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Helmholtz and Schachar held diametrically opposed views of how the ciliary muscle and the zonules work during accommodation. When Helmholtz developed his theory over 155 years ago, he could barely make out the different surfaces of the cornea, the anterior surface of the crystalline lens, and the posterior surface of the crystalline lens. Using candles and a clever screening arrangement, Helmholtz was able to discern that the anterior and posterior surfaces of the lens were moving farther apart during accommodation, indicating that the lens was changing shape and becoming more convex, increasing its focal power. He had no other imaging instruments and had no idea what was going on with the zonules and ciliary muscles during accommodation.
He assumed that the ciliary muscles were in a state of constant tension when the eye was focusing at distance, with the lens relatively flat. He also assumed that the lens had its own elasticity, and when the ciliary muscle contracted or moved in, the tension on all of the zonules (the anterior, equatorial, and posterior zonules) was released simultaneously and equally allowing the lens to round up and become more convex for near vision. Helmholtz was partially right about the lens elasticity, but in fact it is the capsule, not the lens that is under tension, and will allow the lens to change shape when there are no zonules holding them in place. He also knew that with time the lens becomes more brittle and less flexible, at least in some patients. And that this hardness eventually led to cataracts and blindness for some patients. Thus he hypothesized that lens hardness, which eventually prevented the lens from changing shape, was the primary cause of presbyopia in all humans.
Although Helmholtz had relatively crude instruments and was unaware of much of what occurred behind the iris with the ciliary muscles and zonules, and despite significant evidence today that contradicts his mode of action for accommodation and presbyopia, the Helmholtz theory is still the dominant theory prevailing today throughout ophthalmology. Adrian Glasser from the University of Houston and Jean Marie Parel from the Bascom Palmer Eye Institute with the University of Miami are two of the leading proponents of the Helmholtz theory even today, and also two of the leading critics of the Schachar theory. Many start-up companies have been launched to correct presbyopia based on the Helmholtz theory including companies that used femtosecond lasers to soften the lens, companies that extracted the natural lens and attempted to refill the capsule with gels or liquids, and companies that designed IOL’s to refill the capsule that would flex at their haptics like a natural lens. The success of all of these companies and their technologies to date has been quite poor.
A graphical representation of the lens, the zonules and the ciliary muscle at rest (distance) is shown in figure A below. Figure B shows the mode of action proposed by Helmholtz and his followers. The ciliary muscle contracts directly towards the lens equator releasing all of the zonules simultaneously and equally allowing the lens to round up becoming more convex for near vision.
Figure C is a depiction of the proposed mode of action in the Schachar theory. Schachar agrees with Helmholtz in that he believes that the anterior and posterior zonules relax during accommodation, but disagrees with Helmholtz in that he believes that the equatorial zonules, alone, move away from the equator of the lens, flattening the periphery of the lens while causing the center of the lens to expand (since the volume of the lens must remain the same). Schachar correctly focused on the diminishing circumlental space with age, but like Helmholtz, did not have the imaging technology in the early 90’s to discern the actual movements of the ciliary muscles or the zonules during accommodation.
In and around 2004 – 2006 new UBM imaging technology was able to delineate for the first time the action of the ciliary muscle underneath the lens and the sclera during accommodation. Rather than moving either towards the equator or away from the equator, it was discovered and confirmed that the muscle moved up and in completing an asymmetrical arc during accommodation. This motion can be seen in the two OCT images below of a 27 year old male at distance and at near. The ciliary muscle is the dark grey area at the base of the angle (where the cornea and iris meet). In the second image it is clear that the muscle has expanded inward and upward towards the iris during accommodation
Mary Ann Croft and Adrian Glasser were among the first researchers to identify the actual motion of the ciliary muscle, and Mary Ann continues to be in the forefront of the research regarding the functioning of the entire ciliary muscle, zonular and lens system during accommodation. Several years ago, Mary Ann made another ground-breaking development with the discovery of a new zonular system that attached not on the lens capsule, but on the posterior side of the ciliary body at one end and at the ora serrata (the anterior connection of the Hyaloid membrane and the choroid) at the other end. This new structure became known as the vitreous zonule. Mary Ann has outstanding dynamic videos of the entire choroidal system including the vitreous zonule at both attachment points, the ciliary muscle and ciliary processes moving up and in during accommodation, while the equator of the crystalline lens also moves in indicating a decrease in lens diameter and a rounding up at the center of the lens (this video was shown by Mary Ann at our most recent Medcare update meeting in Nashville). She has also recently discovered a “bowing” back of the hyaloid membrane near the vitreous zonules during accommodation, confirming the hydraulic flow of aqueous from the anterior chamber towards the back of the eye during accommodation. A screen shot from one of Mary Ann’s videos presented in Nashville is shown below. “CM” stands for Ciliary Muscle. “CP” stands for Ciliary Processes.
|Choroid / Retina|
The discovery and confirmation of the dynamics of the ciliary muscle during accommodation provides strong evidence against both the Helmholtz theory and the Schachar theory, because the muscle moves up as well as moving in asymmetrically. Both the Helmholtz and Schachar theories envision the ciliary muscle moving laterally directly towards the lens equator (Helmholtz) or away from the lens equator (Schachar – for the equatorial zonules only).
Other significant evidence against the Helmholtz theory of presbyopia (which postulates that the loss of accommodation with age is due to gradual stiffening of the crystalline lens which eventually leads to an inability of the lens to change shape), is that the actual stiffening of the crystalline lens is not linear, but relatively abrupt later in life. There is no substantial hardening of the crystalline lens until a person is in their forties and fifties. However, by the time a person is thirty, as seen in Duane’s curve above, they have already lost more than half of their accommodative amplitude. Moreover, even when lens hardening has started to take place in a person that is in their 50’s and 60’s, the amount of lens stiffness varies considerably by patient. This is evident in the fact that patients having cataract surgery, which generally occurs when people are in the 60’s and 70’s, will require different levels of phako energy to disintegrate the natural crystalline lens containing the cataract before removal (the settings on a phako machine range from 1- 4, with most patients requiring a setting of 1 – 2 to remove their cataracts, indicating less brittleness, but with some patients requiring a higher setting of 3 – 4, indicating greater brittleness). Yet notwithstanding the amount of hardness in the lens at the time their cataracts are removed, most patients will have the same degree of presbyopia.