Presbyopia, the loss of the ability to see clearly at a normal near working distance, while fully corrected for distance vision, affects 100% of the population by the fifth decade of life. Presbyopia is one of the earliest and most predictable signs of middle age; however, until recently, its etiology has been an ophthalmic mystery. 
Presbyopia does not occur suddenly. It is the consequence of the slow and progressive universal decline in the amplitude of accommodation with age.  A young eye, while fully corrected for distance vision, can accommodate as much as 15 diopters (D); ie, see clearly at 6.7 cm (2.6 inches) from the cornea. This remarkable ability of the young eye to focus over a large range from infinity to 2.6 inches within a second is due to a change in shape of the lens.
The lens is a transparent biconvex spheroid suspended at its equator by zonules, which are connected to the ciliary body. When the ciliary muscle, which is located within the ciliary body, contracts, tension on the zonules is altered, resulting in a change in the shape of the lens. There are 2 different theories on how ciliary muscle contraction alters zonular tension to increase the optical power of the lens during accommodation.
Helmholtz Theory of Accommodation
The Helmholtz theory is based on the intuitive thought that, when an equatorial force is applied to a biconvex balloon, the central thickness of the balloon decreases and its central and peripheral surfaces flatten. Using this concept, Helmholtz (1855) proposed that, during accommodation, contraction of the circular ciliary muscle fibers reduces the internal diameter of the ciliary body, causing the zonules to relax and the lens to round-up from the elasticity of the lens capsule; ie, the central thickness of the lens increases and both the peripheral and central surfaces of the lens steepen. 
Importantly, steepening of the peripheral lens surface means that Helmholtz’s theory predicts that spherical aberration shifts in the positive direction during accommodation. In contradiction to this prediction, it has been definitely proven that, during accommodation, spherical aberration shifts in the negative direction as a consequence of peripheral lens surface flattening.
Other experiments undermine Helmholtz’s theory. For example, the eye becomes hyperopic following relaxation of the zonules when the anterior ciliary body is disinserted,  the lens is stable during accommodation when the zonules are predicted to be lax,  there is increased tension on the lens capsule during accommodation,  , and finite element and mathematical analysis demonstrates that the ciliary muscle would have to apply more force than is physiologically possible to simultaneously flatten the central and peripheral lens surfaces for distance vision. [7, 8, 9]
Schachar Mechanism of Accommodation
The Schachar mechanism can readily be demonstrated with an air-, water-, or gel-filled biconvex Mylar balloon.  Counterintuitively, when equatorial force is applied to a balloon with an aspect ratio (minor axis/major axis) of 0.6 or less, its central surfaces steepen and its peripheral surfaces flatten. This shape is called a "steep profile" and is a universal phenomenon, which can be demonstrated in floating oil films, vesicals, and water droplets floating in space.  The formation of a steep profile may explain the evolution of a discy elliptical galaxy into a lenticular galaxy.  Note the image below.
The reflection from its periphery enlarges, as depicted in the image below.
This counterintuitive phenomenon occurs independently of the wall thickness of the balloon.
The human lens, which has an aspect ratio of 0.6 or less throughout life, responds in a similar manner. When an equatorial force is applied to the human lens, its peripheral surfaces flatten, and, surprisingly, its central surfaces steepen (radius of curvature decreases) and its central thickness increases. This shape change makes the accommodative mechanism efficient so that a large increase in central optical power can occur in response to minimum ciliary muscle force (ie, ≤5 g of force).
In vertebrates, the aspect ratio of a lens predicts the accommodative amplitude. Vertebrates with a lens with an aspect ratio of less than 0.6, such as eagles, have a large amplitude of accommodation, while vertebrates with a lens with an aspect ratio of more than 0.6, such as pigs, have small amplitudes of accommodation. 
During accommodation, the equatorial diameter of the lens is increased in response to equatorial zonular tension, as shown in the following image.
As demonstrated mathematically, this unexpected shape change occurs during accommodation to ensure the lens surfaces are in a minimum-energy state. The flattening of the peripheral lens surfaces results in the negative shift in spherical aberration that occurs during accommodation.
In contrast to the Helmholtz theory, the Schachar mechanism states that, during accommodation, there is increased tension applied by the equatorial zonules to the lens while tension applied by the anterior and posterior zonules is reduced. During distance vision, the tension applied by the equatorial zonules is reduced and the tension applied by the anterior and posterior zonular is increased. This ensures that there is always some tension on the lens so that the lens is stable at all times. The equatorial zonules function similarly to the tendons, and the anterior and posterior zonules act like the ligaments of a musculoskeletal joint. For example, when one flexes the elbow to lift a weight, the biceps tendon is under increased tension, while the ligaments of the elbow joint are under reduced tension.
The ciliary muscle controls zonular tension. The difference in tension that occurs in the equatorial zonules compared with the anterior and posterior zonules during ciliary muscle contraction is due to the separate origin of the zonules. The equatorial zonules originate in the valleys of the pars plicata of the ciliary muscle and the anterior and posterior zonules originate in the pars plana of the ciliary body. 
During ciliary muscle contraction, the anterior part of the ciliary muscle moves posteriorly, opening the trabecular meshwork and causing the posterior portion of the valleys of the pars plicata to move toward the sclera. This reduces intraocular pressure and increases tension on the equatorial zonules. Simultaneously, the pars plana moves anteriorly, reducing tension on the anterior and posterior zonules and causing the ciliary processes to swing forward, decreasing the circumlenticular space. The separate effects induced by contraction of the ciliary muscle are due to the distinct orientation and attachments of its constituent longitudinal, radial, and circular muscle fibers. 
Note the images below.
Etiology of Presbyopia
Equatorial lens growth is the etiology of presbyopia. The lens is ectodermal in origin and therefore continues to grow throughout life. New lens fibers are continually formed from mitosis of the anterior subcapsular epithelial cells located near the equator of the lens. This causes the equatorial diameter of the lens to increase approximately 20 µm per year. As the equatorial diameter of the lens increases, the distance between the equatorial edge of the lens and the attachment of the equatorial zonules to the ciliary body decreases. This reduces the stretch and therefore the length of the anterior ciliary muscle fibers with age.
The maximum force a muscle can apply is directly related to its length. Therefore, as the circumlenticular space decreases owing to equatorial lens growth, the maximum force the anterior ciliary muscle fibers can apply to increase tension on the equatorial zonules during accommodation decreases. This results in the age-related decline in accommodative amplitude that leads to presbyopia.
Physiological augmentation of accommodation
With this fundamental understanding of the mechanism of accommodation and the etiology of presbyopia, it is possible to prevent presbyopia and increase the amplitude of accommodation. Any procedure that stops the equatorial growth of the lens will prevent presbyopia. Procedures that increase the circumlenticular space or decrease the length of the equatorial zonules or increase the force the ciliary muscle can apply will increase the amplitude of accommodation.
In the mid-1980s, the author performed scleral expansion based on these concepts by making multiple incisions in the sclera over the anterior ciliary muscle in young presbyopes. The accommodative amplitude was increased, but by only +1.25 D and the effect regressed. In the mid-1990s, Fukusaku made anterior scleral incisions in presbyopes, independently confirming that scleral expansion does increase the amplitude of accommodation. 
In 1992, the first scleral expansion band procedure was performed using a plastic polymethyl methacrylate (PMMA) circular band sutured to the sclera; this band is shown in the image below. 
The results were dramatic. Presbyopic patients had as much as 10 D of accommodation without a change in distance refraction. Since that time, the scleral expansion band procedure was modified by the author to avoid the possibility of anterior segment ischemia. Separate PMMA segments were placed in each of the 4 oblique quadrants of the eye to avoid compression of the anterior ciliary vessels, as shown in the images below.
The scleral expansion band procedure was performed on more than 500 eyes. There was a mean 3.25 D (range, 1.3-7 D) without a change in distance refraction, best corrected visual acuity, or axial length. Common adverse effects that resolve in 6-8 weeks included subconjunctival hemorrhage, transient astigmatism, fluctuating near vision, and dry eyes. The increase in postoperative accommodative amplitude was very dependent on surgical technique. Placement, depth, and orientation of the individual PMMA segments were and remain crucial for maximizing accommodative amplitude improvement.
Comparison of the Helmholtz Theory and the Schachar Theory
The Schachar theory of accommodation has met considerable reaction and discussion, especially from those subscribing to the Helmholtz theory. Many past experiments have been published that are in disagreement with Schachar's conclusions that the crystalline lens diameter increases during accommodation. A careful examination of these experiments reveals that a systematic error exists. Movement occurs between the imaging device and the eye. Measurement of central and peripheral thickness of the cornea in the accommodated and unaccommodated states of these experiments reveals a change in corneal thickness and curvature. Since corneal curvature and corneal thickness do not change during accommodation, these experiments are flawed and cannot be used to reveal the mechanism of accommodation.
Since accommodation involves a small displacement of the lens equator, to make accurate measurements, experiments must have controls, high-resolution instrumentation, triangulation, and eye tacking to ensure image correspondence for proper image registration. Unfortunately, almost all of the experiments involving accommodation have not satisfied these requisites. 
Experiments by Glasser and Kaufman  and their groups are similarly flawed. Although they placed sutures in the cornea as a reference point, neither the sutures nor the corneal Purkinje images of the accommodated and unaccommodated images register, demonstrating that eye movement occurred between the imaging device and the eye. They stated that the small amount of eye movement observed in their experiments cannot account for the changes in the crystalline lens size and configuration during Edinger Westphal or pharmacologic stimulation; however, they offer no controls to prove that this statement is true.
Interestingly, when they fixated the lateral rectus so that eye movement was reduced, they observed that the crystalline lens equator moved toward the sclera, with anterior and posterior zonular relaxation. They state that the movement of the crystalline lens equator toward the temporal sclera is caused by lateral translation of the crystalline lens. This is mechanically impossible. Since the crystalline lens is denser than water and vitreous, when the anterior and posterior zonules are relaxed, the crystalline lens equator can only move toward the temporal sclera by an active generated force (eg, by the pull of the equatorial zonules). Schachar and Kamangar  have used computer image analysis to correct for the spurious eye movement and registered the images from the experiments of Glasser and Kaufman. When this was done, the lens equator moved toward the sclera, less than 100 µm, as predicted by the Schachar theory.
The importance of eye movement relative to the imaging device is exemplified by MRI studies performed on accommodating patients by Strenk et al.  An MRI image of the patient's eye during accommodation revealed that the eye is turned nasally and that a change in the configuration of the orbital bones occurred. Therefore, both the head and eye moved during accommodation. 
Measurement of the transverse diameter of the globe, the corneal diameter, and the equatorial diameter of the crystalline lens in the unaccommodated and accommodated states of the MRI images by Strenk et al demonstrates that all these measurements decrease during accommodation. This means that the image plane of the eye in the unaccommodated and accommodated states was not the same. Their observations are due to artifact, and any conclusion that they make concerning the mechanism of accommodation from this MRI study is not valid. The consequence of not using controls, image correspondence, and registration is that it leads to erroneous conclusions. For example, Strenk et al have claimed that the lens compresses during accommodation and that the equatorial diameter of the lens does not change with age.
The lens consists of 35% water and 65% protein. The speed of ultrasound through the lens in vivo is a constant (approximately 1641 m/s). The bulk modulus is a measure of the compressibility of a material. The higher the bulk modulus, the less the compressibility of the object. Water has a bulk modulus of 2.2 GPa. The bulk modulus is related to the speed of sound by the following formula:
Bulk Modulus = Density of the Lens X (Speed of Sound in the Lens)2
Since the density of the lens is equal to 1064 kg/m3 and the speed of sound in the lens is 1641 m/s, and both remain constant with age, the bulk modulus of the lens is approximately 2.8 GPa throughout life. The speed of ultrasound does not significantly change with accommodation. Therefore, the lens is negligibly compressible with a bulk modulus similar to water.
The lens stroma is totally of ectodermal origin and, therefore, grows throughout life. The stem cells of the lens are located at the lens equator. The embryonic lens is essentially spherical, and, at birth, the lens is a long oval with a minor-to-major axis ratio equal to approximately 0.6, which decreases throughout life as a result of lens equatorial growth. The equatorial diameter of an infant's lens is approximately 7 mm, and the equatorial diameter of a 70-year-old patient's lens is approximately 9.6 mm.
Linear and nonlinear finite element mathematical analyses have been performed on the human crystalline lens. Nonlinear finite element analysis is used routinely to reliably predict reality. The mathematical analysis demonstrates that only equatorial stretching of the equator of the crystalline lens by the equatorial zonules can produce the clinically observed increase in central optical power accompanied by a decrease in spherical aberration. The models demonstrate that the Helmholtz theory of accommodation is physiologically and anatomically impossible. The Helmholtz theory requires more force than is physiologically possible and a larger equatorial circumlental space than is anatomically present. [7, 8]
Scanning electron microscopy has shown the following 3 types of zonules: anterior, posterior, and equatorial. The equatorial zonules act similar to a skeletal muscle tendon and are the components that transduce the force of the ciliary muscle to change the focal power of the crystalline lens during accommodation. The anterior and posterior zonules are tense during distance vision and relax during accommodation. The anterior and posterior zonules act similar to the ligaments of skeletal joints and are stabilizing components, predominately for distance vision. 
Since only the anterior and posterior zonules can be visualized with a slit lamp in vivo during accommodation, it is understandable how incorrect deductions have been made. The equatorial zonules have a separate and distinct insertion into the ciliary body. Because of the increased tension applied by the equatorial zonules, the crystalline lens remains stable during accommodation. Investigators have demonstrated that the crystalline lens is stable and that gravity does not affect the amplitude of accommodation. While experiencing 8 times the Earth's gravitational force (or 8 g's), astronauts have normal accommodative amplitudes. 
In vivo measurements of the position of the crystalline lens equator of young human research subjects during pharmacologically controlled accommodation using high-frequency, high-resolution anterior segment ultrasound revealed that the crystalline lens moves toward the sclera during accommodation. The mean movement was 6.8±1 µm/D. This amount of equatorial movement during accommodation was consistent with the predictions of the nonlinear finite element models and demonstrated that accommodation is a small displacement phenomenon (ie, < 5% change occurs in the equatorial diameter of the crystalline lens during accommodation). 
The small amount of equatorial crystalline lens movement explains the problems and the systemic errors that have occurred during previous experiments that try to determine the position of the crystalline lens equator during accommodation. Eye movements are much larger than the movement of the crystalline lens equator; therefore, proper controls are essential to interpret any measurements.
Table 1. Comparison of the Helmholtz Theory and the Schachar Theory (Open Table in a new window)
|Equatorial traction||Decrease in central optical power||Large increase in central optical power||Large increase in central optical power|
|Spherical aberration with accommodation||Positive shift||Negative shift||Negative shift|
|Gravity effects accommodation||Yes||No||No|
|Refractive change with presbyopia||Myopic||Hyperopic||Hyperopic|
|Anterior disinsertion ciliary muscle||Myopic||Hyperopic||Hyperopic|
|Change in circular ciliary muscle with aging||Atrophy||Hypertrophy||Hypertrophy|
|Change in anterior radial ciliary muscle with aging||No effect||Atrophy||Atrophy|
|Required force||>300 mN, ie, 10 X ciliary muscle capacity||< 50 mN||Ciliary muscle capacity < 50 mN|
|Etiology of presbyopia||Sclerosis||Normal equatorial lens growth||Lenses < 40 years are soft not sclerotic; equatorial lens diameter grows throughout life|
|Theory has widespread applications||No||Yes||Profiles of: balloons, oil films, vesicles, magnetic fluids, ocean tides, spiral galaxy|
Historical Perspective of the Theories of the Mechanism of Accommodation
The healthy, young human (< 40 y) or young primate eye can rapidly focus on near and distant objects (ie, it can change focus or accommodate). The mechanism by which the eye can accomplish this amazing task has been speculated upon for centuries. Initially, it was suggested that the eye was divinely created; therefore, it did not follow known physical laws of optics.
In 1619, Scheiner, a Jesuit priest, proved that accommodation occurred as a result of a change in the optical power of the eye and that the eye obeyed the laws of optics. His experiment easily is duplicated and consists of making 2 vertical pinholes in a card, which are separated by less than the diameter of the pupil of the eye.  The observer looks through both holes simultaneously and focuses on a needle held perpendicular to the plane of the holes. When focusing on the needle, it will appear single; however, if the observer focuses on a more distant or near object, the needle will appear doubled. This simple elegant experiment demonstrates that the eye functions as an optical system.
The explanation of Scheiner's experiment is demonstrated in the image below.
Consider a point source of light as the object. A convex lens converges the rays of light to a point. By placing a card containing 2 holes between the point source and the convex lens, only 2 rays are brought to a focus. If the power of the convex lens is changed, then the 2 rays are brought to a focus at a different distance. The point source appears doubled at all other distances. If the card has 3 or 4 holes, the point source will triple or quadruple.
Some of the most famous philosophers and scientists were interested in how the eye accommodates. In 1611, Kepler and others thought the crystalline lens moved forward and backward.  In 1677, Descartes suggested that the shape of the crystalline lens changed.  In 1742, Lobe postulated that the shape of the cornea changed.  Sturm and Listing believed that the eye elongated. [22, 23]
In 1801, Thomas Young, using ingenious experiments, provided evidence that accommodation occurs as a result of changes in shape of the crystalline lens. He had very prominent eyes. Without anesthesia (which had not been discovered yet) he placed a caliper, that had rings attached to each side, around his eye. With his eye rotated nasally, he placed 1 ring on his cornea and the other ring over his macula. He could see a circular entopic ring induced by the ring on his macula. As he changed his point of focus, the entopic ring did not change size. This proved that the eye does not elongate during accommodation. 
Next, he calculated the amount the cornea would have to move forward to account for his accommodative amplitude. Using candles and a front surface mirror engraved with a scale, he did not observe any corneal movement as he changed his point of focus. He further proved that the radius of curvature of the cornea does not play a role in accommodation. He attached a convex lens possessing the optical power of the cornea to the bottom of an eyecup. He filled the eyecup with saline and placed it over his cornea (the forerunner of contact lenses). The saline in contact with the cornea eliminated the refractive power of the cornea; yet, he was still able to fully accommodate.
Young demonstrated that accommodation did not occur in aphakes. He realized that accommodation had to result from a change in position or shape of the crystalline lens. He was convinced that accommodation could not occur because of forward or backward movement of the crystalline lens. He calculated that the crystalline lens would have to move 10 mm to account for his amplitude of accommodation. This would be impossible.
Young observed that spherical aberration decreased during accommodation. He concluded that accommodation occurs as a result of a change in shape of the crystalline lens. Since the ciliary body had not been discovered yet, he postulated that the change in shape of the crystalline lens is induced by a muscular mechanism within the crystalline lens.
In 1823, Purkinje noted the reflected images of a candle from the anterior and posterior crystalline lens surfaces.  In 1849, Langenbeck was able to observe in a patient that the Purkinje image from the anterior surface of the crystalline lens became smaller during accommodation by using a candle and a magnifying glass. He correctly concluded that the anterior surface of the crystalline lens becomes more convex during accommodation.  He proposed that the ciliary muscle, which had been discovered independently by Bruecke and Bowman in 1847, [27, 28] squeezes the crystalline lens.
In 1851, Cramer followed up on Langenbeck's observation and improved on it by making a device that incorporated a telescope to allow accurate observations of the Purkinje images during accommodation. He observed that the anterior surface of the crystalline lens became more convex, but the posterior surface did not change shape. 
In 1855, Helmholtz improved on the Cramer device by placing crossed glass plates between the patient's eye and the telescope, so that the Purkinje images were doubled and could be measured more accurately. In addition to observing that the anterior and posterior surfaces of the crystalline lens became more convex, he noted that the lens became thicker during accommodation. He hypothesized that the ciliary muscle relaxes during accommodation allowing the lens to become more spherical under the influence of its own elasticity. According to his hypothesis, the equatorial diameter of the lens should decrease as it becomes more spherical during accommodation. 
In 1864, Donders studied the change of the amplitude of accommodation with age. He found that the amplitude of accommodation declined in a linear fashion with age. This decline occurs universally and predictably. If patients are corrected properly for distance vision, their age can be determined within 1.5 years by measuring their amplitude of accommodation. Donders also observed that patients become slightly hyperopic when they become presbyopic.  In 1870, AdamüK and Woinow suggested that presbyopia, the loss of accommodation with age, resulted from lens sclerosis (ie, loss of elasticity of the lens with age). 
In 1904, Tscherning examined the curvature changes of the anterior crystalline lens surface by observing the changes in the Purkinje images when 4 lights are used as objects. He placed the lights so that 2 formed reflected images from the central anterior surface and 2 formed reflected images from the peripheral anterior surface of the crystalline lens. He observed that the central images moved closer together during accommodation, while the peripheral images moved further apart. He concluded that the crystalline lens was becoming more convex centrally but was becoming flatter in the periphery during accommodation.  This was consistent with Young's observation that the spherical aberration of the eye decreases during accommodation.
The Helmholtz theory did not explain the peripheral surface flattening of the crystalline lens without additional assumptions. For example, the iris constricts during accommodation and it was imputed to produce the peripheral flattening of the crystalline lens. However, von Graefe had demonstrated accommodation in a patient with a total iridectomy. 
Tscherning postulated that during accommodation the ciliary muscle exerted tension on the crystalline lens, pressing the crystalline lens against the anterior vitreous. The resistance of the vitreous transmitted sufficient force to effect central bulging of the anterior surface of the crystalline lens. His theory predicts that the central thickness should decrease during accommodation. He did not accept Helmholtz's measurements of increasing crystalline lens thickness with accommodation. Tscherning thought that presbyopia was the result of enlargement of the crystalline lens nucleus.  All subsequent theories Gullstrand (1911), Fincham (1937) used Helmholtz's hypothesis that the zonules are relaxed during accommodation. [33, 34] Helmholtz's hypothesis and subsequent modifications attribute presbyopia to sclerosis of the crystalline lens stroma or capsule, atrophy of the ciliary muscle, or stiffening of the ciliary muscle attachments.
Based on the theory that the decline in the amplitude of accommodation is due to lens sclerosis, the lens stroma of presbyopic monkeys was replaced with a soft pliable material. Postoperatively, the amplitude of accommodation decreased, ie, the monkeys became more presbyopic. This experiment demonstrates that sclerosis of the lens is not the etiology of presbyopia. 
Methods for Treating Presbyopia
Presbyopia initially was treated with near vision optical aids using magnifying lenses, reading glasses, and monocles. Patients were constantly removing reading glasses and losing them because the reading glasses interfere with vision at all other distances. Benjamin Franklin fused the distance lens with the near reading lens to give us bifocals that were later modified to trifocals. The problem with these reading aids is that they only allow sharp near vision at a given distance and the near visual field is limited by the lens. Patients must learn to rotate their eyes downward when reading with bifocals instead of rotating their head. It usually takes 2-3 weeks for patients to get used to wearing bifocals. Trifocals can even be more of a problem for many patients.
To avoid the problems of bifocals and trifocals, bifocal contact lenses have been developed. The bifocal contact lenses generally have been unsuccessful because the distance and near powers of the contact lens must be crowded into an area that can barely cover the pupil. The patient must learn how to shift the contact lens and to ignore the distant or near image according to the visual task.
Multifocal glasses and multifocal contact lenses also are generally not satisfactory. Multifocal lenses produce multiple images at various focal points. Light reflected or emitted by an object must be dispersed by the multifocal lens over all the focal points. Therefore, the intensity at any given focal point will be reduced and the contrast sensitivity diminished. To avoid prismatic effects, the visual field of a multifocal lens is reduced. In addition, the patient must learn to select the appropriate image.
The problems with bifocal and multifocal contact lenses forecast the problems that have, and will continue to occur, with attempts at making a bifocal or multifocal cornea using LASIK or using intracorneal lenses, phakic intraocular lenses, or aphakic intraocular lenses.
The proposed mechanism of action for presently available accommodating intraocular lenses is based on the concept that the vitreous plays a key role in accommodation. For near vision, it is believed that vitreous pressure increases, which causes the intraocular lens to move forward. There are a few problems with this concept. First, patients have normal accommodative amplitude after vitrectomy.  Second, intraocular pressure declines during accommodation.  Third, partial coherent interferometric measurements demonstrate that there is no correlation between accommodating intraocular lens movement and near vision.  Fourth, there is no difference in movement of accommodating and standard intraocular lenses. [38, 39] Finally, the observed refractive change is less than 0.50 D. 
Monovision as a treatment for presbyopia generally is accepted by less than 30% of the population. The loss of stereopsis and learning to ignore a blurry image from one half of the binocular visual field are problems for most patients.
Lasik surgery for presbyopia with or without a hydrogel lens or a corneal pinhole has variable success. Each has advantages and disadvantages. [40, 41, 42, 43, 44, 45] Potential complications include visual distortion, induced corneal ectasia, anisometropia, haze, glare, regression of effect, decrease in uncorrected and/or corrected distance vision, halos around lights at night, and decreased contrast sensitivity. [46, 47, 48]
The Schachar theory of accommodation states the following:
- Increased equatorial zonular tension occurs during accommodation
- Presbyopia is due to a decrease in the effective working distance of the ciliary muscle (space between the ciliary muscle and lens equator) as a result of normal equatorial lens growth
Based on this theory, the accommodative amplitude of presbyopes can be increased.
Any technique that increases the space between the ciliary muscle and the crystalline lens equator will physiologically increase the amplitude of accommodation.