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“Retinal imaging” aberrometry: Author reply
Ophthalmology Volume 109, Number 3, March 2002 ic letter). Invest Ophthalmol Vis Sci Feb 2, 2000; http// www.iovs.org/cgi/eletters/40/6/1162#EL1.
Author reply Dear Editor: The change in asphericity recorded by the Tscherning aberrometer in the author’s left eye during accommodation represents a shift toward negative asphericity (decrease in spherical aberration) from a position of positive asphericity (greater than normal spherical aberration).1 Although the author’s age, manifest, and cycloplegic refractions are not indicated in the article, the aberrometerrecorded refraction without accommodation is listed in Table 1 of the article as ⫹0.60 – 0.70 ⫻ 166 for the central 3 mm; and ⫹0.30 – 0.47 ⫻ 164 for the full pupil diameter of 6.5 mm.1 “Near maximal accommodation” was attempted by the author (age 39) while maintaining a central view of a defocused fixation light during the accommodative mechanism. The author’s near point of accommodation and whether further accommodative amplitude could be achieved is not recorded and is irrelevant for the discussion of asphericity in association with the change in spherical equivalent refraction. The spherical magnitude of accommodation recorded in the central 3 mm is 2.15 diopters (D) and this is even less, 1.38 D, when considering the full 6.5 mm pupillary diameter. The fact that the magnitude at 3 mm did not meet the expected accommodative amplitude of a normal 40-year-old may, in part, be explained by the even smaller pupil diameter (miosis) that is typically achieved during accommodation, which would increase the author’s depth of field. Measuring the near point of accommodation as suggested by Dr. Lothringer is not a true measure of the magnitude of accommodation, but is a functional value aided by the increased depth of field of miosis. The greatest significance of these data in relation to the mechanism of accommodation can be seen by the decrease in spherical aberration (negative asphericity) associated with accommodation viewed through a larger pupil size. The Schachar theory of accommodation assumes a “steepening of the central surfaces with a simultaneous flattening of the peripheral surfaces of the crystalline lens, resulting in a decrease in spherical aberration.”2 If this is to be achieved by equatorial zonular tension (stretching), then the anterior/posterior (polar) dimensions of the lens would also be of decreased magnitude, resulting in an overall negative or zero accommodative amplitude when considering the refractive effect over the full area of a 6.5-mm pupil. To state this more simply, the magnitude of change in asphericity of .77 D is too small to account for the full spherical equivalent refractive change of 2.15 D if the Schachar theory of accommodation were correct in this example. Nevertheless, the change toward decreased spherical aberration brings question to the Helmholtz mechanism of accommodation as to what asphericity changes could be expected by a lens that increased its curvature and anterior/ posterior (polar) thickness with the zonular relaxation. Glasser and Campbell3 answer this asphericity question, in association with accommodation, by demonstrating a shift toward negative asphericity (decrease in spherical aberra-
406
Figure 1. High resolution magnetic resonance imaging scan of the author’s left eye during (A) distance viewing (0 diopters) and (B) fixation on a near target (4 diopters of accommodation).
tion) in prepresbyopic lenses measured with a cadaver experimental scanning laser wavefront system, which stretches and then relaxes the ciliary body/zonule/lens complex to simulate accommodation. This verifies that the spherical aberration pattern observed in the author’s left eye is consistent with the Helmholtz theory of accommodation from an optical and physiologic standpoint. A further illustration of this point can be visualized anatomically by the use of high-resolution magnetic resonance imaging (MRI).4 Diagnostic MRI was performed on the author’s left eye using a GE 1.5 Tesla imager with a
Author reply Dear Editor: The change in asphericity recorded by the Tscherning aberrometer in the author’s left eye during accommodation represents a shift toward negative asphericity (decrease in spherical aberration) from a position of positive asphericity (greater than normal spherical aberration).1 Although the author’s age, manifest, and cycloplegic refractions are not indicated in the article, the aberrometerrecorded refraction without accommodation is listed in Table 1 of the article as ⫹0.60 – 0.70 ⫻ 166 for the central 3 mm; and ⫹0.30 – 0.47 ⫻ 164 for the full pupil diameter of 6.5 mm.1 “Near maximal accommodation” was attempted by the author (age 39) while maintaining a central view of a defocused fixation light during the accommodative mechanism. The author’s near point of accommodation and whether further accommodative amplitude could be achieved is not recorded and is irrelevant for the discussion of asphericity in association with the change in spherical equivalent refraction. The spherical magnitude of accommodation recorded in the central 3 mm is 2.15 diopters (D) and this is even less, 1.38 D, when considering the full 6.5 mm pupillary diameter. The fact that the magnitude at 3 mm did not meet the expected accommodative amplitude of a normal 40-year-old may, in part, be explained by the even smaller pupil diameter (miosis) that is typically achieved during accommodation, which would increase the author’s depth of field. Measuring the near point of accommodation as suggested by Dr. Lothringer is not a true measure of the magnitude of accommodation, but is a functional value aided by the increased depth of field of miosis. The greatest significance of these data in relation to the mechanism of accommodation can be seen by the decrease in spherical aberration (negative asphericity) associated with accommodation viewed through a larger pupil size. The Schachar theory of accommodation assumes a “steepening of the central surfaces with a simultaneous flattening of the peripheral surfaces of the crystalline lens, resulting in a decrease in spherical aberration.”2 If this is to be achieved by equatorial zonular tension (stretching), then the anterior/posterior (polar) dimensions of the lens would also be of decreased magnitude, resulting in an overall negative or zero accommodative amplitude when considering the refractive effect over the full area of a 6.5-mm pupil. To state this more simply, the magnitude of change in asphericity of .77 D is too small to account for the full spherical equivalent refractive change of 2.15 D if the Schachar theory of accommodation were correct in this example. Nevertheless, the change toward decreased spherical aberration brings question to the Helmholtz mechanism of accommodation as to what asphericity changes could be expected by a lens that increased its curvature and anterior/ posterior (polar) thickness with the zonular relaxation. Glasser and Campbell3 answer this asphericity question, in association with accommodation, by demonstrating a shift toward negative asphericity (decrease in spherical aberra-
406
Figure 1. High resolution magnetic resonance imaging scan of the author’s left eye during (A) distance viewing (0 diopters) and (B) fixation on a near target (4 diopters of accommodation).
tion) in prepresbyopic lenses measured with a cadaver experimental scanning laser wavefront system, which stretches and then relaxes the ciliary body/zonule/lens complex to simulate accommodation. This verifies that the spherical aberration pattern observed in the author’s left eye is consistent with the Helmholtz theory of accommodation from an optical and physiologic standpoint. A further illustration of this point can be visualized anatomically by the use of high-resolution magnetic resonance imaging (MRI).4 Diagnostic MRI was performed on the author’s left eye using a GE 1.5 Tesla imager with a