Myopia > Causes of myopia

There are many old and new theories which try to explain the causes of shortsightedness (myopia). In spite of significant scientific effort even now we are not hundred percent sure what is the exact cause behind it. The emphasis is put on various factors such as disproportions in the development and growth of the eye, inferiority of the optic tissue (mainly the sclera), infectious diseases, improper nourishment, excessive eye strain, insufficient illumination but mainly heredity. However, this definitely is not a complete list of possible causes of myopia.

Most of these theories have lack of evidence on which they are based. Probably there is not one common denominator which would be responsible for the development and formation of myopia. Myopia is caused by many various factors which play an important role in this process. It is namely heredity, changes in the development and growth of the eye and external factors. Shortsightedness can be regarded as a phenomenon which depends on various factors and which can occur independently or as a part of other eye changes. Mainly this is the reason why we cannot treat myopia (even though it is actually not a disease) or influence it in some other way. We are only capable of correcting it.

Heritability of myopia

Heritability of myopia has been well-known for centuries but its scientific research has begun only after rediscovery of Mendel´s laws in the beginning of 20th century. Overall refraction of the eye is always the result of mutual combination of following optical factors: curvature of the cornea, corneal thickness, depth of the anterior chamber, curvature of the anterior and posterior lens surface and sagittal length of the eyeball.

Individual factors, which are mentioned above, are independently heritable from various genes and can combine in various ways. This type of heredity, caused by various genes, occurs in the cases of shortsightedness up to -6 diopters. In the cases of myopia higher than - 6 diopters the cause lies only in one pathological gene.

Based on heritability and clinical picture it is possible to divide myopia into three groups:

• low degree myopia (without changes on the eye background),
• high degree myopia (with changes on the eye background),
• congenital myopia

Low degree shortsightedness is an example of heredity of various genes with little effect. Apart from hereditary impacts there are intelligence, ethnic and geographical factors which play an important role and this has been proved by higher occurrence of myopia in intellectuals in certain countries (e.g. Japan). Hereditary risk of transfer of myopia to children equals to the occurrence of shortsightedness in whole population what in general represents 15-30%. This means that here in every healthy family where both parents are not shortsighted there is 15-30% chance that their child will later during puberty suffer from low myopia. In families where one parent suffers from shortsightedness, the risk increases up to 50%. Low degree myopia does not pose a problem for the sight and can reach various levels of diopters which do not need to be the same on both eyes.

High myopia is a serious clinical and genetic problem because its impact on the eye background can lead even to blindness. In our population it occurs in 3 percentages of people. High shortsightedness occurs sporadically approximately in 80% of the cases from this number - both parents have healthy genes and pathological gene of a child was formed spontaneously under certain circumstances which are not known. Parents are transmitters of a pathological gene only in 20% of the cases of high myopia.

Because of the fact that high myopia is a serious threat to the sight, this disorder has exclusively genetic foundation and cannot be influenced neither by certain regimen nor hygienic regime, it is essential to consult it with the department of clinical genetics. This will specify the risk of transmission of shortsightedness to children. Scleroplastic surgeries (see: Surgical treatment of short-sightedness) offer limited possibilities of treatment and can only sporadically stop the progression of shortsightedness.

Congenital myopia is usually nonhereditary and is connected with premature birth. In most cases it is formed during the intrauterine development of an embryo from non-genetic, external causes.

There are many general diseases which affect a human body where short-sightedness can appear as well. Its heritability always depends on particular disease.

Heritability is a crucial factor as far as the causes of short-sightedness are concerned and therefore it is necessary, from the point of diagnostics, treatment and prevention, to go through genetic consultation, even more when it comes to progressive myopia.