Recent insights into the pathophysiology of Guillain-Barré syndrome (GBS) – which affects 1 or 2 persons for every 100,000 people annually, usually post infection – indicate that classic subtypes represent varying manifestations of a shared disease process. This knowledge is yielding new treatment strategies aimed at halting the illness in its tracks. Promising therapies include inhibitors of complement and, perhaps one day, the calcium-activated protease calpain.
Meanwhile, an association between COVID-19 and GBS has been debunked, whereas a small risk of GBS following adenovirus-vectored COVID vaccination is now accepted and quantified. Regardless of cause, the potential severity of GBS and variability in its presentation demand constant vigilance.
Shutting down the disease process
When patients present to an emergency department with sensory symptoms and increasing muscle weakness, “most of the damage has been or is being done,” said Michael P. Lunn, MBBS, MRCP, PhD, professor of clinical neurology, consultant neurologist, and clinical lead in neuroimmunology at University College London Queen Square Institute of Neurology, who spoke at length about GBS with Neurology Reviews 2023 Rare Neurological Disease Special Report. “The crucial reason that GBS treatment has not advanced significantly – and why we’re still slightly stuck where we are in terms of helping people get better more quickly – is that we need something that absolutely turns the disease off as patients come through the door.”
GBS is probably the best-understood autoimmune-mediated neurological disease, in some respects surpassing myasthenia gravis, Dr. Lunn said. “We know very frequently the organisms and stimuli that set off Guillain-Barré syndrome. We understand, to an extent, the immunology and how you break tolerance of the immune system so that an invading organism can provoke an immune response that damages peripheral nerves.”
Dr. Michael P. Lunn
Compared to what was known about GBS in decades past, neurologists now better understand how and where antibodies attack the nerve; how complement then damages the nodes of Ranvier and paranodes; and how an external attack results in sometimes irreparable internal nerve damage. “We’ve got a string, beginning to end, of understanding the disease,” declared Dr. Lunn.
Understanding of differences in the spectrum of pathology of GBS has led to additional diagnostic categories, said Dr. Lunn. Acute inflammatory demyelinating polyradiculoneuropathy, or typical GBS, represents the most common form in affluent Western nations. A motor variant was recognized in the 1980s; in the mid-1990s, Ho and colleagues described a cohort of patients in China who had acute motor axonal neuropathy and acute motor sensory axonal neuropathy1 – two forms that are particularly common throughout Asia and South America.
Shared mechanism
Based on the findings of electrophysiologic studies, Dr. Lunn said, experts traditionally believed that GBS attacked either axons themselves or their myelin sheaths. “That’s where the anti-ganglioside antibodies come in, providing targeting to nerve structures.” The dichotomous classification system, he added, was partially correct.
Then, through the 2010s and 2020s, neurophysiologist Antonio Uncini, MD, recognized, based partly on histologic studies by Ho and colleagues, that the myelin and axonal subtypes are both likely to stem from the same mechanism.2 When antibodies and complement damage the node of Ranvier, Dr. Lunn said, “the myelin gets stripped off and the conduction becomes slow. But then the myelin can return, and patients get better.” But if damage is severe, it severs the axon, resulting in unrecoverable motor axonal neuropathy. “It’s basically all the same spectrum of disease,” Dr. Lunn said. “Anti-ganglioside antibodies may account for different GBS ‘flavors,’ but the immunological attack all occurs at the node of Ranvier in one way or another.”
The foregoing insight has focused development efforts on the shared seminal pathway of all GBS subtypes and given rise to the concept of nodo-paranodopathy, which incorporates damage at either the node of Ranvier or nearby paranodes.3
Simultaneously, Spanish and French researchers began elucidating new antibodies responsible for neuropathology at the node of Ranvier.4 Anti-ganglioside antibodies have long been loosely associated with acute motor axonal neuropathy and poor outcomes, although, Dr. Lunn said, they fail to tell the full story. Anti-GQ1b antibodies are associated with the Miller-Fisher syndrome subtype, well recognized for its medical features: double vision, loss of tendon reflexes, and arm and leg weakness.
However, Dr. Lunn said, most GBS cases lack anti-ganglioside antibodies. In some GBS cases, antibodies attack neurofascin, contactins, and gliomedin, which are mainly adhesion proteins at nodes of Ranvier.
“Therefore,” Dr. Lunn said, “there must be an antibody-mediated attack of the node of Ranvier or the paranode. That’s an important series of discoveries, primarily because it helps us understand the immunological attack at the node of Ranvier, which goes along with what Dr. Uncini was saying. But it also divides off a group of chronic inflammatory demyelinating polyradiculoneuropathies (CIDP) that present acutely and look initially, for all purposes, like GBS.”
Recognizing acute CIDP (A-CIDP) is critically important for clinicians, Dr. Lunn stressed, because it requires treatment with rituximab (the most commonly used option), steroids, or plasma exchange.
Key clues that distinguish A-CIDP from GBS include:
• A high level of cerebrospinal fluid protein.
• Very slow nerve conduction.
• Early muscle wasting (rare in GBS).
Recognizing CIDP and A-CIDP is crucial, said Dr. Lunn, because it begins to bring all the pathology back together to make sense of GBS. Neurologists have known for decades that, if one damages a nerve with antibodies, then binds complement to those antibodies, the complement punches holes in the affected cells, resulting in death. “But it wasn’t quite clear how those cells might die,” Dr. Lunn said.
After complement-induced injury, calcium-activated calpain permanently damages the entire internal axonal structure.5 Perhaps more important, a 2022 mouse study showed that complement-mediated damage could be directed to myelin or axons using the genetically programmed presence or absence of gangliosides to understand subsequent calpain-induced destruction in either axons or myelin.6
Some of the engineered mouse cells included ganglioside; others did not. “So you can have anti-ganglioside antibodies directed at one cell type or the other, which would, or would not, have calpain within them,” Dr. Lunn said. Investigators also showed that a calpain inhibitor (AK295) or overproduction of an endogenous inhibitor, calpastatin, prevented damage to both cell types.6All existing calpain inhibitors are unsuitable for clinical use because they are highly toxic. “But if you could inhibit calpain and stop it from being activated by calcium,” Dr. Lunn explained, “you would have a mechanism for stopping cell degradation during GBS. That would be an important future target for pharmacotherapy. That whole story – from the beginning to the end of GBS – has opened up options for treatment.”
Because complement bound to antibodies, set up by infection, plays a pivotal role, complement inhibitors have become an exciting area of research over the past decade. The 36-patient Japanese Eculizumab Trial for GBS (JET-GBS) trial showed that, after 6 months, significantly more eculizumab-treated patients could run, compared with placebo-treated patients.7
“No other trials of complement inhibitors have yet been completed,” Dr. Lunn said. “But several different complement inhibitors work at different places, in a very complicated immune process. One of the complement inhibitors will become transformative in treating GBS – preventing disability and improving recovery – in the not-very-distant future.”
Additional investigational treatments that have demonstrated early promise in eliminating problem antibodies faster include imlifidase (Idefirix [Hansa Biopharma]), which destroys antibodies, and Fc receptor inhibitors such as efgartigimod alfa-fcab (Vyvgart [argenx]), which push antibodies into the natural catabolic pathway.
“We’ve been stuck with plasma exchange and intravenous immunoglobulin (IVIg) for three or four decades,” Dr. Lunn said. “We now have a series of strategies by which we can completely turn off complement and resulting nerve damage. If we can find a calpain inhibitor that turns off the end of that pathway, we will make dramatic improvements. Our understanding of the immunopathology has changed enormously and influences pharmacotherapy going forward.”