High glaucoma prevalence in blacks leaves questions about treatment

Genetics, biology, environment and socioeconomics are thought to be factors in the high prevalence of the disease in U.S. blacks. But the best methods to address the issue are open to debate.

Glaucoma is the leading cause of blindness among blacks in the United States, but there is little agreement on why that is or what the best method of treatment is. Some even question whether race should be considered in making a diagnosis of glaucoma or designing a treatment regimen.

“The short answer is that no one knows for sure,” M. Cristina Leske, MD, MPH, an epidemiologist at Stony Brook University, told Ocular Surgery News.

Glaucoma occurs in the U.S. black population at a higher rate than in whites and other minorities, with three to four times greater prevalence of the disease in blacks than in whites and a six times-greater risk for blindness.

It is estimated that 50% of people living with the disease are unaware that they have glaucoma.

Scientists, researchers and public health officials are working to find the cause or causes of the higher prevalence of glaucoma in black people. Their work may lead to a better understanding and better prevention or treatment tactics for this at-risk population.

Prevalence documented

Epidemiologic data gathered by the Barbados Eye Studies and the Baltimore Eye Survey have confirmed the higher prevalence of glaucoma in blacks.

“People of African descent have a higher prevalence of glaucoma,” said Dr. Leske, a principal investigator in the Barbados Eye Studies. The Barbados studies produced the most comprehensive collection of data available on the prevalence and incidence of glaucoma in a black Caribbean population, she said.

In the study, more than 4,000 black participants, age 40 to 84 years, had a 7% prevalence of glaucoma. Dr. Leske noted, however, that researchers have found that not all black populations have the same prevalence of glaucoma.

“Black populations from regions of Africa and those that originated in Africa and settled throughout the world have different rates of glaucoma,” Dr. Leske explained. She noted that blacks living in Barbados and in continental North America generally originated in West Africa and share similar origins.

Nevertheless, prevalence can vary among black populations in this part of the world. The Baltimore Eye Survey found a lower rate of glaucoma in black Americans than the Barbados study found in black Caribbeans. The Baltimore study found a prevalence of almost 4% in black Americans in a population-based sample of more than 2,000 participants. (This compared to a 1.7% prevalence in whites.)

In another recent study, Russell W. Read, MD, and colleagues found that some populations of African origins in the United States may not have higher intraocular pressures (IOP) than non-African populations. These researchers found no difference between the mean IOP of a group of patients of East African origin living in the Seattle area and a control group of non-African patients living in the same area. Dr. Read and colleagues noted in their study, published in the Journal of Glaucoma, that “consideration of ethnic and racial origins more specific than ‘African’ should be given when evaluating intraocular pressure in individual patients.”

Incidence important too

“More important than the prevalence of glaucoma is the incidence of the disease,” Dr. Leske explained. She said beneficial data can be gleaned from incidence statistics, which may offer insight into how people develop the disease.

“What’s really interesting is to follow people over time and find out the factors that determine the risk of developing new glaucoma,” Dr. Leske said. By studying people who at baseline do not have the disease, she said — noting family history, lifestyle, eye health and IOP, along with other factors that might contribute to the development of glaucoma — researchers may be able to determine what factors trigger the development of glaucoma.

“Tracking incidence helps us to learn a lot about the course of the disease,” Dr. Leske said. The Barbados Eye Studies are the only source of data on the incidence and risk factors for glaucoma and other eye diseases in a black population, she said. Such studies are difficult to conduct among whites because of their lower glaucoma incidence.

Dr. Leske said a study of the incidence of glaucoma in African Americans would be needed to determine what factors might influence the disease in that population.

Questioning genetics

A family history of glaucoma is a key risk factor for development of the disease. The genetic link can be a threat to persons of any race.

However, some researchers have questioned whether an additional genetic factor — originating in Africa — could be responsible for the greater prevalence of the disease in blacks.

“At this point, no one has a handle on the genetics as to why African Americans have a higher incidence of glaucoma, and why it’s more severe in that population,” said Kenneth Schwartz, MD, of Washington Eye Physicians in Maryland.

Dr. Leske agreed. “It’s a very complicated issue, a difficult question to answer,” she said.

Genetic research in recent years has revealed a relationship between the gene known as myocilin (also known as GLC1A and TIGR) and some forms of open-angle glaucoma.

Researchers at the University of Iowa investigated the possibility that the high prevalence of glaucoma in blacks may be due to a higher prevalence of myocilin mutations. The Iowa researchers investigated this possibility in a racially mixed population of 1,703 people from five populations. The study looked at primarily Caucasian patients from Iowa, Australia and Canada, African-American patients from New York and Asian patients from Japan.

“We thought that myocilin mutations might explain the high prevalence of glaucoma in African Americans,” said Wallace L. M. Alward, MD, a principal investigator in the study and vice chairman of the department of ophthalmology and visual science at the University of Iowa.

The study failed to find this connection. The overall frequency of genetic mutations was similar in all populations, with whites in Iowa having the highest prevalence of mutations, 4.3%, and blacks in New York having the lowest prevalence, 2.6%.

Research led by Barbara Nemesure, PhD, at Stony Brook University, also failed to find a differentiating factor in corneal thickness. Her 2003 analysis of the Barbados Eye Studies data, prompted by the finding of the OHTS and other studies of a correlation between corneal thickness and IOP levels, found no correlation between central corneal thickness and race.

With the data accumulated thus far, Dr. Leske hypothesizes that the cause of a higher glaucoma prevalence in blacks may be an interaction of environmental factors and genetic factors.

“The cause will probably be due to a gene-environment interaction. This happens when a person has a particular genetic composition that, when faced with a certain environmental milieu, is triggered to develop a certain condition,” Dr. Leske said. Her current research is focused on finding what environmental factors may trigger the development of glaucoma in black populations.

“Given the right set of circumstances, the disease may develop,” she said.

Nongenetic factors

Eve J. Higginbotham, MD, professor and chairwoman of the University of Maryland School of Medicine in Baltimore, is among those who question the role of race or ethnicity in determining the prevalence of glaucoma.

“The prevalence and progression of the disease is related to environmental factors that we don’t really understand yet,” she said. According to Dr. Higginbotham, approaching the issue from a “racial” perspective will not lead researchers to valid conclusions.

Screening programs in communities at risk can help to diagnose glaucoma early in the course of the disease.

“The Human Genome Project found that there really isn’t any clear indication that race has any biological basis in the medical sciences,” Dr. Higginbotham said. She cites the monumental work of the genome project as an emerging force that is deconstructing earlier ideas of race classification in medicine.

“Given the genetic evidence that the human species originated from the single continent of Africa, it is inappropriate to use the social and political term ‘race’ in a biological context,” Dr. Higginbotham said. “Furthermore, Africa represents the most diverse pool of genes since this continent is the origin of the human species.”

Additionally, she said, in a country as diverse as the United States, the divisions between races and ethnicities are not clear-cut, but overlapping.

“How can we focus on one population if there is a melting pot of genes mixing African Americans, whites of all European descent, Asian Americans, Native Americans, Hispanic Americans, Alaskan natives?” Dr. Higginbotham asked.

Dr. Schwartz agreed. “All of these people have different incidences of disease and different incidences of primary open-angle and angle-closure glaucoma. This makes it really confusing when you’re trying to evaluate a particular group,” he said.

Dr. Higginbotham said she believes that physicians must assess people on an individual basis, not on preconceived ideas of racial prevalences.

“As a risk factor, family history is more important to glaucoma progression than what some consider race. We need to start evaluating people by their individual risk factors. Yes, glaucoma is a genetic disease, but you can’t assume by a person’s skin color that he or she is at a greater prevalence,” Dr. Higginbotham said.

Socioeconomic factors

Individual risk factors go beyond family history, extending to levels of education and income. Dr. Higginbotham has evaluated the economic risk factors for visual loss from glaucoma.

“There are a great number of socioeconomic factors that significantly influence disease in any population,” she said.

According to Dr. Higginbotham, the Baltimore Eye Survey found that people with a low level of education (0 to 6 years) had a greater chance of visual impairment (5.09%) than people with more than 12 years of schooling (1.94%). Education also made a difference in the prevalence of blindness in these groups, which was 1.8% and 0.64% respectively.

Likewise, people with higher incomes had a lower prevalence of visual impairment than people with lower incomes.

“For every additional $1,000 of median income, the prevalence of visual impairment declined by 0.32%.” Dr. Higginbotham said.

Insurance can also affect access to care, and according to Dr. Higginbotham, blacks in the United States are twice as likely to be uninsured as whites. Lack of medical insurance leads to lower rates of surgery in blacks, which may partly explain the higher prevalence of blindness from glaucoma in this population, she noted.

However, as the Advanced Glaucoma Intervention Study (AGIS) found, blacks and whites react differently to surgical glaucoma treatment. The AGIS found that blacks with advanced glaucoma responded better to laser surgery than trabeculectomy, while whites with advanced glaucoma responded better to trabeculectomy. However, there may be alternate explanations for these findings that are not based on race, Dr. Higginbotham said.

These outcomes suggest that race should be a consideration when planning treatment for patients.

Hispanics at high risk for glaucoma

While blacks may have the highest prevalence of glaucoma, Hispanics also have a higher prevalence than other minority ethnic groups in the United States, according to the Department of Health and Human Services. Currently, Hispanics are the fastest growing minority group in the United States. However, most large-scale studies of primary open-angle glaucoma have focused on black and white patients, leaving little data on the prevalence and incidence of the disease among Hispanics. A statement from the National Eye Institute (NEI) acknowledges this flaw: “Other ethnic and racial groups have been studied less vigorously. There is a dearth of information about the prevalence and incidence of glaucoma in Hispanic and Native American populations; therefore, studies need to be initiated in these populations to obtain this critical information.” Since this call for evaluation, two large, population-based studies on Hispanics have been initiated in Western states. With help from the NEI, Proyecto VER (the Vision Evaluation and Research Project) became the first comprehensive study to assess vision loss among Hispanics in the United States. In a population-based sample of 4,774 Hispanic men and women in Southern California, glaucoma was found to be the leading cause of blindness. Researchers at the University of Arizona stressed the need for further research to identify the obstacles to eye health in this population, particularly women. A second NEI-funded study, the Los Angeles Latino Eye Study, enrolled nearly twice the number of Hispanic patients as Proyecto VER. In approximately 8,500 patients, the study found a high rate of obesity, diabetes and high blood pressure, which increased their chances for glaucoma, diabetic retinopathy and cataract. Publication of results from this study are ongoing. The outcomes of both studies will further the exploration of ocular disease in the Hispanic population and help physicians to identify the risks when treating these patients, researchers say.

Public aid

There are a number of programs, both long-standing and recent, that attempt to screen and treat glaucoma.

“Once you have the disease, the question is — how likely are you to go blind from it?” said Jeffrey Henderer, MD, a glaucoma specialist at Wills Eye Hospital. Dr. Henderer has participated in inner-city screening programs for 5 years.

“Seeking out communities at risk, being able to diagnose the disease early on, encouraging disciplined treatment and actively bringing patients further into the health care system can prevent symptomatic loss of vision,” Dr. Henderer said.

Two ongoing programs currently available to blacks living in urban areas are the Travatan Project Focus, sponsored by Alcon and Prevent Blindness America, and the Partnership for Sight initiative, sponsored by Allergan and a number of glaucoma research organizations.

Through Travatan Project Focus, at-risk patients can receive free screening at their local churches and community centers. Additionally, educational material on glaucoma and Travatan (travoprost, Alcon) is distributed in an effort to educate patients about the disease that often remains without symptoms until nerve damage is advanced.

Through Partnership for Sight, at-risk populations receive free field-of-vision tests and educational material.

For at-risk persons who have poor accessibility to urban areas, the American Academy of Ophthalmology provides free screening through the Glaucoma Eyecare Program.

Patients seeking an ophthalmologist in their area who provides free screenings through this program can call (800) 391-EYES to receive free educational material on glaucoma and to find out if they are eligible for a free screening from one of 7,800 ophthalmologists nationwide.

These programs, which try to target the disease before it damages, are vital to prevention of blindness from glaucoma, researchers say.

For Your Information:

• M. Cristina Leske, MD, MPH, can be reached at the Department of Preventive Medicine’s Division of Epidemiology at Stony Brook University, the State University of New York, Stony Brook, NY 11794; (631) 444-1290; fax: (631) 444-7525; e-mail: mesantoro@notes.cc.sunysb.edu.
• Kenneth Schwartz, MD, can be reached at Washington Eye Physicians, 5454 Wisconsin Ave., Suite 950, Chevy Chase, MD 20815; (301) 654-5114; fax: (301) 654-9132.
• Wallace L. M. Alward, MD, vice chairman of the Department of Ophthalmology and Visual Science at the University of Iowa, can be reached at 200 Hawkins Drive, Iowa City, IA 52242-1091; (319) 356-2864; fax: (319) 356-0363; e-mail: wallace-alward@uiowa.edu.
• Eve J. Higginbotham, MD, chairman of the Department of Ophthalmology, University of Maryland School of Medicine in Baltimore, can be reached at 419 W. Redwood, Suite 580, Baltimore, MD 21201; (410) 328-5929; fax: (410) 328-6346; e-mail: fcwejh6786@aol.com.
• Jeffrey D. Henderer, MD, Assistant Professor of Ophthalmology, Thomas Jefferson University School of Medicine, can be reached at Wills Eye Hospital, 840 Walnut St., Suite 1120; Philadelphia, PA 19107; (215) 928-3342; fax: (215) 928-3903; e-mail: henderer@willsglaucoma.org.

References:

• Kass MA, Heuer DK, Higginbotham EJ, et al. for the Ocular Hypertension Treatment Study Group: The Ocular Hypertension Treatment Study: A Randomized Trial Determines that Topical Ocular Hypotensive Medication Delays or Prevents the Onset of Primary Open-Angle Glaucoma. Arch Ophthalmol. 2002;120: 701-713.
• Tielsch JM, Katz J, Singh K, et al. A population-based evaluation of glaucoma screening: the Baltimore Eye Survey. Am J Epidemiol. 1991;134(10):1102-1110.
• Leske MC, Connell AM, Wu SY, et al. Risk factors for open-angle glaucoma. The Barbados Eye Study. Arch Ophthalmol. 1995;113(7):918-924.