In a recent article,1 Atchison refers to axial movement of the lens as “pseudoaccommodation.” I believe there is some confusion about the difference between what is real and what is pseudo, with misinformation being perpetuated by authors unfamiliar with the several mechanisms of accommodation.
An example of pseudoaccommodation is the simple myopic astigmatism remaining after cataract surgery that Huber spoke and wrote about in the early 1980s.2 Pseudoaccommodation may also be said to result from multifocal and other special IOLs.
The truth is that anterior movement of the lens is an integral component of accommodation. It is not pseudo anything. It seems to be the old story of “My mind is made up. Don't confuse me with facts.” Accommodation includes miosis, convergence, increased sphericity of the lens, and anterior movement of the lens. Another factor that should be considered is the cerebral component. But that is for another time. Let's look at what we know.
Around 1854, Mueller described the circular muscle (Mueller's muscle) of the ciliary body and theorized that contraction of the ciliary muscle resulted in anterior movement of the vitreous, pushing the lens forward and increasing its power. Two years later, in 1856, Helmholtz announced his theory of accommodation, which was based on the increasing sphericity of the lens on relaxation of the ciliary muscle on accommodation. Donders was a contemporary and friend of Helmholtz and favored Helmholtz's theory in his book about accommodation and refraction.3 No one dared question Helmholtz for many years.
In 1924, Johnson4 questioned the rationality of Helmholtz's theory because it ignored the action of the circular fibers of Mueller's muscle and theorized that the lens stayed compressed in the nonaccommodated eye because the ciliary muscles were under constant tension, stretching the zonules. Johnson pointed out the lack of logic in this theory as muscle tension in a relaxed state is not normal.
Johnson described the compression of fluid in the circumlental space on ciliary muscle 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, flowing back into the chamber on relaxation of accommodation. Johnson concluded that the increased curvature of the lens is assisted by hydraulic pressure, not by relaxation of ciliary muscle tension on the zonules, as Helmholtz claimed.3
Johnson's work was largely ignored. Then, in 1970,5 Coleman showed that contraction of the ciliary body brought about a rise in vitreous pressure, which in turn had a hydraulic effect on crystalline lens deformation with anterior displacement, confirming Johnson's theory. In 1986,6 Coleman again verified the anterior displacement of the lens as a component of accommodation with intubation of the vitreous and anterior chamber, demonstrating a rise in vitreous pressure on accommodation.
In 1985 and 1986, I published the results of research on accommodation, demonstrating by real time ultrasonography the anterior movement of the vitreous on the accommodative effort (“An IOL That Gives Accommodation,” Ocular Surgery News, May 1, 1985, October 1, 1985).7 This was the first photographic documentation of a mechanism other than the accepted change in lens shape to account for increased focal power in accommodation.
When I first spoke and wrote about the demonstration of forward movement of the lens (Royal Hawaiian Eye Meeting, January 1985), there was skepticism that the lens could move forward on accommodation despite the photographic evidence. In January 1986, I applied for a patent on an IOL that demonstrated anterior movement on accommodative effort.8 This movement was documented by photographic evidence with real-time A-scan ultrasonography. From 1.8 to 2.5 diopters of accommodation was demonstrated, with shorter (hyperopic) eyes achieving more accommodation with the lens farther back, as verified by Holladay (S. Thornton, MD, “Accommodative IOLs [letter], Ocular Surgery News, October 1, 1985).
Since then, many investigators have verified anterior movement of the lens as a component of accommodation, and most accommodating IOLs are dependent on this for their accommodative effect. When there is increased accommodation by the anterior movement of an IOL, the lens does not increase in sphericity. The increase in focal power results from 3 things only: power of the lens, posterior positioning, and anterior movement.
Most investigators have reported restored accommodation with the forward movement of an IOL, and others have documented the same forward movement with the natural lens. It is this forward movement, along with the remaining elasticity of the natural lens, that provides restoration of accommodation in the phakic presbyopic eye with anterior ciliary sclerotomy (New Focus System, Surgical Specialties Corp.)9 and other approaches to increasing the working space for the ciliary body–zonule–lens complex in the posterior chamber.
Hunkeler has described an IOL that derives its accommodative power from the combined contribution of depth of field and axial travel. He states that “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” (C. Guttman, “IOL Design Mimics Optic Shape Change of Natural Lens,” Ophthalmology Times, August 1, 2003, page 11).
Nawa et al.10 refer to Holladay's calculation that 1.0 mm of forward movement corresponds to 1.9 D of myopic change based on a study of normal eyes. They conclude that short eyes accommodate better with forward IOL movement and define the major components of apparent accommodation with IOLs as astigmatism, depth of focus, multifocality of the cornea, and forward IOL movement.
Rana and coauthors11 describe a multifocal IOL with a pinhole aperture giving increased depth of field and state, “The other mechanism to achieve good distance and near vision in pseudophakic eyes is a moveable IOL.”
In summary, most investigators have come to the realization that Mueller and Helmholtz were describing different but complimentary components of accommodation. Together, increased sphericity and forward movement of the lens produces accommodation. Taken separately, each component produces partial, but not pseudo, accommodation.
Spencer P. Thornton MD
Nashville, Tennessee, USA
1. Atchison DA. Pseudoaccommodation with forward movement of IOLs [letter]. J Cataract Refract Surg 2005; 31:11
2. Huber C. Myopic astigmatism as a substitute for accommodation in pseudophakia. Doc Ophthalmol 1981; 52:123-178
3. Donders FC. On the anomalies of accommodation and refraction of the eye; translated from the author's manuscript by WD Moore. London, New Sydenham Society, 1864
4. Johnson L. A new theory of accommodation. Arch Ophthalmol (old series) 1924; 53:426-430
5. Coleman DL. Unified model for accommodative mechanism. Am J Ophthalmol 1970; 69:1063-1079
6. Coleman DJ. On the hydraulic suspension theory of accommodation. Trans Am Soc Ophthalmol 1986; 84:846-868
7. Thornton SP. Lens implantation with restored accommodation. Curr Canadian Ophthalmic Pract 1986; 4:60, 62, 82
8. Thornton SP, inventor; Eye Technology, Inc., assignee. Intraocular lens for capsular bag implantation. US patent 4 718 904. January 12, 1998
9. Thornton SP. Anterior ciliary sclerotomy with tissue barriers; the scleral approach to presbyopia treatment. Highlights Ophthalmol 2005; 33(1):19-20
10. Nawa Y, Ueda T, Nakatsuka M, et al. Accommodation obtained per 1.0 mm forward movement of a posterior chamber intraocular lens. J Cataract Refract Surg 2003; 29:2069-2072
11. Rana A, Milller D, Magnante P. Understanding the accommodating intraocular lens. J Cataract Refract Surg 2003; 29:2284-2287