exponentially with age); the average estimated prevalence in those older than 70 years of age was 6% in white populations, 16% in black populations, and 3% in Asian populations. African or African origin populations had the highest POAG prevalence at all ages but the increase in prevalence of POAG is steeper for white populations; increases with age in black and Asian populations are similar (Rudnicka et al., 2006).

PACG is commonest in Asian ethnic groups, with the exception of the Japanese. Only 8% of all cases of glaucoma occur in this population (Yamamoto et al., 2005), whereas LTG is quite common in the Japanese population and accounts for 92% of all cases of POAG (Iwase et al., 2004).

Over 80% of those with PACG live in Asia, while POAG disproportionately affects those of African derivation. Women are more affected by glaucoma because of their greater prevalence of PACG, as well as their relatively greater longevity (Quigley and Broman, 2006).

In 2010, there will be 60.5 million people with glaucoma, and this rate is expected to increase to 79.6 million in 2020. Seventy-four percent of these affected individuals will have POAG. Fifty-five percent of POAG cases, 70% of PACG cases, and 59% of all glaucoma cases in 2010 will occur in women. Asians will represent 47% of those with glaucoma and 87% of those with PACG (Quigley and Broman, 2006).

The costs of treating glaucoma are considerable: in 1988 the United States healthcare system spends an estimated $2.5 billion a year for this disease ($1.9 billion in direct costs and $0.6 billion in indirect costs) (Glick et al., 1994). A retrospective analysis conducted in 1988 (Kobelt-Nguyen et al., 1998) estimated that the annual cost of treating newly diagnosed open-angle glaucoma was $1055. In Europe, the direct cost of treatment increased by an estimated 86 euros for each incremental step ranging from 455 euro per person year for stage 0 (HIOP only) to 969 euro per person year for stage 4 (severe glaucoma) disease; medication costs ranged from 42% to 56% of total direct cost for all stages of disease (Traverso et al., 2005). If we multiply the number of patients with glaucoma in Europe estimated by Quigley and Broman (2006) — 12 million — by a mean annual cost of 750 euros (for

9

moderate stage 2 glaucoma) we find that the total cost for 2010 is substantial: 9 billion euros.

Incidence of glaucoma

Incidence rates reflect the rate at which a disease develops in a population within a defined time period. Incidence rates for glaucoma reveal more about the natural history and current causes of this disease, and this information is essential if effective corrective measures are to be adopted. Very few incidence studies have been completed, because the cost of examining large samples is high.

The first study on the incidence of glaucoma was conducted in 1966 on inhabitants of Skowde, Sweden, who were 55–64 years old at baseline and had an IOPo21 mmHg. Over the next 5 years not one new case was observed (Linnir and Stromberg, 1966). However, this finding is not very reliable due to the low rate of participation in the followup study and the diagnostic criteria used. The first reliable data are those collected in Dalby, Sweden, by Bengtsson (1989). This study included an initial examination and two follow-up visits over the next 10 years, and the estimated incidence rate was 0.25% in the population aged 55–69 years. This was the first longitudinal study conducted in an acceptable manner with visual-field testing at baseline and at follow-up.

In 1983, given the unreliability of the available data on glaucoma incidence, Leske et al. (1981) and Podgor et al. (1983) decided to estimate the incidence of this disease based on the prevalence data collected in the Ferndale (Leske et al., 1981) and Framingham (Podgor et al., 1983) studies and on survival tables. They calculated 5-year POAG incidence rates of 0.20% at age 55 and 1.1% at age 75. These studies were later repeated in the United States (Quigley and Vitale, 1997) and in the United Kingdom (Minassian et al., 2000).

The medical charts of patients with glaucoma diagnosed between 1980 and 1982 and confirmed by Goldmann perimetry were analyzed to determine the incidence of glaucoma in the Swedish town of Halsingland (Lindblom and Thorburn, 1984). However, the data collected in this study

10

merely reflect the frequency with which glaucoma was diagnosed in the population by the healthcare system. The mean annual frequency of POAG diagnosis reported by these authors ranged from 0.02% in 51–55-year-olds to 0.44% in those aged 91–95 years. The medical histories of the residents of Olmsted County, Minnesota were also used to calculate the incidence of POAG in the United States (Schoff et al., 2001).

However, more reliable data were collected in two recent studies conducted on persons of African descent in Barbados (West Indies) and on white inhabitants of Rotterdam (Netherlands) (Leske et al., 2001; de Voogd et al., 2005). The 4-year incidence of POAG in Barbados was 2.2%. Incidence rates increased from 1.2% at ages 40–49 years to 4.2% at ages of 70 years or more, tending to be higher in men than women (2.7% vs. 1.9%). Of the 67 incident cases of POAG, 52% had higher pressures at baseline. Risk of incident glaucoma was highest among persons classified as having suspect POAG at baseline (26.1%), followed by those with ocular hypertension (4.9%) and lowest in the remaining population (0.8%) (Leske et al., 2001). The 5-year risk of definite POAG was 0.6% among the Rotterdam study survivors aged 55 years and over at base line; the rate rose significantly from 1% at age 60 years to approximately 3% at age 80 years. No difference in incidence of POAG between men and women was found. HIOP at baseline or use of IOP-lowering treatment, gave a three times higher risk for incident POAG. Normal fellow eyes of POAG eyes had a fivefold higher risk for developing POAG compared with fellow eyes of normal eyes (de Voogd et al., 2005). In both studies, most of the incident cases (half of those observed in Barbados, two thirds of those in Rotterdam) had not been previously diagnosed and were not receiving treatment.

Blinding effects of glaucoma

The percentage of cases associated with binocular blindness has been reported only in the more recent studies on the prevalence of glaucoma, and in many of these studies the criteria used to define

blindness were not specified; in others the definition of ‘‘legal blindness’’ was used (VAo20/200), which is less restrictive than that recommended by the WHO (VAo20/500).

According to a WHO report, in which correction factors were used to determine the prevalence of best-corrected VA in studies using non-WHO definitions, the estimated number of visually impaired people in 2002 was 161 million, and 37 million of these were blind (Resnikoff et al., 2004). With regard to correction factors to determine prevalence according to ICD-10 from different definitions of visual impairment, there were a sufficient number of studies reporting data with both definitions to enable a table of conversion to be calculated. Glaucoma accounts for 12.3% of the cases of best-corrected blindness in the world (4.5 million in the year 2002).

In contrast, in 2000, the number of people with primary glaucoma in the world is estimated to be nearly 66.8 million, with 6.7 million suffering from bilateral blindness (Quigley, 1996). A later study (Quigley and Broman, 2006) estimated that there will be 60.5 million people with primary glaucoma in 2010 and 79.6 million by 2020. Bilateral blindness will be present in 8.4 million people in 2010 and 11.2 million people in 2020. These authors concluded that ‘‘the two estimates differ because of methodological issues; blindness prevalence surveys often assign the most treatable disease as the primary cause of blindness. It is often assumed that cataract is more treatable than glaucoma. This leads to underestimation of glaucoma blindness.’’

It is possible that rates of blinding glaucoma are underestimated because this disease is not a major cause of central visual loss, which is the sole criterion used to define blindness in most of the studies. However, the more recent studies which included routine visual-field testing reveal rates of blinding glaucomao10% in many countries, including those that are developing (Table 4). No cases of bilateral blindness caused by glaucoma were found in India, China, and Brazil (Raychaudhuri et al., 2005; He et al., 2006; Sakata et al., 2007). The Los Angeles Latino Eye Study found a rate of 1% of legal blindness due to glaucoma (Varma et al., 2004). Other prevalence studies in India revealed rates ranging from 1.6% to

3.1% (Ramakrishnan et al., 2003; Vijaya et al., 2005, 2006; Vijaya et al., 2008a, b). A blinding rate of 3.7% was found in Thailand (Bourne et al., 2003). In the Salisbury Eye Evaluation Glaucoma Study (Friedman et al., 2006), the rate of blindness in black patients with glaucoma was 5.3% (vs. 1.3% in whites), but this difference was not statistically significant. Blinding rate of 6–7% were found in Tanzania and Singapore (Buhrmann et al., 2000; Foster et al., 2000). The highest blinding rate were found only in India, South Africa, Bangladesh, and Myanmar but visual impairment cases (VAo20/200, and/or VFo201 around central fixation) were included in Bangladesh, and Myanmar studies (Dandona et al., 2000a, b; Rotchford and Johnson, 2002; Rahman et al., 2004; Casson et al., 2007).

Finally, changes between 1988 and 2000 in the prevalence and main causes of blindness and low vision in Ponza, Italy was investigated (Cedrone et al., 2007). The prevalence of all types of glaucoma among subjects aged 40 years and over in Ponza was quite high (1988: 3.6%; 2000: 4.8%), and this may be a reflection of inbreeding in this island community, but the rate of bilaterally blinding glaucoma (visual fieldo101 around central fixation) dropped from 5.4% to 2.5%. It is interesting to note that the mean age of patients

with glaucomatous impairment of at least in one eye also increased considerably (1988: 68.2710.7; 2000: 75.576.3, p ¼ 0.030), whereas the mean age of the total glaucoma subgroup remained essentially stable (1988: 66.8, 2000: 68.2, pW0.05).

Acknowledgment

The authors thank Marian Everett Kent, who received payment for her assistance in writing and editing the manuscript.

12

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