018 Role of transcallosal inhibition in disease spread in ALS

Introduction

There is substantial evidence for the cortical origin of amyotrophic lateral sclerosis (ALS). Impaired function of the corpus callosum has been demonstrated in ALS patients and may play a role in disease spread, potentially mediating the spread of cortical hyperexcitability between hemispheres. We assessed transcallosal inhibition, utilising the threshold tracking transcranial magnetic stimulation (TMS) technique to assess transcallosal inhibition and related the changes to disease involvement.

Methods

Threshold tracking TMS was undertaken in 15 ALS patients and results were compared to 16 healthy controls. Interhemispheric inhibition was assessed using a figure of eight coil over each hemisphere across interstimulus intervals ranging from 8 to 40ms.

Results

Transcallosal inhibition was reduced in ALS patients (0.9 ± 1.0%) when compared to controls (6.6 ± 1.0%, P=0.03). Importantly differences in transcallosal inhibition between hemispheres were evident in ALS patients. Specifically, transcallosal inhibition projecting from the motor cortex contralateral to disease onset (0.9 ± 1.0%) was significantly lower when compared to projection form the ipsilateral motor cortex (3.2 ± 1.0%, P=0.036). Abnormalities of transcallosal inhibition correlated with upper motor neurone dysfunction and greater functional disability in ALS.

Conclusion

Abnormalities of transcallosal inhibition were demonstrated in ALS patients and were associated with clinical features. Consequently, dysfunction of transcallosal fibres may contribute to development of cortical hyperexcitability, a pathogenic mechanism in ALS. Strategies aimed at modulating dysfunction may prove therapeutically useful in ALS.

Conduction block in immune-mediated neuropathy: paranodopathy versus axonopathy

BACKGROUND AND PURPOSE

Conduction block is a pathognomonic feature of immune-mediated neuropathies. The aim of this study was to advance understanding of pathophysiology and conduction block in chronic inflammatory demyelinating polyneuropathy (CIDP) and multifocal motor neuropathy (MMN).

METHODS

A multimodal approach was used, incorporating clinical phenotyping, neurophysiology, immunohistochemistry and structural assessments.

RESULTS

Of 49 CIDP and 14 MMN patients, 25% and 79% had median nerve forearm block, respectively. Clinical scores were similar in CIDP patients with and without block. CIDP patients with median nerve block demonstrated markedly elevated thresholds and greater threshold changes in threshold electrotonus, whilst those without did not differ from healthy controls in electrotonus parameters. In contrast, MMN patients exhibited marked increases in superexcitability. Nerve size was similar in both CIDP groups at the site of axonal excitability. However, CIDP patients with block demonstrated more frequent paranodal serum binding to teased rat nerve fibres. In keeping with these findings, mathematical modelling of nerve excitability recordings in CIDP patients with block support the role of paranodal dysfunction and enhanced leakage of current between the node and internode. In contrast, changes in MMN probably resulted from a reduction in ion channel density along axons.

CONCLUSIONS

The underlying pathologies in CIDP and MMN are distinct. Conduction block in CIDP is associated with paranodal dysfunction which may be antibody-mediated in a subset of patients. In contrast, MMN is characterized by channel dysfunction downstream from the site of block.

Amyotrophic lateral sclerosis diagnostic index: Toward a personalized diagnosis of ALS

OBJECTIVE

The aim of the study was to assess the utility of a novel amyotrophic lateral sclerosis (ALS) diagnostic index (ALSDI).

METHODS

A prospective multicenter study was undertaken on patients presenting with suspected ALS. The reference standard (Awaji criteria) was applied to all patients at recruitment. Patients were randomly assigned to a training (75%) and a test (25%) cohort. The ALSDI was developed in the training cohort and its diagnostic utility was subsequently assessed in the test cohort.

RESULTS

A total of 407 patients were recruited, with 305 patients subsequently diagnosed with ALS and 102 with a non-ALS mimicking disorder. The ALSDI reliably differentiated ALS from neuromuscular disorders in the training cohort (area under the curve 0.92, 95% confidence interval 0.89-0.95), with ALSDI ≥4 exhibiting 81.6% sensitivity, 89.6% specificity, and 83.5% diagnostic accuracy. The ALSDI diagnostic utility was confirmed in the test cohort (area under the curve 0.90, 95% confidence interval 0.84-0.97), with ALSDI ≥4 exhibiting 83.3% sensitivity, 84% specificity, and 83.5% diagnostic accuracy. In addition, the diagnostic utility of the ALSDI was confirmed in patients who were Awaji negative at recruitment and in those exhibiting a predominantly lower motor neuron phenotype.

CONCLUSION

The ALSDI reliably differentiates ALS from mimicking disorders at an early stage in the disease process.

CLASSIFICATION OF EVIDENCE

This study provides Class I evidence that for patients with suspected ALS, the ALSDI distinguished ALS from neuromuscular mimicking disorders.

Ih contributes to increased motoneuron excitability in restless legs syndrome

KEY POINTS

Restless legs patients complain about sensory and motor symptoms leading to sleep disturbances. Symptoms include painful sensations, an urge to move and involuntary leg movements. The responsible mechanisms of restless legs syndrome are still not known, although current studies indicate an increased neuronal network excitability. Reflex studies indicate the involvement of spinal structures. Peripheral mechanisms have not been investigated so far. In the present study, we provide evidence of increased hyperpolarization-activated cyclic nucleotide-gated (HCN) channel-mediated inward rectification in motor axons. The excitability of sensory axons was not changed. We conclude that, in restless legs syndrome, an increased HCN current in motoneurons may play a pathophysiological role, such that these channels could represent a valuable target for pharmaceutical intervention.

ABSTRACT

Restless legs syndrome is a sensorimotor network disorder. So far, the responsible pathophysiological mechanisms are poorly understood. In the present study, we provide evidence that the excitability of peripheral motoneurons contributes to the pathophysiology of restless legs syndrome. In vivo excitability studies on motor and sensory axons of the median nerve were performed on patients with idiopathic restless legs syndrome (iRLS) who were not currently on treatment. The iRLS patients had greater accommodation in motor but not sensory axons to long-lasting hyperpolarization compared to age-matched healthy subjects, indicating greater inward rectification in iRLS. The most reasonable explanation is that hyperpolarization-activated cyclic nucleotide-gated (HCN) channels open at less hyperpolarized membrane potentials, a view supported by mathematical modelling. The half-activation potential for HCN channels (Bq) was the single best parameter that accounted for the difference between normal controls and iRLS data. A 6 mV depolarization of Bq reduced the discrepancy between the normal control model and the iRLS data by 92.1%. Taken together, our results suggest an increase in the excitability of motor units in iRLS that could enhance the likelihood of leg movements. The abnormal axonal properties are consistent with other findings indicating that the peripheral system is part of the network involved in iRLS.

Utility of threshold tracking transcranial magnetic stimulation in ALS

Upper motor neuron [UMN] and lower motor neuron [LMN] dysfunction, in the absence of sensory features, is a pathognomonic feature of amyotrophic lateral sclerosis [ALS]. Although the precise mechanisms have yet to be elucidated, one leading hypothesis is that UMN precede LMN dysfunction, which is induced by anterograde glutamatergic excitotoxicity. Transcranial magnetic stimulation (TMS) is a neurophysiological tool that provides a non-invasive and painless assessment of cortical function. Threshold tracking methodologies have been recently adopted for TMS, whereby changes in threshold rather than motor evoked potential (MEP) amplitude serve as outcome measures. This technique is reliable and provides a rapid assessment of cortical function in ALS. Utilisng the threshold tracking TMS technique, cortical hyperexcitability was demonstrated as an early feature in sporadic ALS preceding the onset of LMN dysfunction and possibly contributing to disease spread. Separately, cortical hyperexcitability was reported to precede the clinical onset of familial ALS. Of further relevance, the threshold tracking TMS technique was proven to reliably distinguish ALS from mimicking disorders, even in the presence of a comparable degree of LMN dysfunction, suggesting a diagnostic utility of TMS. Taken in total, threshold tracking TMS has provided support for a cortical involvement at the earliest detectable stages of ALS, underscoring the utility of the technique for probing the underlying pathophysiology. The present review will discuss the physiological processes underlying TMS parameters, while further evaluating the pathophysiological and diagnostic utility of threshold tracking TMS in ALS.

In vivo evidence for reduced ion channel expression in motor axons of patients with amyotrophic lateral sclerosis

KEY POINTS

The progressive loss of motor units in amyotrophic lateral sclerosis (ALS) is initially compensated for by the reinnervation of denervated muscle fibres by surviving motor axons. A disruption in protein homeostasis is thought to play a critical role in the pathogenesis of ALS. The changes in surviving motor neurons were studied by comparing the nerve excitability properties of moderately and severely affected single motor axons from patients with ALS with those from single motor axons in control subjects. A mathematical model indicated that approximately 99% of the differences between the ALS and control units could be explained by a non-selective reduction in the expression of all ion channels. These changes in ALS patients are best explained by a failure in the supply of ion channel and other membrane proteins from the diseased motor neuron.

ABSTRACT

Amyotrophic lateral sclerosis (ALS) is characterised by a progressive loss of motor units and the reinnervation of denervated muscle fibres by surviving motor axons. This reinnervation preserves muscle function until symptom onset, when some 60-80% of motor units have been lost. We have studied the changes in surviving motor neurons by comparing the nerve excitability properties of 31 single motor axons from patients with ALS with those from 21 single motor axons in control subjects. ALS motor axons were classified as coming from moderately or severely affected muscles according to the compound muscle action potential amplitude of the parent muscle. Compared with control units, thresholds were increased, and there was reduced inward and outward rectification and greater superexcitability following a conditioning impulse. These abnormalities were greater in axons from severely affected muscles, and were correlated with loss of fine motor skills. A mathematical model indicated that 99.1% of the differences between the moderately affected ALS and control units could be explained by a reduction in the expression of all ion channels. For the severely affected units, modelling required, in addition, an increase in the current leak through and under the myelin sheath. This might be expected if the anchoring proteins responsible for the paranodal seal were reduced. We conclude that changes in axonal excitability identified in ALS patients are best explained by a failure in the supply of ion channel and other membrane proteins from the diseased motor neuron, a conclusion consistent with recent animal and in vitro human data.

Anti-MAG neuropathy: Role of IgM antibodies, the paranodal junction and juxtaparanodal potassium channels

OBJECTIVE

To improve understanding of disease pathophysiology in anti-myelin-associated glycoprotein (anti-MAG) neuropathy to guide further treatment approaches.

METHODS

Anti-MAG neuropathy patients underwent clinical assessments, nerve conduction and excitability studies, and ultrasound assessment.

RESULTS

Patients demonstrated a distinctive axonal excitability profile characterised by a reduction in superexcitability [MAG: -14.2 ± 1.6% vs healthy controls (HC): -21.8 ± 1.2%; p < 0.01] without alterations in most other excitability parameters. Mathematical modelling of nerve excitability recordings suggested that changes in axonal function could be explained by a 72.5% increase in juxtaparanodal fast potassium channel activation and an accompanying hyperpolarization of resting membrane potential (by 0.3 mV) resulting in a 94.2% reduction in discrepancy between anti-MAG data and the healthy control model. Superexcitability changes correlated strongly with clinical and neurophysiological parameters. Furthermore, structural assessments demonstrated a proximal pattern of nerve enlargement (C6 nerve root cross-sectional area: 15.9 ± 8.1 mm² vs HC: 9.1 ± 2.3 mm²; p < 0.05).

CONCLUSIONS

The imaging and neurophysiological results support the pathogenicity of anti-MAG IgM. Widening between adjacent loops of paranodal myelin due to antibodies would expand the pathway from the node to the juxtaparanode, increasing activation of juxtaparanodal fast potassium channels, thereby impairing saltatory conduction.

SIGNIFICANCE

Potassium channel blockers may prove beneficial in restoring conduction closer to its normal state and improving nerve function in anti-MAG neuropathy.

Targeted lung cancer screening selects individuals at high risk of cardiovascular disease

BACKGROUND

Cardiovascular disease (CVD) is a major cause of morbidity and mortality in populations eligible for lung cancer screening. The aim of this study was to determine whether a brief CV risk assessment, delivered as part of a targeted community-based lung cancer screening programme, was effective in identifying individuals at high risk who might benefit from primary prevention.

METHODS

The Manchester Lung Screening Pilot consisted of annual low dose CT (LDCT) over 2 screening rounds, targeted at individuals in deprived areas at high risk of lung cancer (age 55-74 and 6-year risk ≥1.51%, using PLCO_M2012 risk model). All participants of the second screening round were eligible to take part in the study. Ten-year CV risk was estimated using QRISK2 in participants without CVD and compared to age (±5 years) and sex matched Health Survey for England (HSE) controls; high risk was defined as QRISK2 score ≥10%. Coronary artery calcification (CAC) was assessed on LDCT scans and compared to QRISK2 score.

RESULTS

Seventy-seven percent (n=920/1,194) of screening attendees were included in the analysis; mean age 65.6 ± 5.4 and 50.4% female. QRISK2 and lung cancer risk (PLCO_M2012) scores were correlated (r = 0.26, p < 0.001). Median QRISK2 score was 21.1% (IQR 14.9-29.6) in those without established CVD (77.6%, n = 714/920), double that of HSE controls (10.3%, IQR 6.6-16.2; n = 714) (p < 0.001). QRISK2 score was significantly higher in those with CAC (p < 0.001). Screening attendees were 10-fold more likely to be classified high risk (OR 10.2 [95% CI 7.3-14.0]). One third (33.7%, n = 310/920) of all study participants were high risk but not receiving statin therapy for primary CVD prevention.

DISCUSSION

Opportunistic CVD risk assessment within a targeted lung cancer screening programme is feasible and is likely to identify a very large number of individuals suitable for primary prevention.

004 Mechanisms of nerve dysfunction in inflammatory neuropathies

Introduction

Immune-mediated neuropathies are a cause of disability and an immense cost to the healthcare system. They include chronic inflammatory demyelinating polyneuropathy (CIDP), multifocal motor neuropathy (MMN) and the neuropathy associated with IgM antibodies against myelin-associated glycoprotein (MAG). CIDP is extremely heterogeneous with marked variability in treatment responsiveness. Patients with MMN often respond to treatment but progressive weakness and wasting typically ensues over time. No therapy has consistently proven effective in anti-MAG neuropathy. The present series of studies were undertaken to improve understanding of disease mechanisms in these neuropathies, a critical step before targeted treatment approaches can be developed.

Methods

Patients fulfilling Peripheral Nerve Society criteria for CIDP or MMN and patients positive for anti-MAG IgM underwent comprehensive clinical assessments, neurophysiology, serological testing and structural assessments.

Results

The patient cohort consisted of 80 patients (51 CIDP, 14 MMN, 15 MAG). 6% of CIDP patients tested positive for anti-neurofascin 155 (NF155) and 4% for anti-contactin 1 IgG4. Anti-NF155 neuropathy was characterised by diffuse nerve enlargement and an axonal excitability profile consistent with severe disruption of the paranodal seal. CIDP patients testing negative for IgG4 antibodies also demonstrated significant nerve enlargement compared with healthy subjects. Axonal excitability profiles differed in those with and without median nerve conduction block. MMN was characterised by patchy nerve enlargement, marked increases in super-excitability and enlarged motor unit size. In contrast, anti-MAG neuropathy patients demonstrated a proximal pattern of nerve enlargement and an axonal excitability profile characterised by reduced super-excitability consistent with increased juxta-paranodal fast potassium channel conductance.

Conclusion

Patterns of nerve enlargement and neurophysiological profiles differ in the immune-mediated neuropathies providing insights into molecular mechanisms. These results provide templates that can guide treatment approaches. The combination of directed autoantibody assays and measures of axonal function can be used to monitor disease progression and therapeutic response.

Physiological Processes Underlying Short Interval Intracortical Facilitation in the Human Motor Cortex

Short interval intracortical facilitation (SICF) may be elicited by a paired pulse transcranial magnetic stimulation (TMS) paradigm, whereby a suprathreshold first stimulus (S1) precedes a perithreshold second stimulus (S2). Other facilitatory circuits can be probed by TMS such as intracranial facilitation, however the cortical contributions to these circuits may lie partially outside of M1. SICF as such represents a unique analog to M1 inhibitory circuits such as short interveal intracortical circuits. The aim of the present study was to provide insight into the physiological processes underlying the development of SICF using the threshold tracking TMS technique which was recently demonstrated to have significant reliability. TMS studies were undertaken on 35 healthy controls, using either a 90 mm circular and 70 mm figure of eight coil, and one of two targets (0.2 and 1.0 mV) tracked. The motor evoked potential (MEP) responses were recorded from the abductor pollicis brevis. SICF was consistently evident between interstimulus intervals (ISI) of 1–5 ms (P < 0.001), with two peaks occurring ISIs 1.5 and 3 ms when using the circular coil. A significant SICF reduction (F = 5.631, P < 0.05) was evident with the higher tracking target, while SICF increased when stimulating with the figure of eight coil. While there was a correlation between SICF and CSP duration, there was no relationship between SICF and SICI or ICF. Age appeared to have no influence on SICF, SICI, or ICF. Findings from the present work suggest that SICF appears to be mediated by I-wave facilitation.

Excitability of sensory axons in amyotrophic lateral sclerosis

OBJECTIVE

To evaluate the excitability of sensory axons in patients with amyotrophic lateral sclerosis (ALS).

METHODS

Comprehensive sensory nerve excitability studies were prospectively performed on 28 sporadic ALS patients, compared to age-matched controls. Sensory nerve action potentials were recorded from digit 2 following median nerve stimulation at the wrist. Disease severity was measured using motor unit number estimation (MUNE), the revised ALS Functional Rating Scale (ALSFRS-R) and the MRC scale.

RESULTS

There were no significant differences in standard and extended measures of nerve excitability between ALS patients and controls. These unchanged excitability measures included accommodation to long-lasting hyperpolarization and the threshold changes after two supramaximal stimuli during the recovery cycle. Excitability parameters did not correlate with MUNE, ALSFRS-R, APB MRC scale or disease duration.

CONCLUSIONS

This cross-sectional study has identified normal axonal membrane properties in myelinated sensory axons of ALS patients. Previously described sensory abnormalities could be the result of axonal fallout, possibly due to a ganglionopathy, or to involvement of central sensory pathways rostral to gracile and cuneate nuclei.

SIGNIFICANCE

These results demonstrate the absence of generalized dysfunction of the membrane properties of sensory axons in ALS in the face of substantial deficits in motor function.

Inter-session reliability of short-interval intracortical inhibition measured by threshold tracking TMS

Paired-pulse transcranial magnetic stimulation (TMS) using fixed test stimuli suffers from marked variability of the motor evoked potential (MEP) amplitude. Threshold tracking TMS (TT-TMS) was introduced to overcome this problem. The aim of this work was to describe the absolute and relative reliability of short-interval intracortical inhibition (SICI) using TT-TMS. Cortical excitability studies were performed on twenty-six healthy subjects over three sessions (two recordings on the same day and one seven days apart), with MEPs recorded over abductor pollicis brevis. Reliability was established by calculating the standard error of the measurements (SEm), minimal detectable change (MDC) and intraclass correlation coefficient (ICC). Resting motor threshold and averaged SICI presented the lowest SEm and highest ICCs. SICI at 1 ms showed a higher SEm than SICI at 3 ms, suggesting different physiological processes, but averaging SICI over a number of intervals greatly increases the reproducibility. The variability was lower for tests undertaken at the same time of day seven days apart compared to tests performed on the same day, and in both instances the ICC for averaged SICI was ≥0.81. The MDC in averaged SICI was reduced from 6.7% to 2% if the number of subjects was increased from one to eleven. In conclusion, averaged SICI is the most reproducible variable across paired-pulse TT-TMS measures, showing an excellent ICC. It is recommended that, in longitudinal studies, testing be performed at the same time of day and that changes in cortical excitability should be measured and averaged over a number of interstimulus intervals to minimise variability.