The ability of cannabis to improve the symptoms of glaucoma has been demonstrated in various studies, but doubts still remain over its efficacy and consistency within patient populations. Despite this, it is one of the most common reasons to request medicinal cannabis. We review the research and the limitations of cannabis as a medicine for glaucoma.
Glaucoma is the second leading cause of irreversible blindness worldwide. It has been known since the 1970s that cannabis can be an effective treatment, but concerns about side effects have made the medical community reluctant to endorse it. On top of this, the dosage required for a notable therapeutic effect may make cannabis treatment unaffordable and inefficient. Let’s explore cannabis as a treatment for glaucoma and compare it with conventional pharmaceutical treatment.
What is glaucoma?
Glaucoma affects over 70 million people worldwide, of which 10 % are completely blind. This insidious condition can remain symptomless until it is well advanced, making it likely that the 70 million sufferers represent the tip of an iceberg of undiagnosed glaucoma. The condition is most prevalent in older people, in those with a family history of glaucoma, and in people of African ancestry.
Glaucoma is usually associated with high pressure in the eye, or intra-ocular pressure (IOP). This pressure can cause stress and strain on the components of the eye, leading to the death of retinal ganglion cells, which are responsible for transmitting information from the retina to the brain. Patients experience irreversible vision loss once this link between the eye and the brain is severed.
Two kinds of glaucoma: open-angle and angle-closure
There are two categories of glaucoma, open-angle glaucoma and angle-closure glaucoma. Open-angle glaucoma accounts for at least 90% of all glaucoma cases, but angle-closure glaucoma is more severe, and more likely to lead to complete vision loss.
Both open-angle and angle-closure glaucoma are associated with high pressure in the front part of the eye which contains the cornea, iris and lens. It is normal for the eye to be under pressure – this is how the eye maintains its roughly spherical shape and keeps its walls taut.
The pressure is caused by a fluid in the eye called the aqueous humour, which is like blood plasma but with less protein in it. In a healthy eye, this fluid flows from the edges of the eye into the space between the iris and the lens. The fluid passes out through the pupil into a space between the iris and the cornea, before draining out at the edge of the eye through structures known as the trabecular network.
In open-angle glaucoma, the iris is correctly located, but the trabecular network does not drain properly. In closed-angle glaucoma, the iris is pressed up against the cornea, and this blocks the trabecular network from draining. These are the two ways that intraocular pressure increases as a result of glaucoma.
Much remains to be discovered about glaucoma. It is clear is that intraocular pressure is related to retinal ganglion cell death. However, the biological basis of glaucoma and the factors contributing to its progression are poorly understood. Many people remain asymptomatic until late in the disease progression, while others have intraocular pressure in the normal range but still develop glaucomatous optic neuropathy.
Effect of cannabis on glaucoma
The efficacy of cannabis in lowering intra-ocular pressure is well known, having been first reported in 1971. Pharmacologist, Prof. Manley West and his ophthalmologist colleague, Dr Albert Lockhart, reported the use of cannabis in the treatment of glaucoma in 1978.
West went on to develop cannabis-based eye-drops for the treatment of glaucoma and in 1987 gained approval to market them in Jamaica under the name “Canasol”. Canasol contains no psychoactive cannabinoids. Though approved for use in Jamaica only, it has reportedly also been prescribed in a number of countries around the world.
West has further developed Canasol into Cantimol, a combination of Canasol and the beta-blocker Timolol Maleate. Animal and clinical trials of Cantimol have been carried out in Jamaica, and the drug will be available in Jamaican pharmacies once it has cleared the local approval and registration process. Like Canasol, Cantimol is likely to remain a Caribbean secret, for there are no plans to submit the drug for the prohibitively expensive FDA approval process required to market it in the USA.
Why have the medical community been reluctant to accept cannabis as a treatment for glaucoma?
The medical understanding of cannabis and its effects on intraocular pressure have not changed since the 1970s. It is typically accepted by the medical community that cannabis reduces intraocular pressure. However, much of West’s research went unnoticed, probably because it was published in obscure journals without much exposure.
Glaucoma is one of the most commonly cited reasons for using cannabis medicinally, and it was the first indication for which the US federal government approved medicinal cannabis. However, as research continues into the topic, the medical community is reluctant to call cannabis an effective medicine for glaucoma. This is because, like many other glaucoma medications, the risks seem to outweigh the benefits. Cannabis’ effects on intraocular pressure are short-lived, for example, requiring heavy and frequent dosing.
To reduce intraocular pressure to the normal range and maintain that range, patients would have to ingest 18-20 mg of THC six-eight times a day, every day. This is an exceptionally high dose of THC to ingest, let alone up to eight times a day. This would dramatically interfere with a person’s ability to carry out normal activities such as driving and working. On top of this, there may be side effects relating to sleep, mood, and psychology.
Tolerance has been another reluctance of the medical community, although this is less justified. A study compared the ophthalmological characteristics of Costa Ricans who had used cannabis for ten years or more with those of non-using controls. If significant tolerance to the intra-ocular pressure lowering properties of cannabis had developed in the cannabis users, the two groups would be expected to have the same IOP.
Despite enforcing a 3-10 hour abstinence prior to ophthalmological testing, cannabis users exhibited lower intraocular pressure than non-users. This suggests that the intraocular pressure lowing properties of cannabis were still apparent after ten years of daily use.
For glaucoma treatment, the difference between THC and CBD is enormous
CBD has become increasingly popular in the last 5 years, and many of those seeking the therapeutic benefits of cannabis have turned to CBD. Unfortunately, this isn’t a viable option for those with glaucoma.
The intraocular pressure reducing properties of cannabis can be attributed almost entirely to THC. In one study, CBD did not reduce intraocular pressure, but at a dose of 40 mg, actually produced a transient increase in intraocular pressure. In another study, researchers found that CBD actually prevented THC from fully exerting its reducing effects on intraocular pressure.
Thus, it seems that it is virtually impossible to mitigate the negative side effects of THC by using CBD instead. In fact, even a full-spectrum cannabis extract may not suffice in the treatment of glaucoma, given the study findings that CBD reduces THC ability to reduce intraocular pressure.
A comparison with pharmaceutical treatments
At present, lowering intraocular pressure is the only proven therapy for glaucoma. There have been several experimental treatments such as treatment with neuroprotective agent, memantine. However, this treatment was unsuccessful at slowing the progression of glaucoma.
Conventional treatments focus exclusively on lowering intra-ocular pressure, despite the fact that not all sufferers have high intra-ocular pressure and some people with high intra-ocular pressure remain asymptomatic. The treatment regime varies depending on the type of glaucoma.
For open-angle glaucoma, Prostaglandin analogues are the primary line of treatment. For those who don’t respond to this treatment, a range of options exist: β-Adrenergic blockers, α-Adrenergic agonists, carbonic anhydrase inhibitors and cholinergic agonists, though all are less effective than prostaglandin analogues.
In the event that a patient doesn’t respond to any treatment, there is an option for surgery. For open-angle glaucoma, laser trabeculoplasty is performed. It is effective at reducing intraocular pressure, although the effects don’t last forever.
For angle-closure glaucoma, the first line of treatment is to make a hole in the iris using laser peripheral iridotomy. If this doesn’t reduce pressure, drug treatments similar to those used in open-angle glaucoma can be used.
Acute angle-closure glaucoma is treated as a medical emergency due to the risk of vision loss. First, medications are used to affect an immediate reduction in pressure. An iridotomy is then performed.
Something that greatly separates pharmaceutical intervention from cannabis treatment is cost. Research has suggested that a patient would need to smoke 1.5 g of cannabis per day to maintain low intraocular pressure, at a total cost of $8,820 per year. This compares to an annual cost of anywhere between $150 and $873 for topical glaucoma medication in the USA.
On the contrary, Cantimol produced in Jamaica is expected to retail for about $250 per vial. This makes the annual cost of Cantimol very competitive with pharmaceutical options.
Recent years have seen the demonstration of endocannabinoid intraocular pressure mechanisms and the discovery of CB1 receptors within the eye. This suggests that cannabinoids are involved in the regulation of normal ocular tension, and has led to widespread interest in the development of cannabinoid-based glaucoma drugs.
Topical delivery of such drugs directly to the site of action should eliminate psychoactivity. Armed with such new medicines, the management of glaucoma will be able to move beyond the simple intra-ocular pressure reduction offered by conventional therapies, unlocking the following additional mechanisms through which the development of glaucoma may be managed:
Such new therapies will be particularly welcomed by individuals who develop glaucomatous optic neuropathy despite having intraocular pressures within the normal range. Provided such new therapies are supported by high quality clinical trials, there may be an entirely new angle through which to tackle the problem of glaucoma.
- Disclaimer:This article is not a substitute for professional medical advice, diagnosis, or treatment. Always consult with your doctor or other licensed medical professional. Do not delay seeking medical advice or disregard medical advice due to something you have read on this website.