page_title: Presbyopia
section_title: Mechanism
hierarchical_section_title: Presbyopia / Mechanism
caption_attribution_description: English: Presbyopia. See a full animation of this medical topic
context_page_description: Presbyopia is physiological insufficiency of accommodation associated with the aging of the eye that results in progressively worsening ability to focus clearly on close objects. Symptoms include difficulty reading small print, having to hold reading material farther away, headaches, and eyestrain. Different people will have different degrees of problems. Other types of refractive errors may exist at the same time as presbyopia.
  Presbyopia is a normal part of the aging process. It occurs due to age related changes in lens and ciliary muscle power of the eye, causing the eye to focus light behind rather than on the retina when looking at close objects. It is a type of refractive error along with nearsightedness, farsightedness, and astigmatism. Diagnosis is by an eye examination.
  Presbyopia can be corrected using glasses, contact lenses, multifocal intraocular lenses, or LASIK surgery. Most common treatment is glass correction using appropriate convex lens. Glasses used to correct presbyopia may be simple reading glasses, bifocals, trifocals, or progressive lens.
  People over 40 are at risk for developing presbyopia and all people become affected to some degree.
context_section_description: The cause of presbyopia is lens hardening by decreasing levels of α-crystallin, a process which may be sped up by higher temperatures. It results in a near point greater than 25 cm (or equivalently, less than 4 diopters).
  In optics, the closest point at which an object can be brought into focus by the eye is called the eye's near point. A standard near point distance of 25 cm is typically assumed in the design of optical instruments, and in characterizing optical devices such as magnifying glasses.
  There is some confusion over how the focusing mechanism of the eye works. In the 1977 book, Eye and Brain, for example, the lens is said to be suspended by a membrane, the 'zonula', which holds it under tension. The tension is released, by contraction of the ciliary muscle, to allow the lens to become more round, for close vision. This implies the ciliary muscle, which is outside the zonula, must be circumferential, contracting like a sphincter, to slacken the tension of the zonula pulling outwards on the lens. This is consistent with the fact that our eyes seem to be in the 'relaxed' state when focusing at infinity, and also explains why no amount of effort seems to enable a myopic person to see farther away.
  The ability to focus on near objects declines throughout life, from an accommodation of about 20 dioptres (ability to focus at 50 mm away) in a child, to 10 dioptres at age 25 (100 mm), and levels off at 0.5 to 1 dioptre at age 60 (ability to focus down to 1–2 meters only). The expected, maximum, and minimum amplitudes of accommodation in diopters (D) for a corrected patient of a given age can be estimated using Hofstetter's formulas: expected amplitude (D) = 18.5 - 0.3 × (age in years), maximum amplitude (D) = 25 - 0.4 × (age in years), minimum amplitude (D) = 15 - 0.25 × (age in years).
caption: Presbyopia
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