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The Incyclorotator: A Unique Case of Self-Induced Myopia and Monocular Diplopia |
Experiments aimed at causing myopia have centered mainly on various methods of form deprivation using animal subjects such as monkeys, rabbits and chickens.
For obvious reasons, experiments designed to elicit myopia in humans have been practically non-existent. The project described here is an anomaly: an experiment designed to produce myopia in a human subject.
It is well established that elongation of the globe is a significant factor in myopia. Consequently, if an emmetropic eye is made to elongate, it should become myopic, and an experiment was devised to test this.
The method chosen to elongate the globe was to compress it in the general area of the equator by forcing contraction of the superior oblique muscles.
Because the eye muscles are not subject to individual voluntary control, it was necessary to devise some means to make the superior obliques contract while maintaining relative relaxation of the other extraocular muscles. The natural tendency of the eyes to fuse two disparate images was utilized for this purpose.
A viewing device was constructed which contained two identical photographic transparencies depicting a visually rich pattern. When the subject looked through the device, each eye viewed one of the transparencies; the visual cortex then fuses the two images to form a single scene.
The transparencies were then incyclorotated, i.e. as seen by the subject, the right-side image was rotated counterclockwise and the left-side image was rotated clockwise. In order to maintain fusion of the two images, each eye must then rotate in the same direction as the image it is viewing, i.e. the upper end of the vertical meridian of each eye leans nasalwards.
The movement of incyclorotation is effected principally by the superior oblique muscles, but there is a limit as to how far the globe can rotate, since this is opposed by the check ligaments and other fascial structures of the orbit. If an effort is made to maintain fusion, the traction of the superior obliques, which wrap part way around the globe, will exert pressure in the general area of the equatorial meridian.
The device was later modified for
portable use to facilitate long-term viewing. Instead of viewing
transparencies, the subject looked through a system of mirrors that
tilted in like manner any scene viewed. The amount of tilt
(incyclorotation) varied between
6 and 12 degrees.
This is not to say that if the images are rotated, say, 8 degrees, each eye will also rotate exactly 8 degrees; eye rotation can be as much as 2 degrees less. This is because of Panum's fusional area, which in stereopsis allows the image to be pulled apart by some 2 degrees before being broken up into two separate images. The images are actually pulled apart on the retina, but a supra-retinal function maintains perception of a single image. In order to eliminate any stimulus to accommodation, distance fixation of at least six meters was maintained. (Fender, 1967).
Since I was unable to find an emmetrope willing to risk becoming a myope, the subject of the experiment necessarily had to be myself. However, because I was already myopic, the objective of the experiment was changed: to determine if elongating the globe would increase the existing degree of myopia.
Because I was unable to make axial length measurements, I had to rely on changes in the visual acuity. Thus, if my visual acuity deteriorated in the course of the experiment, this might indicate that the globe had elongated.
I began to wear the device, strapped to my head, for three to four hours per day, and used it for distance vision only.
Within three weeks, two phenomena were observed:
1. A myopic increase of 5 D.
2. A high degree of monocular diplopia
At the start of the experiment the refraction was O.D. -7.5 -1.25 ; -5.50 -1.50. The increase was later determined to have reached O.D. -11.75 -2.25 ; O.S. -9.0 -2.00.
The monocular diplopia was so pronounced that the uncorrected acuity improved to the extent that I had become almost emmetropic--I could easily read the 20/25 line of a Snellen chart. However, this was not normal near-emmetropic vision because blur was still present.
The subjective experience of this effect of dual vision was as if two photographic transparencies, one blurred and one sharp, were superimposed one on the other (I use the term "dual vision" because "monocular diplopia" seems inadequate to describe the phenomenon).
It seemed obvious that this remarkable change in acuity had been caused by something to do with the experiment, but by what means? It should be made clear that this was monocular, and approximately to the same degree in each eye.
The crucial question was, how had squeezing the eye produced dual vision? The presence of dual vision suggests strongly that there were two separate focal points. There are a few reports in the literature on double focal points resulting from cataracts, but this case was clearly different.
The most significant point, however, is that these changes in acuity must have come principally not from elongation of the globe (although the globe had probably elongated to some extent), but rather from changes in the shape and power of the crystalline lens.
A possible explanation is that contraction of the superior obliques had exerted pressure on the globe, which was transmitted through the sclera to the vitreous, forcing the vitreous against the back of the lens and flattening its periphery.
Rays passing through this outer region of the lens came to a focus at a point very close to the retina, which produced the secondary image (clear vision), while the rays passing through the central region of the lens came to a focus in front of the retina, which produced the primary image, which was severely blurred. The 5 D. increase in the degree of myopia suggests that the vitreous pressure had accommodated the lens to an extreme degree. It is important to note the that the subject, who was 35 years old at the time, was far beyond the age at which myopia increases normally occur.
Ivanoff (1956) and others have shown that when the eye is at rest the spherical aberration is positive, which means that the rays passing through the periphery of the lens come to a focus in front of rays passing through the axial region of the lens.
As the lens accommodates to view a near object and begins to change its shape, the spherical aberration decreases, and at around 3 diopters there is almost no aberration at all, i.e. all the rays come to a focus at the same point.
If the eye accommodates further, the aberration begins to reverse, in which case the peripheral rays come to a focus at a point behind the axial rays.
Apparently this condition had reached an extreme degree, as visualized in Figure 1.
Figure 1.
It is questionable that these results could be extrapolated to explain myopia in normal subjects. However, the presence of negative spherical aberration with increased lens power is well-established, and a number of studies have shown that spherical aberration is more common in high myopia. (Hu, et al, 2004).
"A high proportion of the aberroscope grids photographed in myopic eyes were too highly distorted to permit analysis. This was not the case for emmetropic subjects…". (Collins et al, 1995).
. Roorda and Glasser (2004) state that "The increasing negative aberration of the accommodating lens arises from a more pronounced increase in optical power near the central region of the lens compared to the peripheral region. In other words, as the lens accommodates, the central curvature steepens while the peripheral curvature flattens".
The possible role of increased lens power in myopia is directly contradicted by decades of well-documented studies by researchers from around the world.
The principal objections are based on the following:
1. Study after study has shown that principal cause of myopia is increased axial length, not increased lens power.
2. The hypothesis that vitreous pressure could cause the eye to accommodate is contradicted by the widely accepted Helmholtz-Fincham zonular relaxation hypothesis.
3. The vitreous is not required for accommodation to occur.
If, in the experiment described here, the increase in lens power were caused by vitreous pressure, it would be difficult to explain how this could have occurred without tensing the zonule. The case for accommodation caused by relaxation of the zonule is exceptionally strong. In what is undoubtedly the demonstration that clinched the case for zonular relaxation, Fincham (1937) showed conclusively that without the tension of the zonule, the lens becomes more spherical.
An eye was made to accommodate for distance viewing by the instillation of atropine and then removed from the orbit and pointed upward after dissection of the cornea and iris. The profile of the lens can then be photographed, and in this condition it demonstrates the characteristic shape of the lens when the eye is looking at a distance.
However, when the fibers of the zonule are severed all around by the sharp edge of a knife, the curvature of the anterior surface increases markedly and "assumes the shape that it has under maximum accommodation," i.e. the lens becomes thicker, as is clearly seen in the photographs taken by Fincham.
This appears to be an unassailable argument. To recapitulate: when the zonules that hold the lens in place are cut, the lens immediately becomes more spherical, thereby increasing its power.
To counter this argument requires rejection, not of Fincham's observation (the photographic evidence is too strong for that) but of his interpretation. When the zonules were cut and the lens became more spherical, he assumed that the consequent change in the shape of the lens was the same as that which occurs in accommodation. Could this be a non sequitur?
It is not inconceivable that the shape he observed was not the shape that occurs in accommodation, but merely resembled it. It is possible that the lens could increase its power under two different conditions: 1) When released from the tension of the zonule, and 2) When molded by vitreous pressure, with only the latter being true accommodation.
Even today, the belief that accommodation simply makes the lens become more spherical is widespread.. According to Roorda and Glasser, writing in 2004, "The prevailing view is that the lens becomes more spherical with accommodation due to the molding force of the capsule". "Spherical" does not adequately describe the shape of the accommodated lens because the peripheral area of the posterior surface of the lens is quite different from the anterior surface.
The phenomenon of spherical aberration seems to be widely ignored. Even in basic works such as Adler's Physiology of the Eye, The Myopias (Curtin) and Visual Optics and Refraction (Michaels) and The Physiology of the Eye (Davson) the question of spherical aberration is almost completely neglected.
The many studies retinal defocus apparently fail to include the possible effect of spherical aberration. If negative spherical aberration begins at a certain point in accommodation, the presence of two focal points could conceivably "confuse" the input from the autonomic nervous system. In the case of a very near point of fixation, one focal point might be situated in front of the retina, and one behind, especially in the case of a myope.
The phenomenon of spherical aberration and its relation to lens shape is supported by a number of researchers who have reported highly unusual changes in the shape of the accommodating lens, particularly the anterior aspect.
Lowe
(1972) reported that "During examination of a large series of eyes that
had pupils dilated after peripheral iridectomy...I was struck by the
marked curvature of the anterior lens surface within the enlarged
pupil. The lens frequently appeared as though it were herniating
through the enlarged pupil, with the pupillary margin of the iris
seeming to grip the lens."
Jampel and
Mindel (1967), in a report on stimulation of the oculomotor nucleus in
monkeys, observed changes "... characterized by a conspicuous forward
bulging of the pupillary or central portion of the iris which produced
a marked convexity of the iris diaphragm
and a marked increase in
the depth of the anterior chamber...On observation of the eye from the
side during iris-bulge, the central portion of the lens appeared to
become conoidal and to
move forward into the anterior chamber."
Burian
and Allen (1955) reported that "The most remarkable change was seen in
the middle one-third of the body of the iris. This part of the iris
bowed backward during active accommodation, forming a deep hollow, and
returned to its normal position when the eye was
relaxed."
And Suzuki (1971) states that "Concerning the iris, its silhouette was a slightly curved line, being convex anteriorly in the form of a physiological iris bombé. On stimulation, the iris showed a peculiar change. That is, besides the change of the contraction of he pupil, the iris was bent reversely to the posterior chamber, so that the central half of the iris was held in contact with the anterior surface of the lens and the iris-lens apposition became tighter over a much larger area."
In a study of accommodation in the rhesus monkey, Bito et al state that "A possible iridial contribution was also observed during carbachol-induced accommodation in young animals: development of full miosis was prevented by occlusion of the pupil by the anterior-central portion of the lens. Thus it appears that the pupillary margin and/or the sphincter muscle can apply a force to the lens which may steepen the curvature of its anterior-lenticular central portion thus increasing total dioptric power".
All these reports describe the iris as being pressed against the lens, and two of them note that the conoid form of the lens appears to be the result of bulging through the pupil.
Could the iris play a major role in accommodation after all? The iris/lens mechanism is well documented in certain birds, although there are significant differences from human eyes.
According to Walls (1967), "The avian iris is always of material assistance during accommodation in holding back the lens against which it presses, and in inhibiting the peripheral part of the anterior surface of the lens from bulging, thus concentrating the change-of-curvature in the part of the surface opposite the pupil."
It is highly improbable that with the vast amount of research done on the iris, such an important function as counterpressure on the lens could remain undetected. Yet these reports strongly suggest an iris/lens connection, and it is interesting that the researchers themselves seem surprised by their findings.
Another candidate for lens molding is the capsule: In regard to the conoidal form of the anterior lens surface, when Helmholtz first proposed his relaxation theory of accommodation it was criticized on the ground that relaxation of the zonule failed to explain how this molding was achieved.
Tscherning claimed that this could only be produced by pressure from the vitreous, which he believed molded the softer cortex of the lens around the harder nucleus. Fincham thought he found an answer in evidence that the thickness of the lens capsule varies, and he believed that these minute differences in thickness were sufficient to impose a conoidal shape on the anterior surface of the lens.
Although it is conceivable that the capsule could mold the lens to a slight degree in this manner, the evidence from the experiment described her indicates that this explanation is insufficient.
Because the degree of spherical
aberration was undoubtedly so extreme, which indicated extreme
flattening of the periphery of the lens, it is difficult to believe
that it could have been
produced by such minute differences in capsule thickness.
I
initially assumed that the alteration in the shape of the lens had
occurred principally because of pressure by the vitreous against the
lens. However, Roorda and Glasser (2004) showed that in in vitro
experiments with macaque eyes, accommodation was accompanied by
spherical aberration even without the presence of the vitreous.
According to Suzuki (1971), "During accommodation the posterior valley became swollen toward the inner direction of the eyeball. This could account for the relaxation of the zonules attached to the anterior surface of the ciliary muscle.
"During more advanced accommodation, the anterior valley sank toward the outer direction of the eyeball. This could account for the contraction of the zonules attached to the posterior surface of the lens (italics added).
An experiment by Araki (1965) showed that "electric recordings of the changes in tension of the ciliary zonules suggested relaxation of the zonules which was (sic) stretched to the anterior surface of the lens and on the contrary, increased tension of that stretching to the posterior surface (cat and dog eyes).
In more recent investigations, Schachar (2001) has proposed that there are three sets of fibers: anterior, posterior and equatorial, and that when the ciliary contracts there is increased tension of the equatorial fibers which reduces the tension of the other two sets. When the eye is in the unaccommodated state, the anterior and posterior zonules are taut, and there is reduced tension on the equatorial zonules.
A major objection to the vitreous/lens hypothesis is the contention that the vitreous is not required for the eye to accommodate. This is proven by experiments in which accommodation occurs even without the presence of the vitreous in cases of vitrectomized eyes.
Almost all eye researchers support this view. For example, Fisher (1982) states that "The vitreous plays a negligible role during accommodation in modifying the position or shape of the lens." Burian and Allen (1955) state that "...our observations on the periphery of the vitreous surface strongly suggest that the vitreous body, far from pressing on the periphery of the lens, was actually under reduced tension during accommodation."
As happens all too frequently in scientific research, however, there is contrary evidence:
Araki (1965) reported that in experiments on pig, dog and cat eyes, "...it is suggested that tension of the ciliary muscle/zonules stretching from the posterior surface of the lens was increased by forward movement of the ciliary body and consequently it resulted in pressure to the posterior peripheral (my emphasis) part of the lens...the increase in pressure of the vitreous body due to contraction of the accommodative muscle is considered to be the most important factor for the transformation of the lens."
Suzuki (1971) performed an experiment in which he injected radiopaque material into the vitreous of a cat's eye, which during accommodation moved in a direction indicating that the vitreous was forced against the back of the lens and also somewhat toward the posterior pole of the lens."
An experiment by Koke (1942) produced a similar result. He injected cat eyes with radiopaque material and took X-rays during miosis and mydriasis, which showed that during accommodation the vitreous moved toward the lens and inward toward the optic axis.
The experiment that is most closely related to the one described in this paper, because it involved external pressure on the globe, is that of von Pflugk (1035). He cut windows in the equatorial region of bovine eyes and injected a drop of dye into the anterior vitreous, midway between the ciliary body and the posterior pole of the lens. Pressing against the ciliary body from the outside in a radial direction made the dye move toward the lens capsule.
The hypothesis that the extraocular muscles play a role in the causation of myopia is certainly not new. It has been suggested by numerous investigators over the years. A major difference, however, is that in none of these hypotheses has it been proposed that they have any effect on the lens. All are limited to the concept of elongation of the globe by elevation of the intraocular pressure, scleral weaknes or other means.
One of the earliest to investigate this phenomenon was Lancaster (1952), who stated that "...if the accommodation is maintained a few minutes at the maximum, the near point does get nearer and the eye may become accommodated 20% to 30% or more, nearer than at the first. If the near point at the start was 6 D. it may become 7, 8, or 9 D. This...is due to the viscosity of the lens substance. An immediate rapid (about one second) change takes place when the lens adjusts itself for a near object, but if a maximum effort of accommodation continues to be made, the lens slowly (5 to 10 minutes) goes on changing its shape and becoming more strongly refractive.
"Commonly,
when the eye, after such an intense effort of accommodation, is shifted
to a distant object, although the ciliary muscle may promptly relax, it
takes time (a few seconds to
a few minutes depending on how long the
near effort was continued) for the lens to regain its normal shape
adapted to a distance. This is due to the viscosity which makes a
change in the shape slow."
Ong and Ciufredda (1995) in their studies of nearwork-induced transient myopia (NITM) state that (after Lancaster)"little was done in this field for the next seven decades, until computer display terminals became commonplace, and symptoms related to their use became prevalent". However, these more recent studies have attributed this slow recovery time only to the ciliary muscle in the form of changes in tonic accommodation; the possible role of the viscosity of the lens substance seems to be neglected.
They state that "with the continuous high level of accommodative effort necessary to maintain accurate focus, the accommodative hysteresis that was reflected in the (presumed) increased level of tonic accommodation developed as a consequence of increased innervation due to gradual fatiguing of the accommodative system. This myopic increase would then be carried over to distant viewing".
It is significant that NITM appears to be more strongly affected by task duration (my emphasis) than either the initial magnitude of NITM or the task demand. Task durations of 10 to 20 minutes generally result in a rapid to baseline values, usually within seconds, while tasks with a duration of >40 minutes require longer to return to baseline (minutes).
The slowness of lens changes has been studied by other investigators as a function of lens viscosity. According to Kikkawa and Sato (1963), "Application of an external force to the lens caused a rapid deformation followed by a second phase of slow deformation. On removal of the force, a rapid partial reversal of the deformation occurred and was followed by a gradual restoration; complete recovery was not achieved."
Kabe 1967) reported a similar result from his investigations. He showed that when accommodation is increasing, the change in the apparent curvature of the anterior surface of the lens is slow and continuous, but when accommodation is decreasing, there is a prompt, followed by a slow phase.
Because my uncorrected acuity was so close to emmetropia (I thought), I decided to
increase
the degree of rotation of the images to the maximum possible while
still maintaining a fused image, and to increase the wearing time of
the device; I thought that if this produced further flattening of the
lens periphery it might push the secondary focal to anteriorly so as to
contact the retina.
The results were disappointing. The uncorrected acuity began to deteriorate; I could no longer read the 20/25 line, and the 20/30 line was still legible but more blurred than previously. Most notably, the acuity with corrective lenses had deteriorated even further.
Testing the acuity with plus lenses
gave a surprising result: with +4.00 lenses I could still read the
20/30 line. In other words, the condition could be described as being
simultaneously a high myope and a hyperope: vision with the primary
image, from rays passing through the axial region, had further
shortened, which increased my myopia even further, while rays passing
through the peripheral region of the lens had apparently shifted to a
point behind the retina.
Figure 2. (Exaggerated to clarify focal points and blur circles).
The idea that myopia could be the result of increased lens power has always been countered by a strong argument. If in myopic eyes the lenses are accommodated, they would tend to be thicker than the lenses of emmetropes. Not only is this not true, but, in general, myopic eyes tend to have even thinner lenses than emmetropes, and in fact most researchers rank the importance of the lens in myopia as very low:
"Three variables, then, the axial length, the shape of the cornea, and the power of the crystalline lens, exert the greatest effect upon refraction. There is good agreement among authors as to the relative influence which each of these exerts, the axial length being the greatest, followed by the cornea and lens in that order. There are minor disagreements among investigators as to the relative importance of the least of these three elements, the crystalline lens: Van Alphen's work suggests perhaps the lowest estimate of the importance of the lens. However, all investigators arrive at the same order of importance, and at relative values not too different from those obtained by others". (Hirsch, 1967).
Sorsby (1967) seemed to be
puzzled by the existence of thin lenses in myopes and tried to find a
way out of the difficulty by speculating about the tension of the
zonule. He stated that,
"Obviously, a large fairly spherical eye
will have not only a long anteropsterior axis but also a flatter
cornea. Flattening of the lens in a large eye is more difficult to
understand, but a more marked tension on the suspensory ligaments may
be a possible factor."
The barrier to a resolution of this contradiction seems to be the belief that a thin lens is necessarily a low power lens. But if NITM is a proven fact, then it is difficult to explain how the myopic lens could not be accommodated.
In the case of a typical myope with a history of nearwork, e.g. with hour after hour of reading over months and years, it could very well be that with repeated periods of prolonged accommodation the lens, with its slow recovery time, would never return completely to the unaccommodated state.
Ong and Ciufredda in their studies of NITM have a similar view, although they attribute it to changes in tonic accommodation, not lens viscosity: "The presence of a relation between the time course of NITM and decay and task duration supports the notion that repeated occurrences of transient myopia may lead to more permanent forms of myopia".
The possibility that a lens subjected to frequent prolonged accommodation for months and years may not have the same shape as a lens that is accommodated only briefly has apparently not been considered.
That a flat lens is not necessarily a lens of low power is suggested by a number of studies on internal lens changes:
"The increasing negative spherical aberration of the accommodating lens arises from a more pronounced increase in the optical power near the central region (my emphasis) of the lens compared to the peripheral region….this is different to the generally accepted notion that the lens simply becomes more spherical with accommodation….The increase in negative spherical aberration is likely due to the effect of the structure of the lens substance…but may also be due in part to accommodative variations in gradient refractive index of the lens. (Roorda and Glasser, 2004).
The conventional wisdom that the principal changes occur in the anterior lens was challenged by Patnaik (1967), who wrote that "...the often stated and commonly accepted statement, that it is the anterior lens surface which moves forward while the posterior surface remains stationary and that it is only the anterior surface which changes its curvature during accommodation seems not to be correct.
Patnaik also commented on the possibility of nuclear changes: "Our observations strongly indicate that during accommodation the increase in the thickness of the anterior cortex is minimal, and that the change in the posterior cortex is greater, and that in the nuclear thickness change is greatest" (my emphasis).
Otsuka (1970) commented on the difficulty of studying the posterior surface of the lens: "...the exact radius of the posterior lens surface is impossible to determine because of the lack of knowledge regarding the internal change of the lens substances."
He also suggested the possibility of a thin, yet high power, lens: "the thicker the lens became during accommodation, the thinner the lens became annually." This is intriguing, but unfortunately he did not elaborate.
The hypothesis of vitreous pressure suggests that such prolonged pressure might produce a lens with a flattened periphery but with a high degree of curvature in the axial region, i.e. a lens that is thin yet accommodated.
It may be that a single factor, external
pressure on the globe, produces two separate effects, in opposite
directions: anteriorly it accommodates the lens, and posteriorly it
elongates the globe. One consequence of this dual effect could be that
axial elongation has masked the role of the lens.
The
experiment described here was highly artificial, but it may indicate
that the vitreous plays a part in normal accommodation, i.e. that the
experiment mimicked what happens in normal accommodation but to a
greater degree. It is possible that in normal accommodation,
contraction of the ciliary muscle pulls the vitreous forward against
the lens, again, not a new idea; this was suggested by Cramer in 1851,
and later by Tscherning. In the present experiment, however,
indications are that the vitreous was pushed forward by external
pressure on the globe ( see reference to von Pflugk above).
If significant lens changes do occur with long-term accommodation that produces two focal points, this might go undetected in routine visual examinations. In a myope with a moderate degree of myopia, the posterior surface of the lens might be flattened just enough to have created a second focal point, but one that does not actually reach the retina. In a routine eye examination, the ophthalmologist or optometrist would never discover this for two reasons: He will probably not look for something whose existence he is unaware of; and because the second focal point would not reach all the way to the retina, no clear secondary image is formed. Only the appropriate lens power would move the secondary focal point to the retina and thus form a clear secondary image.
In order to simplify this discussion, it has been limited to the primary and secondary focal points and their images, i.e. diplopia. It would be more accurate to say polyopia, because there are other focal points situated between the primary and secondary focal points. Testing a number of myopes with different lens powers revealed that there are often other images, fainter "ghost" images that are more difficult to detect. The origin of these could be the various isoindicial surfaces within the lens. There is evidence for this in the fact that many myopes, especially high myopes, when viewing a scene such as a full moon, without corrective lenses, see, instead of a uniform diffuse blur, multiple moon images.
Although I didn't appreciate it at the time, it was fortunate that the first subject for the experiment was myself. If I had found a willing emmetrope, or a subject with only a small degree of myopia, the outcome would probably been very different. The experiment would probably produced a small degree of myopia, partly from axial elongation and partly from lens changes (just as I believe occurs in normal myopia). The significant point, however, is that I would never have suspected the lens, but would have attributed the increase in myopia to axial elongation alone. Because I was a myope of fairly high degree, flattening of the periphery of the lens was probably quite advanced, so that the secondary focal point was already located very close to the retina. It then required very little additional flattening to push it all the way, or very close to, the retina, at which time I became aware of the secondary image.
Could the almost universally accepted view that lens is unimportant in myopia be wrong? It is difficult to even conceive of this. Nevertheless, it is possible, however remote. Textbooks on ophthalmology give the impression of a solid edifice of knowledge built on firm foundations. Yet at least one researcher, Ludlam (1967), suggests that some of the most basic facts about the eye are based on faulty data and should be re-evaluated.
These include invalid mathematical assumptions, mixed sampling, inadequate experimental technique, and oversimplified models of the refractive system, some of these dating from the nineteenth century:
"Nevertheless, the analyses and conclusions drawn from such studies can be no better than either the methods of acquisition of the basic data or the validity of the assumptions underlying the mathematical formulation of the ocular model. "It is well to note that in all of these studies the model of the ocular system utilized has consisted of:
1. Spherical refracting surfaces (my emphasis) causing a systematic under-estimation of the paraxial refracting power of each surface. -----
4.
Schematic refractive indices, invariant in the population--which
assumes that all variability in the ocular refracting system derives
only from differences in curvature and spacing of the ocular elements,
thus again underestimating both the number of variables in a given eye
and the true variability for each component actually existing in the
population
5. A homogeneous monoindicial lens (my emphasis). This places a high order of importance on the accuracy and precision of the measures of curvature of both the anterior and posterior surfaces of the lens and concomitantly increases the potential effects of spherical assumption. " In addition, in none of these studies have all the refractive components of any given eye been measured. There has always been at least one component whose value was calculated from the other measured elements, so that the measurement errors would all tend to accumulate in the non-measured element. Since the measurement errors have not always been stated with sufficient clarity to enable the effects of these errors to be assessed, the probability exists that measurement errors have contributed substantially to spurious correlations of measured and calculated elements, as for example between the lens and axial length."
The lens/vitreous hypothesis provides a possible explanation for the failure, or at least disappointing results, of therapeutic measures aimed at preventing or slowing the progress of myopia, such as cycloplegics, and progressive addition spectaclelenses. In both cases, they undoubtedly reduce ciliary muscle contraction, but for only a few hours at a time, while the lens may require far more time for a significant reduction in the degree of accommodation. More importantly, in both these regimens the subjects are permitted to continue doing nearwork, so that even if complete relaxation of the ciliary muscle were achieved, accommodation could still have been maintained, at least partly: reading, with continuous back-and-forth scanning movements with depressed gaze, requires a constant contraction/relaxation of the superior obliques, and this could compress the globe sufficiently to force the vitreous against the lens.
The use of base-in prisms to prevent convergence and, consequently, accommodation has, may have failed because of the optical distortion inherent in such prisms.
I used base-in prisms extensively in various experiments aimed at reducing accommodation and was surprised to find that in some cases the degree of myopia increased. Strong base-in prisms produce considerable distortion, and a possible explanation is that in trying to fuse the distorted images, the eyes were forced to cyclorotate in antagonism to each other, and this in turn required the oblique muscles to maintain contraction as long as the image was fused, thereby exerting pressure on the globe and maintaining accommodation.
1. An experiment in long-term compression of the globe of the eye produced a large increase in myopia, and in monocular diplopia; the two separate images were superimposed, one on the other, one being highly blurred and one sharp.
2. It is hypothesized that the cause of this effect was spherical aberration of the crystalline lens resulting from pressure of the superior oblique muscles, transmitted through the sclera, which forced the vitreous forward, pressing it against the posterior surface of the lens.
3. This suggests a possible role of the vitreous in normal accommodation, that ciliary contraction pulls the vitreous forward to mold the lens. Alternatively, the vitreous may play a role only when accommodation is combined with convergence.
4. The extremely slow recovery from the spherical aberration/accommodation strongly implicates the lens in the etiology of myopia. Because of the slowness of recovery from accommodation, long periods of accommodation with insufficient intervals of rest could result in a lens that becomes permanently accommodated.
5. The argument that myopic lenses are not accommodated because they tend to be thin could be explained by long-term compression. This could produce an accommodated lens with either a flattened periphery and convex axial region, or a thin lens with accommodative changes in the nucleus.
If the hypothesis of the oblique muscle/vitreous/lens connection is confirmed, it could open the way to prevent or slow the progression of myopia by preventing accommodation and. convergence while performing nearwork. A more radical approach would be the use of invasive techniques to reshape the curvature of the lens.
Araki, M. Changes of the ciliary region and the ciliary zonule in accommodation. (1965) Jpn. J. Ophthalmol. 9: 50-58.
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