Glaucoma

                                             Glaucoma

Open vs. Closed-Angle Glaucoma
There are two categories of glaucoma and they have very different mechanisms. Open-angle glaucoma is the most common type in our country. It occurs from decreased aqueous drainage caused by an unidentified dysfunction or microscopic clogging of the trabecular meshwork. This leads to chronically elevated eye pressure, and over many years, gradual vision loss.
This differs from closed-angle glaucoma, also called “acute glaucoma,” which occurs when the angle between the cornea and iris closes abruptly. With this closure, aqueous fluid can’t access the drainage pathway entirely, causing ocular pressure to increase rapidly. This is an ophthalmological emergency and patients can lose all vision in their eye within hours.
Let’s examine each of these types of glaucoma in more detail. 

Open-Angle Glaucoma
The majority of glaucoma patients (about 80% ) have chronic open angle glaucoma. Most patients are over the age of 40. This condition is more common in African Americans, and has a strong familial inheritance. The major risk factors are family history, age, race, high eye pressure, and large vertical nerve cupping. More recently, thin-corneas have been found to be a major risk factor, though this mechanism is not well understood.
The underlying mechanism for open-angle glaucoma involves degeneration of the trabecular meshwork filter, usually by unknown causes, that leads to aqueous backup and chronically elevated eye pressure. With prolonged high pressure, the ganglion nerves in the retina (the same nerves that form the optic nerve) atrophy. The exact mechanism for this nerve damage is poorly understood and proposed mechanisms include stretching, vascular compromise, and glutamate transmitter pathways. As the ganglion nerves are progressively destroyed, vision is gradually lost.
Open-angle glaucoma has the reputation of being the “sneaky thief of sight” because the visual loss occurs so slowly that many patients don’t realize they have the disease until it is far advanced.
Because the disease is otherwise asymptomatic, detecting open-angle glaucoma requires early pressure screening. Free screening clinics also use different types of automated visual-field testing to detect subtle peripheral vision loss. 

Presentation
Open-angle glaucoma patients usually present with three exam findings: elevated eye pressures, optic disk changes, and repeatable visual field loss patterns.

1. Pressure: The most accurate way to measure eye pressure is with the Goldman applanation tonometer. This is a device mounted on the slit-lamp that measures the force required to flatten a fixed area of the cornea. Normal pressures range from 10 to 20 mm Hg, while glaucoma patients can measure over 21 mm Hg. Keep in mind that eye pressure can fluctuate throughout the day (typically highest in the morning) so the pressure should be checked with each visit and the time of measurement should be noted. Also, some glaucomatous eyes have a “normal” pressure. In other words, a “good pressure” doesn’t rule out glaucoma, nor does a high pressure necessarily indicate glaucoma. 


2. Fundus Exam: The optic disk looks striking in advanced glaucoma. In normal patients, the optic disk has a physiological indentation or “cup” that is less than one-third the disk diameter. With glaucoma, the ganglion nerve layer slowly dies away, and, as fewer ganglion nerves course through the optic disk, the amount of cupping increases. A cup to disk ratio greater than 0.5 or an asymmetry between the eyes suggests ganglion atrophy caused by glaucoma.

3. Visual Loss: The vision loss from chronic glaucoma occurs in characteristic patterns that can be followed by automated perimetry (machines that map out the peripheral vision). The central vision is typically spared – in fact, late stage patients may have 20/20 central vision, but be otherwise legally blind because of peripheral blindness.

Treatment:
Since IOP is the only risk factor we can treat, the primary treatment of glaucoma focuses on decreasing eye pressure to less than 20 mm Hg or even lower depending upon the severity of disease. Treatment may be either medical or surgical. 

Medical Treatment
Topical beta-blockers are the traditional therapy for these patients and have been around for decades. Beta-blockers work by decreasing aqueous humor production at the ciliary body. Unfortunately, systemic side effects can occur from nasal absorption, making it especially important to ask your patients about history of asthma, COPD, and cardiac problems.
These days, many physicians are using newer drugs like topical CAIs, alpha-agonists, and prostaglandin analogues for first-line therapy, as they have fewer systemic side effects.
Prostaglandin analogues like latanoprost (Xalatan™) are the newest of these glaucoma drugs, and they are very popular as a first-line agent. They work by increasing aqueous humor outflow. They do have some side effects, though. They can make eyelashes grow longer (many patients actually like this), and in a few patients may darken the iris color, turning green and blue eyes brown. 

Surgical Treatment for Chronic Glaucoma
If eyedrops aren’t working, there are several surgical techniques available to relieve eye pressure. One common surgery is the trabeculectomy, where an alternate drainage pathway is surgically created. A small hole is cut through the superior limbus, creating a drainage tract from the anterior chamber to a space under the conjunctiva. This can be very effective in decreasing pressure, but if the patient is a rapid healer the shunt can scar down and close, so anti-metabolites like mitomycin-C are often applied to the site. If this surgery doesn’t work, a plastic tube-shunt can be inserted into the anterior chamber that drains to a plate fixed under the conjunctiva further back.

Several laser procedures can also help. Argon laser trabeculoplasty (ALT) can be used to burn portions of the trabecular meshwork itself. The resulting scarring opens up the meshwork and increases outflow. A laser can also be used to burn the ciliary body to decrease aqueous production at its source. 

                                                                            Acute Glaucoma

Acute glaucoma is a medical emergency. The most common mechanism is pupillary block. This occurs when the lens plasters up against the back of the iris, blocking aqueous flow through the pupil. This resistance produces a pressure gradient (this is a good buzz word to memorize) across the iris that forces the iris and lens to move anteriorly. When the iris moves forward, the irido-corneal angle closes, blocking the trabecular meshwork. Without an exit pathway, aqueous fluid builds up, eye pressure increases rapidly, and the retina is damaged from stretching and decreased blood supply.
The outflow angle can close for many reasons, and people with naturally shallow anterior chambers such as hyperopes (far-sighted people with small eyes) and Asians are predisposed to developing angle closure. One inciting condition that is typical in acute glaucoma is pupil dilation — many patients describe onset of their symptoms occurring while in the dark or during stressful situations. When the iris dilates, the iris muscle gets thicker and the irido-corneal angle becomes smaller, making it more likely to spontaneously close. Along those lines, medications that dilate the eye, such as over-the-counter antihistamines and cold medications, also predispose angle closure. 

Presentation
These patients will present with an extremely red and painful eye, often complaining of nausea and vomiting. On exam, you’ll find their pupil sluggish and mid-dilated. Pressures in the affected eye can be very high, often 60 mm Hg or higher. The eye will feel rock hard, and you can actually palpate the difference between the eyes with your fingers. One classic sign that patients often describe is seeing halos around lights. This occurs because the cornea swells as water is pushed under high pressure through the endothelium into the corneal stroma. This corneal swelling also makes it hard for you to see into the eye, further complicating diagnosis and treatment.

Acute Glaucoma Exam Techniques:
Ophthalmologic examination for acute glaucoma involves measuring the eye pressure, accessing the anterior chamber angle, and a fundus exam.
One trick to determine whether an angle is shallow is to shine a simple penlight across the eyes. If the iris is pushed forward, it will cast a shadow. Additionally, an ophthalmologist can visualize the angle directly through gonioscopy. Here’s how it works: 

Gonioscopy:
Normally, the inside angle cannot be seen with a microscope because the cornea-air interface creates “total internal reflection.” However, we can use a goniolens, which is a special glass lens with mirrors on its sides, to look directly at the angle. When the glass lens is placed directly onto the cornea, the cornea-air interface reflection is broken and light from the angle can escape and be seen through the mirrors.

Acute Glaucoma Treatment
In cases of acute glaucoma, you want to decrease the pressure in the eye as quickly as possible. A “kitchen sink” approach is often used, throwing many treatments on at once. You can decrease aqueous production using a topical beta-blocker like Timolol and a carbonic anhydrase inhibitor like Diamox. Also, osmotic agents such as oral glycerin or IV mannitol (even ethanol, in a bind) can be given systemically to draw fluid out of the eye and back into the bloodstream. Finally, a miotic such as pilocarpine may be helpful in certain cases to constrict the pupil and thus open up the outflow angle. You can also use topical glycerin to transiently dehydrate/clear the cornea to aid with examination.
Ultimately, these patients need surgical treatment to avoid recurrence of their angle closure. A high intensity laser can burn a hole through the iris and create a communication between the posterior and anterior chambers, relieving the pressure gradient  across the iris, and allowing it to move back into a normal position. This opens up the trabecular meshwork and allows aqueous fluid to flow freely out of the eye. This procedure is typically performed on both eyes because these patients are predisposed to having attacks in the other eye as well.

Other types of glaucoma

1. Neovascular Glaucoma:
This can occur in diabetic patients or those with a retinal vein occlusion.  

2. Pigment Dispersion Syndrome (PDS):
Occurs when the pigmented back-surface of the iris rubs against the radial zonules supporting the lens.

3. Pseudoexfoliation Syndrome (PXF)
In this systemic condition, basement membrane-like material is deposited throughout the body. This material adheres to the anterior lens capsule, creating a rough surface. As the overlying iris dilates and contracts with daily activity, pigment is rubbed off and clogs the trabecular drain. These patients also suffer from zonular instability, making cataract operations difficult.