What is Glaucoma?
Glaucoma is a group of diseases that affect and cause damage to the optic nerve that lies in the back of the eye. This damage is often progressive, leading to irreversible damage to the eye, and this leads to the progressive loss of vision. That is why glaucoma is known as the leading cause of irreversible blindness all over the world. In 2020, glaucoma was diagnosed in approximately 80 million individuals globally.
Glaucoma is often referred to as the “silent thief of sight” because most people are often undiagnosed, and the majority of patients who present with symptoms are at late or severe stages, where the damage done to the eyes becomes irreversible. The normal eyes are divided into two chambers by the lens. The anterior chamber contains fluid known as aqueous humor, while the posterior chamber contains jelly-like material known as the vitreous humor. Normally, the aqueous humor is produced by specific cells behind the iris (the colored part of the eye) and drained at the angle between the cornea and the iris. When the fluid (aqueous humor) in the anterior chamber is increased above the normal level, glaucoma occurs. This could happen because the production of the aqueous humor is increased or because its drainage is affected. This buildup of fluid in the eyes causes increased pressure on the optic nerve leading to its damage.
When glaucoma was first diagnosed, it was thought that the underlying cause for it is the increase in intraocular pressure (the pressure of the fluid inside the eyes), which is referred to as ocular hypertension. However, as the knowledge of glaucoma and its pathogenesis has advanced, it became clear that the increase in intraocular pressure is a risk factor and not a cause for glaucoma and that many cases of glaucoma have normal intraocular pressure. Other researchers highlighted that poor blood supply to the eyes or degenerative diseases that include the eye might cause glaucoma.
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What are the Types of Glaucoma?
There are many types of glaucoma, two of which are the most common:
This is the most common type of glaucoma, affecting nearly 90% of all glaucoma patients. Open-angle glaucoma occurs when there is a problem with the drainage of the aqueous humor. This, subsequently, leads to an increase in the pressure in the eyes and eventually harming the optic nerve. Unfortunately, there are no warning signs, and vision becomes gradually affected over the next few years, without any pain or discomfort. Meanwhile, if it is detected early in the course of the disease, the prognosis becomes much better.
This type is totally different from open-angle glaucoma, where the pressure inside the eyes becomes elevated very rapidly; however, this type is not as common. In this type of glaucoma, the peripheral parts of the iris become approximated towards the drainage angle of the aqueous. This causes a blockage to the draining canal, leading to a rapid buildup of fluid in the anterior chamber. This type of glaucoma is considered a medical emergency and needs immediate care.
Although glaucoma is often silent with no apparent symptoms except for late stages, there are many symptoms that patients with glaucoma can present with, including the following:
- Blurry vision
- Seeing ‘halos’ under objects
- Tunnel vision. Patients with tunnel vision see things that are right in front of them, while the things that are to the sides of the eyes can’t be seen. This occurs when the eyes become damaged in the long-term.
How Is Glaucoma Treated?
There are many treatment options for glaucoma. However, most of them have side-effects, and this eventually affects the compliance of the patients. These treatment options include:
This is the first treatment option for patients with glaucoma. They often include prostaglandins, beta-blockers, and alpha agonists. However, there are many side-effects to their usage, including burning sensation, change in the eye color, interaction with various drugs that are taken for diabetes or heart conditions, headache, fatigue, or drowsiness.
Carbonic anhydrase inhibitor (CAI) is the most commonly used drug in conjunction with the previously mentioned eye drops. However, CAI causes various systemic side-effects, including myopia (short-sightedness), an increase in the frequency of urination, and lightheadedness.
This method is used when the previous two steps are ineffective. However, this approach is often associated with mild side-effects such as redness of the eyes, soreness, and blurry vision. Other serious yet uncommon side-effects include elevation in the intraocular pressure, which may further worsen the eyes, and synechia (attachment) between the iris and cornea.
This method is used in emergency cases and when the previous approaches become ineffective in managing glaucoma. That being said, this surgery is associated with many adverse events, including postoperative infection of the eye, double vision, swelling of the eyelids, bleeding, scarring, and remarkable reduction in intraocular pressure (hypotony).
Since all of the mentioned treatment options for glaucoma have many side-effects and are often ineffective, researchers have become interested in investigating other potential therapeutic options. Of particular interest, cannabidiol received much attention for its various therapeutic properties in preventing, managing, and treating many health problems. And based on preliminary research, both cannabidiol (CBD) and tetrahydrocannabinol (THC) have been shown to reduce the pressure inside the eyes of glaucoma patients. This suggests that CBD can play a significant role in treating glaucoma.
Is CBD Beneficial for Glaucoma?
Based on previous studies, CBD has been shown to be effective in glaucoma by reducing intraocular pressure and by promoting the health of the optic nerve. Since then, many preclinical, animal, and human studies have been conducted to confirm the CBD’s effects on glaucoma.
In an animal study of mice with glaucoma, the efficacy of a topically applied drug containing both tetrahydrocannabinol (THC) and CBD was studied. Surprisingly, a single application of the topical drug (THC and CBD) resulted in a significant reduction in intraocular pressure. However, the researchers were not confident whether this effect was caused mainly by THC or CBD.
A randomized, double-blinded, placebo-controlled clinical trial investigated the efficacy of a medicinal extract containing both delta-9-THC and CBD in six patients with ocular hypertension or open-angle glaucoma. Patients received the medicinal extract sublingually at different doses: 5mg of THC, 20 mg of CBD, 40 mg of CBD, or placebo. Within two hours after the administration of the previously mentioned drugs, it was shown that the sublingual administration of 5 mg THC significantly reduced intraocular pressure compared to placebo. At 4 hours, the intraocular pressure returned back to the normal range. However, CBD did not significantly reduce the intraocular pressure at any time-point. In contrast, the highest dose of CBD (40 mg) resulted in the elevation of intraocular pressure at 4 hours. The safety of all treatments was assured except for a mild panic-like attack in one patient in the THC group.
Based on the available evidence, CBD should not be used as an alternative option for glaucoma, particularly acute closed-angle glaucoma. Until more studies are conducted to further elucidate the effects of CBD on intraocular pressure, CBD should not be used without consulting an ophthalmologist.
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