by Seshata on 11/02/2013 | Cannabis News

Cannabis and Glaucoma

In 1971, Hepler et al. reported in a letter to the respected journal JAMA Ophthalmology that use of smoked cannabis reduced IOP by 25-30%; however, opinions within the medical community remain divided

Glaucoma is a degenerative disease that affects the optic nerve, leading to permanent damage to the vision and possible blindness if untreated. The retinal ganglion cells of the optic nerve are progressively destroyed, for reasons that are not fully clear: genetics, diet and environment play as yet undetermined roles in the development of the disease.

High intraocular pressure

A diagram illustrating the progression of glaucoma
A diagram illustrating the progression of glaucoma

The disease is associated with increased intraocular pressure (IOP), caused by a build-up of the aqueous humour, one of the two main fluids found within the eye. However, although high IOP is an important risk factor, many sufferers of glaucoma-related optic neuropathy (nerve damage) exhibit normal levels.

In 1971, Hepler et al. reported in a letter to the respected journal JAMA Ophthalmology that use of smoked cannabis reduced IOP by 25-30%; however, opinions within the medical community remain divided, as many question whether cannabis is any more effective than currently-available treatments. Since then, various studies have been performed that have confirmed these findings, but their results have often proved inconclusive due to small sample size or other constraints.

The aqueous humour & the trabecular meshwork

Glaucoma may occur in various forms. Developmental glaucoma (in which the sufferer manifests symptoms from birth or during childhood) occurs in just one in 10,000 births, and results in blindness in around 10% of cases. The two main types of primary adult-onset glaucoma – open angle glaucoma (OAG) and angle closure glaucoma (ACG) – affect one in 200 adults below the age of fifty, and one in ten adults aged eighty and above.

The ‘angle’ referred to in this nomenclature is the area between the cornea and the iris, through which the aqueous humour flows on its way to the trabecular meshwork, the porous, spongy tissue responsible for draining the humour into the bloodstream. IOP associated with OAG results when outflow through the meshwork decreases due to degeneration of the trabecular tissue. In cases of ACG, forward displacement of the iris against the cornea completely blocks the flow of aqueous humour, leading to rapid fluid build-up and intense pain.

Open Angle Glaucoma and Angle Closure Glaucoma

OAG is a chronic condition that can take years to develop, and which commonly leads to severe vision loss as the disease painlessly progresses to an advanced stage before being noticeable to the sufferer—acute attacks are unknown. This form of glaucoma is the most prevalent by far, accounting for 74% of cases worldwide and over 90% in the USA.

High rates of ACG are associated with female, Asian and African populations
High rates of ACG are associated with female, Asian and African populations

ACG is more commonly an acute condition which progresses rapidly, but causes severe enough symptoms that sufferers usually seek emergency medical attention, leading to less overall damage than is often found in cases of OAG. ACG disproportionately affects women, East Asians and individuals of African descent: 87% of all ACG cases are found in Asian populations. It is thought that this discrepancy is due to the shallow anterior chamber (the cavity within the eye that contains the aqueous humour) typically found within these demographics.

Controversy regarding cannabis as a glaucoma treatment

Although several studies have been conducted since Hepler et al. (1971), it has not yet been possible to reach consensus due to the need for further, more exhaustive investigations of the complex biochemistry at work. Of the existing studies, results have been somewhat inconsistent, and much information of questionable validity has found its way to the patient. Dr. Keith Green, a notable researcher at the Medical College of Georgia, stated in 1998 that management of IOP would necessitate the smoking of “at least 3300 cigarettes per year.”

Dr. Green based this estimate on the results of past studies including Hepler et al., which had shown that the cannabis-induced decrease in IOP experienced by the majority of glaucoma sufferers lasted for just three hours. Green predicted that as treatment of glaucoma necessitates constant reduction of IOP, repeated re-administration of cannabis would be required as the effect faded.

The importance of the method of consumption

This alarming proclamation may have led to many sufferers feeling that cannabis was not a suitable treatment option, a conclusion which is not necessarily accurate. There are various problems with the given estimate, some of which are addressed within the article – such as the variable ratio of cannabinoids found in herbal cannabis, and the method of ingestion. As to the latter, far more research has gone into the effects of smoked herbal cannabis than to the effects of the individual cannabinoids, although THC at least has been studied separately.

While the negative effects of smoking cannabis are controversial, it is undisputed that dose-control is more problematic and that ratios of absorbed cannabinoids can vary wildly. There may also be more side-effects associated with smoking cannabis than with ingestion of the drug via sublingual applications, cannabis vaporizers or other alternative methods. Further, the amount required to successfully reduce IOP, as well as the duration of the effect, may vary between patients.

However, it is, in Dr. Green’s own words, “undisputed that smoking of marijuana plant material causes a fall in intraocular pressure (IOP) in 60% to 65% of users”. Despite the need for more precise research into just how this reduction is achieved, the bottom line is that cannabis has the potential to prove very useful indeed in the fight against glaucoma – which, as a disease that is estimated to affect over 60 million adults worldwide, is a condition well worth fighting.

The relationship between cannabis and glaucoma requires clarification

A typical case of acute glaucoma, showing vasodilation of the capillaries and an irregularly-shaped eyeball
A typical case of acute glaucoma, showing vasodilation of the capillaries and an irregularly-shaped eyeball

Despite ongoing controversy, several studies into the effects of THC and other cannabinoids on IOP have been undertaken. In 1979, a study showed that THC caused greater and more enduring drops in IOP in hypertensive glaucoma sufferers than in normotensive patients. A study from 1981 found that while topically-applied THC in various concentrations had been shown to reduce IOP in animal models, the findings could not be replicated in humans. A 1984 study on rabbits which tested thirty-two different cannabinoids concluded that certain derivatives of ?9- and ?8-THC were more active in lowering IOP than the parent cannabinoids.

A 2002 study that investigated the effects of orally-administered THC capsules alongside smoked cannabis found a general decrease in IOP with both methods of ingestion, but the small sample size of just 9 people rendered more accurate results impossible to achieve. A 2009 study into ?9-THC and cannabigerol in cats demonstrated that both cannabinoids markedly reduced IOP, but these findings are still to be replicated in humans.

Clearly, there is a definite link between the endocannabinoid system and the development of glaucoma. As our knowledge expands, we will no doubt learn a great deal more of this important relationship.

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