Any research related to cannabis is bound to be sensationalized or politicized because people have strong emotional and political views about its usage. A few months ago, my fellow Scilogs blogger Suzi Gage wrote an excellent blog post about a study that investigated the link between cannabis usage and intelligence. That study had many critical flaws which were often ignored when the research was reported and discussed in the media. All research should be conducted and reported cautiously. However, when research touches on highly controversial topics, it is even more important that researchers clarify whether their research establishes statistically rigorous associations and true cause-effect relationships or whether they are merely pointing out important observations that require further research to derive definitive conclusions.

The recent paper entitled “Cannabis-related Stroke: Myth or Reality?” published by Wolff and colleagues in the journal Stroke investigates whether cannabis usage is related to stroke, concludes:

“In regard to the literature, cannabis-related stroke is not a myth, and a likely mechanism of stroke in most cannabis users is the presence of reversible MIS induced by this drug. The reality of the relationship between cannabis and stroke is, however, complex because other confounding factors have to be considered (ie, lifestyle and genetic factors). To confirm that cannabis may be a precipitating factor of RCVS with severe complications, an epidemiological study to determine the incidence of MIS, complicated or not by stroke, in the general population and in the cannabis users is necessary.”

The abbreviation MIS stands for multifocal intracranial stenosis, and refers to the presence of multiple blockages in the blood vessels of the brain that are impeding the blood flow and causing the stroke. RCVS stands for reversible cerebral vasoconstriction syndrome and describes a transient spasm of the blood vessels that briefly interrupts the blood flow to the brain, thus causing the stroke. The novelty of the paper by Wolff and colleagues is their idea that cannabis usage causes reversible MIS, i.e. that cannabis transiently causes multiple blockages that are reversed when cannabis is removed.

This is indeed an interesting idea, but unfortunately, they do not present any convincing data to back up this intriguing hypothesis. The data in their paper consists of a table listing 59 cases of stroke in patients who used cannabis, combining their own data with data that has been published by others. The authors admit that many of the patients who used cannabis were also active users of tobacco and alcohol, which makes it difficult to attribute the stroke to cannabis as opposed to these other confounding factors.

Most of the patients had some sort of brain imaging performed to diagnose the stroke, but less than half of them had a follow up scan later on to see if the blockage was still present. In those few cases where follow up imaging was performed, most of them did show some degree of reversibility of the blockages in the brain blood vessels. However, they do not present data on whether strokes in non-cannabis users also show similar reversibility patterns.

Wolff and colleagues also reference an older 2001 paper “Triggering Myocardial Infarction by Marijuana” by Mittleman et al and state that their current findings are in accordance with the conclusions of the 2001 paper. The paper by Mittleman and colleagues studied heart attacks and not strokes, but both diseases are caused by reduced blood flow, so it is not unreasonable to compare the data. The 2001 paper stated that the risk of having a heart attack is increased 4.8 fold within an hour of using cannabis. However, one has to bear in mind that the 2001 paper studied heart attacks in 3882 patients, of whom only 3.2% used cannabis and only 9 patients had used cannabis within an hour of the heart attack. The 4.8 fold risk determination was therefore based on this tiny sample of 9 patients!

In summary, the paper by Wolff and colleagues does not really answer the question of whether cannabis-related stroke is myth or reality. The small sample size, the observational nature of the data, the lack of follow up imaging on all the patients and the lack of controlling for confounding risk factors such as tobacco (which has a very strong association with stroke) make it difficult to draw definitive conclusions. All we can say is that Wolff and colleagues have presented an intriguing hypothesis that cannabis might cause strokes by inducing transient blockages or spasms of blood vessels in the brain. We need more definitive data to determine how cannabis usage is “related” to strokes: Is it a true cause of stroke or is it just an indicator of other more established risk factors, such as tobacco usage.

Wolff, V., Armspach, J., Lauer, V., Rouyer, O., Bataillard, M., Marescaux, C., & Geny, B. (2012). Cannabis-related Stroke: Myth or Reality? Stroke, 44 (2), 558-563 DOI: 10.1161/STROKEAHA.112.671347