Increased intraocular pressure (ocular hypertension) is the risk factor in the majority of glaucoma cases, although it is absent in certain populations in almost 50% of individuals suffering from open-angle glaucoma (OAG)—the most common type, comprising around 90% of all cases. Intraocular pressure is determined by the production of aqueous humour in the eye coupled with the rate of drainage of the aqueous humour via the trabecular meshwork, a spongy tissue located at the base of the cornea. Consistently high intraocular pressure can cause progressive damage to the optic nerve and the retinal ganglion cells that contain light-sensitive photoreceptors. If damage is severe enough, total blindness can result.
During his decades-long tenure at the University of the West Indies, the respected pharmacologist Professor Manley West conducted landmark research into cannabis as a potential treatment for glaucoma alongside the noted ophthalmologist Dr. Albert Lockhart. As a result of their work, which began in 1964 and investigated traditional cannabis use in Jamaican communities, they developed cannabis-based eye-drops and gained approval to market their product in Jamaica under the name “Canasol”, in 1987. Professor West sadly died in 2012, and will be much missed by the medical cannabis community in Jamaica and beyond.
Canasol has been demonstrated to be effective in lowering intraocular pressure, and has also been shown to potentiate the effectiveness of other widely-used glaucoma drugs such as Timolol. Recently, a new and improved drug was developed by the same research team; this drug is known as Cantimol, and contains Canasol and Timolol, but has not yet been approval for market. Canasol contains no psychoactive cannabinoids and has also been widely prescribed by physicians in the USA and Canada—and has apparently been used by patients in the UK and Australia too, despite not having market approval or a legal precedent for use.
A common phenomenon found in glaucoma patients is mydriasis, in which the pupil becomes dilated. In fact, it is thought that extreme mydriasis may actually cause the characteristic bulging-out of the iris in attacks of closed-angle glaucoma—in order for the pupil to dilate, muscle tissue in the iris (known as the dilator pupillae) must contract. When muscle tissue contracts, it bulges, and in extreme cases this bulging tissue can expand into the anterior chamber (the fluid-filled space between the iris and cornea) and press up against the cornea, totally preventing escape of aqueous humour via the anterior chamber and into the trabecular meshwork.
The opposite of mydriasis is miosis (not to be confused with the form of cell division known as meiosis). Miotic drugs are highly useful in glaucoma treatment, not only CAG, as constriction of the pupil allows increased drainage of aqueous humour into the trabecular meshwork. Cannabis has been shown to possess miotic properties on various occasions, including one notable case of apparent acute cannabis poisoning in a 20-month-old infant.
While the majority of glaucoma cases are painless, the rarer form of the disease (known as closed-angle glaucoma or CAG) is characterized by sharp, stabbing pains in the eyeball and rapid loss of vision, which may be permanent if not given medical attention. Closed angle glaucoma differs from the more common form in that it produces acute symptoms instead of chronic (and generally painless) symptoms. An attack of CAG is considered a medical emergency, and occurs due to a total blockage of the trabecular network (rather than a progressively reduced ability to drain aqueous humour, as with OAG) caused by a sudden bulging forward of the iris.
Although not specifically tested as an analgesic in glaucoma cases, cannabis’ wide-spectrum efficacy as an analgesic, mood-enhancer and muscle relaxant may provide subjective relief to individuals suffering from acute attacks of closed-angle glaucoma.
On the basis of recent research, some medical professionals are beginning to suspect that persistent, low-level inflammation in the trabecular meshwork may have an important role to play in the progressive, chronic form of glaucoma, OAG. As well as this, a further type of glaucoma that is often found in patients suffering from uveitis (inflammation of the uvea, the area of the eye that contains the iris and the ciliary body) is known as inflammatory glaucoma as it has been proven that inflammation of the trabecular meshwork is to blame. In most respects, inflammatory glaucoma resembles OAG, but is characterized as occurring as a secondary symptom of uveitis.
Cannabis has been proven time and time again to be a useful and effective anti-inflammatory for a range of different conditions, although no formal studies have been conducted on the potential of cannabis to reduced glaucoma-related inflammation. As understanding of the role inflammation has to play in the progression of the disease increases, it may well prove to be the case that cannabis also provides relief to glaucoma patients due to its anti-inflammatory properties.
Attacks of closed-angle glaucoma are often accompanied by secondary symptoms of nausea and vomiting, which is believed to arise as a result of a phenomenon known as the oculoemetic reflex. Various past studies have shown an association with ophthalmic surgery and post-operative vomiting (particularly squint surgery, which causes vomiting in 41% of patients), which gave rise to the possibility of the oculoemetic reflex existing.
It is believed that noxious (painful or unpleasant) stimulation of the orbital nerves that surround the eye sends signals to the area postrema of the medulla oblongata, the section of the brain that is often simply known as the vomiting centre. The brain then sends signals to the gastrointestinal tract via the vagus nerve (a fundamental part of the parasympathetic nervous system, which controls cardiac and gastrointestinal function), which in turn causes the vomiting reflex by stimulating retroperistalsis (backward/upward movement of GI tract contents).
It is not known what precise role the endocannabinoid system has to play in regulation of vomiting, but it has been shown that cannabinoid receptor agonists such as THC appear to directly suppress vomiting and nausea by agonizing the CB₁-receptor –while antagonists of the CB receptors such as CBD are neutral, and inverse agonists actually cause nausea.