Studying Nerve Function and Structure in Charcot-Marie-Tooth Disease, Anti-MAG Neuropathy and CIDP

Overview

The project aims to perform both conventional nerve-conduction studies and axonal-excitability assessments using the TRONDF protocol in patients with selected forms of Charcot-Marie-Tooth disease, with comparison to individuals affected by dysimmune, acquired neuropathies, specifically chronic inflammatory demyelinating polyneuropathy (CIDP) and anti-MAG-neuropathy. The study further includes the analysis of nerve fibers obtained from skin biopsy in patients with CMT, as well as ultrasound evaluation of nerves (from the wrist to the axilla) and of intrinsic hand muscles. Axonal-excitability techniques involve the delivery of two electrical stimuli to the nerve under investigation; both stimuli vary in intensity, whereas only the first, known as the conditioning stimulus, varies in duration. Changes in response amplitude are then measured as these stimulation parameters are systematically adjusted. Some preliminary studies have already suggested the effectiveness of this method in distinguishing CMT1A from certain forms of acquired demyelinating disease, including acute inflammatory demyelinating polyradiculoneuropathy (AIDP) and CIDP. Despite the promising results, only a limited number of studies have so far been conducted in humans and mice, and no comprehensive and systematic study has yet been carried out describing the changes in axonal excitability in the various CMT subtypes, either in humans or in mouse models.

Charcot-Marie-Tooth neuropathy (CMT) is the most prevalent hereditary neuromuscular disorder, estimated to affect 11.8 to 82.3 individuals per 100,000 in Europe. While genetic assessment is increasingly gaining significance, neurophysiology continues to play a crucial role in classifying CMT into axonal, intermediate or demyelinating forms. Despite its diagnostic importance, standard neurophysiology techniques may prove inadequate to reliably capture disease progression, advocating for the adoption of newer methodologies in the near future. Among these, axonal excitability testing, providing information about the properties of axonal membranes insight into the behaviour of voltage-gated ion channels, pump and exchangers involved in impulse conduction, could have potential additional diagnostic and prognostic value in neuromuscular disorder. Axonal excitability techniques provide insights into ion channel function, serving as an in vivo surrogate markers of axonal membrane potential in human axons. Recent advancements in this field have standardized procedures, increased execution speed, and minimized patients' discomfort enabling routine clinical application. These advancements include the development of standardized semi-automated recording setups and the publication of international guidelines. It is plausible that progressive ion-channel dysfunction and spontaneous baseline depolarization of nerves maybe associated with disease severity and response to therapy (in patients with acquired polyneuropathies). Investigators plan to study a series of patients with different types of CMT with this technique and compare results with diseased controls affected by dysimmune neuropathies and corresponding mouse models, by conventional nerve conduction studies, by electrical nerve stimulation to study neuronal excitability as below depicted, and by examining myelinated skin nerves by performing skin biopsy in a subset of patients. Moreover, literature data demonstrate that nerve ultrasound supports the diagnosis and follow-up of neuromuscular disorders. In the context of neuropathies, nerve ultrasound enables an anatomical and structural evaluation that provides useful data for the differential diagnosis between hereditary and acquired forms. In particular, in Charcot-Marie-Tooth disease, nerve ultrasound allows the identification of pathognomonic patterns that can significantly guide genetic testing (e.g CMT1A). In severe acquired forms, where the pathological process is so extensive that clear electrophysiological findings are not always present, nerve ultrasound can detect specific alterations that may help guiding the diagnosis. Moreover, nerve ultrasound can identify certain features that may suggest (and correlate with) the severity of the disease. Muscle ultrasound in neuromuscular disorders is instead a more recent application. The detection of specific patterns of muscle involvement can suggest a possible regional distribution, described in some nosological entities. Furthermore, literature data show that the structural and ultrasound characteristics of the muscle correlate with the degree of strength deficit. Despite the increasing amount of literature in the field, no nerve or muscle ultrasound studies have yet been published on some acquired forms and rare forms of Charcot-Marie-Tooth disease. The collection and analysis of ultrasound data in these cases would allow for a broader understanding of the related pathological processes, potentially identifying patterns that support diagnosis and follow-up. Collaborators for this project are the following: Inherited Neuropathy Consortium (INC); Prof. Christian Krarup (Dept of Clin. Neurophys., Rigshospitalet, Copenhagen, DK; Dept of Neuroscience, Univ. of Copenhagen, Copenhagen, DK); Prof. Mihai Moldovan (Dept of Neurol., North Zealand Hospital, Hillerød, DK; Dept of Clin. Neurophys., Rigshospitalet, Copenhagen, DK; Dept of Neuroscience, Univ. of Copenhagen, Copenhagen, DK); Prof. Hatice Tankisi (Aarhus University Hospital, Department of Clinical Neurophysiology, Aarhus, Denmark; Aarhus University, Department of Clinical Medicine, Aarhus, Denmark)