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Researchers have developed a hypothesis that may explain how chronic neuroinflammation contributes to conditions such as myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and postacute sequelae of SARS-CoV-2 infection through a continuing relapse-recovery cycle.
ME/CFS has been established as resulting from infections, environmental exposures, stressors, and surgery. Similarities have been drawn during the COVID-19 pandemic between ME/CFS and a large subgroup of patients with post-acute sequelae of SARS-CoV-2 infection – also known as post-COVID conditions, or long COVID – who continue to have viral fatigue and other lingering symptoms after their infection resolves.
What has been less clearly understood, the researchers said, is the reason behind why ME/CFS and other postviral fatigue tends to be chronic and can sometime develop into a lifelong condition.
“These diseases are very closely related, and it is clear the biological basis of long COVID is unequivocally connected to the original COVID infection – so there should no longer be any debate and doubt about the fact that postviral fatigue syndromes like ME/CFS are biologically based and involve much disturbed physiology,” Warren Tate, MSc, PhD, emeritus professor in the department of biochemistry at the University of Otago in Dunedin, New Zealand, stated in a press release.
Their hypothesis, set forth in a study published in Frontiers of Neurology, proposes that the systemic immune/inflammatory response that occurs after an infection or stressful event does not revolve, which results in a “fluctuating chronic neuroinflammation that sustains and controls the complex neurological symptoms of ME/CFS and long COVID and facilitates frequent more serious relapses in response to life stress, as evidenced from a comprehensive disruption to the cellular molecular biology and body’s physiological pathways.”
Tate and colleagues said that it is still unclear how the neuroinflammation occurs, why it’s persistent in ME/CFS, and how it causes symptoms associated with ME/CFS. In their hypothesis, “abnormal signaling or transport of molecules/cells occurs through one or both of neurovascular pathways and/or a dysfunctional blood brain barrier,” they said, noting “the normally separate and contained brain/CNS compartment in the healthy person becomes more porous.” The neurological symptoms associated with ME/CFS occur due to strong signals sent because of persistent “inflammatory signals or immune cells/molecules migrating into the brain,” they explained.
This results in a continuous loop where the central nervous system sends signals back to the body through the hypothalamus/paraventricular nucleus and the brain stem. “The resulting symptoms and the neurologically driven ‘sickness response’ for the ME/CFS patient would persist, preventing healing and a return to the preinfectious/stress-related state,” Tate and colleagues said.
Lingering Inflammation May Be the Culprit
Commenting on the study, Achillefs Ntranos, MD, a board-certified neurologist in private practice in Scarsdale, N.Y., who was not involved with the research, said previous studies have shown that long COVID is linked to chronic activation of microglia in the brain, which has also been seen to activate in patients with ME/CFS.
“The hypothesis that lingering inflammation in the brain is the culprit behind the neurological symptoms of long COVID and ME/CFS is valid,” he said. “If these cells remain activated in the brain, they can cause a state of increased and lingering inflammation, which can interfere with the function of neurons, thus producing neurological symptoms. Since the neurological symptoms are similar between these entities, the mechanisms that produce them might also be similar.”
While the exact cause of ME/CFS is still unclear, it is often tied to the aftereffects of a flu-like illness, Ntranos said. “This has led researchers to propose that it arises after a viral infection, with many different types of viruses being associated with it. Other ways researchers think ME/CFS is being brought on after a viral illness is via changes in the immune system, such as chronic production of cytokines, neuroinflammation, and disruption of the hypothalamic-pituitary-adrenal axis, which regulates the body’s response to stress,” he explained.
While a newer condition, long COVID is not all that different from ME/CFS, Ntranos noted, sharing the catalyst of a viral infection and core neurological symptoms such as fatigue, postexertional malaise, a “brain fog” that makes thinking or concentrating difficult, sleep problems, and lightheadedness, but there are differences that set it apart from ME/CFS.
“Long COVID is unique in having additional symptoms that are specific to the SARS-CoV-2 virus, such as respiratory and cardiovascular symptoms and loss of smell and taste. However most central nervous system effects are the same between these two entities,” he said.
Ntranos said long COVID’s neurological symptoms are similar to that of multiple sclerosis (MS), such as “brain fog” and postexertional malaise. “Since MS only affects the brain and spinal cord, there are no symptoms from other organ systems, such as the lungs, heart, or digestive system, contrary to long COVID. Furthermore, MS rarely affects smell and taste, making these symptoms unique to COVID,” he said.
However, he pointed out that brain fog and fatigue symptoms on their own can be nonspecific and attributed to many different conditions, such as obstructive sleep apnea, migraines, depression, anxiety, thyroid problems, vitamin deficiencies, dehydration, sleep disorders, and side effects of medications.
“More research needs to be done to understand how these cells are being activated, how they interfere with neuronal function, and why they remain in that state in some people, who then go on to develop fatigue and brain fog,” he said.
This study was funded by the Healthcare Otago Charitable Trust, the Associated New Zealand Myalgic Encephalomyelitis Society, and donations from families of patients with ME/CFS. The authors and Ntranos report no relevant financial disclosures.
This article originally appeared on MDedge.com, part of the Medscape Professional Network.