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8. The therapeutic effects of cannabinoids
8.3 Cannabinoids as anti-glaucoma agents
Glaucoma is the leading cause of blindness in the United States, affecting two million people and producing 300,000 new cases each year (Adler and Geller, 1986). It is a condition "which is generally characterised by an increase in intraocular pressure ... that progressively impairs vision and may lead to absolute blindness" (Adler and Geller, 1986, p54). Although its causes are not understood, it is believed to involve an obstruction to the outflow of the aqueous humour in the eye leading to a gradual increase in intraocular pressure (IOP) which, if untreated, may damage the optic nerve, resulting in blindness. Its incidence increases over the age of 35, especially among individuals who are myopic (i.e. short-sighted). Although various drugs are available which reduce IOP, all possess unwanted side-effects and patients may become tolerant to their therapeutic effects.
The effects of cannabis in reducing IOP were discovered serendipitously by researchers and patients in the early and middle 1970s. Hepler and his colleagues (1971, 1976) observed a substantial decrease in IOP while researching the effects of cannabis intoxication on pupil dilation. They demonstrated that both cannabis and oral THC produced substantial reductions in IOP in both normal volunteers and patients with glaucoma (Hepler and Petrus, 1976; Hepler et al, 1976). Subsequent research identified THC as the agent responsible for producing this effect (Adler and Geller, 1986).
Around the same time, patients with glaucoma who had used cannabis recreationally also discovered its therapeutic effects. One such patient, Robert Randall, used cannabis daily to control his glaucoma. When arrested for possession and cultivation of cannabis, he successfully used the defence of "medical necessity" arguing, with the support of his physicians, that he would go blind if he stopped his cannabis use. He subsequently was given legal access to cannabis for medical purposes (Randall Affidavit, in Randall, 1988).
Although there have been a number of case reports of the successful use of cannabis in the management of glaucoma (e.g. Grinspoon and Bakalar, 1993; Randall, 1990), there have not been any controlled clinical studies of its effectiveness and safety in the long-term management of glaucoma. Informed clinical opinion has been that THC is an effective anti-glaucoma agent when used acutely, but there are doubts about its effectiveness with chronic use because of the development of tolerance to its effects on IOP (Jones et al, 1981). Ophthalmologists who are opposed to the clinical use of THC point to a number of major disadvantages. First, because THC is not water-soluble, it cannot, unlike other anti-glaucoma agents, be applied topically to the eye to ensure that enough is absorbed to produce a clinically significant reduction in IOP. Second, as a consequence, THC must be absorbed systemically in order to produce a therapeutic effect on IOP, which means that patients must experience the psychoactive effects of THC in order to derive its therapeutic benefits against glaucoma. Third, because glaucoma is a chronic condition, THC or cannabis would need to be taken in substantial doses on a daily basis over long periods of time, if not for the remainder of adult life. There has been an understandable concern about the health risks of chronic daily cannabis use (e.g. Hepler, 1990; American Academy of Ophthalmology, 1990).
The position adopted by the American Academy of Ophthalmology has been to insist that cannabis has no accepted medical use in the management of glaucoma, and cannot have such medical use until a large controlled trial has been conducted into its safety and effectiveness in daily chronic use. There has been no evidence that the Academy has any interest in, or has given any encouragement to, the conduct of such a trial. Consequently, its position is that THC and other cannabinoids should not be used be in the management of glaucoma.
A contrary position has been taken by Randall, who has argued that patients should be allowed to make the choice between the uncertain health risks of chronic cannabis use and the more certain risks to sight of poorly controlled glaucoma: "People with life- and sense-threatening diseases are routinely confronted by stark choices ... [between] the devastating consequences of a debilitating, progressive disease ... [and] often highly damaging biological and mental consequences of the toxic chemicals required to check the progression of disease. .. Viewed in this medical context, marihuana is more benign and far less damaging that the synthetic toxins routinely prescribed by physicians" (cited in Grinspoon and Bakalar, 1993, p153)