2. Accommodation is?
• A. The Motel 6
• B. Southern Hospitality
• C. Mechanism by which the eye changes
refractive power by altering the shape of the
lens.¹
3. Accommodative effort
• Parasympathetic nerve fibers carried in
cranial nerve III cause the ciliary muscle to
contract. This contraction causes the
zonules to relax. Tension on the lens
capsule is decreased and the lens becomes
more round, shifting the point of focus
closer to the eye.
4. Parasympatholytics
• Atropine, cyclopentolate, tropicamide are
antagonists of the parasympathetic nerves
and thus cause cycloplegia (paralysis of
ciliary muscle)
• Parasympathomimetic - pilocarpine is an
agonist of the parasympathetic system and
cause ciliary spasm (hence the brow ache)
5. Diopter
• 1/focal length measured in meters
– 1 D = 1/1m
– 2 D = 1/0.5m
– 0.5 D = 1/2m
• Amount of accommodation can be
measured in diopters.
6. Amplitude of Accommodation
•
•
•
•
Measured in diopters.
This is the amount of change in lens power.
Decreases with age.
What is the closest a patient w/ emmetropia
and an accommodative amplitude of 2
diopters can see without corrective lenses?
8. Range of Accommodation
• Similar to amplitude of accommodation, but
measured in meters or centimeters.
• Previous example pt had 2 diopter
amplitude of accommodation, but his range
is from infinity to 50 centimeters in front of
the eye.
• This is from the patients far point to the
near point.
9. Far point²
• Far point = the object point which forms a
focused image on the retina in the
unaccommodated eye.
• Emmetropia = far point is at infinity
• Myopia = far point is closer than infinity
• Hyperopia = far point only exists
theoretically, because image is imaged
behind the retina
10. Near Point
• Object point that is in focus when the eye is
fully accommodated.
• Can be found mathematically by subtracting
the pt’s refractive error from their amplitude
of accommodation.
• What is the near point of a +5.00D
hyperope with a 9 diopter amplitude of
accommodation?
11. Answer
• Pt is a hyperope so to get his focus to
infinity he has to accommodate 5 diopters.
That leaves him 4 diopters of useful range,
so he can see up to 25 centimeters in front
of him.
• 1/4m = .25 m or 25 cm
12. Myopic example
• What is the range of accommodation in a pt
w/ a refractive error of -1.00D and an
amplitude of accommodation of 4 diopters?
• A. 100cm to 20cm
• B. 100cm to 25cm
• C. Infinity to 25cm
• D. Infinity to 20cm
13. Answer
• This patient’s far point is 100cm due to his
uncorrected myopia. Add this one diopter
to the 4 diopters he can accommodate and
you get 5 diopters. So his near point is 20
cm in front of him. (1/5D = 20cm). Hence
his range is from 100cm to 20cm in front of
his eye.
14. Accommodation and Lenses
• Myopes must accommodate more when
using contact lenses than if wearing
spectacles. Hyperopes are the opposite.
They may be able to initially forego bifocals
if wearing contact lenses. Hyperopes must
accommodate more when wearing
spectacles.³
15. Presbyopia
• Lens becomes less flexible with age.
Ability to accommodate decreases with age.
• When prescribing glasses keep one half of
the amplitude of accommodation in reserve.
For example if a patient needs to see 40 cm
(2.5D) and as a total amplitude of
accommodation of 2 diopters allow the
patient to accommodate 1 diopter on his
own. Hence the patient will need reading
glasses of 1.5 diopters.
17. References
•
•
•
•
1. Accomodation - From American Academy of Ophthalmology: Basic
and Clinical Science Course. Section 3 Optics, Refraction, and Contact
Lenses, 2004-2005, pg. 119.
2. Albert DM, Jakobiec FA, Azar DT, Gragoudas ES(eds): Principles
and Practice of Ophthalmology, pgs. 5337-8. Philadelphia, W.B.
Saunders, 2000.
3. Vander JF, Gault JA: Ophthalmology Secrets, pg. 12. Philadelphia,
Hanley & Belfus, 2002.
4. Wilson FM (ed): Practical Ophthalmology: A Manual for Beginning
Residents pg. 88. 4th edition. American Academy of Ophthalmology,
1996.
