Introduction

Glaucoma is an ocular condition; usually caused by elevated intraocular pressure (IOP); which can lead to blindness. This article will explore how medical cannabis interacts with the body's systems to reduce IOP and its potential role in managing glaucoma.

Understanding Glaucoma and IOP

Glaucoma is an eye condition that affects the optic nerve, leading to the loss of retinal ganglion cells and a gradual deterioration of the optic nerve.²¹ The eye constantly produces a fluid called aqueous humor. This fluid normally drains out of the drainage angle of the eye to maintain stable eye pressure.¹ If the drainage system is blocked or not working properly, the fluid builds up which can raise eye pressure. This increased pressure can damage the optic nerve, which consists of many small nerve fibres. As the optic nerve fibres die, they can cause blind spots in an individual’s vision. If all the fibers are lost, it can result in blindness. In Glaucoma the damage usually progresses without early symptoms, making regular eye checkups important.

Mechanism and limitations of medical cannabis in IOP reduction

Medical cannabis is legal in various regions around the world, with regulations varying significantly by country and state.^2,3,5,22 Medical cannabis refers to cannabis-based products that can be used to alleviate symptoms fof medical conditions.³ Cannabis contains over 480 compounds, of which 65 are phytocannabinoids.wo most active compounds are; cannabidiol (CBD) and tetrahydrocannabinol (TCH).² Phytocannabinoids are natural compounds found in the cannabis plant that can interact with the body’s endocannabinoid system.⁴ 

The body’s endocannabinoid system helps to balance mood, sleep and appetite in an individual’s body.⁴ Endocannabinoids are the chemicals made naturally in an individual's body that help to regulate the body’s endocannabinoid system.^4,5 Endocannabinoids interact with specific cannabinoid receptors (CB1 and CB2).^4,5 CB1 and CB2 receptors are found in many parts of the eye. Both types of receptors play a role in eye function, and their distribution suggests they might influence processes related to vision.^7,8,9

TCH has been recognised as the cannabinoid compound associated with lowering an individual’s IOP.^{10, 11} It is thought that THC could help protect the optic nerves in an individual’s eyes by improving blood flow to the optic nerve. Increased blood flow can promote better fluid outflow and decrease fluid buildup, thereby helping to maintain IOP at a healthy level in an individual’s eye.^12,13 It has been suggested that when CB1 and CB2 receptors are activated (by TCH), it can help stop the release of glutamate. Glutamate is a compound that can be harmful to retinal ganglion cells.¹⁴ These cells are found in the retina (the light-sensitive layer at the back of the eye), and play a crucial role in vision. Therefore by activating receptors CB1 and CB2, it might be possible to protect retinal ganglion cells, thereby protecting an individual’s vision.¹⁴ 

Cannabinoids (e.g CBD and TCH) can be administered topically (e.g. creams), orally (e.g. tablets) or sublingually (e.g. skin patches).⁵ They can be utilised to influence the body’s endocannabinoid system. Unlike CBD, TCH causes a euphoric ‘high’ that can lead to a possible addiction.^5,6 Compared to cannabinoids from cannabis, the body’s naturally occurring endocannabinoids help the body respond to different physiological conditions.⁵ 

An individual’s IOP changes throughout the day and does not stay consistent. In patients with glaucoma, fluctuations in IOP within a 24 hour period can be a significant risk factor for disease progression.^{16,17, 18,19} Glaucoma patients are usually advised to use eye drops (such as beta-blockers or prostaglandins) in the first instance as a treatment option to control the fluctuation in the 24-hour day IOP cycle.^{16,17,18,19,20} However, the use of TCH has been limited because it does not last longer than a few hours in the body and does not show to lower IOP for a significant portion of time.^2,15 Additionally, topical cannabis drops have not been effective due to their poor ability to pass through the cornea (the transparent layer at the front of the eye that covers the iris and pupil).² 

Summary

In conclusion, Glaucoma is an eye condition marked by elevated IOP. The cannabinoid compound TCH within medical cannabis has been shown to lower IOP by increasing blood flow to the optic nerve. However, there are concerns regarding the psychotropic effects of the cannabinoid compound THC and its short duration of action. Further research is needed to explore the role of medical cannabis in managing glaucoma.