Axonal dysfunction, dysmyelination, and conduction failure in hereditary neuropathy with liability to pressure palsies: Axonal Dysfunction in HNPP

Peripheral nerve excitability abnormalities in Neuronal Intranuclear Inclusion Disease: Assessment with histopathological analysis

OBJECTIVE

Neuronal Intranuclear Inclusion Disease (NIID) is a neurodegenerative disease affecting the central and peripheral nerves. We aimed to assess the pathophysiological features of peripheral nerve dysfunction in NIID.

METHODS

We observed six unrelated NIID patients through clinical records, nerve conduction studies, and multiple measures of motor nerve excitability. Additionally, we reviewed one NIID patientt who underwent a nerve biopsy. Control measures were obtained from 22 age-matched normal subjects.

RESULTS

The NIID patients exhibited mild conduction slowing and distinct nerve excitability abnormalities, including a significant decrease in excitability through hyperpolarizing threshold electrotonus (TE) and increased overshoots in both depolarizing and hyperpolarizing conditions. Histopathology revealed thinly myelinated fibers and axonal degeneration. Mathematical modeling suggested that reduced leak conductance was the key factor contributing to the observed excitability changes.

CONCLUSIONS

The findings indicate that NIID involves a complex interplay of axonal degeneration and myelin dysfunction, leading to unique peripheral nerve excitability changes. These results provide new insights into the pathophysiology of NIID.

SIGNIFICANCE

Nerve excitability testing offers insight into particular axonal excitability abnormalities especially combined with histopathologic studies.

Literature review of clinical analysis of hereditary neuropathy with liability to pressure palsies

This review summarizes the clinical and electromyography (EMG) characteristics and peripheral myelin protein 22 (PMP22) gene-related diseases of hereditary neuropathy with liability to pressure palsies (HNPP). Clinical, EMG, and laboratory data of patients diagnosed with HNPP at our institution from 2022 to 2023 were retrospectively reviewed. Relevant literature from January 2003 to June 2024 was retrieved from PubMed using the keywords "hereditary neuropathy with liability to pressure palsies" and "HNPP." Clinical manifestations, EMG characteristics, and gene detection results of HNPP were summarized. All patients exhibited transient neurological symptoms and tested positive for the PMP22 deletion. EMG revealed multiple peripheral nerve abnormalities. Sixty-eight studies meeting the inclusion and exclusion criteria were included, comprising 124 HNPP cases (including six from our study), with 67 males and 57 females. The mean age of onset and diagnosis for the 124 cases were 26.5 ± 18 years and 32.7 ± 18.9 years, respectively, with a maximum onset-to-diagnosis interval of 40 years. Typical weakness and numbness in vulnerable areas were observed in 63.7% of cases, with 62% experiencing recurrent episodes. Atypical symptoms were present in 29.8%, while 6.5% were asymptomatic. Patients exhibited pain and muscular dystrophy (17.7%), pes cavus (12.1%), and a family history of HNPP (64.5%). Among symptomatic patients, triggers were traction or compression (57.8%), temperature changes (3.4%), or unclear (38.8%). Heterozygous PMP22 deletions and other PMP22 gene mutations were found in 77.4% and 22.6% of cases, respectively.

Hereditary neuropathy associated with liability to pressure palsies: a 24-year experience with carpal and cubital tunnel surgery

The aim of this single-centre retrospective study was to evaluate the outcomes of carpal tunnel release surgery in patients with hereditary neuropathy with pressure palsies (HNPP). The secondary aims were to identify prognostic factors for the outcome of carpal tunnel release and to assess the outcome of cubital tunnel release. Our primary hypothesis was postoperative improvement. In total, 18 patients (26 carpal tunnel releases) with at least one symptomatic carpal tunnel syndrome were included. At a median follow-up of 8.5 years, more than 73% of the patients were satisfied with the results. The visual analogue scale (0 to 10) for discomfort decreased by 2.2 points (p < 0.001). The Boston Carpal Tunnel Questionnaire symptom severity scale decreased by 1.3 points (p < 0.001). The decrease in the Functional Status Scale was not significant. No significant prognostic factor for outcome was identified. A total of 12 patients also underwent cubital tunnel release, and three patients underwent just this procedure (23 procedures). Despite the lack of preoperative data, cubital tunnel release provided encouraging results. Level of evidence: III.

Heterogeneous Presentation of Hereditary Neuropathy With Liability to Pressure Palsies: Clinical and Electrodiagnostic Findings in Three Patients

Hereditary neuropathy with liability to pressure palsies (HNPP) is characterized by the acute onset of focal sensory and/or motor deficits when the peripheral nerves are stressed by a mechanical force. Caused by a deletion in the PMP22 gene, this condition is often underdiagnosed or misdiagnosed due to the heterogeneity of clinical and electrophysiological presentation. This case series describes the clinical and electrodiagnostic (EDX) features of three patients presenting with clinically heterogenous phenotypes of HNPP. A retrospective analysis of patients referred to our facility for EDX studies identified three patients with genetically confirmed HNPP. The clinical presentations were unique in each patient, and the referring physicians did not consider HNPP in their differential diagnosis. The patients underwent comprehensive clinical and EDX evaluations, which led to the diagnosis of HNPP with subsequent confirmation by genetic studies. The reasons for referral for EDX studies were foot drop in one patient, carpal tunnel syndrome (CTS) in another, and wrist drop in the third patient. Patient #1 was initially diagnosed with chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) at another facility and developed a foot drop while recovering from a cervical discectomy and fusion. A positive family history of neuropathy led to the suspicion of hereditary neuropathy, and further studies confirmed a PMP22 gene deletion. Patient #2 gave a typical history suggestive of CTS. EDX studies showed focal slowing of conduction at potential areas of entrapment/compression. A history of foot drop in the patient and many other family members led to the confirmation of HNPP by genetic testing. US studies aided in confirming CTS by documenting a significant drop in the diameter of the median nerve within the carpal tunnel. The third patient developed radial nerve palsy after using axillary crutches. The possibility of HNPP was considered due to the pattern of nerve conduction slowing and a family history of foot drop. Physicians should be cognizant of the varied presentation of HNPP and the potential for misdiagnosis and underdiagnosis of this condition. In each case, elicitation of a detailed family history led to the suspicion of HNPP. A high index of suspicion is necessary for patients with multiple episodes of compressive neuropathies, mono/multi mononeuropathies, unexplained polyneuropathy, and a family history of neuropathy. The EDX pattern in patients presenting with acute focal neuropathies, consisting of a global increase in distal motor latencies, moderately decreased motor nerve conduction velocities, and focal slowing of conduction in potential sites of entrapment, should trigger genetic testing for HNPP.

New evidence for secondary axonal degeneration in demyelinating neuropathies

Development of peripheral nervous system (PNS) myelin involves a coordinated series of events between growing axons and the Schwann cell (SC) progenitors that will eventually ensheath them. Myelin sheaths have evolved out of necessity to maintain rapid impulse propagation while accounting for body space constraints. However, myelinating SCs perform additional critical functions that are required to preserve axonal integrity including mitigating energy consumption by establishing the nodal architecture, regulating axon caliber by organizing axonal cytoskeleton networks, providing trophic and potentially metabolic support, possibly supplying genetic translation materials and protecting axons from toxic insults. The intermediate steps between the loss of these functions and the initiation of axon degeneration are unknown but the importance of these processes provides insightful clues. Prevalent demyelinating diseases of the PNS include the inherited neuropathies Charcot-Marie-Tooth Disease, Type 1 (CMT1) and Hereditary Neuropathy with Liability to Pressure Palsies (HNPP) and the inflammatory diseases Acute Inflammatory Demyelinating Polyneuropathy (AIDP) and Chronic Inflammatory Demyelinating Polyneuropathy (CIDP). Secondary axon degeneration is a common feature of demyelinating neuropathies and this process is often correlated with clinical deficits and long-lasting disability in patients. There is abundant electrophysiological and histological evidence for secondary axon degeneration in patients and rodent models of PNS demyelinating diseases. Fully understanding the involvement of secondary axon degeneration in these diseases is essential for expanding our knowledge of disease pathogenesis and prognosis, which will be essential for developing novel therapeutic strategies.

Myelin protein zero gene dose dependent axonal ion-channel dysfunction in a family with Charcot-Marie-Tooth disease

OBJECTIVE

The myelin impairment in demyelinating Charcot-Marie-Tooth (CMT) disease leads to various degrees of axonal degeneration, the ultimate cause of disability. We aimed to assess the pathophysiological changes in axonal function related to the neuropathy severity in hypo-/demyelinating CMT patients associated with myelin protein zero gene (MPZ) deficiency.

METHODS

We investigated four family members (two parents and two sons) harboring a frameshift mutation (c.306delA, p.Asp104ThrfsTer14) in the MPZ gene, predicted to result in a nonfunctional P₀, by conventional conduction studies and multiple measures of motor axon excitability. In addition to the conventional excitability studies of the median nerve at the wrist, we tested the spinal accessory nerves. Control measures were obtained from 14 healthy volunteers.

RESULTS

The heterozygous parents (aged 56 and 63) had a mild CMT1B whereas their two homozygous sons (aged 31 and 39 years) had a severe Dejerine-Sottas disease phenotype. The spinal accessory nerve excitability could be measured in all patients. The sons showed reduced deviations during depolarizing threshold electrotonus and other depolarizing features which were not apparent in the accessory and median nerve studies of the parents. Mathematical modeling indicated impairment in voltage-gated sodium channels. This interpretation was supported by comparative modeling of excitability measurements in MPZ deficient mice.

CONCLUSION

Our data suggest that axonal depolarization in the context of abnormal voltage-gated sodium channels precedes axonal degeneration in severely hypo-/demyelinating CMT as previously reported in the mouse models.

SIGNIFICANCE

Measures of the accessory nerve excitability could provide pathophysiological markers of neurotoxicity in severe demyelinating neuropathies.

Measurement of axonal excitability: Consensus guidelines

Measurement of axonal excitability provides an in vivo indication of the properties of the nerve membrane and of the ion channels expressed on these axons. Axonal excitability techniques have been utilised to investigate the pathophysiological mechanisms underlying neurological diseases. This document presents guidelines derived for such studies, based on a consensus of international experts, and highlights the potential difficulties when interpreting abnormalities in diseased axons. The present manuscript provides a state-of-the-art review of the findings of axonal excitability studies and their interpretation, in addition to suggesting guidelines for the optimal performance of excitability studies.

Burning pain: axonal dysfunction in erythromelalgia

To gain insights into erythromelalgia disease pathophysiology, this study elucidated changes in peripheral axonal excitability and influences of temperature and mexiletine on axonal function.

Erythromelalgia (EM) is a rare neurovascular disorder characterized by intermittent severe burning pain, erythema, and warmth in the extremities on heat stimuli. To investigate the underlying pathophysiology, peripheral axonal excitability studies were performed and changes with heating and therapy explored. Multiple excitability indices (stimulus–response curve, strength–duration time constant (SDTC), threshold electrotonus, and recovery cycle) were investigated in 23 (9 EMSCN9A+ and 14 EMSCN9A−) genetically characterized patients with EM stimulating median motor and sensory axons at the wrist. At rest, patients with EM showed a higher threshold and rheobase (P < 0.001) compared with controls. Threshold electrotonus and current–voltage relationships demonstrated greater changes of thresholds in both depolarizing and hyperpolarizing preconditioning electrotonus in both EM cohorts compared with controls in sensory axons (P < 0.005). When average temperature was raised from 31.5°C to 36.3°C in EMSCN9A+ patients, excitability changes showed depolarization, specifically SDTC significantly increased, in contrast to the effects of temperature previously established in healthy subjects (P < 0.05). With treatment, 4 EMSCN9A+ patients (4/9) reported improvement with mexiletine, associated with reduction in SDTC in motor and sensory axons. This is the first study of primary EM using threshold tracking techniques to demonstrate alterations in peripheral axonal membrane function. Taken together, these changes may be attributed to systemic neurovascular abnormalities in EM, with chronic postischaemic resting membrane potential hyperpolarization due to Na⁺/K⁺ pump overactivity. With heating, a trigger of acute symptoms, axonal depolarization developed, corresponding to acute axonal ischaemia. This study has provided novel insights into EM pathophysiology.

The spread of EBV to ectopic lymphoid aggregates may be the final common pathway in the pathogenesis of ME/CFS

According to the hypothesis presented here, myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) develops over 3 steps: Step 1 is characterized by the aggregation of lymphoid cells in dorsal root ganglia or other nervous structures. The cause of this formation of ectopic lymphoid aggregates may be an acute infection, asymptomatic reactivations of a common neurotropic virus, exposure to a neurotoxin, or physical injury to peripheral nerves. In step 2, Epstein-Barr virus (EBV)-infected lymphocytes or monocytes bring EBV from the circulation to one or several of these lymphoid aggregates, whereupon cell-to-cell transmission of EBV and proliferation of latently EBV-infected lymphocytes lead to the presence of many EBV-infected cells in the lymphoid aggregates. The EBV-infected cells in the aggregates ignite an inflammation in the surrounding nervous tissue. This local inflammation elicits, in turn, a wave of glial cell activation that spreads from the EBV-infected area to parts of the nervous system that are not EBV-infected, disturbing the neuron-glial interaction in both the peripheral - and central nervous system. In step 3, immune cell exhaustion contributes to a consolidation of the pathological processes. There might be a cure: Infusions of autologous EBV-specific T-lymphocytes can perhaps remove the EBV-infected cells from the nervous system.

[Molecular genetic diagnosis and clinical features of hereditary neuropathy with liability to pressure palsies in Belarusian patients]

OBJECTIVE

To analyze the molecular defect, a phenotype of hereditary neuropathy with liability to pressure palsies (HNPP, OMIM 162500), in patients with PMP22 gene mutation caused by 1.5 Mb deletion at 17p11.2. and present the principles of diagnosis and genetic counselling.

MATERIAL AND METHODS

Patients were selected on the basis of the results of the clinical/genealogical analysis, neurological examination and ENMG study. Genomic DNA was isolated from peripheral blood leukocytes.

RESULTS AND CONCLUSION

DNA diagnosis was performed in 5 families (the PMP22 deletion was found in 9 patients). The authors described clinical and electrophysiological characteristics and presented a diagnostic protocol. Identification of the mutation makes it possible to confirm the clinical diagnosis, assess genetic risks for the outcome and perform a prenatal DNA diagnosis in HNPP families.

Excitability properties of motor axons in adults with cerebral palsy

Cerebral palsy (CP) is a permanent disorder caused by a lesion to the developing brain that significantly impairs motor function. The neurophysiological mechanisms underlying motor impairment are not well understood. Specifically, few have addressed whether motoneuron or peripheral axon properties are altered in CP, even though disruption of descending inputs to the spinal cord may cause them to change. In the present study, we have compared nerve excitability properties in seven adults with CP and fourteen healthy controls using threshold tracking techniques by stimulating the median nerve at the wrist and recording the compound muscle action potential over the abductor pollicis brevis. The excitability properties in the CP subjects were found to be abnormal. Early and late depolarizing and hyperpolarizing threshold electrotonus was significantly larger (i.e., fanning out), and resting current-threshold (I/V) slope was smaller, in CP compared to control. In addition resting threshold and rheobase tended to be larger in CP. According to a modeling analysis of the data, an increase in leakage current under or through the myelin sheath, i.e., the Barrett-Barrett conductance, combined with a slight hyperpolarization of the resting membrane potential, best explained the group differences in excitability properties. There was a trend for those with greater impairment in gross motor function to have more abnormal axon properties. The findings indicate plasticity of motor axon properties far removed from the site of the lesion. We suspect that this plasticity is caused by disruption of descending inputs to the motoneurons at an early age around the time of their injury.

Pathophysiology of motor dysfunction in a childhood motor neuron disease caused by mutations in the riboflavin transporter

OBJECTIVE

Brown-Vialetto-Van Laere (BVVL) syndrome is a progressive motor and sensory neuronopathy secondary to mutations in SLC52A2 encoding the riboflavin transporter type 2 (RFVT2). The phenotype is characterized by early childhood onset hearing loss and sensory ataxia followed by progressive upper limb weakness, optic atrophy, bulbar weakness and respiratory failure. To gain further insight into disease pathophysiology and response to riboflavin supplementation, the present study investigated whether axonal ion channel or membrane abnormalities were a feature of BVVL.

METHODS

Axonal excitability studies and clinical assessments were prospectively undertaken on six patients with BVVL secondary to riboflavin transporter deficiency type 2 (age range 10-21 years) at baseline and after 12 months of riboflavin (1000 mg daily) therapy.

RESULTS

At baseline, depolarizing and hyperpolarizing threshold electrotonus was 'fanned out' and superexcitability was increased, while the resting current-threshold gradient and refractoriness were significantly reduced in BVVL patients when compared to controls. Mathematical modeling suggested that functional alterations of myelin underlay these findings with an increase in myelin permeability. Riboflavin therapy resulted in partial normalization of the axonal excitability findings, paralleled by maintenance of muscle strength.

CONCLUSIONS

The present study established that abnormalities in myelin permeability at the paranode was a feature of BVVL and were partially normalized with riboflavin therapy.

SIGNIFICANCE

This study reveals a novel pathophysiological process for motor nerve dysfunction in BVVL. It also indicates that nerve excitability studies may be further developed in larger cohorts as a potential biomarker to identify treatment response for BVVL patients.