Myopia is a condition whereby images come into focus in front of the eye, resulting in a blurred image on the retina. The more severe the nearsightedness, the farther the image is from the retina, which results in more blurry vision in the distance.
Myopia causes light rays to focus on the front of the retina. As a result, close objects are seen clearly, while distant objects appear blurred. Near vision, however, can deteriorate to a level where reading even close to the face can become difficult.
There are three ways for an eye to become myopic:
The front surface of the eye (the cornea) is too curved and, therefore, too powerful.
The eyeball itself is too long.
A combination of both of the above.
In many cases, myopia will stabilize when the growth process has been completed, and glasses can offer normal vision. Higher levels of myopia, however, tend to be hereditary, meaning that if there is a moderately or highly nearsighted parent, the odds are higher for one of the children to be myopic.
Myopia that develops in childhood is often called juvenile onset myopia, which almost always increases in severity with the progression into adulthood.
In the more severe chronic cases ("degenerative" or "pathological" myopia), there is the possibility of sight loss. The deformation of the eye creates stress on the retina, which can become damaged or detached, and this can then provoke additional changes. This is especially true in degenerative myopia, which can lead to macula problems (not to be confused with age-related macular degeneration).
There are at least four other clinical types of myopia: simple, nocturnal, pseudo, and induced. For the purposes of this article, degenerative myopia is described here.
Degenerative myopia is the seventh leading cause of legal blindness, occurring in about 2% of the U.S. population. It is most often seen in people of Chinese, Japanese, Middle Eastern and Jewish descent. This condition can start at birth, but most often starts during the pre-teen years. It is believed to be hereditary.
Degenerative myopia is more severe than other forms of myopia and is associated with retina changes, potentially causing severe vision loss. It progresses rapidly, and visual outcome depends largely on the extent of fundus and lenticular changes. The diagnosis of degenerative myopia is accompanied by characteristic chorioretinal degenerations. Pathologic myopes, particularly those with higher refractive errors, are at risk for retinal detachment and macular changes.
Patients with degenerative myopia typically complain of decreased vision, headaches, and sensitivity to light. If retinal degeneration or detachment is present, patients may also report light flashes and floaters, which are associated with retina changes. Those with degenerative myopia have an increased incidence of cataract formation (nuclear cataracts are most typical).
Some of the most typical features of degenerative myopia are:
Vitreous liquefaction and posterior vitreous detachment
Peripapillary atrophy appearing as temporal choroidal or scleral crescents or rings around the optic disc
Lattice degeneration in the peripheral retina
Tilting or malinsertion of the optic disc, usually associated with myopic conus
Thinning of the retinal pigment epithelium with resulting atrophic appearance of the fundus
Ectasia of the sclera posteriorly (posterior staphyloma)
Breaks in Bruch's membrane and choriocapillaris, resulting in lines across the fundus called "lacquer cracks"
Fuch's spot in the macular area.
Myopic degeneration is similar to age-related macular degeneration AMD) in that it causes loss of central vision due to degeneration of the photoreceptor cells. This is caused by separation of the retina as a result of abnormal elongation of the eyeball. This usually happens because the back of the eye is larger than normal when the eye is very nearsighted. Marked thinning and stretching may lead to break down of the macula, surrounding retina and it's underlying tissue. This will cause a varying amount of blurred vision.
Even after the eyeball has fully grown (by adulthood), weakness in the sclera (the white outer shell) can lead to development of a posterior staphyloma. This is a distention of the sclera at the back of the globe where the optic disc and macula are located. Breaks in Bruch's membrane and atrophy of the choroid layer of the retina (where the blood vessels are) can create lesions known as lacquer cracks. Blood vessels may protrude through the cracks and leak into the subretinal space beneath the photoreceptor cells. Known as "choroidal neovascularization," this hemorrhaging can lead to scarring, retinal separation, and profound sight loss in the central field. If this occurs, immediate treatment is needed, which may be in the form of photodynamic therapy, antiangiogenic drug treatment or both.
Treatment for Prevention of Degenerative Myopia
Two treatments are showing promise. One is scleral buckling, which can be read about in the article, "Degenerative Myopia: a Review of its Nature and its Treatment", on this site. The other is early systemic treatment with 7-methylxanthine, which has been shown to normalize the abnormal growth pattern of myopic eyes in children aged 8-13.
In the abstract, "Effect of 7-methylxanthine on eye growth in myopic children" (Ophthalmic and Physiological Optics, Vol. 26, Suppl. 1, August 2006), researchers Klaus Trier and Soren Munk Ribel-Madsen reported that "axial growth in the 7-methylxanthine group was reduced by 22% in the low axial growth stratification layer and by 8% in the high axial growth layer compared with placebo. The myopia progression in the two layers was reduced by 21% and 12%." This study was based upon the success of earlier research, in which 7-methylxanthine increased the content of collagen and proteoglycans (connective tissue components of the sclera) as well as the diameter of collagen fibrils in rabbits. Excessive eye elongation is related to abnormal organization and reduced content of these components.
Stem Cell Treatment
Advanced Cell Technology, Inc. announced on February 11, 2013 that they had gained approval from the FDA to begin safety trials to evaluate the safety and tolerability of embryonic stem cell replacement in people with severe myopia. The company's press release revealed that "the primary focus of the study will be to evaluate the safety in patients with severe myopia of the type that causes fissures in the RPE layer of the eye." This refers specifically to degenerative myopia (aka "myopic macular degeneration"), offering hope for people who have lost vision to this condition.
The trial will enroll a total of 12 patients, with cohorts of three patients in an ascending dosage format. The trial is a prospective, open-label study designed to determine the safety and tolerability of hESC-derived RPE cells following sub-retinal transplantation into patients with myopia at 12 months, the study’s primary endpoint. For more information about stem cell therapy, select this link.
Principal source: www.revoptom.com
For further information, see Eye Diseases and Glossary