Another treatable eye disease is glaucoma, which is characterized by the following triad: the IOP increases ( >21 mmHg); this leads to progressive excavation of the optic disc, the site where the optic nerve leaves the eye; and cupping of the disc. This leads to ischemic damage to the optic nerve fibers, which may result in blindness. Two types of glaucoma are known, chronic open-angle glaucoma (COAG), which is the frequently diagnosed type, and the closed-angle type, which is rare (7-10). The incidence of COAG in the population increases rapidly with age, from a low of 0.2% in the fifth decade to about 1% in the seventh decade, 3% in the eighth decade, and 10% in the ninth decade. OAG is characterized by an insidious and slow onset, initially asymptomatic and then gradually leading to blindness if unchecked. Closed-angle glaucoma is rare and is characterized by an acute attack of severe eye pain and marked loss in vision due to a rapid increase in IOP compressing the entire retina.
Systemic blood pressure and hypertension correlate with IOP and high-tension glaucoma, but no link appears to exist between blood pressure and normal-tension glaucoma (NTG) (84). Prospects of genetic intervention in primary open-angle glaucoma (POAG) have been investigated, and five primary open-angle genes have been mapped. Understanding these genes and their products may help in finding better treatments
(92). Collagen degradation appears to play a role in glaucoma
(93). Imbalance in rates of extracellular matrix production and turnover may be important in OAG, and matrix metalloprotei-nases are likely to be used in its treatment (93).
According to Bergen et al. (94), POAG is a group of multifactorial diseases that affects 1.5% of the population. Important risk factors for POAG are older age, elevated IOP, the presence of POAG in relatives, and still largely unknown molecular genetic factors. The three genes known to be involved in POAG are MYOC, CYP1B1, and OPTN, and they account for up to 18% of the POAG cases.
In a report based on the data of the "Rotterdam study," de Voogd et al. (95) reveal that diabetes mellitus in people 55 years or older is not a risk factor of OAG. Also neither atherosclerosis nor serum C-reactive protein level, which is an index of heart disease, was an important risk factor for OAG (96). A recent study by Friedman et al. (97) emphasized the higher incidence of OAG in older people and also finds that the incidence of this eye disorder is higher among older blacks than age-matched whites. Varma et al. (98) found that the prevalence of OAG and ocular hypertension is high among Latinos of Mexican ancestry, with no gender-related differences.
In relation to genes as risk factors, Tang et al. (99) has reported that the optineurin (OPTN) gene has been identified as a causative factor for NTG. Alterations in this gene were found in Caucasian families with NTG. While no glaucoma-specific mutations were found in the OPTN gene in Japanese glaucoma patients, some novel single-nucleotide polymorphisms (SNPs) in the exons and introns were reported (99). Fan et al. (100) identified disease-causing mutations in MYOC and OPTN in 1.75% and 1% of POAG Chinese patients, respectively. Common polymorphisms in MYOC, OPTN, and APOE might interactively contribute to POAG, indicating a polygenic etiology. Similarly, Sripriya et al. (101) report that SNPs rather than mutations in OPTN may play a role in POAG pathology in the Indian population.
Whereas cataracts are usually treated surgically, glaucoma is often treated medically with eye drops containing various drugs. However, for patients who do not respond to anti-glaucoma medications, surgery and, more commonly today, laser therapy are used to lower IOP (Box 2).
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