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1.7 Consequences and Risks of Higher Myopia

Some numbers from the statistics about the consequences of higher myopia⁶:

• England, 1966: Myopia was responsible for 8.8% of blind registrations.
• England, 1972, age between 50 and 59: Myopia was responsible for 18.2% of blind registrations, only behind diabetic retinopathy.
• Bavaria/Germany, 1992, up to age 18: Myopia was responsible for 11% of blind registrations ⁴⁰.
• Myopic macular degeneration is the seventh greatest cause of registered blindness in adults in Europe and in the United States, but has become the leading cause of blindness in Taiwan⁴¹.
• Myopia accounted for 5% of the causes for blindness of people aged 20 to 59 years in Denmark⁴².
• 2% of Americans have pathologic myopia⁴³.

As the rate of myopic people is still increasing significantly today, the problem of resulting blindness can also be expected to rise further.

A basic effect of high myopia is that the posterior sclera shows substantial thinning by the elongation of the eye. The normal sclera has a thickness of about 1.35 mm on the back of the eye. A highly myopic eye has a typically reduced thickness of the sclera of about 0.2 to 0.5 mm,⁷ and a thinned choroid as well. It is, however, still an open question, whether the thinning of the sclera is due to:

• An optically regulated mechanism,

• An excessive mechanical stretching force,

• A defective connective tissue of the sclera.

Section 3 contains more information about these issues. Basic reasons of most of the serious consequences of myopia are vitreochorioretinal dystrophies, i.e. disturbed structures of the layers of vitreous, choroid, and retina. It was found that 52.6% of people with weak myopia and 86.4% of people with high myopia had this disorder⁴⁴.

Some basic pathological consequences of high myopia can be^{6, 45}:

• Retinal detachment:

There is an elevated risk for retinal detachment, i.e. the retina is separated from the choroid and the sclera, often accompanied with tearing of the retina. Retinal lattice degeneration and retinal breaks are often early signs of later retinal detachment⁴⁶. Some publications, however, are contradictory with respect to an increasing risk with the degree of myopia. Some people are saying that there is an increased risk for myopes, which is, however, not dependant on the degree of myopia⁹. Other sources state a risk for retinal detachment e.g. for 0 D to – 4.75 D a risk of 1/6662, for – 5.00 D to – 9.75 D a risk of 1/1,335, and for more than – 10.00 D a risk of 1/1486. In other words, an additional risk factor of 3 for low myopia, and up to 300 for high myopia⁴⁷ was reported.

• Vitreous liquefaction and detachment:

The vitreous body between lens and retina consists of 98% water and 2% fibers of collagen. It gradually becomes liquefied with age, and especially in myopic eyes^{46, 48}. This is due to a loss in the regular arrangement of the fibers. In early stages, small objects can be seen when looking at bright and uniform backgrounds (called fleeting flies, or floaters): "Most of the time they are nothing to worry about, but sometimes they can be a symptom of a retinal tear."⁴⁹. In later stages, the vitreous body can collapse and lose its connection to the retina. This separation is connected with the risk of retinal detachment and corresponding damage of the retina. Immediate medical examination is necessary if symptoms like flashing lights or a rain of soot can be seen. No treatment is available for vitreous detachment by itself. About 6% of "normal" people between age 54 to 65 and 65% of the people between age 65 to 85 have a vitreous detachment. The higher rate of vitreous detachment of myopic people is sometimes explained by the larger volume, which has to be filled by the vitreous body.

It was concluded that the liquefaction is caused by the functional disorder of the bloodretinal barrier in myopia⁵⁰.

• Various Myopic maculopathies:

There can be a thinning of the choroid and the retina and a loss of capillary vessels in eyes with high myopia² and as a consequence an atrophic loss of retinal cells (i.e. cells are dying), resulting in a loss of vision in this area¹³ (visual field defect). In pathological myopia the death of retina cells (apoptosis) can occur due to various biochemical processes, e.g. related to oxidative events (see section 3.12).

Furthermore, there can be bleedings in the retina and the choroid, leading to a partial loss of vision¹³.

Choroidal neovascularization (CNV) / myopic macular degeneration is also a consequence of "normal" macular degeneration, and it is caused by abnormal blood vessels that grow under the center of the retina. It generally occurs among people over 30 and can result in a progressive loss of vision. The worldwide incidence of CNV due to pathologic myopia is estimated to be 50,000 new cases per year excluding Asia, where the rate may be even greater due to a higher prevalence of pathologic myopia⁵¹ (see section 3.17 about a treatment for CNV).

Myopic macular degeneration is said to be the seventh greatest cause of blindness of adults in Europe and the USA, and has become the leading cause of blindness in Taiwan⁴¹.

• Posterior Staphyloma:

In pathologic myopia there can be a herniation-like deformation ("out-pouching") of a thinned sclera, which can hardly be corrected with lenses. It also leads to other complications.

• Glaucoma:

The increased pressure within the eye that often accompanies myopia (see section 3.6.2), can damage the optic nerve. The results of older techniques for measuring the intraocular pressure of myopes were wrong: even when the pressure was high, the softer myopic tissue was interfering with the measuring process in a way that the result was a normal, i.e. lower pressure than in reality. Later a measurement called "applanation tonometry" was invented. This technique is claimed to be independent from the rigidity of the sclera. It is said that open-angle glaucoma occurs twice as often with the myopic eye as with the normal eye⁶.

• Cataract:

The lens is loses its transparency. It is reported that myopia induces an earlier onset of cataracts. Statistical data are lacking⁴⁷.

• Chui TY et al. reported52: "… data indicated that approximately 15 D of refractive error doubles the spacing between retinal neurons, thereby halving peripheral resolution acuity relative to the emmetropic eye."

Soft contact lenses, and the complications of refractive surgery, can lead to infections, which may cause blindness⁴⁷.

Each myopic person is strongly advised to see an ophthalmologist at the slightest sign of visual abnormities, and people with high grade of myopia should have the background of their eyes checked regularly!

On the other hand, even if there is a high degree of myopia at a young age already, there is still hope, as Goldschmidt and Fledelius found⁵³ after the observation of the development of the myopia of 14 year olds with at least – 6 D over 40 years "...the adult visual prognosis for working age appeared better than usually claimed. There seems to be a correlation between degree of myopia at age 14 and consecutive visual loss, but it was not possible to identify subjects at high risk at that early age."

Notes:
- In many cases, a general systemic problem may cause one of these diseases, and also myopia. This then explains the noted association, without implying that myopia is the cause of the disease. In these cases the therapy should focus more on the systemic problem, and not primarily on the optical myopia problem only (which should be positively influenced by the therapy as well).
- As progressive, pathological myopia is based on defects of the connective tissue, the connective tissue related coronary problem mitral valve prolapse (MVP) might have an increased probability, which makes preemptive measures still more appropriate, as stated by Yeo et al.⁵⁴: "Although most patients with MVP are asymptomatic or have minor symptoms, it is associated with significant morbidity."

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