What is Glaucoma?

© Dr. Harry A Quigley, used with permission
Johnson Laboratory at the Johns Hopkins Wilmer Eye Institute

What is Glaucoma?

Glaucoma is an eye disease that causes irreversible vision loss, and potentially blindness, by destroying nerve cells of the retina which carry visual information from the eye to the brain (called retinal ganglion cells, or RGCs).  RGCs send impulses that encode vision through their fibers (called axons) within the optic nerve, and as RGCs are lost over time the optic nerve degenerates.  Therefore, glaucoma is called an optic neuropathy.

Glaucoma is the most common form of optic nerve disease in the world, affecting more than 100 million people.  If glaucoma is detected early, most patients have a very good prognosis.  All currently available treatments for glaucoma focus on lowering the eye pressure (intraocular pressure, or IOP) which has been clinically proven to slow or prevent further loss of vision. This can be achieved with eye drops, laser procedures, and operating room surgery.  So if glaucoma can be diagnosed at an early stage, the vast majority of patients will maintain good vision for the rest of their lives.  Unfortunately, about half of people living with glaucoma do not know they have the disease.  This is because glaucoma is notoriously insidious – it is generally painless, slowly progressive, and tends to affect the peripheral vision well before it starts to affect central vision.  Therefore, in the absence of routine eye examinations by an eye doctor, patients can live with glaucoma for years before enough vision is lost that they begin to notice it.  At that point, the vision loss can be quite severe.  For this reason, glaucoma is sometimes referred to as the “sneak thief of sight”.

How do eye doctors diagnose and monitor glaucoma?  There are several components of the eye examination that tell an eye doctor about a patient’s glaucoma.  Though measurement of the eye pressure (called tonometry) is important, because all treatment modalities are based on reducing the IOP to a safe level, the eye pressure is not the entire story.  The doctor can directly visualize the optic nerve head (the point at which the optic nerve enters the back of the eyeball) using specialized lenses.  A normal, healthy optic nerve head appears almost like a donut, with a rim of pink/orange nerve tissue (the dough) and a depression in the center called the cup.  As glaucoma worsens and nerve cells die, the healthy nerve tissue on the periphery of the optic nerve head becomes thinner and the cup in the center becomes larger.  In very severe glaucoma, there may be no remaining rim tissue.  Doctors can also obtain a scan of the back of the eye to measure the thickness of the nerve fibers entering the optic nerve (called an optical coherence tomograph, or OCT).  A healthy optic nerve will typically show two thick nerve fiber bundles entering near the top and bottom of the optic nerve head (shown in the figure as orange/red on the OCT on the left).  As glaucoma progresses and those nerve fibers degenerate, the bundles become thinner (indicated by the yellow/blue color in the corresponding OCT on the right).  Ultimately this causes loss of vision.  Since glaucoma typically does not affect central vision until the very end stages of the disease, patients with even severe glaucoma may have normal visual acuity and be able to read the 20/20 line on the eye chart.  But doctors can map out the peripheral visual field using a test called perimetry.  The bottom of the figure shows a normal visual field on the left, where a patient can see even dim lights presented far off into the periphery.  Patients with glaucoma require that these peripheral lights be much brighter in order to recognize them, or cannot recognize them at all (as indicated by grey and black regions on the peripheral vision map on the bottom right).

For more information about glaucoma, please refer to the free online book by Dr. Harry Quigley and Dr. Mona Kaleem called Glaucoma: What every patient should knowhttps://learn.wilmer.jhu.edu/glaucomabook/

In order to reduce the global burden of glaucoma blindness, three important things need to happen. 1) We need better methods of screening people for the disease, that are highly accurate and can reach patients even in underserved parts of the world. 2) We need to continue developing better treatments for the disease to halt vision loss once it is recognized. 3) We need to develop regenerative therapies that are capable of actually restoring lost vision in glaucoma.

The Johnson laboratory contributes to worldwide efforts to address all three of these important goals.  We are involved in research and service initiatives that aim to screen at-risk people to determine if they have glaucoma, and if so, to get them prompt care by ophthalmologists regardless of their insurance status or ability to pay.  We have a clinical research program that studies IOP fluctuation in patients with glaucoma, to better understand how this important risk factor leads to optic nerve degeneration. Moreover, we conduct laboratory research and are involved in clinical trials seeking to develop new neuroprotective therapies for the disease, which could better prevent vision loss in a manner that is complimentary to traditional IOP lowering.  Finally, the Johnson Laboratory is at the forefront of translational neuroscience research aiming to repopulate RGCs within the retina through transplantation of stem cell derived neurons into the eye.

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