Understanding Glaucoma: Epidemiology and Pathophysiology
Glaucoma comprises a group of diseases characterized by progressive optic nerve injury that results in visual field loss and potentially permanent blindness. Initially the damage to neural cells may go undetected as the disease is typically asymptomatic, but, left untreated, it can cause severe vision loss. The damage to the optic nerve is irreversible: so far, regenerative attempts have been unsuccessful, so early diagnosis is essential.
More than 64 million people globally are believed to have glaucoma,1 and it is expected that more than 111 million will have it by 2040, due to population aging. In the United States approximately 2.9 million individuals have glaucoma. 2Throughout the world, approximately 8.4 million cases of irreversible bilateral blindness are attributed to glaucoma,3and it accounts for an estimated 9% to 12% of all blindness in the United States.4
The traditional paradigm of glaucomatous damage is that biomechanical damage, namely from elevated intraocular pressure (IOP), occurs to the optic nerve head.5 Pressure on the optic nerve head appears to be the mechanism through which pressure in the front of the eye causes damage to the nerve itself.5However, several studies have provided evidence of mechanisms other than IOP that may contribute to nerve damage in glaucoma.6–8According to Dr. Grace Richter, Assistant Professor of Ophthalmology in the Glaucoma Division at the USC Roski Eye Institute, “Reduction or fluctuations in ocular blood flow, increased susceptibility to nerve damage from inflammatory diseases, and having an abnormally low intracranial pressure have all been implicated in contributing to glaucomatous damage.”
Types of glaucoma
Elevated IOP is the most common precipitating factor in glaucoma.8 In the normal eye, a balance exists between the amount of aqueous humor produced within the eye, and the amount that drains out of the eye; an increase in fluid build-up caused by a failure to drain properly can increase IOP. According to Dr. Richter, “The range of IOP for people without glaucoma is 10-21 mm Hg, but in fact, over 50% of patients with open angle glaucoma (OAG) actually have baseline IOPs in the ‘normal’ range.”
Primary OAG accounts for the majority of cases.9 The drainage area (“drainage angle”) in eyes with OAG appears anatomically normal, despite elevated IOP, but it may not drain fluid efficiently.4 This is opposed to chronic angle closure glaucoma (ACG) where the drainage angle is visibly abnormal. OAG and chronic ACG are both painless, but have different treatment patterns, so seeing a specialist is paramount. In contrast to chronic ACG, acute ACG occurs in eyes in which the drainage canals are blocked by the iris and is considered a medical emergency.4 A sudden, acute blockage may be accompanied by severe pain, blurred vision, excessive tearing, halos around lights, and/or headache, nausea, and vomiting.4 Acute ACG may have no warning symptoms and requires immediate treatment, as blindness can occur over hours.4 ACG is responsible for half of all glaucoma-related blindness.10
Exfoliation glaucoma is a systemic condition in which grayish-white exfoliation material accumulates in ocular tissues.11 It is associated with elevated IOP and is the most common identifiable cause of OAG.
Regardless of precipitating factor, IOP reduction is still the only proven treatment to slow glaucomatous damage for all types of glaucoma.8
Numerous risk factors for glaucoma exist, including age >60 years, hypertension, diabetes, hypothyroidism, and African or Hispanic heritage. Among African Americans, blindness from glaucoma is 4 to 5 times more common than among Caucasians.12 Type 2 diabetes is associated with an 82% higher risk of primary OAG.9 Individuals with trauma to the eye, severe myopia, inflammation, or previous eye surgeries may be at greater risk for glaucoma. Some types of tumor or a detached retina are known to increase risk. Some genetic variants may be associated with elevated IOP; thus family history is an important risk factor.9
ACG is more common in women, older individuals, persons of Asian descent, and those with a family history of this condition.4,10 Small eyes and far-sightedness are also risk factors for ACG. 4,10
The etiology of normal-pressure glaucoma may be multifactorial, but persons with a family history, vascular dysfunction, and/or Japanese ancestry appear to be at greater risk for this condition.13
Although glaucoma mainly occurs with aging, younger adults can also get the disease. Moreover, both juvenile and congenital glaucoma exist: according to the American Glaucoma Foundation, approximately 1 of every 10,000 babies born in the United States has glaucoma at birth.4
In short, while some individuals may be at higher risk for glaucoma, no one is exempt from risk. This is why regular eye examinations are critical. Techniques for diagnosing glaucoma and available treatments, if it is detected, will be discussed in a subsequent article.