mm 0 <-- ¥ Fsp +4.00 --> f’sp +250 -d -14 (ocular ref) +4.24 +236 Near Vergence D mm Ls -2.98 <-- ls -336 +Fsp +4.00 +1.02 --> f’sp +980.4 (Verg. at PP +1.03 <-- +966.4 for near) Ocular accommodation = +4.24 - 1.03 = 3.21D Therefore hyperope accommodates more wearing spectacles.

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

Thus, if working distance = -400mm from spectacle plane and amplitude of accommodation = 3 dioptres Add = (2.50) - 1.50 = +1.00D Need to check reading range as well. Patients with poorer VA can obtain greater magnification by prescribing higher Adds and making them read closer.

Correction Methods Single Vision Readers Bifocals Varifocals Bifocal IOL

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