A Follow-Up on a Post About Whether Anxiety and Depression Perpetuate Functional Limb Weakness

By David Tuller, DrPH

Zachary Grin, a physical therapist in New York City who specializes in treating patients with functional neurological disorder (FND), commented on my most recent blog after I posted a link to it on X, the social media platform formerly known as Twitter. As he noted in his series of tweets (or his series of Xs?), he found my post to be “very misleading.” Per his interpretation, the study I discussed was not investigating whether depression and anxiety were perpetuating factors in functional limb paralysis, or “FND-par,” but whether they had prognostic value. (The study also examined personality disorders, but the authors ultimately concluded that there was insufficient data on them to draw conclusions.)

Dr Grin quoted this passage from the abstract to support his point: “The impact of these conditions on the prognosis of FND-par has not been systematically reviewed. The aim of this study was to identify a potential prognostic effect of comorbid depression, anxiety, and/or personality disorder on prognosis in patients with FND-par.” (The use of both “prognostic effect” and “prognosis” in the same sentence is clunky and seems redundant; perhaps the study, like so many academic papers, could have benefited from a rigorous copy-editing process.)

As Dr Grin further noted: “Prognosis and perpetuating factors are not the same thing and this study does not show anxiety and depression aren’t perpetuating factors. You are misrepresenting this study.”

I have written many posts about FND, and Dr Grin has often challenged them. As in this case, he has done so politely, which I greatly appreciate because it differs from how many people—including sometimes me–approach such discussions on X. Although we rarely agree on these matters, I have generally enjoyed exchanging views with him. In this case, in the interests of dialogue, I invited him to submit a comment to the post on Virology Blog; I promised to highlight it while adding my response. So far, he hasn’t taken me up on this offer.

But I thought Dr Grin made an interesting point. And since I assume others might agree with him, it seemed worthwhile to write this follow-up to the post. It is certainly true, as he noted, that some factors or indicators might have value in predicting outcomes—in other words, prognostic value—without being directly implicated in perpetuating the illness of concern. Perhaps the factors, whatever they might be, are associated with or accompany the mechanism that is actually perpetuating an illness without being directly implicated in that perpetuation.

The flaw in Dr Grin’s argument, as I see it, is that the description he cited of the study’s goals was not the only way in which the authors themselves framed their research. The headline itself, for example, did not mention “prognosis” or “prognostic effect.” It stated explicitly that the study is about the “impact” of anxiety, depression, and personality disorders “on the outcome of patients with functional limb weakness.” The abstract’s conclusion indicated that the study “found no evidence that depression or anxiety influenced outcomes”—again without mentioning prognosis or prognostic effect. Notwithstanding Dr Grin’s concerns, these phrases indicate in plain English—at least to me–that the study examined whether anxiety and depression were perpetuating factors.

In other words, if the authors intended to make a subtle but clear distinction between the constructs of prognostic effect and perpetuation, they failed. Perhaps they would agree with Dr Grin’s reading of what they wrote; if so, they should have done a better job of explaining themselves. While Dr Grin suggested that I was conflating these two constructs in a problematic manner, my response is that the authors themselves were the ones responsible for any such conflation. I was only quoting their words.

In his series of tweets, Dr Grin included the following: “I hope others actually read the paper rather than take your word for it.” I definitely agree! I think, if others actually read the study, they will find that it does indeed appear to state unequivocally that, according to the available data, anxiety and depression were not shown to be perpetuating factors—not just that they have no prognostic value. If that was not the intended meaning, I would advise the authors to submit a clarification or corrigendum. From my perspective, Dr Grin’s interpretation does not hold water.