Presbyopia is the gradual age- related loss of accommodative amplitude which begins early in life and culminates in complete loss of accommodation by about 50 years of age. The word presbyopia is derived from the Greek words presbus (meaning old man) and ops (eye).
Accommodation is a dynamic, optical change in the dioptric power of the eye allowing the point of focus of the eye to be changed from distant to near objects. Accommodation is the ability to increase the refractive power of the optical system of eye. It occurs to produce a clear image of near objects. For accommodation, the ciliary body contracts, the lens zonules relax, and the crystalline lens assumes a more spherical shape, which increases its refractive power. Other than presbyopia, disorders that affect accommodation are quite rare. Regardless of the cause, it causes blurred vision for near objects and there is eyestrain with prolonged near work.
In an un-accommodated, emmetropic (with no refractive error) eye, distant objects at or beyond what is considered optical infinity for the eye (6 meters) are focused on the retina. For an object closer to the eye, the eye accommodates for a clearly focused image on the retina. Myopic eyes, being too long for the optical power, are unable to attain a sharply focused image for objects at optical infinity. Myopes can focus clearly on objects closer to the eye than optical infinity without accommodation (i.e. objects at their far point). In contrast, young hypermetropes are only able to focus clearly on objects at optical infinity through an accommodative increase in the optical power, provided their accommodative amplitude exceeds the amount of hypermetropia.
Optical requirements of accommodation: The optical power of the crystalline lens increases (i.e. the focal length of lens decreases) during accommodation. As a result, the eye changes focus from distance to near so that the image of near object focuses on retina. The dioptric change in power of the eye defines accommodation and accommodation is measured in units of dioptres (D). A dioptre is reciprocal of meter and is a measure of the vergence of light. Light rays converging towards a point image are designated to have positive vergence. An object at optical infinity subtends zero vergence at the cornea. Cornea and crystalline lens add positive vergence to bring light towards a focus on the retina. If the eye accommodates from an object at optical infinity to an object 1.0 meters in front of the eye, this represents 1.0 D of accommodation. If the eye accommodates from infinity to 0.5 meters in front of the eye, this is 2 D of accommodation. Thus, the accommodative response is the increase in optical power the eye undergoes, to change focus from an object at optical infinity to the near object.
The accommodative apparatus of the eye consists of ciliary muscle, ciliary body, choroid, anterior and posterior zonular fibers, the lens capsule and the crystalline lens.
Although presbyopia is age related, the age of onset varies around the world. For example, presbyopia develops earlier in people who live closer to the equator. Specifically, the age of onset of presbyopia is noted to be 37 years in India. Further studies show that the important variable is ambient temperature rather than latitude. Higher the ambient temperature, earlier is the onset of presbyopia.
Presbyopia (‘old- age sight’) occurs when the near point of accommodation recedes to a point where it is difficult or impossible to accommodate sufficiently for reading or other close work. For most people, close work becomes difficult when the amplitude of accommodation is less than 5 dioptres (corresponding to a near point of accommodation of 20 cm).
Once presbyopia occurs, it gradually increases over a period of 10 to 12 years and then stabilises. A presbyope usually requires a + 1.00 D (dioptres) of reading addition in the age of early forties, levelling off to about + 2.50 D at about the age of 55 years.
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Patients with latent hypermetropia (far- sightedness) use a portion of their accommodative reserve to focus at distance, and may present with premature presbyopia. Mild myopia (short- sightedness), in contrast, may have delayed onset of symptoms of presbyopia.
Most disorders of accommodation are bilateral. Thus, if a patient is corrected to emmetropia, the amount of near blurring should be similar in each eye. Disorders that present as a unilateral loss of accommodation localise to the infra-nuclear third nerve, ciliary ganglion (Adie’s syndrome), effector ciliary body itself (pharmacological cycloplegia or paralysis of accommodation).
Accommodative disorders typically present with symptoms of:
These symptoms are intensified under inadequate light conditions, poor contrast and are exaggerated at the end of the day.
Presbyopia is caused due to loss of the fine balance of forces that permit the accommodative structures to bring a change in optical power of the lens in the young eye.
With advancing age, there is decline in the ability of the eye to accommodate or change its focus.
Neural pathway for accommodation:
Accommodation is associated with convergence and miosis forming the triad of ‘near reflex’.
The neural pathway for accommodation probably originates in the caudal part of the Edinger- Westphal or third nerve parasympathetic nucleus in the midbrain. These areas receive input from the cerebral cortex and pretectum. Fibers from the third nerve nucleus, responsible for accommodation, then travel to the ciliary ganglion, where they synapse with postganglionic parasympathetic fibers going to ciliary body and iris sphincter. Then via short ciliary nerves, they reach to the intrinsic muscles of the eye.
Parasympathetic control is of much greater clinical importance. However, experiments have shown that the ciliary body receives sympathetic input as well. Since accommodation is mediated almost exclusively via parasympathetic pathways, it is antagonised best with muscarinic receptor (type of acetylcholine receptors) blockers. With muscarinic antagonists, paralysis of accommodation (cycloplegia) is always associated with dilatation of pupil (mydriasis).
Muscarinic antagonists commonly used are:
Homatropine, scopolamine (hyoscine) and atropine are generally used for therapeutic (treatment) purposes.
There is great variability in the normal levels of accommodation. Accommodative ability decreases with age. Children have great accommodative capabilities and presbyopia is rare before thirty five years of age.
Mechanism of accommodation:
Helmholtz in 1855 provided the description of accommodative mechanism. When the young eye is un-accommodated and focused for distance, the ciliary muscle is relaxed. Resting tension on anterior zonular fibers inserting around the lens equator, apply an outward directed tension to hold the lens in a relatively flattened and un-accommodated state. For near focus, the ciliary muscle contracts, the inner apex of the ciliary body moves forward and towards the axis of the eye. This stretches the posterior attachment of the ciliary muscle and releases resting tension on all zonular fibers around the lens equator. The lens capsule then moulds and provides the force to cause the lens to become accommodated. The accommodative increase in optical power of the lens comes about from an increase in the lens anterior surface curvature, and to lesser extent, of the posterior surface curvatures.
When accommodative effort is over, the ciliary muscle relaxes and the elasticity of the posterior attachment of the choroid pulls the ciliary muscle back into its flattened and un-accommodated state. The outward movement of the apex of the ciliary body once again increases the tension on the anterior zonular fibers around the lens equator to pull the lens via the capsule into a flattened and un-accommodated form.
Alternative theory of mechanism of accommodation:
Schachar (1993) proposed that zonular tension is increased during accommodation in young subjects in contrast with the classic Helmholtz’s theory. The basic assumptions are:
- During near accommodation, the lens equator moves outward causing an increase in lens diameter.
- The equatorial lens diameter increases with age due to natural growth of the lens.
- Presbyopia is caused by decrease of the distance between lens equator and ciliary muscle leaving not enough space for the lens equator to move outward with accommodation.
Ocular changes resulting in presbyopia:
Changes that occur in the eye resulting in presbyopia are broadly divided into three categories:
Other causes of diminished near vision are:
Diagnosing presbyopia is straightforward and age of the patient is a major consideration. A simple reading test using well- illuminated text of graded sizes at a standard distance may be helpful both in characterising the degree of near- vision and in determining the appropriate refractive power.
Accommodation may be measured by determining the accommodative amplitude or the accommodative range.
Monocular accommodative amplitude may be measured by following methods:
Accurate objective measurements of accommodation may be done statically or dynamically with auto-refractors, refractometers or aberrometers. Dynamic optometers provide a real-time graphic display of the accommodative response and can provide an indication of how much the accommodative response varies over time. Objective methods provide a true measure of accommodative amplitude of the eye.
Accommodation may also be stimulated by topically applied muscarinic agonists (e.g. pilocarpine) and the resulting accommodative response measured periodically over 30- 45 minutes using a refractometer or an auto-refractor until the maximal accommodative response is attained.
Assessment of accommodative range:
Presbyopia is the most common cause of accommodative dysfunction.
Presbyopia should be differentiated from other conditions with decreased accommodation like:
Unlike presbyopia, increased accommodation is rarely seen as in:
Management should be carried out under medical supervision.
It is important to correct refractive error properly to put the far point at infinity so that ocular system is emmetropic for distance.
Management of presbyopia involves use of plus lenses for near work either in a bifocal or as reading glasses only. Most problems with reading addition of plus glasses result from over- correction for the near distance.
Correction to emmetropia and the use of trial frames to determine the proper power give good results.
Surgical refractive presbyopia management:
Natural accommodation has both accommodative and pseudo-accommodative components. Accommodation (i.e. real change in optical power) should be distinguished from the ability to see clearly for near. Depth of focus bifocality or multifocality (e.g. multifocal intraocular lens) may provide a variable degree of functional near vision. Such vision is known as pseudo-accommodation.
Treatment may be performed by replacing the crystalline lens with a multifocal lens after cataract surgery. If the crystalline lens is still clear, it may be preferable to treat presbyopia in a way that allows for preserving lens.
I. Pseudo-accommodative procedures:
- Refractive multifocal IOLs: Refractive bifocal or multifocal IOLs have on their optic two or more ring- shaped spherical zones of different refraction. The near part is usually located in the center of the lens optic. With accommodative miosis, the near part comes into play when looking at near targets, and the distance part when looking at far points. Efficacy of this depends on the centration of the IOL and diameter of the pupil.
- Diffractive multifocal IOLs: Diffractive multifocal IOLs part away the light to two focus points, one for near and one for distance. They have a diffractive anterior or posterior surface with concentric rings for the diffraction of incoming light rays, which are focused onto a far and a near point. The bifocality is largely independent of pupil diameter and centration.
Some modern multifocal IOLs use a combination of refractive and diffractive principles.
II. Accommodative procedures:
Most patients with symptoms of eye strain do well with plus power glasses for reading. Plus power requirements increase until patients attain the age of early sixties, and then stabilise.