Accommodation

1
Accommodation and Presbyopia
Prof. Roger S. Anderson University of Ulster,
Coleraine UCL Institute of Ophthalmology, London
2

• Role of the lens
• Makes up remaining 1/3 refractive power of eye
• Accommodation
• Compensation for aberrations of cornea

3
Control of Accommodation of utmost importance in
refraction. A hypermetrope can bring the focal
point of the eye forward by accommodation.
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. gt PRESBYOPIA (old
eye) Binocular Amplitude of Accommodation is
usually slightly greater than monocular. Also
dependent on pupil size.
5
Anatomy of Accommodation the ciliary muscle
n1.40
n1.375
n1.385
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 gt 0.6
Peripheral thinning but central
thickening. Increased central curvature. Decrease
d 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
(No Transcript)
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
1_______ 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 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)
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
s
P
o
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. gt Ocular Acc.
16
Myope. e.g. -4.00DS (spec ref), d
14mm object at 350mm from PP Distance
Vergence D mm 0 lt--
Fsp -4.00 --gt fsp -250 -d -14 (ocular
ref) -3.79 -264 Near Vergence D
mm Ls -2.98 lt-- ls -336 Fsp -4.00
-6.98 --gt fsp -143.27 -d -14 (Verg.
at PP -6.36 lt-- -157.27 for near) Ocular
Accommodation -3.79 - (-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 lt--
Fsp 4.00 --gt fsp 250 -d -14 (ocular
ref) 4.24 236 Near Vergence D
mm Ls -2.98 lt-- ls -336 Fsp 4.00
1.02 --gt fsp 980.4 -d -14 (Verg.
at PP 1.03 lt-- 966.4 for near) Ocular
accommodation 4.24 - 1.03 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) - 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.
20
Correction Methods
Single Vision Readers Bifocals Varifocals
Bifocal IOL
21
Thank You