There are several ways to classify the different types of glaucoma. Traditionally, they have been classified as open angle glaucoma and closed angle glaucoma. This terminology describes the way the drainage system of the eye appears; to understand this, one needs to understand a little bit of anatomy. Inside the eye there is a constant flow of fluid that nourishes the front part of the eye. The area through which the fluid leaves the eye joins the iris at an angle and it is called the trabecular meshwork.
In open angle glaucoma, the iris is not blocking the trabecular meshwork. Open angle glaucoma is more common and will often go undetected because the rise in pressure tends to be slow and mild and as a result there are usually no initial symptoms. The vision worsens slowly over time which is hardly noticeable but without treatment, blindness can ensue. The American Academy of Ophthalmology estimates that 50% of people affected by glaucoma may not even know they have the disease.
Closed angle glaucoma is far less common than open angle glaucoma, but the damage to the optic nerve can be faster and more severe. With closed angle glaucoma, the iris blocks the drainage canals when the pupil enlarges. in dim illumination or as a result of stress or medications. When the drainage gets blocked, the pressure can rise quickly and the patient may notice symptoms such as headaches, eye pain, rainbows around lights at night, blurred vision, and nausea. As a note of importance,there is a subacute (slower) form of closed angle glaucoma where the symptoms are not as intense and may go unchecked; patients sometimes mistake their symptoms for those of migraines. Closed angles or narrow angles can easily be diagnosed with a simple in office test and could save your vision. If you have narrow angle glaucoma you must consult your doctor before taking over the counter cold medicines, allergy medications, antidepressants, and of course always read all package inserts for warnings regarding glaucoma.
There are many other classifications of glaucoma. Normal tension glaucoma occurs when the typical optic nerve damage happens at normal intraocular pressures (IOP, the pressure inside the eye). In patients with normal tension glaucoma, the optic nerve appears more sensitive to pressure and undergoes damage at what are normal pressures for others. Most people think glaucoma means increased intraocular pressure, but while we know intraocular pressure is a risk factor, glaucoma could also happen with normal pressures and some patients with elevated pressures will never develop the disease.
Other classifications depend on whether or not there is a cause for the elevation in pressure. Primary glaucoma occurs when open angle glaucoma pressure is elevated without an identifiable cause. Secondary glaucoma occurs when there is an abnormality causing an obstruction and a rise in pressures.
Regardless of the type of glaucoma, the results are damage to the optic nerve. The optic nerve is the cable that sends all the images from the eye to the brain. When one looks at the optic nerve through the pupil, it resembles a doughnut composed of very fine strands (nerve fibers). Each of these strands transmits impulses back to the brain. As the disease progresses, the optic nerve fibers are damaged, causing the nerve to look increasingly “cupped” which means some of the doughnut tissue has been lost and the central empty space becomes bigger. As the loss of tissue progresses, it becomes more visible to the doctor while examining the eye and is easier to diagnose. The loss of tissue in the optic nerve causes the vision to deteriorate over time. First, peripheral vision is affected, followed by progressive loss of central vision which could ultimately progress to blindness. However, glaucoma usually responds to medication if it is detected and treated early.