Friday, June 16, 2006

Section 5-A: Risk Factors for Primary Open Angle Glaucoma

Primary open angle glaucoma (POAG) has several well-known risk factors. They include elevated intraocular pressure (IOP), older age, black race and positive family history.

  1. Elevated IOP: While elevated IOP is not necessary for having glaucoma (for example, normal tension glaucoma), it is clear that elevated IOP increases the chance of developing glaucoma (Table 5-1). For example, if you have an IOP of 30 mmHg, your chance of having glaucoma is 25.4%, and your risk of developing glaucoma is 39 times higher than those with IOP <15mmhg.

Table 5-1. Increasing risk of developing POAG according to increasing intraocular pressure (IOP).

IOP (mmHg)Prevalence in population (%)Relative Risk (fold)
<150.71.0
16-181.32.0
19-211.82.8
22-248.312.8
25-298.312.8
30-34 25.439.0
≥ 3526.140.1

We now know that measurement of IOP is at least partly dependent on the corneal thickness. In thicker corneas, the true IOP is lower than the measured IOP; in thinner corneas, the true IOP is higher than the measured IOP. Therefore, the measurement of corneal thickness is an important part of measuring IOP (Fig. 5-1).

Figure 5-1 (same as 4-4). A patient undergoes corneal thickness measurement (pachymetry).

  1. Older age: Like many other diseases, glaucoma occurs more frequently in older population (Table 5-2). For example, the prevalence of POAG increases from 0.6% in a 40-year old to 7.3% in an 80 year-old white person. This is > 12 fold increase in risk in the 40-year span.

Table 5-2. Increasing risk of developing POAG according to increasing age and race.

Age (years)WhiteBlackHispanicAsian
40-490.61.20.50.3
50-591.54.11.61.6
60-692.75.51.71.8
70-795.19.25.72.9
80 7.311.312.6N/A
OverallN/A4.72.01.2

  1. Black race: The prevalence is generally higher in blacks than in whites (Table 5-2). Hispanics had similar prevalence as whites, except for the oldest age group (80 years) whose prevalence (12.6%) was comparable to that of blacks. Asians had similar prevalence as whites. Blacks are not only more likely to develop glaucoma, they are also more likely to lose sight and become blind once glaucoma develops.
  1. Positive family history of glaucoma: Like many other diseases, there is hereditary component to glaucoma. If you have a first-degree relative (parent, sibling or child) with glaucoma, your risk of developing glaucoma is 2-4 times higher than those without family history. This strongly suggests genetic component to the disease. Several genes (and their location in the chromosome or DNA) have been implicated in POAG. The most well-studied is the Myocilin gene on chromosome 1, which accounts for 3-5% of POAG patients. Other, yet to be discovered, genes are suspected to play a role in the remaining POAG population.
  1. Additional minor risk factors for POAG include: myopia (near-sightedness), diabetes, and elevated blood pressure. While elevated blood pressure is associated with the increase in the risk of POAG in a population study, there is no direct relationship between the two in individual patients. In other words, the IOP and blood pressure are not directly correlated (linked) in a given patient. Migraine headaches and Raynaud’s disease (poor circulation in hands and feet) have been associated with normal tension glaucoma.

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