THEORIES
Von Helmholz and Müeller
One recent article referred to an increased depth of focus at near after surgery as pseudoaccommodation.1 Another one stated that accommodation “should be based upon the Helmholtz theory of accommodation” and, “in accordance with the Helmholtz theory ... the flexible haptic would allow the optic to move anteriorly during the accommodative effort and thereby increase the effective power of the lens.”2
Credit should be given where credit is due. Von Helmholz was not the first to describe the crystalline lens' anterior movement during accommodative effort.3 This concept was first proposed by Johannes Müeller, 2 years before von Helmholtz announced his theory.
Around 1854, Müeller described the circular muscle (Müeller's muscle) of the ciliary body. He theorized that the contraction of the ciliary muscle caused the vitreous to move anteriorly, pushing the lens forward and increasing its power, based purely on anterior axial movement of a converging lens alone. Two years later, in 1856, von Helmholtz announced his theory of accommodation, which was based on the lens' increasing sphericity upon relaxation of the ciliary muscle during accommodative effort.3,4
Young
Thomas Young is also of historical interest. His pioneering work on the accommodative mechanism of the eye, which he reported in 1801, gave the proofs for the capsular or Helmholz theory, as described by von Helmholz some 50 years later.4-7 Technically, Young should also be credited with observing the anterior movement of the lens, but this discovery should be taken in context. His observation that the anterior crystalline surface approaches the cornea was secondary to the crystalline len's anterior surface becoming more convex. Müeller's, not von Helmholz's, specific contribution was the theory that the lens mass shifts anteriorly or toward the cornea in the eye's effort to accommodate. Von Helmholtz promoted Young's conclusion “that the power of accommodation depends on a change of form in the lens.”4 He also emphasized Young's finding of the lens fibers' contractility.8
Donders
As a contemporary and friend of von Helmholtz, Frans Cornelius Donders favored von Helmholtz's theory when he wrote The Anomalies of Accommodation and Refraction in 1864.4 According to Donders, regarding the mechanism of accommodation, “the change consists in an alteration of form of the lens: above all, its anterior surface becomes more convex and approaches the cornea.” Thanks to Donders' authoritative text, no one dared question von Helmholtz for many years.
Tscherning
As an aside, it is worth noting that another early theory of accommodation is the vitreous lens model or Tscherning theory, which was first proposed by Cramer in 18519 and subsequently by Tscherning in 190010 and von Pflugk in 1935.11 This theory in essence stated that if the lens is pushed forward, the anterior lens surface contour would become more “conoidal” and focus the eye for near. One capsular theorist argued that this could not be the case, because the zonules would have to become tense to compress the lens equator and it had already been directly observed that the zonules were relaxed during accommodation.12
Regarding the concept of pseudoaccommodation, the term is correct when astigmatism increases focal depth. Huber first described this phenomenon in 1981.13 Others subsequently determined that slight against-the-rule astigmatism gave better accommodative results.14 Pseudoaccommodation may also be said to occur after implantation of multifocal and other special IOLs.
One confounding factor in assessing the success of any currently available approach in restoring the eye's ability to focus at near is determining what accommodation is and what it is not.
The truth is that the anterior movement of the lens is integral to accommodation. It is not “pseudo” anything. Accommodation includes miosis, convergence, the increased sphericity of the lens, and the anterior movement of the lens.
Johnson
In 1924, Lindsay Johnson, MD, questioned the rationality of the Helmholtz theory, because it ignored the action of the circular fibers of Müeller. Von Helmholtz had theorized that the lens stayed compressed in the eye when it was not accommodating, because the ciliary muscles were under constant tension, stretching the zonules. Johnson acknowledged the lack of logic in this theory. He pointed out that muscular tension in a relaxed state is not normal.15
Johnson described the compression of fluid in the circumlental space upon the ciliary muscle's contraction, with anterior bulging of the anterior lens surface and anterior movement of the lens. Compressed aqueous is forced into the spaces of Fontana during accommodation, and it flows back into the chamber upon the relaxation of accommodation. Johnson concluded that the increased curvature of the lens is assisted by hydraulic pressure, not by the relaxation of the ciliary muscle's tension on the zonules, as von Helmholtz claimed.4
Coleman
Johnson's work was largely ignored. Then, in 1970, Jackson Coleman showed that the contraction of the ciliary body brought about a rise in vitreous pressure, which in turn had a hydraulic effect on the crystalline lens' deformation with anterior displacement. He thus confirmed Johnson's theory.12
Again, in 1986, Coleman verified the anterior displacement of the lens as a component of accommodation. Through intubation of the vitreous and anterior chambers, he demonstrated a rise in vitreous pressure upon accommodation.16
Thornton
In 1985 and 1986, Spencer Thornton published the results of his research in accommodation, in which he demonstrated via real-time ultrasonography the anterior movement of the vitreous upon accommodative effort.17,18 His was the first photographic documentation of a mechanism other than the accepted change in lenticular shape to account for increased focal power in accommodation.
When Thornton initially spoke and wrote about the demonstration of the lens' forward movement,17,19 many ophthalmologists expressed skepticism that the lens could move forward upon accommodation, despite the photographic evidence. In January 1986, Thornton applied for a patent (US Patent No. 4,718,904) on an IOL that demonstrated anterior movement upon accommodative effort. Real-time A-scan ultrasonography documented this movement. Photographic evidence demonstrated accommodation from 1.80 to 2.50D, with shorter (hyperopic) eyes achieving more accommodation with the lens placed farther back, as verified by Jack Holladay, MD.18
MECHANISM OF ACCOMMODATION
Anterior Movement
Many investigators have since verified that the anterior movement of the lens is a component of accommodation, and most accommodating IOLs depend on this movement for their accommodative effect.20 When ophthalmologists observe increased accommodation upon the anterior movement of an IOL, we know that the lens does not increase in sphericity. The increase in focal power can only be the result of three things: (1) the lens' power; (2) its posterior position; and (3) its anterior movement.
Forward Movement
Most investigators have reported restored accommodation with the forward movement of IOLs, and others have documented the same forward movement with the natural lens.21-23 It is this forward movement (New Focus System; Surgical Specialties Corp., Reading, PA), along with remaining elasticity of the natural lens, that restores accommodation in the phakic presbyopic eye after anterior ciliary sclerotomy by increasing the working space in the posterior chamber for the ciliary body/zonule/lens complex.24
Depth of Field
John Hunkeler, MD, described an IOL that derives its accommodative power from the combined contribution of depth of field and axial travel. He stated, “While lens sclerosis causes loss of accommodation with aging, accommodative ability is preserved because the other elements of the eye's accommodative apparatus remain functional.”21
Forward Movement and Myopic Alteration
Nawa et al22 referred to Holladay's calculation that 1mm of forward movement corresponds to 1.90D of myopic change based on their study of normal eyes. They concluded that short eyes accommodate better with forward movement of the IOL, and they define the major components of apparent accommodation with IOLs as astigmatism, depth of focus, multifocality of the cornea, and forward movement of the IOL.
Moveable IOL
Rana and Miller23 described a multifocal IOL with a pinhole aperture that increased depth of field. They stated, “The other mechanism to achieve good distance and near vision in pseudophakic eyes is a moveable IOL.”
CONCLUSION
Most investigators have realized that Müeller and von Helmholtz were describing different but complementary components of accommodation. Together, increased sphericity and forward movement of the lens produce accommodation. Separately, each component produces partial, but not “pseudo,” accommodation.
Spencer Thornton, MD, is Clinical Professor of Ophthalmology, University of Tennessee Health Science Center, Memphis, Tennessee. He states that he has a financial interest in the New Focus System mentioned herein. Dr. Thornton may be reached at (615) 373-1236;
thornton@eyecareusa.org.
John F. Doane, MD, FACS, is in private practice with Discover Vision Centers in Kansas City, Missouri, and is Clinical Assistant Professor of the Department of Ophthalmology, Kansas University Medical Center. He states that he does not hold a financial interest in any product or company mentioned herein. Dr. Doane may be reached at (816) 350-4539; jdoane@discovervision.com.
1. Atchison DA. Pseudoaccommodation with forward movement of IOLs. J Cataract Refract Surg. 2005;31:11.
2. Masket S. Accommodating IOLs: emerging concepts and designs. Cataract & Refractive Surgery Today. 2004;4;7:32–37.
3. Donders FC. On the Anomalies of Accommodation and Refraction of the Eye. London: The New Sydenham Society; 1864:10-27.
4. von Helmholz H. Uber die akkommodation des auges. Albrecht Von Graefes Arch Klin Exp Ophthalmol. 1855;1:1-89.
5. Young T. On the mechanism of the eye. Vol 92. London: Philosophical Transactions; 1801:23-88.
6. Young T, Peacock G, Leitch J. Miscellaneous Works of the late Thomas Young. Vol 1. London: J. Murray; 1855:12.
7. Katz M. The human eye as an optical system. In: Tasman W, Jaeger EA, eds. Duane's Clinical Ophthalmology. Vol 1. Philadelphia, Pa: Lippincott Williams & Wilkins; 2002:49-50.
8. Von G. Karsten. Allgemeine Encyclopaedie der Physik. Erste lieferung: 112 et seq.
9. Cramer A. Tijdschrift der maatschappij vor geneeskunde. Nederlandisch. Lancet. 1851;1:529-541.
10. Tscherning M. Dioptrics of the Eye, Accommodation. In: Weiland C, ed. Physiologic Optics. Philadelphia, PA: Keystone Publishing Co.; 1900:166-189.
11. Von Pflugk A. Neve wege zur erforshung der lehre von der akkommodation, der glasskorper im akkommodierenden auge. Albrech Von Graefes Arch Klin Exp Ophthalmol. 1935;133:454-571. (Addendum, 1935;134:188).
12. Coleman DJ. On the hydraulic suspension theory of accommodation. Trans Am Soc Ophthalmol. 1986;84:846-868.
13. Huber C. Myopic astigmatism as a substitute for accommodation in pseudophakia. Doc Ophthalmol. 1981;52:123-178.
14. Trindade F, Oliveira A, Frasson M. Benefit of against-the-rule astigmatism to uncorrected near acuity. J Cataract Refract Surg. 1997;23:82-85.
15. Johnson L. A new theory of accommodation. Arch Ophthal. 1924;53:221-223.
16. Coleman DL. Unified model for accommodative mechanism. Ophthalmology. 1970;69:1063-1079.
17. An IOL that gives accommodation? Ocular Surgery News. 1985;3:9.
18. Thornton SP. Lens implantation with restored accommodation. Curr Canadian Ophth Prac. 1986;4:2:60-62.
19. Thornton S. Letters to the editor: accommodative IOLs. Ocular Surgery News. 1985;3:19.
20. Cumming S. Accommodating IOLs. In: Sher N, ed. Surgery For Hyperopia. Thorofare, NJ: Slack, Inc. 2004:251-260.
21. Guttman C, Hunkeler J. IOL design mimics optic shape. Ophthalmology Times. 2003; 28:15:11.
22. Nawa Y, Ueda T, Nakatsuka M, et al. Accommodation obtained per 1.0mm forward movement of a posterior chamber intraocular lens. J Cataract Refract Surg. 2003;29:2069-2072.
23. Rana A, Milller D, Magnante P. Understanding the accommodating intraocular lens. J Cataract Refract Surg. 2003;29:2284-2287.
24. Thornton SP. Anterior ciliary sclerotomy with tissue barriers. Highlights Ophthalmol. 2005;33;1:19-20.
Advertisement - Issue Continues Below
