Presbyopia

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:

• Blurring of vision for near in both eyes.
• Eyestrain.
• Headache after prolonged focus.
• Stinging.
• Watering.
• Increase in comfortable reading distance.
• Trouble in focusing when tired.
• Slow response to change in focus distance.
• Running of letters and lines.

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:

• Tropicamide: Tropicamide has a very short half- life and should not be used to conduct cycloplegic refraction.
• Cyclopentolate: Cyclopentolate has sufficient half- life and may be used for cycloplegic refraction, even in children.
• Homatropine.
• Scopolamine (hyoscine).
• Atropine.

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:

• Lens and lens capsule- based theories: Lens and lens capsule- based theories consider changes in the elasticity and compliance of the lens and its capsule. There is continued deposition of the lens fibers within the lens as it ages, causing the lens to become compact and stiff. A major factor in the loss of accommodation may be the increased stiffness of the ageing lens with inability to respond to accommodative stimuli. The near point of accommodation gradually recedes from about 7 cm at the age of 10 years to about 20 cm at the age of 40 years and it recedes further to about 40 cm at about the age of 50 years.
• Extralenticular theories: Extralenticular theories consider changes in the ciliary muscle and choroid.
• Geometric theories: Geometric theories consider changes in the geometry of the zonular attachments to the lens.

Other causes of diminished near vision are:

• Uncorrected hypermetropia where all the accommodation is used for clear distant vision.
• Use of local or systemic parasympatholytic drugs (e.g.atropine).
• Aphakia and pseudo-phakia.
• Internal ophthalmoplegia.

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:

• Subjective push- up method: Subjective push- up method involves a small target (like near letter chart) being brought forward toward the eye until it blurs. It is the near point of accommodation. The reciprocal of the distance at which the target blurs is the accommodative amplitude in dioptres.
• Prince rule: Prince rule combines a scaled ruler with a near addition of + 3 D (dioptres), and it puts the far point of an emmetrope (a person with no refractive error) at 33 cm. The target on Prince rule is brought forward until it blurs. Taking account of addition of + 3, this distance is converted into dioptres of accommodative amplitude.
• Negative powered trial lenses: The test uses a distance letter chart and lenses of increasing minus spherical power to induce accommodation. More minus sphere is added until the subject can no longer overcome the minus lenses with accommodation. The amount of minus sphere that can be overcome represents the accommodative amplitude.

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:

• Accommodative range: Accommodative range refers to the range of distances which can be viewed clearly by using accommodation. It is expressed without correction for emmetropia. A + 2 hypermetrope with 4 D of accommodation would have an uncorrected accommodative range from infinity to 50 cm. A minus 2 myope with similar accommodative amplitude would have an accommodative range from 50 cm to about 17 cm.

Differential Diagnosis:

Presbyopia is the most common cause of accommodative dysfunction.

Presbyopia should be differentiated from other conditions with decreased accommodation like:

• Trauma along neuro-anatomical pathway: Trauma to the parasympathetic nuclei in the midbrain, to supra-nuclear structures, or to the third nerve may produce symptoms of asthenopia (eye strain).
• Adie’s syndrome: Adie’s syndrome is characterised by one eye with a pupil that is larger than normal and constricts slowly in bright light (tonic pupil), along with the absence of deep tendon reflexes, usually in the Achilles tendon. Adie’s syndrome is usually associated with decrease in accommodation.
• Pharmacological cycloplegia: Pharmacological cycloplegia also produces temporary accommodative dysfunction.
• Systemic medications: Systemic medications may cause decrease in accommodation. These medicines often have anti-cholinergic side effects. Examples include phenothiazines and anti-parkinsonian drugs.

Unlike presbyopia, increased accommodation is rarely seen as in:

• Horner Syndrome: Several experiments have shown that ciliary body also receives sympathetic input. This is shown clinically by the increased accommodative amplitude seen in the affected eye of patients with Horner Syndrome.
• Miotic agents: Miotic agents such as pilocarpine may cause an abnormally proximal near point.
• Accommodative spasm: Accommodative spasm is a functional disorder characterised by blurred vision for distance, diplopia, episodic esotropia (inward turning of eye), miosis (small pupil), and an abnormal near point of accommodation.

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.

Medical therapy:

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.

• Spectacle correction: Reading glasses, bifocals, progressive glasses, and even trifocals, are the most common methods of correction for eyes that can no longer focus on near objects, and are the most versatile option.
• Contact lenses: Different contact lenses are available for the presbyopes. These are either bifocal or multifocal contact lenses, but these have received a mixed response by the users. An additional alternative is mono-vision contact lenses, where the refractive power of one eye is set for focus at a distance and the other eye for focusing near objects. Although image is clear for each eye, there is concomitant loss of depth perception. A general drawback to the use of contact lens for presbyopia is the decrease in tear production with ageing.

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:

• Mono-vision: Mono-vision refers to correcting dominant one eye to emmetropia and the other to myopia between minus 1 and minus 2 D, which can be achieved by excimer laser surgery. In hypermetropia, a steepening of the cornea with overcorrection of one eye may be performed by laser thermokeratoplasty (LTK). The limitation of mono-vision is the lack of a possibility for correction of intermediate distance vision without simultaneously affecting near and distance vision.
• Multifocal excimer laser: Multifocal corneal ablation with laser-assisted in situ keratomileusis (LASIK) is currently used almost exclusively in hypermetropic and emmetropic eyes.
• Intra-corneal implants: Exact control of the lamellar corneal cut is obtained with microkeratome and femtosecond laser technologies. Benefit of these intra-corneal implants is their reversibility. If the implant is not well tolerated, or cataract surgery with intraocular lens (IOL) implantation is to be performed later, it can be removed, thereby restoring the original shape of the cornea.
• Multifocal intraocular (IOL) lenses:

-       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:

• Anterior ciliary sclerotomy and sclera expansion: Anterior ciliary sclerotomy and sclera expansion procedure is based on alternative theory of the mechanism of accommodation, proposed by Schachar. Accordingly, it seems logical to increase this space for the lens equator to move outward.
• Potentially accommodating IOLs: Accommodating IOLs are designed to transform the movement of the ciliary muscle into a dynamic change of ocular dioptric power. Accommodative IOLs are based on the principle of optic shift. Haptic (which hold the lens in place) contraction of the ciliary muscle is supposed to shift the IOL optic anteriorly. This allows for only small accommodative amplitude.

Prognosis:

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.