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Glaucoma

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Glaucoma

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USMLE® Step 2 style questions USMLE

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Preview

A 73-year-old woman comes to the office because of red eyes, and darkening eyelashes and irises for 6 weeks. She has primary open-angle glaucoma and was prescribed a new medication for it 8 weeks ago. Examination of her visual fields shows gross deficits in each eye. Fundus examination shows a cup-to-disc ratio of approximately 0.7. Automated perimetry confirms peripheral, asymmetric, bilateral visual field loss. Her intraocular pressure is 19mmHg (normal range 8-21 mmHg). Which of the following medical treatments did this patient most likely receive?

Transcript

Content Reviewers:

Rishi Desai, MD, MPH

Contributors:

Tanner Marshall, MS

Glaucoma is actually a group of eye diseases that are usually due to intraocular hypertension, or increased pressure in the eye, which damages the optic nerve and if left untreated can lead to blindness.

Taking a closer look at this cross section of the eye, you can see that it’s split up into different chambers: The anterior chamber includes the area from the cornea to the iris, the posterior chamber is this really narrow space between the iris and the lens.

And then this larger vitreous chamber includes the space between the lens and the back of the eye.

Not to be too confusing, but both the anterior and posterior chambers are located in the anterior segment of the eye, while the vitreous chamber is part of the posterior segment of the eye.

Typically all of the chambers in the eye are filled with fluid.

The chambers in the anterior section are filled with a liquid called aqueous humor, and the posterior section is filled with vitreous humor.

The aqueous humor is a transparent, watery fluid that is secreted by the ciliary epithelium, which in addition to secreting aqueous humor and providing nutrients to the lens and cornea, it provides structural support and helps to keep the shape of the eye.

So that fluid’s secreted into the posterior chamber, and then flows through a narrow space between the front of the lens and the back of the iris through the pupil to the anterior chamber.

From there the fluid flows out of the eye through the trabecular meshwork, which is a spongy tissue that acts like a drain, and this allows the fluid to go down into a circular channel called the canal of Schlemm and finally into aqueous veins that are part of the episcleral venous system—the veins around the sclera of the eye.

In glaucoma, part of this aqueous humor drainage pathway becomes partially or completely blocked, so that fluid can’t easily drain out.

This causes the pressure within the fixed space of the anterior chamber to quickly build up causing intraocular hypertension, which is defined as pressure greater than 21 mmHg or 2.8kPa.

The high pressure affects all of the structures in the eye, including the optic nerve, which is the nerve that carries visual information from the eyes to the brain, and this means that over time as the optic nerve gets damaged, glaucoma leads to vision loss.

Now, there’re a couple types of glaucoma, first there’s open angle glaucoma, which is actually the most common and it has this name because the angle between the cornea and the iris is “open.”

In this type, the drainage system slowly gets clogged over time, so there’s a gradual increase in pressure on the optic nerve.

This increase in pressure initially results in atrophy of the outer rim of the nerve, resulting in a decrease in peripheral vision.

As that pressure increases even more though, there’s continued damage to the optic nerve which eventually leads to a loss in central vision as well.