A2 Presbyopia: Loss of Accommodation

Topic

To focus on a nearby object, the eye requires more than the fixed optical power provided by the cornea and the lens in their resting state: the ciliary muscle—a ring of muscle surrounding the lens—contracts and releases tension on the zonules, the thin filaments that hold the lens suspended in place. As this tension is released, the elastic lens naturally becomes more rounded, momentarily increasing its optical power; this adjustment is known as accommodation, and it allows the eye to briefly add the extra optical power needed to bring the focal point of a nearby object back onto the retina.

The amount of additional optical power the lens can generate when the ciliary muscle fully releases zonular tension depends on the lens's elasticity: the more elastic the lens, the more it rounds out when released from zonular tension, and the more additional optical power it can provide. The maximum amount of additional optical power the lens can achieve through accommodation is known as the amplitude of accommodation.

Over the years, the elasticity of the lens progressively decreases; consequently, even if the ciliary muscle continues to release zonular tension just as effectively as before, the lens can no longer round out to the same extent, and the amplitude of accommodation diminishes year by year. The lower the amplitude of accommodation, the less additional optical power the lens can generate, and the further away an object must be from the eye to remain in focus on the retina; the closest distance at which an object can still be focused is called the near point, and as the amplitude of accommodation decreases with age, this near point moves progressively further away from the eye. This reduction in the amplitude of accommodation—and the consequent recession of the near point—occurs regardless of whether the resting eye's optical power is matched to its axial length; an eye free of myopia, hyperopia, and astigmatism experiences this same age-related loss of accommodative amplitude because lens elasticity declines irrespective of the eye's fixed optical power. This condition is known as presbyopia; it is superimposed upon—rather than replacing—any pre-existing ametropia, as presbyopia does not alter the resting eye's fixed optical power but merely limits the amount of additional power that accommodation can temporarily provide.

Since presbyopia specifically reduces the extra optical power available for near vision without affecting the fixed power that corrects (or fails to correct) any underlying ametropia, its correction involves an optical "addition": extra power applied solely for near vision, externally compensating for the additional power the crystalline lens can no longer generate on its own through accommodation. Bifocals provide this addition in a distinct lens zone; progressive lenses provide it via a continuous gradient between the distance and near zones; and multifocal contact lenses offer multiple superimposed power zones, allowing the brain to select the sharpest image from among them. Corneal or lens surgery, by contrast, aims to achieve this same effect permanently—either by reshaping the cornea or by replacing the natural lens with an artificial one that does not rely on the ciliary muscle or zonules to provide the extra optical power that natural accommodation can no longer supply.

ID:4317