Survival analysis in stroke

Stimulated by Sangtin et al 2020.[1]

Photo by Robin Noguier on Unsplash. This image popped up when I put ‘survival’ and ‘Thailand’ in the search box on Unsplash.

This paper was published online in our very own Acupuncture in Medicine just the other day. I was drawn in by the term survival analysis, which has never appeared before in the title of a paper in our journal.

Another unusual feature is that it comes from Thailand, and I cannot recall many papers from this region before. We learn from the paper that acupuncture is used as an adjunct to routine post-stroke rehabilitation in this part of the world, and that up to one session per day can be reimbursed by the government for therapeutic or rehabilitative purposes as part of 1-month treatment plans. The value of reimbursement is 150-200 Thai Baht per day, which equates to 3.75 to 5 British Pounds.

I thought I better make that clear before all you keen acupuncture advocates who feel constrained by NHS rationing make plans to emigrate.

We do not see survival analysis written in the title of papers on acupuncture, indeed this may be the first time. I guess the reason for this is that by definition, this statistical method is generally applied to survival or more correctly the length thereof.[2]

Survival Analysis:

the outcome variable of interest is the time until an event occurs

In Survival Analysis the outcome variable of interest is the time until an event occurs. The authors here have selected an event that would not normally be associated with the word survival, but the statistical methods can still be applied. The event they chose was an improvement of 10 points or more in a scale (0–100) often used for functional assessment in stroke rehabilitation – the Barthel Index (BI).[3]

This is a retrospective observational study set over a one-year period. The main data used was the number of acupuncture sessions and the BI score, which appears to have been measured after each session.

Well they are not really curves

The results show the probability of significant improvement against the number of acupuncture sessions on Kaplan–Meier curves. Well they are not really curves unless you stand a long way back, and cannot see that they are actually more like staircases with irregular sized steps. Each step represents one or more event, ie one or more patients reaching the 10 (or more) point improvement on the BI.

Kaplan–Meier ‘curves’ from Sangtin et al 2020.[1]

So how do we know whether or not we are just observing the natural history of recovery after stroke. In short, we don’t. But there were some differences, in particular, those that started acupuncture within 3 months of stroke reached the event horizon (I have always wanted to use that phrase – it sounds so important) in fewer sessions than those that started after 3 months. Of course, there can be confounding explanations, but this is merely observation research.

Interestingly the severity at baseline did not seem to affect the probability of improvement against the number of acupuncture sessions.

The bottom line is that most stroke patients that were going to improve saw a significant improvement by the 16th acupuncture session.

Why aren’t we seeing a greater use of acupuncture in stroke rehabilitation in the West? Well this is because controlled trial data seems to show no advantage in using needles in the context of an optimum sensory environment. Acupuncture does appear to have an advantage when added to conventional stroke rehabilitation,[4] but not when compared to non-needling sham.[5] Having said that, I have just checked the most recent Cochrane review and it is not as negative as I was expecting.[6]

References

1         Sangtin S, Supasiri T, Tangsathitporn R, et al. Application of survival analysis techniques to determine the optimal number of acupuncture therapy sessions for stroke patients. Acupunct Med Published Online First: 20 January 2020. doi:10.1177/0964528419883268

2         Singh R, Mukhopadhyay K. Survival analysis in clinical trials: Basics and must know areas. Perspect Clin Res 2011;2:145–8. doi:10.4103/2229-3485.86872

3         Quinn TJ, Langhorne P, Stott DJ. Barthel index for stroke trials: development, properties, and application. Stroke 2011;42:1146–51. doi:10.1161/STROKEAHA.110.598540

4         Johansson K, Lindgren I, Widner H, et al. Can sensory stimulation improve the functional outcome in stroke patients? Neurology 1993;43:2189–2189. doi:10.1212/WNL.43.11.2189

5         Johansson BB, Haker E, von Arbin M, et al. Acupuncture and transcutaneous nerve stimulation in stroke rehabilitation: a randomized, controlled trial. Stroke 2001;32:707–13. doi:10.1161/01.str.32.3.707

6         Yang A, Wu HM, Tang J-L, et al. Acupuncture for stroke rehabilitation. Cochrane Database Syst Rev Published Online First: 26 August 2016. doi:10.1002/14651858.CD004131.pub3


Declaration of interests MC