Long Covid — the name adopted for cases of prolonged symptoms after an acute bout of Covid-19 — is an umbrella diagnosis covering a broad range of clinical presentations and abnormal biological processes. Researchers haven’t yet identified a single or defining cause for some of the most debilitating symptoms associated with long Covid, which parallel those routinely seen in other post-acute infection syndromes. These include overwhelming fatigue, post-exertional malaise, cognitive deficits (often referred to as brain fog), and extreme dizziness.
Given the current gaps in knowledge, some neurologists, psychiatrists, and other clinicians in the United States, United Kingdom, and elsewhere have suggested that an existing diagnosis known as functional neurological disorder (FND) could offer the best explanation for many cases of this devastating illness.
We strongly disagree. Although prominent news outlets such as The New Republic and Slate have promoted this perspective, it is unwarranted to view long Covid through the lens of functional neurological disorder. Despite assertions of robust evidence from those most invested in promoting it, the FND construct is based largely on speculation and assumption. Successful treatments for long Covid are much more likely to emerge from investigations into the kinds of immunological, neurological, hormonal, and vascular differences that have already been documented than from the inappropriate imposition of an often ill-fitting diagnosis onto the broad swath of people with these prolonged symptoms.
Functional neurological disorder carries a lot of historical baggage. It is the updated name for the hoary Freudian diagnosis of conversion disorder, in which people are said to be “converting” psychological trauma into physical ailments like arm or leg paralysis. (Once upon a time not so long ago, some people with these conditions — especially women — would have been diagnosed with hysteria.) In the last two decades, clinicians have tried to rebrand conversion disorder, recognizing that their patients often disliked being told their symptoms were psychosomatic.
In 2013, psychiatry’s Diagnostic and Statistical Manual of Mental Disorders officially adopted the more neutral-sounding term “functional neurological symptom disorder” as an alternate name for conversion disorder. (In practice, the word “symptom” is usually dropped.) This update was accompanied by new diagnostic criteria. Beside the lack of a better explanation for the symptoms, a diagnosis of FND now also required the presence of positive clinical signs said to be incompatible with recognized neurological and other medical conditions, such as physical reflexes in an apparently paralyzed limb.
These days, neurologists and other experts frequently describe FND as a “brain network” disorder, with symptoms purportedly rooted in disruptions to “the predictive machinery of the brain,” faulty perceptions of self-agency, and hypersensitivity to bodily sensations, among other factors. FND is also described as a problem exclusively with the brain’s “software” (how the brain functions) rather than with its “hardware” (or structural elements). Where biological changes at the cellular, intracellular, or extracellular levels — such as possible alterations to mitochondria or to epigenetic profiles — fit within this schema, or if they do at all, remains extremely murky.
Following this line of thinking, recommended FND treatments include forms of psychotherapy and physical therapy specifically designed to address the presumed problems with the brain’s “predictive machinery” or “software.” Some people diagnosed with FND report improvement, with or without treatment. For many, the prognosis is poor.
Many people experience troubling symptoms that resist easy explanation. This fact has always been known to medical professionals, but the pandemic and the parallel wave of extended post-Covid-19 illness have heightened public awareness of the phenomenon. Individuals with such symptoms, whether or not they are related to long Covid, often face dismissal and neglect from the medical system and individual doctors. A diagnosis of FND from a compassionate health care provider can be a source of relief for many.
Yet many people with long Covid, or with prolonged symptoms from other causes, reject a diagnosis of FND, or complain that it has been given with only a cursory examination. Many find that getting it removed from their medical records can be difficult, if not impossible.
Depression, anxiety, and related mood disorders, which have been widespread during the pandemic, can obviously play a role in triggering and exacerbating symptoms. And in long Covid, these neuropsychiatric features might themselves be rooted in immunological or other biological aspects of the illness. In any event, like conversion disorder before it, FND as currently framed and as applied to those with long Covid is essentially impossible to prove or disprove.
Brain scan studies show that those with and without FND diagnoses differ in patterns of cerebral activity — or “software” usage, per the analogy preferred by FND proponents. But these studies document associations, not causal relationships. Research has also found differences in brain structures — what would be called “hardware” — between those with and without FND diagnoses. What all of these brain scan findings mean remains unclear, and is open to interpretation.
The 2013 rebranding of conversion disorder as functional neurological disorder focused awareness on a problem: the dearth of reliable evidence that the newly required clinical signs for FND accurately identify the condition. As a 2014 review noted, “These positive signs are well known to all trained neurologists but their validity is still not established.” Here’s our translation of that statement: “All neurologists are trained to associate certain clinical signs with conversion disorder or FND, but this association cannot be supported with data.” The situation hasn’t changed much in the years since.
The positive clinical signs represent an apparent mismatch between an individual’s symptoms — whether or not they arise from long Covid — and how the body normally behaves. But this mismatch does not indicate that the symptoms are caused by issues with the brain’s “predictive machinery,” faulty perceptions of self-agency, or the other hypothesized mechanisms of FND. Such unproven explanations are best interpreted as fancy ways of saying — or of avoiding saying — “we really don’t know.”
Even before the emergence of the Covid-19 pandemic, functional neurological disorder had entered a phase marked by the phenomenon known as diagnostic creep. Neurology papers routinely declared FND to be “common.” The website FND Guide, a popular resource for patients and clinicians, highlights the recent expansion of the diagnosis to include cases of “dizziness” and “cognitive problems.”
These emerging FND subtypes seem especially poised for growth in the coronavirus era. Many people with long Covid struggle with dizziness, which is often demonstrably related to problems with the autonomic nervous system. Many also experience profound cognitive changes, such as an inability to concentrate and frequent memory lapses, which has been demonstrably related to such factors as changes in blood supply to the brain and neuroinflammation.
However, it is mainly FND proponents who insist that the broad domains of “dizziness” and “cognitive symptoms” fall within their purview. Other scientists and clinicians dismiss the categorization of these and other complex manifestations of long Covid as forms of FND and are investigating pathobiological processes for answers. A recent study in a Nature journal, for example, reported a link between greater cognitive symptoms in long Covid patients and higher levels of a biomarker also found in multiple neurodegenerative diseases.
Papers on FND caution physicians that the clinical signs, while necessary for a definitive diagnosis, are not perfect and should be assessed alongside other information. However, if a patient is subsequently found to have Parkinson’s or multiple sclerosis or another disease, their earlier FND diagnosis is not necessarily rendered obsolete; rather, the patient is often said to have an FND “overlay” or co-morbidity. Ultimately, if clinicians rely on inflated claims about FND prevalence and about the accuracy and specificity of the required signs, they risk overlooking or ignoring other potential diagnoses or abnormal biological mechanisms.
With long Covid continuing to be a leading cause of disability, it is essential that physicians, researchers, and policymakers follow the science — and the science does not point to functional neurological disorder as a driving factor in this wave of illness. New studies document daily the long-term impacts of an acute coronavirus infection on multiple organ systems in the body, including the central nervous system. Individuals and clinicians are slowly learning how to manage some of the complex and life-altering symptoms, and how not to. To address this urgent medical issue, it is unnecessary and unhelpful to resort to categorical diagnostic assertions largely based — as was conversion disorder — on questionable arguments and unconvincing research claims.
David Tuller, Dr.P.H., is a senior fellow in public health and journalism at the Center for Global Public Health at the University of California, Berkeley; his academic position is supported by crowdfunded donations to the university, many from people with ME/CFS, long Covid, and related illnesses. Mady Hornig, M.D., is a psychiatrist, physician-scientist, and president of CORe Community, Inc. David Putrino, Ph.D., is the Nash Family Director of the Cohen Center for Recovery from Complex Chronic Illness, and a professor in the Department of Rehabilitation and Human Performance at the Icahn School of Medicine at Mount Sinai.