Accommodation turner

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Accommodation turner

  1. 1. Accommodation Scott D. Turner, MD Jones Eye Institute University of Arkansas for Medical Sciences
  2. 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. 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. 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. 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. 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?
  7. 7. Answer • • • • A. 24 inches B. 2 meters C. 50 centimeters D. Depends on lighting.
  8. 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. 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. 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. 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. 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. 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. 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. 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.
  16. 16. Presbyopia Chart4 Age 36 40 44 48 52 56 60 7.0D (+ 2) 6.0 D (+ 2) 4.5 D (+ 1.5) 3.0 D (+ 1.5) 2.5 D (+ 1.5) 2.0 D (+ 1.0) 1.5 D (+ 1.0) 64 1.0 D (+ 0.5)
  17. 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.

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