Postinfectious myalgic encephalomyelitis/chronic fatigue syndrome (PI-ME/CFS) is a distinct, centrally mediated condition, with evidence of autonomic, immune, and metabolic dysfunction, new "deep phenotyping" data suggested.
The study was initiated in 2016 at the US National Institutes of Health. Its aim was to better elucidate the underlying pathophysiology of ME/CFS, a multisystem disorder characterized by persistent and disabling fatigue, post-exertional malaise, cognitive complaints, and other physical symptoms. A total of 17 carefully selected individuals with PI-ME/CFS onset within the prior 5 years were compared with 21 healthy volunteers on a more extensive set of biologic measurements than has been examined in any prior study of the condition.
Overall, the findings suggested that ME/CFS is "a distinct entity characterized by somatic and cognitive complaints that are centrally mediated," with fatigue that is "defined by effort preferences and central autonomic dysfunction," wrote Brian T. Walitt, MD, of the National Institute of Neurological Disorders and Stroke (NINDS), Bethesda, Maryland, and colleagues in the paper, published on February 21 in Nature Communications.
In addition, "there are distinct sex signatures of immune and metabolic dysregulation which suggest persistent antigenic stimulation." Physical deconditioning over time, while not the source of the condition, "is an important consequence," the authors added.
Asked to comment, Hector Bonilla, MD, director of the ME/CFS Clinic and codirector of the Stanford Post-Acute COVID-19 Syndrome Clinic, Atherton, California, pointed out that the sample was small and the study was cross-sectional and therefore likely missed dynamic changes in the patients.
Nonetheless, Bonilla told Medscape Medical News, "they have shown clear objective changes in patients with ME/CFS not seen in the controls. These are present in the microbiome, in the immune system, and in metabolites, especially in spinal fluid, that lead to a neuroinflammatory condition. And these are linked with autonomic dysfunction that can explain many of the symptoms that patients experience…The symptoms are not manufactured by them."
Thus far, the only treatments for ME/CFS are symptomatic. Understanding the pathophysiology is essential to identifying disease-modifying therapy, study lead author Avindra Nath, MD, Senior Investigator and Clinical Director of Intramural Research at NINDS, told Medscape Medical News.
"The disease is real. But our medical profession is limited in what they can do to diagnose or impact them…The first thing we need to do is try to understand the pathophysiology. So that's why the study was put together," Nath said.
Postinfectious syndromes including ME/CFS have been given many names, including post-Lyme disease, Gulf War illness, and more recently, long COVID. With ME/CFS, the Epstein-Barr virus has historically been one of the most commonly associated triggers, although several other viral, bacterial, and environmental toxins have been implicated.
"There are a whole host of these things that have very similar symptoms or overlapping symptoms…It's quite possible that the underlying pathophysiology overlaps between all these syndromes," Nath noted.
Another ME/CFS expert not involved in the study, researcher Michael VanElzakker, PhD, of the Neurotherapeutics Division at Harvard Medical School and Massachusetts General Hospital, Boston, Massachusetts, said that the possibility of antigen persistence of the infectious pathogen arising from the immune system profiling conducted in the study is noteworthy and merits further study.
"To me, the obvious next step would be techniques like tissue-based assays and T-cell sequencing to try and understand what exactly those antigens are and what their source might be. Importantly, it is probably not the same antigen or pathogen source in all patients, but that's a question that needs an answer," VanElzakker said.
Of note, the 17 study participants had been adjudicated by an expert panel from an initial 484 inquiries and 217 who underwent detailed case reviews. They had to meet at least one of three published ME/CFS criteria and to have moderate to severe clinical symptom severity as determined by several fatigue scores. None met the criteria for psychiatric diagnoses.
Yet, even in the cases that met study criteria, underlying causes emerged in 20% of the participants over time, suggesting diagnostic misattribution. "This misclassification bias has important ramifications on the interpretation of the existing ME/CFS research literature," the authors wrote.
VanElzakker commented, "The fact that this research study was probably the most detailed workup many of these patients had ever gotten is a serious indictment of our current profit-based healthcare system's prioritization of 15-minute doctor's appointments. It is almost certain that other patients would also benefit from an intensive detailed workup."
Multiple Abnormalities Identified
There were no differences between the PI-ME/CFS and control groups in ventilatory function, muscle oxygenation, mechanical efficiency, resting energy expenditure, basal mitochondrial function of immune cells, muscle fiber composition, or body composition, suggesting the absence of a resting low-energy state, the authors said.
In 40-minute head-up tilt-table testing, there were no differences between the ME/CFS and control groups in frequency or orthostatic hypotension or extensive orthostatic tachycardia. However, a 24-hour ambulatory electrocardiogram showed that the patients with PI-ME/CFS had diminished heart rate variability. They also showed increased heart rate throughout the day, suggesting increased sympathetic activity, and a diminished drop in nighttime heart rate, suggesting decreased parasympathetic activity.
"Considered together, these data suggest that there is an alteration in autonomic tone, implying central nervous system regulatory change," Walitt and colleagues wrote.
On the "Effort-Expenditure for Rewards Task," the participants with PI-ME/CFS showed significant differences in "effort preference," or a tendency to avoid the harder tasks, as well as a slowing of button-pushing over time, compared with the controls, even with easier tasks. This pattern suggests that those with PI-ME/CFS were "pacing to limit exertion and associated feelings of discomfort," the authors wrote.
Nath describes this behavior as akin to "if you develop a flu, you feel that you just want to lay down in bed and not hurt yourself. It's not that you're not capable of doing [the task], but your body tells you don't do it. Your body just wants to fight the infection…these people just never bounce back."
Compared with the controls, the participants with PI-ME/CFS failed to maintain a moderate grip force even though there was no difference in maximum grip strength or arm muscle mass. This performance difference correlated with decreased activity of the right temporal-parietal junction, a novel observation suggesting that the fatigue in the PI-ME/CFS group "is due to dysfunction of integrative brain regions that drive the motor cortex, the cause of which needs to be further explored," Walitt and colleagues wrote.
On cardiopulmonary testing, peak power, peak respiratory rate, peak heart rate, and peak VO2 were all lower in the PI-ME/CFS group, correlating to a difference of approximately 3.3 metabolic equivalent of task units. The differential cardiorespiratory performance relates to "autonomic function, hypothalamic-pituitary-adrenal axis hyporesponsiveness, and muscular deconditioning from disuse that clinically impacts activities of daily life," they said.
In the participants with PI-ME/CFS, catechol levels in cerebrospinal fluid correlated with grip strength and effort preference, and several metabolites of the dopamine pathway correlated with several cognitive symptoms.
"This suggests that central nervous system catechol pathways are dysregulated in PI-ME/CFS and may play a role in effort preference and cognitive complaints," as well as decreased central catecholamine biosynthesis. Similar findings have been seen in patients with long COVID, the authors noted.
There were increased naive B cells and decreased switched memory B cells in blood of participants with PI-ME/CFS. Contrary to prior studies, there was no consistent pattern of autoimmunity across all participants with PI-ME/CFS, and no previously undescribed antibodies were identified.
However, programmed cell death protein 1, a marker of T-cell exhaustion and activation, was elevated in the cerebrospinal fluid of the patients with PI-ME/CFS.
Several sex-based differences were noted, including in immune cell expression in cerebrospinal fluid, peripheral blood mononuclear cell gene expression, and muscle gene expression. Males and females also differed in the cerebrospinal metabolomics that distinguished the participants with PI-ME/CFS from controls.
What Do These Findings Suggest About Treatment?
The data point to several treatment implications. For one, the finding of possible immune exhaustion suggests that immune checkpoint inhibitors may be therapeutic by promoting clearance of foreign antigens. Immune dysfunction leads to neurochemical alterations that affect neuronal circuits, which may be another point of intervention, the authors suggested.
On the other hand, "attempting to target downstream mechanisms with exercise, cognitive behavioral therapy, or autonomic directed therapies may have limited impact on symptom burden, as it would not address the root cause of PI-ME/CFS," they noted.
Combination therapy targeting multiple pathways along with a personalized medicine approach should be considered, they said.
Nath told Medscape Medical News, "I think the most important thing is not to discount these patients…They have a real disease, and we need to be empathetic towards them. We also need to make sure that they don't have something underlying that is treatable, and then treat them symptomatically the best that you can. If not, then refer them to ME/CFS studies or clinics where people specialize in these conditions and work with them."
The study authors and VanElzakker reported no relevant financial relationships. Bonilla consults for United Health and Resverlogix.