Download English (4) Origin of Ametropia

THE ETIOLOGY OF AMETROPIA: WHAT IS EMMETROPIA?*
(Spanish: EL ORIGEN DE LAS AMETROPÍAS: ¿QUÉ ES
EMETROPÍA?)
MEDINA A
ARCH. SOC. ESP. OFTALMOL., 1980; 40
This English translation is reviewed and supported by Otis Brown. Otis clarification’s [in parenthesis.]
SUMMARY
The mystery of emmetropization, that strange force squeezing the frequency distribution of the
total refractive errors into its sharply peaked shape has been, to-date, unsolved. Let alone the cause of
ametropia.
After analyzing refraction with age in any subject it is clear that a servo mechanism exists and
furthermore it can be measured. This servo can explain theoretically all kind of ametropias, the effect of
glasses and the eye refraction of any subject from intrauterine life.
Experiments already conducted support the theory, and the results show that they are in perfect
agreement. From a practical point of view, both theoretical and experimental considerations would allow
us to treat ametropic patients in order to reduce their dioptric value.
INTRODUCTION
The mystery of emmetropization, that strange force squeezing the frequency distribution of the total
refractive errors into its sharply peaked shape as age progresses has been, to-date, unsolved.
Some theories attempting to explain emmetropization are known. Some of them are unsupported
while others only demonstrate the existence of emmetropization (1).
Analyzing the development of refraction with age in any subject, it is clearly observed the response of
a servo-mechanism to a command signal. Based on such observation we developed the diagram of the
servo which explains all kind of ametropias and ocular refraction of any individual from intrauterine life.
Some important work already conducted can be considered as support of the validity of this new theory
because they are in perfect agreement with its predictions.
DESCRIPTION OF THE THEORY
Development of ocular refraction with age
Fig. 1 depicts the curve refraction versus age representative of an emmetropic individual. This curve
resembles the response of a servo system to a step input, where the step is 0 diopters or emmetropia.
Figs. 2 and 3 are representatives of the development of refraction in myopic and hyperopic
individuals. We can see that the only difference from Fig. 1 is a shift keeping the horizontal shape. The
curve that in Fig. 1 tends to 0 diopters tends in Figs. 2 and 3 to other values, plus or minus. In terms of
control engineering this would simply correspond to a change in the command signal (step level).
From these observations we can conclude that there is a mechanism for emmetropization that is
nothing else but a feedback control -- following a command signal, possibly determined hereditarily.
This signal is not necessarily “zero diopters” in all cases, for example, genetic programming may
have indicated a negative command signal (myopia).
This is the reasoning could be thought of to imply that the cause of myopia is genetic.
However a feedback system is responsible for the inherited refraction to be revealed in the individual.
Therefore any interference with or failure of the feedback mechanism could lead to different refractive
states. Ametropia can then be caused by this contingency, as it is the case of school myopia which will be
discussed below.
*This work is a summary of author’s doctoral thesis proposed at The City University, department of Ophthalmic Optics.
Correspondence:
Islas Filipinas, 46
Madrid 3
The Servo-mechanism of Emmetropization
The feedback diagram is presented in Fig. 4.
The information that the feedback system has of the refractive state of the eye may be deduced from
the states of accommodation from the ciliary muscle. This is possible as shown in Fig. 5, which
represents the possible states of accommodation in a subject of any refraction.
In this figure, which is very simplified, it can be seen that the central or feedback system would be
capable of detecting ametropia in the interval +/- 5 diopters.
This is done in block “H” of the feedback loop in Fig. 4. The circle [block] that is marked as “-”
calculates the difference between this input signal from block H, which is between -5 and +5 D, and the
control signal. If the difference is zero the eye is emmetropic or has the genetically advised refraction, so
the servo takes no further action.
If the difference is non-zero, the error detector delivers a signal that, once processed and amplified by
block, “A” is an indicator that the eye should start a myopizing [negative-changing] or hyperopizing
[positive-changing] trend depending on the signal being + or – respectively and with the intensity
determined by the amplitude of the signal. The eye begins to change its parameters which determine
refraction. Those that are more easily changed are the curvature of the lens and length of the
anteroposterior [front-to-back] axis.
By varying any of these parameters and therefore the refraction as ordered by the control signal, the
block “H” in the feedback loop begins to deliver an error signal to the error detector closer to the input
value. Once achieved equality, the error signal is zero and the eye stops changing its parameters.
This paragraph explains, in a clear way, the behavior of the servo controlled eye. To work with it
quantitatively, to predict a patient's ametropia after a number of years for example, we need the servo in
detail, which is not described in these lines, and all the parameters of the differential equation of the
servo, which must be measured from the patient.
As an example let’s see the cause of school myopia in the light of the above lines. An individual with
no genetic predisposition for myopia has little chance of acquiring myopia. However, a combination of
factors, such as small genetic predisposition and continued use of accommodation, especially with
artificial light (remember that the interval of accommodation decreases with the level of illumination)
causes a progression of myopia to about 3 to 5 Diopters.
This is because the block “H” in the feedback loop of Fig. 4, which deduces the eye’s refraction from
the use of accommodation, takes it for hyperopic [positive] because of its frequent accommodation, there
is an error signal that directs myopizing [negative-changing].
The prescription of glasses in these cases not only does not benefit from a standpoint of reduction of
myopia, but aggravates it by the increase in the error signal. A similar, and with more accurate reasoning,
can be done with an improved, Fig. 5, tracing with accurate actual data where the simple diamond
straight lines are modified, under these conditions.
PREDICTIONS OF THE THEORY
Based on all the above, and in some cases following similar arguments, we can see that this theory
satisfactorily explains:
1. Ametropia development with age.
2. Why some people are ametropic and other are not.
3. Progressive myopia.
4. Why myopia tends to increase while hyperopia tends to decrease.
5. The reason for the difference in refraction in racial groups.
It also predicts:
6. The effect on refraction of wearing corrective spectacles or contact lenses.
7. Near work with low illumination increases myopia and reduces hyperopia.
8. Continuous use of cycloplegics increases hyperopia and reduces myopia.
This study offers non-invasive treatments to correct ametropia acting directly on its origin such as:
1. Continuous wear of opposite correction.
2. Continuous wear of an opposite correction on stenopeic contact lenses to improve visual acuity.
3. Use of drugs, such as cycloplegics In the case of myopes
.
It has to be pointed that many of the predictions listed here have been already observed by other
investigators [2-5].
REFERENCES
1. Arch Ophth. 63, 1806.
2. Prob. visuales de la infancia, Gil del Río.
3. Am J Opt. 46, 250.
4. Ar Soc Esp Of. 39, 6, 754.
5. Am J Opt. 41, 60.
LEGENDS
(added to original text):
Fig. 1. Refraction versus age for a typical emmetropic eye.
Fig. 2. Refraction versus age for a typical myopic eye.
Fig. 3. Refraction versus age for a typical hyperopic eye.
Fig. 4 Feedback loop that controls the long term refraction of the eye.
Fig. 5. Diagram showing the accommodation used (vertical axis) by an uncorrected adult of any degree
of ametropia (horizontal axis). Values are approximate averages and depend on the subject and age. It
shows that a myope of -5 D or more will essentially not use accommodation. A hyperope of +5 D or more
will use accommodation continuously and an emmetrope will accommodate between 0 and 4 D. The 4 D
accommodation interval is variable with age and it was arbitrarily chosen for illustration.