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2.1 Is Myopia Inherited?
In the past, the official medical view considered myopia to be simply inherited.
No question, heredity plays a key role66, 67 everybody is determined by heredity and environment.
In the same way, however, as there are talents of various levels, which can be compensated for to a
great extent by personal energy, hard work, and methodology, all the possibilities should be tried
to influence the "chemical factory" of the body by optimizing external parameters, e.g. by appropriate
nutrition and adequate behavior.
Fact is,
Myopia is dramatically increasing compared to the past1, 7, 68, 69, and many children are becoming
progressively myopic with none of the parents or grandparents being myopic,
and even populations like Eskimos, where myopia was extremely rare before, are getting
highly myopic.
Myopia can be easily created artificially in experiments with animals70.
Myopia occurs in different frequencies in different regions of a country71.
Myopic parents have more often myopic children72: if both parents are myopic, the risk
for the children to become myopic is sevenfold.
Two arguments about this controversy:
Goldschmidt states73, " Twin studies indicate a strong genetic influence and a weak environmental
impact, while extreme myopia prevalences among selected population groups
(university students) point to the opposite."
Note:
In case myopia is caused primarily by personal habits (which includes e.g. reading), a reported
genetic influence might be more related to identical habits of twins than to a biochemical
cause.
Morgan states74, "... evidence for low prevalences of myopia in Indians growing up in India,
while Indians in Singapore have much higher prevalence rates."
The conclusion is that myopia itself is not inherited. Rather, biochemistry is, and it reacts differently
to various environments.
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As Lyhne et al.75, who did research with twins expressed very well:
"... results indicate a high heritability for ocular refraction and its determiners and thus suggest
that environmental impact on refraction is not significant. However, the epidemiological association
between educational length (near work) and myopia, the evidence on increasing myopia
prevalence within a few generations, and the theory of gene-environment interaction imply
that some individuals might be genetically liable to develop myopia if exposed to certain environmental
factors".
In other words, it is often not the myopia, which is inherited, but the susceptibility to specific
environmental conditions like excessive near work.
Note:
The fact that some people do not develop myopia in spite of very adverse conditions is a clear
indication that heredity is involved in any case, the question is just to what extent.
Morgan has a different view76:
"Overall, while there may be a small genetic contribution to school myopia, detectable under
conditions of low environmental variation, environmental change appears to be the major
factor increasing the prevalence of myopia around the world. There is, moreover, little evidence
to support the idea that individuals or populations differ in their susceptibility to environmental
risk factors."
Note:
Morgan refers explicitly to school myopia and not to the higher grades of myopia and progressive
myopia. |
A reader of this Myopia Manual expressed another likely possibility with respect to the inheriting
of myopia: "The parents probably become myopic because they live in similar conditions as their
offspring."77
Another point of view stated by Morgan74: "Thus, parents with longer than average eyes would
tend to have children with longer than average eyes ... however, if average eye size is increasing
due to environmental effects, a high proportion of children would become myopic."
In principle almost all the facts about myopia that will be presented in the next sections are either
connected with the way of living of people, or with biochemical processes. Largely, however not
completely, these biochemical processes are determined by heredity.
A few authors have reported a connection of few specific chromosomes with myopia78, but contradiction
with other results followed immediately79, 80. One expert states: Future genetic therapies
of myopia are highly unlikely, because it appears that numerous genes are having an impact on
myopia72.
On the other hand, many papers show the fact that myopia often runs "in the family"81, 82, and that
myopia is very clearly more common in some populations than in others (see section 3.1).
By the way: Influences like nutrition during pregnancy have a high congenital impact on the
child's health. In addition, the "individual biochemical factory" is built very early in life and cannot
be changed later. An example is the copper metabolism - which might have an impact on
myopia83, 84.
Basically any biochemical heritage can become effective via one of these two alternatives:
Via a biochemical process which cannot be modified, and whose results cannot be modified
either.
Via a biochemical process which can be modified, or whose results can be modified. An
example: People suffering from favism, an enzyme defect (a variant of G6PD deficiency)
which leads to acute anemia after eating e.g. fava beans. Simple advice after somebody is
diagnosed with this deficiency: Do not eat these beans. The literature gives further examples
of "Nutritional Regulation of Gene Expression85".
It is my aim to offer assistance in the second case and to motivate people not to become fatalistic.
Further, biochemistry is not in itself good or bad: An effect that has a disadvantage in one respect,
can have an advantage in another respect (e.g. the G6PD deficiency mentioned above is said to offer
protection against Malaria).
A simple analogy: If the engine of a car is prone to overheating through a design fault, the damage
can be avoided in most cases by carefully watching the cooling water level, selecting top grade
motor oil, and avoiding mechanical overload. Why not apply the same strategy to the human
body, instead of hiding behind the word heredity?
Some facts, which can contribute to the answering of the question whether myopia is inherited
can be found in section 3.1 ("Distribution of Myopia by Region, Age, Gender and Ethnicity").
As some people do not become myopic even under very adverse conditions, it could always be
claimed that myopia is caused by genetic heritage, even if it was demonstrated that without
these adverse condition myopia did not occur.
The reason for this conflict is that the three potential causes for myopia, are in one way or another influenced by the genetic heritage of the individual person.
See also section 4.1.7. These 3 potential causes of myopia are:
connective tissue disorders (see section 2.2)
active growth by imaging effects (see section 2.3)
mechanical effects (see section 2.4)
2.2 Connective Tissue Disorders
In the case of connective tissue disorders, the structure of components of the myopic eye shows
significant deficits. These structural deficits can have various origins:
Clearly inherited (like in case of the Marfan syndrome).
Appearing as a form of genetic variation, which happened after birth.
An acquired disorder, e.g. caused by interactions of the immune system, or by inappropriate
nutrition, or by other environmental or behavioral circumstances.
An example for the impact of the connective tissue on the highly myopic eye by Chang et al.86:
"... that the fiber bundles of highly myopic eye were of thinner lamellar arrangement, with looser
collagen matrix distribution and decreased collagen fiber diameter."
Computer tomography measurements showed that the more severe the myopia is the more reduced
is the density of the sclera87.
Section 3.3.2 describes some results with respect to the connective tissue.
2.3 Active Growth by Imaging Effects
After birth, the eye is too short for an optically correct focus. During growth, the eye expands by
some feedback mechanisms to the proper length. This process is called emmetropization88. Animal
experiments have investigated this process in detail, but there are still a lot of open questions
about what goes wrong when some people become myopic, when others in the same environment
don't. This active growth mechanism will be described in more detail in section 3.3.
2.4 Mechanical Effects
There is quite general agreement that myopia has some connection with accommodation. One
explanations is the influence of the strain of the various muscles which are involved when doing
near work not just only the ciliary muscle for accommodation, but also various extraocular
muscles which are controlling the position of the axis of the eye (see sections 1.3.3 and 3.6).
In this context, there is the interesting result that the thickness of the ciliary body increases with
increasing myopia and axial length89.
2.5 General Overview of the Causes of Myopia
A very schematic overview of
possible paths to myopia is
shown in Figure 6. Each of
these possibilities could be
greatly expanded.-->
Figure 6 The potential paths to myopia
From the fact that under the same external conditions (i.e. near work, illumination etc.) not everybody
gets myopic it can be concluded that the influence of the personal biochemistry / personal
anatomy plays a deciding role.
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