The very first article on a subject related to alternative medicine with a 2015 date that I came across is a case-report. I am afraid it will not delight our chiropractic friends who tend to deny that their main therapy can cause serious problems.
In this paper, US doctors tell the story of a young woman who developed headache, vomiting, diplopia, dizziness, and ataxia following a neck manipulation by her chiropractor. A computed tomography scan of the head was ordered and it revealed an infarct in the inferior half of the left cerebellar hemisphere and compression of the fourth ventricle causing moderately severe, acute obstructive hydrocephalus. Magnetic resonance angiography showed severe narrowing and low flow in the intracranial segment of the left distal vertebral artery. The patient was treated with mannitol and a ventriculostomy. Following these interventions, she made an excellent functional recovery.
The authors of the case-report draw the following conclusions: This report illustrates the potential hazards associated with neck trauma, including chiropractic manipulation. The vertebral arteries are at risk for aneurysm formation and/or dissection, which can cause acute stroke.
I can already hear the counter-arguments: this is not evidence, it’s an anecdote; the evidence from the Cassidy study shows there is no such risk!
Indeed the Cassidy study concluded that vertebral artery accident (VBA) stroke is a very rare event in the population. The increased risks of VBA stroke associated with chiropractic and primary care physician visits is likely due to patients with headache and neck pain from VBA dissection seeking care before their stroke. We found no evidence of excess risk of VBA stroke associated chiropractic care compared to primary care. That, of course, was what chiropractors longed to hear (and it is the main basis for their denial of risk) – so much so that Cassidy et al published the same results a second time (most experts feel that this is a violation of publication ethics).
But repeating arguments does not make them more true. What we should not forget is that the Cassidy study was but one of several case-control studies investigating this subject. And the totality of all such studies does not deny an association between neck manipulation and stroke.
Much more important is the fact that a re-analysis of the Cassidy data found that prior studies grossly misclassified cases of cervical dissection and mistakenly dismissed a causal association with manipulation. The authors of this new paper found a classification error of cases by Cassidy et al and they re-analysed the Cassidy data, which reported no association between spinal manipulation and cervical artery dissection (odds ratio [OR] 5 1.12, 95% CI .77-1.63). These re-calculated results reveal an odds ratio of 2.15 (95% CI.98-4.69). For patients less than 45 years of age, the OR was 6.91 (95% CI 2.59-13.74). The authors of the re-analysis conclude as follows: If our estimates of case misclassification are applicable outside the VA population, ORs for the association between SMT exposure and CAD are likely to be higher than those reported using the Rothwell/Cassidy strategy, particularly among younger populations. Future epidemiologic studies of this association should prioritize the accurate classification of cases and SMT exposure.
I think they are correct; but my conclusion of all this would be more pragmatic and much simpler: UNTIL WE HAVE CONVINCING EVIDENCE TO THE CONTRARY, WE HAVE TO ASSUME THAT CHIROPRACTIC NECK MANIPULATION CAN CAUSE A STROKE.
Here’s the full text to the case report cited above:
Ref: Jones J, Jones C, Nugent K. Vertebral artery dissection after a chiropractor neck manipulation. Proc (Bayl Univ Med Cent). 2015 Jan;28(1):88-90.
Re the classification error of cases by Cassidy et al, I hope that the American Chiropractic Association will be mailing the 16,600 neurologists – to whom it sent the original paper – notifying them of that error:
since chiropractic seems to be rather unknown in our parts, how does the “parent discipline” osteopathy fare?
I would be interested if the proven benefits with back pain (wikipedia) are superior to say, a regular massage (in place of a placebo). This for example could be added to regular medicine if it turns out favorable.
(Sorry to bring up something potentially off-topic / off-quack)
This is nothing new. Unfortunately, as with many medical procedures, there are risks. You can go into surgery to have a kidney stone removed and you can die. You can be sedated during a dental tooth extraction and never awake. There are some risks with chiropractic manipulation, but the reality is there are far far fewer risks with manipulation than other medical procedures. You can probably count on one hand how many people have a stroke following chiropractic manipulation each year. It’s not something to take lightly in any way, but the risks are incredibly low. I have been a doctor of chiropractic since 2008 and have adjusted thousands of people, and in that time I have “injured” two or three people, the worse being muscle strain.
Students are taught have to adjust the spine in a very gentle and careful way. Methods have changed significantly, and the forces applied for spinal manipulation have be minimized greatly.
Millions of chiropractic adjustments occur daily throughout the U.S., and few experience even a muscle strain. On the other hand, botched medical procedures kill in the tens of thousands of people each year. There is barely a reason to even be discussing this.
“This is nothing new”…the re-analysis and the fact that the Cassidy results are wrong are both new.
“You can go into surgery to have a kidney stone removed and you can die. You can be sedated during a dental tooth extraction and never awake.” … in both instances, there is evidence that the interventions normally generate more good than harm. Not so with chiropractic upper spinal manipulation.
” the reality is there are far far fewer risks with manipulation than other medical procedures”…how do you know? there is no monitoring system!
” I have been a doctor of chiropractic since 2008 and have adjusted thousands of people, and in that time I have “injured” two or three people, the worse being muscle strain”… the plural of anecdote is anecdotes, not evidence.
“Students are taught have to adjust the spine in a very gentle and careful way. Methods have changed significantly, and the forces applied for spinal manipulation have be minimized greatly”….any evidence for that?
“. On the other hand, botched medical procedures kill in the tens of thousands of people each year”…classical fallacy.
The difference is obvious. Someone who dies during a science-based medical procedure has undergone real treatment, the person who has a stroke after chiropractic “treatment” is simply the victim of pseudo-scientific nonsense. Of course there are cases of malpractice, but real medicine has systems in place to address these.
With all due respect, Jayson, I think you are committing a category error along the lines: All animals make mistakes that cause harm to other animals; the few mistakes made in chiropractic adjustments are so insignificant that there is barely a reason to even be discussing them.
Air travel is the safest means of transport simply because each and every accident is fully investigated and (usually) the whole industry is mandated to make upgrades to their products and services based on inescapable evidence. Your argument is similar to: Well, cars cause so many more deaths and injuries than aircraft therefore why even bother to investigate or discuss aircraft accidents?
The focus should never be placed on all the things that happened to turn out well; the primary focus must remain firmly placed on reducing the number of things that go catastrophically wrong. Please read:
Jayson, the problem here is that this is a “medical procedure” with no proven curative effect, thus there is no benefit to offset the risk. It’s like giving an active drug as a placebo: it won’t do any good, and it has the potential to cause harm.
“You can probably count on one hand how many people have a stroke following chiropractic manipulation each year.” When I had my vertebral artery dissection due to a chiropractic adjustment (at 25 years old, no pre-existing conditions, still no medical conditions that would indicate such a thing), my doctor in the hospital I stayed at for 2 weeks, told me that he had seen 8 people already that year who had had stroke due to chiropractic manipulation. I was the 9th. It was June. The year was only half over. This was in a semi-small city/hospital. Pretty sure the numbers are MUCH higher than that. Just go look at the facebook groups created for survivors…
Even if they don’t cause stroke(which they can). There are many other adverse events they can cause such as nerve and arterial stretches, herniated discs, and brachial plexus injuries. I speak from personal experience. Cervical thrust manipulation ruined my life. I went to see a chiropractor for low back pain and all he did was turn my head to one side and pulled then the other side. I asked why he was pulling on my head when it was my back that I was having trouble with? He responded that the type of therapy he used treats the whole spine. When I left and had been home for about an hour I experienced the worst headache I’d ever had. He called to find out why I didn’t return for my next treatment and when I told him why, he told me to please come in so he could talk to me. He told me that my subluxations in all areas of my spine were so severe that things would get worse before they got better. After the 4th visit I could hardly walk due to dizziness, had the worst head and neck pain in my life, was vomiting, and felt like I had a virus attacking every muscle in my body. Called my G.P. Who sent me straight to the e.r. I had an arterial and nerve stretch, herniated disc with annular tear at c-6-c7, and a brachial plexus injury. I went through horrific treatments and surgeries and had my life turned upside down. Every Chiropractor who has a degree from the Chiropractic college in Georgia, whose founding member is a crook and a half and belongs in prison, should have their licenses stripped. There are reputable chiropractors out there but the quacks from this group need sent to prison. They make false claims and mislead the public. They are also predatory and many are scamming medicaid and regular insurance companies. The problem is that most sue these quacks but have to sign hush waivers basically.
Jayson, unless you provide evidence, that your students are really taught to be careful …, and I do not mean only manipulation itself, but also ability to recognise serious conditions, like, e.g. kidney problems in young persons (they may have worse osteoporosis than an old lady), Marfan’s syndrome, cancers (primary or secondary), because these may lead to trauma….
And then: if kidney stone can be removed only by operation (depends on composition, form, state of the kidney), then operation must be done, whereas neck manipulations is far from the only thing you can subject your body to.
And, by the way, do you teach your students to leave alone patients who are simply to stressed and need some rest?
My story on this in Washington Post: How safe are the vigorous neck manipulations done by chiropractors?
Of course there is risk! Here are some recent articles on the topic from 2014:
This should then lead to the importance of Informed Consent:
It’s my understanding that osteopaths and chiropractors in Australia routinely seek informed consent. Does this differ elsewhere?
In Australia it is a mandatory common law requirement for all health professions. It came into force around 2000! It is checked by the national registration board and my professional indemnity insurer every year!
Unfortunately this study does not give any details of why this patient consulted with her chiropractor or the nature of the treatment provided. Poor quality case reports do little to help extend our knowledge of association and causation see http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3604043/.
I don’t argue with the intent of this piece, nor with your conclusion as written, but it is, as you say, a single case study with no indication of risk to the population at large. It illustrates the potential harm and is particularly useful because the injury is explored in depth, and I agree that it seems very likely that the high-velocity thrust caused the complications, but it is still just a case study. Yes, better studies are required to gauge the risks and cost-effectiveness of the treatment. Yes, chiropractors and other proponents of SMT should take more care not to overstate the benefits nor understate the risks. However, the fact remains that a great many people appear to derive significant relief from neck and head pain via SMT. If you can use a case study to demonstrate harm then anyone can use umpteen case studies to demonstrate benefit and put their conclusions in bold capital letters besides.
no, my post referred to 1) a case-report and 2) a re-analysis of a large case-control study.
where is the evidence that upper SM is effective for any condition?
Your post leads with and emphasizes the case study and the re-analysis says only that the risk is likely to be higher than the original analysis estimated. My point on case studies stands. As for the evidence: this is why I chose the word ‘appear’, carefully. Appearances aren’t everything but nor should they be disregarded, especially when in apparently large numbers.
informed consent is an ethical obligation for all clinicians. however, there is evidence that UK chiropractors tend to disregard it.
you appear to be just a tinge biased.
An interesting response. Well, we all have our biases. No one, not even the best scientist-advocate, is free of subjectivity, born of experience and values. Those values and experiences don’t necessarily invalidate their views, do they? I’m not a practitioner, but I am supportive of manual medicine, generally and not without reservations. I fully and freely admit that I have derived benefit from upper-cervical manipulation and that I know many others who have too. Whilst these experiences aren’t on the same par as, say, an RCT, they help to inform my views, as do your views together with the scientific literature.
And you? How are you biased?
As something of an aside, you may find this interesting: http://bos.sagepub.com/content/71/1/70.long
I have studied the evidence for > 20 years and tried my best to minimise bias.
I wasn’t questioning your expertise only asking after your values, experiences, and how these have shaped your advocacy and study. Perhaps one day, in another forum, we can have a chat about it.
Well at least you tried!
To you, the only clinical correlations within chiropractic that deserve recognition are those between SMT and VAD. Please understand that this is bias. You do not give credence to literature detailing those successes derived from chiropractic care and you do not hesitate to support every study that attempts to link SMT and VAD.
it’s called the ‘precautionary principle’ – one of the most important issues in health care
The most amusing part of our difference in opinion is that we extend our same precautionary disbelief to opposite realms of health care. You fear that people who believe that the body possesses the ability to heal itself will eat, drink and smoke whatever they like because a simple “adjustment” or “detox pill” will make them healthy again. I extend that same opinion to those who take multiple pharmaceuticals throughout the day.
Now to this article: if the transverse process of C1 is used as the lever to afford movement of the vertebra, the vertebral artery is not at risk for compression against the posterior arch. If a gentle, vectored force is applied to the transverse process, the patient is at no more risk to let loose a thrombus than if the patient were to move his or her cervical spine through the full ranges of motion.
Whether it is 1:400,000 or 1:1,000,000 or 1:5,850,000 which is a very rare incidences under medical classification, there is still risk and it must be discussed with patients. I have been utilising an informed consent form since 1995 and it is on its 6th revision. This rare incidence also makes research problematical when it comes to establishing causation, but having said that, extreme caution and a valid clinical reason when it comes to the upper cervical spine is always warranted!
… and a risk/benefit ratio that justifies the procedure. This ratio should be weighted positively for the potentially disastrous severity of a vascular event.
I just took a look at the Cai et al. paper. I’m not sure we can say that these authors performed a re-analysis of the Cassidy data. They used a different data base from the VA and “conducted a sensitivity analysis by applying the positive predictive value measured in the VA data to aggregated data reported in the Cassidy study.” They did not have data on the exposure to SMT for patients in the VA data base. I am no statistical expert so I am not sure how accurate or proper the methods performed by Cai are. The gist of all of this, in my mind, is that Cassidy et al. likely underestimated the OR of VAD for patients under 45 who were treated by chiropractic. So we can perhaps say that the OR is likely higher then Cassidy et al. report but we really don’t have a definitive number? There is a big difference between an OR of 2 reported by Cassidy versus 7 estimated by Cai. There are some other limitations such as the VA data base being ~ 90% male. It would be helpful if Cassidy and colleagues re-analyzed their data from Ontario using the same ICD-9 codes as Cai et al.
The obvious way for chiropractors to address this constant criticism would be to institute proper systematic recording of adverse events. They appear to reject this on the basis that they deny there is a problem, but without systematic recording they actually have nothing on which to base that view other than blind faith.
But the chiros already have such a system! Chiropractic Patient Incident Reporting and Learning System has been running for a while, building up an anonymous database of incidents, the harms and what caused them so that chiros can be learn from mistakes made and bad practices and patients can be better protected in the future.
Oh! Wait. The data are confidential and not made public and I don’t think it’s used much. As I understand it, not even the statutory regulator has access to it…
Indeed. It looks like it’s next to useless:
Even the CEO of the ‘Royal’ College of Chiropractors says the full benefits of the system can only be realised with more use and participation:
Alan Henness wrote: “As I understand it, not even the statutory regulator has access to it…”
That’s my understanding, too. Also, the regulator (the General Chiropractic Council – GCC) seems to have dug a hole for itself when it claimed the following in 2006:
“*IF* chiropractors are to provide a safe clinical experience for patients then a reporting procedure needs to be put in place, within the clinics and within the profession as a whole, which allows for adverse events and near misses to be shared on an anonymised basis so that we can all learn from them.”
The above is quoted from Item 7 of the Minutes of the GCC’s 2nd March 2006 council meeting which could previously be found here: http://www.gcc-uk.org/files/link_file/C-020306-Open1.pdf
Since we know that the CPIRLS is being under-utilised, it would appear that the GCC’s recommendations for patient safety are not being fully met. Further, since a large part of the GCC’s remit is to protect patients, you have to wonder why it’s not developed its own robust adverse event event reporting system during the 14 years that it has existed.
Is this the same case history? Sounds the same except Ernst, Jones (the author) and Blue Wode left a few things out. By the way, the case report is over 3 years old, not new for 2015.Just dredging old injuries up. The woman had a connective tissue disease that should have been a red flag toward aggressive SMT. Jones doesn’t mention that the second time around.
#207 VERTEBRAL ARTERY DISSECTION AFTER CHIROPRACTOR VISIT
Jones J TTUHSC, Lubbock, TX. Case Report: Vertebral artery aneurysms/dissections occur in both men and women at an average age of 48. They are more common in patients with a history of connective tissue diseases, such as Marfan’s Syndrome or Ehlers- Danlos. They are also associated with neck trauma or manipulation, such as chiropractic maneuvers, sports, yoga, coughing, falls, and ceiling painting. One in 20,000 spinal manipulations results in an aneurysm/dissection and cerebrovascular accident. There is coexistent subarachnoid hemorrhage in 50-66% of adult cases. We present the case of a young woman with a ver- tebral artery dissection after a chiropractor visit. A 38-yr-old woman with a past medical history of Poland’s Syndrome presented with complaints of headache, nausea/vomiting, blurry vision, diplopia, dizziness, and ataxia for two to three weeks which developed after a recent visit to her chiropractor. Her level of consciousness was also decreased. She was not taking any medications on admission and denied tobacco, alcohol, and illicit drug use. On physical exam, vital signs stable. The patient was drowsy but aroused to sternal rub. She was not oriented and followed simple commands poorly. She demonstrated nystagmus to the left and left sided weakness. CBC and elec- trolyte panel were normal. Urine drug screen and a hypercoagulable workup were negative. CT showed an acute left cerebellar process with extensive edema, mass effect, obstructive hydrocephalus, and possible tonsillar herni- ation. A VP shunt was placed. Subsequent MRI showed an acute left cere- bellar infarct, involving the posterior inferior cerebellar artery territory and the anterior inferior cerebral artery territory with hydrocephalus and pneu- moventricle. MRA of the head and neck showed low flow and severe nar- rowing at the intracranial segment of the left distal vertebral artery near the basilar artery. The patient received mannitol for 7 days and gradually became more alert and responsive. At discharge, her limb strength and sensation were at baseline, but had some residual left-sided facial weakness and impaired sensation. Vertebral artery aneurysms/dissections are known complications of spinal manipulation procedures. The differential diagnosis for patients who present with headache, nausea, diplopia, and ataxia should always in- clude vertebral artery aneurysms, especially if there is a recent history of a chiropractic visit.
Chiropractors have been warned by national organisations of risks associated with CMT long before this old case history was recycled.
An old warning.
Do you still think all DCs are unaware that bad things can happen?
In relpy to Alan and Blue:
The CPiRLS adverse event reporting should not be the job of the college. That is the responsibility of the registration board and legislators. We had a similar discussion here in The Medical Journal of Australia early last year and it highlighted a lack of adverse event reporting across the board for chiro’s, physio’s, osteo’s, GP’s in private practice etc. The Orthopod John Cunningham made a very good point in the comments section, where he proposed a standardised reporting system in hospital Emergency departments that then gets sent to the relevant registration board for investigation.
The physio’s have dodged a bullet in regards to this issue and they know it, they even discuss it here http://www.jospt.org/doi/abs/10.2519/jospt.2010.3229
Why no reply to my post? Your very first article on a subject related to alternative medicine with a 2015 date that you came across is a case-report, that’s over three years old and been ” edited “.
Sorry if You are afraid it will not delight our chiropractic friends who tend to deny that their main therapy can cause serious problems.
The patient in your new report had a connective tissue disorder that should not have had manipulation, especially to her neck. That this happened is an error by the practitioner and not the profession.
By the way, how many thousands have probably died or had strokes by chiropractors since that original article?
If you have followed the blog for the past days you may have noticed that Dr. Ernst has been busy with something more interesting than answering your arguments, which are groundless.
The woman had Poland’s syndrome according to the older text. This is not a connective tissue disorder. It is a congenital malformation involving unilateral non-development of the pectoral musculature and hand deformity. It is not related to a detectable genetic defect and only very rarely seems familial. The most likely cause being a disturbance of the ipsilateral subclavian vasculature in the developing fetus. It is not associated with neck or head abnormalities. Therefore it is quite adequate to state in the 2015 definitive publication that the woman had no significant previous medical history.
I see no problem with this.
Dr. Ernst did not state that this was a “new” case history. He only wrote that the paper he came across was dated 2015. The publication date is January 2015. Of course the case happened before that. It is quite common for clinicians to present interesting cases first as posters or short talks at conferences. A booklet is usually published beforehand with the different presentations numbered for reference and an abstract of the poster or talk. This text can often also be found online as you seem to have done The work is then usually written up as an article, changes made to reflect points of critique and suggestions from the audience (one form of peer review) and published in a journal or, as in this case, in the institution’s proceedings. The text you cut and paste is obviously from the prior list of conference abstracts. It is easy to imagine the bombardment of questions after the presentation, regarding the significance of Poland’s syndrome to the injury and the author decided, after reading up on the matter, that it was not significant to the case history.
I see no problem with this either.
Thank you for this article. I am 34 and suffered a stroke at 31 immediately following neck manipulation. I have had some people actually tell me that that is a load of bs, and that in can in no way cause a stroke. I have now “met” dozens of people who have had this happen. I am thankful more and more science is supporting this..even if it is a rare occurrence, people need to be made aware that it is possible.
I am also 31 and had a stroke immediately after a neck manipulation. I literally had a stroke on the chiropractic table. Before this I was 10% bodyfat, ate a really clean diet, all my blood was was good (was approved for life insurance just before) worked out 5 days a week etc.
The people in ICU at the hopistal told me that they see 1-2 cases a month of stroke after visit a chiropractor. I have experience pain relief from chiropractic for my lower back etc… I tell people not get their neck adjusted anymore.
If this is such b.s. then why are there young and healthy people that have this happen? I have no study to back it up, besides the fact that I had a stroke in the office immediately after the adjustment.
very sorry to hear your story; has this ever been published in full detail in a medical journal?
I, too, am sorry to hear about the devastating consequences arising from your chiropractic encounter. FYI, there is a new website in Australia that’s just been set up which is inviting victims of neck manipulation injuries to contact it: http://www.neckdamage.org/
The site also carries a survey aimed at medical doctors and surgeons in an effort to find out how many people are being injured by neck manipulation.
Dear Dr. Ernst
In Manitoba Canada, we have formed a Group called the Manitoba Chiropractic Stroke Survivors and are petitioning our provincial government to apply conditions, restrictions and limitations on high velocity low amplitude manipulation of the vertebrae – in particular the cervical vertebrae. Our provincial government is in the process of assessing and allocating “reserved acts” to profession specific health care providers through profession specific regulations. Reserved acts are considered to be acts that pose a significant risk of harm to the public and high velocity low amplitude manipulation of the spinal vertebrae is one of those “reserved acts”. The government has assured our Group that we will be given the opportunity to present our position. If it is at all possible that you might be of some assistance in formulating and presenting out position it would be very much appreciated.
please send me an email via this blog.
And taking an aspirin can cause gastrointestinal hemorrhage.
Almost every intervention carries risks. We have to inform ourselves of the risks to reduce the likelihood of any serious complications from whatever intervention that we make.
As you know there are thousands of medical mishaps that happen every day due to reactions to medicine or due to surgical interventions which have been deemed necessary and low risk.
It does not mean that we do not do those interventions anymore but it does mean that we must drive to ensure that the risks are as low as possible and that the patient is aware of the risks however low
“And taking an aspirin can cause gastrointestinal hemorrhage.
Almost every intervention carries risks.”
But the difference is that Aspirin is effective and manipulation isn’t.
Aspirin is effective?
“We found high-quality evidence that paracetamol (4 g per day) is no better than placebo for relieving acute LBP in either the short or longer term. It also worked no better than placebo on the other aspects studied, such as quality of life and sleep quality.”
you are surpassing yourself!
1) aspirin is not paracetamol
2) neck pain is not low back pain
ANY NONSENSE TO DEFEND THE INDEFENSIBLE?
Please share a meta-analysis of aspirin for neck pain.
This paper makes one problem with the Cassidy paper clear: they diluted any signal from chiropractors causing strokes, with strokes that weren’t caused by a cervical artery dissection.
However it doesn’t explain why the Cassidy paper found that the younger people who’d had a stroke were a lot more likely to have seen their PCP shortly before.
They did say
but this is a lot less likely to apply to young people.
It would have been good if they had also re-analyzed the data used in the Cassidy study to see if the young people who had strokes caused by a dissection were also a lot more likely to have seen their PCP shortly before than the controls who hadn’t had a stroke.
The Cassidy paper used the people who’d seen their PCP shortly before a stroke, as a kind of control for people who’d seen a chiro shortly before a stroke. And it turned out that young people were ALSO a lot more likely to have seen their PCP shortly before the stroke. This has been used to argue that the chiropractors aren’t actually causing strokes, they’re just seeing people who had a stroke in progress.
So it would have been good to re-analyze the Cassidy data, to find out if the young people who had strokes were also more likely to have seen their PCP shortly before.
As I stated elsewhere, the Cassidy study has issues. What i find interesting is how some go on about how bad of a study it was but then they pull out one little piece of data and claim that finding is valid. I suppose it pads to their confirmation bias.
How the odds ratios in the Cassidy study were re-estimated to account for the strokes that were misclassified as CAD strokes:
So this was a rough attempt to clean up the Cassidy data, which didn’t apply to PCP visits before a stroke.
They also say
So that explains why they didn’t do the same analysis for PCP visits before a stroke: they knew the patients who were at risk for stroke due to atherosclerosis would be seeing their PCP more often than controls.