Transcortical and cervical magnetic stimulation with recording of the diaphragm

Critical illness-associated weakness and related motor disorders

Weakness of limb and respiratory muscles that occurs in the course of critical illness has become an increasingly common and serious complication of adult and pediatric intensive care unit patients and a cause of prolonged ventilatory support, morbidity, and prolonged hospitalization. Two motor disorders that occur singly or together, namely critical illness polyneuropathy and critical illness myopathy, cause weakness of limb and of breathing muscles, making it difficult to be weaned from ventilatory support, commencing rehabilitation, and extending the length of stay in the intensive care unit, with higher rates of morbidity and mortality. Recovery can take weeks or months and in severe cases, and may be incomplete or absent. Recent findings suggest an improved prognosis of critical illness myopathy compared to polyneuropathy. Prevention and treatment are therefore very important. Its management requires an integrated team approach commencing with neurologic consultation, creatine kinase (CK) measurement, detailed electrodiagnostic, respiratory and neuroimaging studies, and potentially muscle biopsy to elucidate the etiopathogenesis of the weakness in the peripheral and/or central nervous system, for which there may be a variety of causes. These tenets of care are being applied to new cases and survivors of the coronavirus-2 disease pandemic of 2019. This chapter provides an update to the understanding and approach to critical illness motor disorders.

Electrophysiological assessment of respiratory function

While the traditional lung function tests are used to assess lung capacity and pulmonary function, they cannot evaluate respiratory driving function and the integrity of the conduction pathway from the central nervous system to the respiratory motor neuron in the spinal cord and to the diaphragm. The inspiratory trigger is sent from the central nervous system through the phrenic nerve and drives the diaphragm to generate inspiratory movement. Therefore, phrenic nerve stimulation and diaphragmatic electromyography are two fundamental methods to assess respiratory function. There are several useful tools to assess respiratory motor system including electrical or magnetic phrenic nerve stimulation, diaphragmatic needle electromyography, and diaphragmatic ultrasound. By these means, physicians can assess current respiratory status in different neurological diseases that affect respiratory muscles, follow-up of the severity of respiratory impairment, help to predict the chance of successfully weaning from ventilatory support, and confirm clinical diagnoses such as diaphragmatic myoclonus. Although some of these tests require special training, applying these neurophysiological assessments in clinical practice is highly recommended.

Diaphragm Motor-Evoked Potential Induced by Cervical Magnetic Stimulation following Cervical Spinal Cord Contusion in the Rat

Cervical spinal injury is typically associated with respiratory impairments due to damage to bulbospinal respiratory pathways and phrenic motoneurons. Magnetic stimulation is a non-invasive approach for the evaluation and modulation of the nervous system. The present study was designed to examine whether cervical magnetic stimulation can be applied to evaluate diaphragmatic motor outputs in a pre-clinical rat model of cervical spinal injury. The bilateral diaphragm was monitored in anesthetized rats using electromyogram at the acute, subchronic, and chronic stages following left mid-cervical contusion. The center of a figure-of-eight coil was placed 20 mm caudal to bregma to stimulate the cervical spinal cord. The results demonstrated that a single magnetic stimulation can evoke significant motor-evoked potentials in the diaphragms of uninjured animals when the animal's head was placed 30 mm right or left from the center of the coil. The spontaneous bursting of the diaphragm was significantly attenuated by contusion injury at all-time-points post-injury. However, the threshold of the diaphragmatic motor-evoked potential was reduced, and the amplitude of the diaphragmatic motor-evoked potential was enhanced in response to cervical magnetic stimulation at the acute injury stage. Moreover, the motor-evoked potentials of the bilateral diaphragm in animals with contusions were generally larger when the coil was placed at the left spinal cord at the subchronic and chronic injury stages. These results suggested that cervical magnetic stimulation can be used to examine the excitability of phrenic motor outputs post-injury, and magnetic stimulation applied more laterally may be more effective for triggering diaphragmatic motor-evoked potentials.

Reliability of diaphragmatic motor-evoked potentials induced by transcranial magnetic stimulation

The diaphragmatic motor-evoked potential (MEP) induced by transcranial magnetic stimulation (TMS) permits electrophysiological assessment of the cortico-diaphragmatic pathway. Despite the value of TMS for investigating diaphragm motor integrity in health and disease, reliability of the technique has not been established. The study aim was to determine within- and between-session reproducibility of surface electromyogram recordings of TMS-evoked diaphragm potentials. Fifteen healthy young adults participated (6 females, age = 29 ± 7 yr). Diaphragm activation was determined by gradually increasing the stimulus intensity from 60 to 100% of maximal stimulator output (MSO). A minimum of seven stimulations were performed at each intensity. A second block of stimuli was delivered 30 min later for within-day comparisons, and a third block was performed on a separate day for between-day comparisons. Reliability of diaphragm MEPs was assessed at 100% MSO using intraclass correlation coefficients (ICC) and 95% limits of agreement (LOA). MEP latency (ICC = 0.984, P < 0.001), duration (ICC = 0.958, P < 0.001), amplitude (ICC = 0.950, P < 0.001), and area (ICC = 0.956, P < 0.001) were highly reproducible within-day. Between-day reproducibility was good to excellent for all MEP characteristics (latency ICC = 0.953, P < 0.001; duration ICC = 0.879, P = 0.002; amplitude ICC = 0.789, P = 0.019; area ICC = 0.815, P = 0.012). Data revealed less precision between-day versus within-day, as evidenced by wider LOA for all MEP characteristics. Large within- and between-subject variability in MEP amplitude and area was observed. In conclusion, TMS is a reliable means of inducing diaphragm potentials in most healthy individuals.NEW & NOTEWORTHY Transcranial magnetic stimulation (TMS) is a noninvasive technique to assess neural impulse conduction along the cortico-diaphragmatic pathway. The reliability of diaphragm motor-evoked potentials (MEP) induced by TMS is unknown. Notwithstanding large variability in MEP amplitude, we found good-to-excellent reproducibility of all MEP characteristics (latency, duration, amplitude, and area) both within- and between-day in healthy adult men and women. Our findings support the use of TMS and surface EMG to assess diaphragm activation in humans.

Diaphragmatic motor cortex hyperexcitability in patients with chronic obstructive pulmonary disease

Background and objectives

Respiratory muscles dysfunction has been reported in COPD. Transcranial magnetic stimulation (TMS) has been used for assessing the respiratory corticospinal pathways particularly of diaphragm. We aimed to study the cortico-diaphragmatic motor system changes in COPD using TMS and to correlate the findings with the pulmonary function.

Methods

A case control study recruited 30 stable COPD from the out-patient respiratory clinic of Main Alexandria University hospital- Egypt and 17 healthy control subjects who were subjected to spirometry. Cortical conduction of the diaphragm was performed by TMS to all participants followed by cervical magnetic stimulation of the phrenic nerve roots. Diaphragmatic resting motor threshold (DRMT), cortical motor evoked potential latency (CMEPL), CMEP amplitude (CMEPA), peripheral motor evoked potential latency (PMEPL), PMEP amplitude (PMEPA) and central motor conduction time (CMCT) were measured.

Results

66.7% of COPD patients had severe and very severe COPD with median age of 59 (55–63) years. There was statistically significant bilateral decrease in DRMT, CMEPA and PMEPA in COPD group versus healthy subjects and significant increase in CMEPL and PMEPL (p <0.01). Left CMCT was significantly prolonged in COPD group versus healthy subjects (p <0.0001) but not right CMCT. Further, there was significant increase in CMEPL and CMCT of left versus right diaphragm in COPD group (p = 0.003 and 0.001 respectively) that inversely correlated with FEV₁% and FVC% predicted. Right and left DRMT were insignificantly different in COPD group (p >0.05) but positively correlated with FEV₁/FVC, FEV₁% and FVC% predicted.

Conclusion

Central cortico-diaphragmatic motor system is affected in COPD patients with heterogeneity of both sides that is correlated with pulmonary function.

Significance

Coticospinal pathway affection could be a factor for development of diaphragmatic dysfunction in COPD patients accordingly its evaluation could help in personalization of COPD management especially pulmonary rehabilitation programs.

Noninvasive Prediction of Twitch Transdiaphragmatic Pressure: Insights from Spirometry, Diaphragm Ultrasound, and Phrenic Nerve Stimulation Studies

BACKGROUND

Twitch transdiaphragmatic pressure (twPdi) following magnetic stimulation (MS) of the phrenic nerves is the gold standard for non-volitional assessment of diaphragm strength. Expiratory muscle function can be investigated using MS of the abdominal muscles and measurement of twitch gastric pressure (twPgas).

OBJECTIVES

To investigate whether twitch pressures following MS of the phrenic and lower thoracic nerve roots can be predicted noninvasively by diaphragm ultrasound parameters and volitional tests of respiratory muscle strength.

METHODS

Sixty-three healthy subjects underwent standard spirometry, measurement of maximum inspiratory (PImax) and expiratory pressure (PEmax), and diaphragm ultrasound. TwPdi following cervical MS of the phrenic nerve roots and twPgas after lower thoracic MS (twPgas-Thor) were measured using esophageal and gastric balloon catheters inserted transnasally. Using surface electrodes, compound muscle action potentials (CMAP) were simultaneously recorded from the diaphragm or obliquus abdominis muscles, respectively.

RESULTS

Forced expiratory flow (FEF25-75) was significantly correlated with twPdi (r = 0.37; p = 0.003) and its components (twPgas and twitch esophageal pressure, twPes). Diaphragm excursion velocity during tidal breathing was correlated to twPes (r = 0.44; p = 0.02). No prediction of twitch pressures was possible from CMAP amplitude, forced vital capacity (FVC), or PImax. TwPgas-Thor was correlated with FEF25-75 (r = 0.46; p = 0.05) and diaphragm thickness at total lung capacity (r = 0.38; p = 0.04) but could not be predicted from CMAP amplitude, FVC, or PEmax.

CONCLUSIONS

TwPdi and twPgas-Thor cannot be predicted from volitional measures of respiratory muscle strength, diaphragm and abdominal CMAP, or diaphragm ultrasound. Invasive recording of esophageal and gastric pressures following MS remains indispensable for objective assessment of respiratory muscle strength.

The nature of respiratory muscle weakness in patients with late-onset Pompe disease

Late-onset Pompe disease (LOPD) causes myopathy of skeletal and respiratory muscles, and phrenic nerve pathology putatively contributes to diaphragm weakness. The aim of this study was to investigate neural contributions to diaphragm dysfunction, usefulness of diaphragm ultrasound, and involvement of expiratory abdominal muscles in LOPD. Thirteen patients with LOPD (7 male, 51±17 years) and 13 age- and gender-matched controls underwent respiratory muscle strength testing, ultrasound evaluation of diaphragm excursion and thickness, cortical and cervical magnetic stimulation (MS) of the diaphragm with simultaneous recording of surface electromyogram and twitch transdiaphragmatic pressure (twPdi; n = 6), and MS of the abdominal muscles with recording of twitch gastric pressure (twPgas; n = 6). The following parameters were significantly reduced in LOPD patients versus controls: forced vital capacity (p<0.01), maximum inspiratory and expiratory pressure (both p<0.001), diaphragm excursion velocity (p<0.05), diaphragm thickening ratio (1.8 ± 0.4 vs. 2.6 ± 0.6, p<0.01), twPdi following cervical MS (12.0 ± 6.2 vs. 19.4 ± 4.8 cmH₂O, p<0.05), and twPgas following abdominal muscle stimulation (8.8 ± 8.1 vs. 34.6 ± 17.1 cmH₂O, p<0.01). Diaphragm motor evoked potentials and compound muscle action potentials showed no between-group differences. In conclusion, phrenic nerve involvement in LOPD could not be electrophysiologically confirmed. Ultrasound supports assessment of diaphragm function. Abdominal expiratory muscles are functionally involved in LOPD.

Electrophysiological Properties of the Human Diaphragm Assessed by Magnetic Phrenic Nerve Stimulation: Normal Values and Theoretical Considerations in Healthy Adults

PURPOSE

This study determined normal values for motor evoked potentials (MEPs) and compound muscle action potentials (CMAPs) of the diaphragm following cortical and cervical magnetic stimulation (COMS and CEMS) of the phrenic nerves in healthy adults.

METHODS

Using surface electrodes, diaphragmatic MEP and CMAP were recorded in 70 subjects (34 ± 13 years, 25 men) following supramaximal cortical magnetic stimulation and CEMS at functional residual capacity and using a standardized inspiratory pressure trigger (-0.5 kPa). All healthy volunteers underwent standard spirometry and measurement of maximum inspiratory and expiratory pressure.

RESULTS

At functional residual capacity, upper limit of normal for MEP latency was 25 ms in men and 23 ms in women (p < 0.05), and upper limit of normal for CMAP latency was 6 ms. In contrast to MEP and CMAP amplitude, corresponding latencies showed little interindividual and intraindividual variability. Use of an inspiratory pressure trigger enhanced reproducibility and amplitude of diaphragm MEP. Diaphragm responses to both cortical and cervical magnetic stimulation were symmetrical and independent of age (in our cohort), with higher values for latency and amplitude in men (each p < 0.05). Diaphragm CMAP amplitude showed weak-moderate correlations with forced vital capacity (r = 0.47; p < 0.01), maximum inspiratory pressure (r = 0.39; p < 0.01), and maximum expiratory pressure (r = 0.32; p < 0.01).

CONCLUSIONS

Combination of cortical magnetic stimulation and CEMS of the phrenic nerves is feasible and allows noninvasive assessment of both central and peripheral conductivity of the diaphragm and the inspiratory pathway.

Reliability of the diaphragmatic compound muscle action potential evoked by cervical magnetic stimulation and recorded via chest wall surface EMG

INTRODUCTION

Stimulation of the phrenic nerve via cervical magnetic stimulation (CMS) elicits a compound muscle action potential (CMAP) that allows for assessment of diaphragm activation. The reliability of CMS to evoke the CMAP recorded by chest wall surface EMG has yet to be comprehensively examined.

METHODS

CMS was performed on healthy young males (n=10) and females (n=10). Surface EMG electrodes were placed on the right and left hemi-diaphragm between the 6-8th intercostal spaces. CMAPs were analysed for: latency, duration, peak-to-peak amplitude, and area. Reliability within and between experimental sessions was assessed using intraclass correlation coefficients (ICC). Bilateral (right-left) and sex-based (male-female) comparisons were also made (independent samples t-test).

RESULTS

All CMAP characteristics demonstrated high reproducibility within (ICCs>0.96) and between (ICCs>0.89) experimental sessions. No statistically significant bilateral or sex-based differences were found (p>0.05).

DISCUSSION

CMS is a reliable and non-invasive method to evaluate phrenic nerve conduction.

Interdisciplinary approaches of transcranial magnetic stimulation applied to a respiratory neuronal circuitry model

Respiratory related diseases associated with the neuronal control of breathing represent life-threatening issues and to date, no effective therapeutics are available to enhance the impaired function. The aim of this study was to determine whether a preclinical respiratory model could be used for further studies to develop a non-invasive therapeutic tool applied to rat diaphragmatic neuronal circuitry. Transcranial magnetic stimulation (TMS) was performed on adult male Sprague-Dawley rats using a human figure-of-eight coil. The largest diaphragmatic motor evoked potentials (MEPdia) were recorded when the center of the coil was positioned 6 mm caudal from Bregma, involving a stimulation of respiratory supraspinal pathways. Magnetic shielding of the coil with mu metal reduced magnetic field intensities and improved focality with increased motor threshold and lower amplitude recruitment curve. Moreover, transynaptic neuroanatomical tracing with pseudorabies virus (applied to the diaphragm) suggest that connections exist between the motor cortex, the periaqueductal grey cell regions, several brainstem neurons and spinal phrenic motoneurons (distributed in the C3-4 spinal cord). These results reveal the anatomical substrate through which supraspinal stimulation can convey descending action potential volleys to the spinal motoneurons (directly or indirectly). We conclude that MEPdia following a single pulse of TMS can be successfully recorded in the rat and may be used in the assessment of respiratory supraspinal plasticity. Supraspinal non-invasive stimulations aimed to neuromodulate respiratory circuitry will enable new avenues of research into neuroplasticity and the development of therapies for respiratory dysfunction associated with neural injury and disease (e.g. spinal cord injury, amyotrophic lateral sclerosis).

Descending motor pathways and cortical physiology after spinal cord injury assessed by transcranial magnetic stimulation: a systematic review

We performed here a systematic review of the studies using transcranial magnetic stimulation (TMS) as a research and clinical tool in patients with spinal cord injury (SCI). Motor evoked potentials (MEPs) elicited by TMS represent a highly accurate diagnostic test that can supplement clinical examination and neuroimaging findings in the assessment of SCI functional level. MEPs allows to monitor the changes in motor function and evaluate the effects of the different therapeutic approaches. Moreover, TMS represents a useful non-invasive approach for studying cortical physiology, and may be helpful in elucidating the pathophysiological mechanisms of brain reorganization after SCI. Measures of motor cortex reactivity, e.g., the short interval intracortical inhibition and the cortical silent period, seem to point to an increased cortical excitability. However, the results of TMS studies are sometimes contradictory or divergent, and should be replicated in a larger sample of subjects. Understanding the functional changes at brain level and defining their effects on clinical outcome is of crucial importance for development of evidence-based rehabilitation therapy. TMS techniques may help in identifying neurophysiological biomarkers that can reliably assess the extent of neural damage, elucidate the mechanisms of neural repair, predict clinical outcome, and identify therapeutic targets. Some researchers have begun to therapeutically use repetitive TMS (rTMS) in patients with SCI. Initial studies revealed that rTMS can induce acute and short duration beneficial effects especially on spasticity and neuropathic pain, but the evidence is to date still very preliminary and well-designed clinical trials are warranted. This article is part of a Special Issue entitled SI: Spinal cord injury.

Transcranial magnetic stimulation after spinal cord injury

OBJECTIVE

To review the basic principles and techniques of transcranial magnetic stimulation (TMS) and provide information and evidence regarding its applications in spinal cord injury clinical rehabilitation.

METHODS

A review of the available current and historical literature regarding TMS was conducted, and a discussion of its potential use in spinal cord injury rehabilitation is presented.

RESULTS

TMS provides reliable information about the functional integrity and conduction properties of the corticospinal tracts and motor control in the diagnostic and prognostic assessment of various neurological disorders. It allows one to follow the evolution of motor control and to evaluate the effects of different therapeutic procedures. Motor-evoked potentials can be useful in follow-up evaluation of motor function during treatment and rehabilitation, specifically in patients with spinal cord injury and stroke. Although studies regarding somatomotor functional recovery after spinal cord injury have shown promise, more trials are required to provide strong and substantial evidence.

CONCLUSIONS

TMS is a promising noninvasive tool for the treatment of spasticity, neuropathic pain, and somatomotor deficit after spinal cord injury. Further investigation is needed to demonstrate whether different protocols and applications of stimulation, as well as alternative cortical sites of stimulation, may induce more pronounced and beneficial clinical effects.

Diaphragm electromyography using an oesophageal catheter: current concepts

The usefulness of diaphragm electromyography recorded from an oesophageal electrode depends on a reliable signal which is free of artefact. The diaphragm EMG (electromyogram) recorded from chest wall surface electrodes may be unreliable because of signal contamination from muscle activity other than the diaphragm. Initially, the oesophageal electrode catheter for human studies had only one electrode pair, which could be difficult to position accurately and was influenced by a change in lung volume. Recently, a multipair oesophageal electrode has been developed which allows a high-quality EMG to be recorded. In the present review, the progress of oesophageal electrode design is outlined. The effects of signal contamination, electrode movement and particularly the effect of change in lung volume on the diaphragm EMG are discussed. The diaphragm EMG, recorded from a multipair oesophageal electrode, is useful to assess neural respiratory drive and diaphragm function in different groups of patients with respiratory disease, including patients with neuromuscular disease and sleep-disordered breathing, and those in the intensive care unit. When combined with cervical and cranial magnetic stimulation, an oesophageal electrode can be used to partition the central respiratory response time and phrenic nerve conduction time.

Phrenic nerve stimulation

Weakness of the limbs and respiratory muscles has increasingly been found to be a frequent event that complicates the medical history of patients in Intensive Care. The problem normally affects more serious cases and presents as muscular weakness leading to flaccid paralysis and difficulty in weaning patients off mechanical ventilation. This latter sign leads the intensivist to suspect possible involvement of the neuromuscular respiratory system. Unfortunately, in-depth clinical assessment of the neuromuscular respiratory system is difficult with critically ill patients, and electrophysiological studies have been used instead to overcome this problem. Of these latter, electric and electromagnetic stimulation of the phrenic nerve have been successful (along with needle electromyography of the diaphragm) in identifying the causes of neuromuscular respiratory insufficiency, especially in Intensive Care. In this brief chapter, we will be discussing the technique of electric stimulation of the phrenic nerve and neuromuscular respiratory insufficiency within the field of critical illness polyneuropathy.

Interaction between genioglossus and diaphragm responses to transcranial magnetic stimulation in awake humans

The modulation of activity of the upper airway dilator and respiratory muscles plays a key role in the regulation of ventilation, but little is known about the link between their neuromuscular activation processes in vivo. This study investigated genioglossus and diaphragm responses to transcranial magnetic stimulation applied in different facilitatory conditions. The amplitude and latency of motor-evoked potential responses and the stimulation intensity threshold leading to a motor response (motor threshold) were recorded with stimulation applied at the vertex and anterolateral area in 13 awake normal subjects. Stimuli were applied during inspiration with and without resistance, during expiration with and without maximal tongue protrusion and during deep inspiration. In each stimulation location and condition, no diaphragmatic response was obtained without previous genioglossus activity (diaphragmatic and genioglossus responses latencies during expiration: 18.1 +/- 2.9 and 6.3 +/- 2.6 ms, respectively, mean +/- s.d., P < 0.01). Genioglossus motor-evoked potential amplitude, latency and motor threshold were significantly modified with tongue protrusion with a maximal effect observed for stimulation in the anterolateral area. Deep inspiration was associated with a significant facilitatory effect on both genioglossus and diaphragm motor responses. The facilitatory effects of respiratory and non-respiratory manoeuvres were also observed during focal stimulation where isolated genioglossus responses were observed. Genioglossus and diaphragm differed in their motor threshold both at baseline and following facilitatory manoeuvres.

CONCLUSIONS

(1) transcranial magnetic stimulation-induced genioglossus response systematically precedes that of diaphragm; (2) this sequence of activation is not modified by respiratory and non-respiratory manoeuvres; and (3) the genioglossus and diaphragm are differently influenced by these manoeuvres in terms of latency of the motor response and of motor threshold.

The cortico-diaphragmatic pathway involvement in amyotrophic lateral sclerosis: neurophysiological, respiratory and clinical considerations

Cortico-diaphragmatic pathway was investigated by means of transcranial magnetic stimulation (TMS), in 14 patients affected by definite amyotrophic lateral sclerosis (ALS) without clinical signs of respiratory impairment. Spirometry, gas analysis, and measurement of static inspiratory and expiratory pressures were performed in all patients. Forced vital capacity, forced expiratory volume at the first and second peak expiratory flow, sniff effort from FRC level (SNIP), maximal inspiratory and expiratory pressure at mouth (MIP/MEP), maximal transdiaphragmatic pressure (Pdimx) were considered. TMS was performed, recording by surface electrodes from hemidiaphragm, bilaterally. Latency of cortical and spinal motor-evoked potentials (Cx-MEP/Sp-MEP) and central motor conduction time (CMCT) were measured. None of the patients showed altered spirometry and gas levels. Seven patients showed decreased Pdimx and eight of MEP values. Four patients showed a delayed Sp-MEP. In one patient the Cx-MEP was abolished while the mean values of both Cx-MEP and CMCT were significantly increased (19.2+/-4.1 ms, P<0.0001; 10.8+/-4.8 ms, P<0.0001). Cx-MEP and CMCT did not show significant correlations with any of the respiratory measures. The patients with prolonged Sp-MEP, showed longer disease duration, lower Norris score, lower Pdimx and MEP values. In conclusion, cortico-diaphragmatic study is a sensitive measure to reveal subclinical diaphragmatic impairment although not correlated to respiratory measures.

Demonstration of a second rapidly conducting cortico-diaphragmatic pathway in humans

Functional imaging studies in normal humans have shown that the supplementary motor area (SMA) and the primary motor cortex (PMC) are coactivated during various breathing tasks. It is not known whether a direct pathway from the SMA to the diaphragm exists, and if so what properties it has. Using transcranial magnetic stimulation (TMS) a site at the vertex, representing the diaphragm primary motor cortex, has been identified. TMS mapping revealed a second area 3 cm anterior to the vertex overlying the SMA, which had a rapidly conducting pathway to the diaphragm (mean latency 16.7 +/- 2.4 ms). In comparison to the vertex, the anterior position was characterized by a higher diaphragm motor threshold, a greater proportional increase in motor-evoked potential (MEP) amplitude with voluntary facilitation and a shorter silent period. Stimulus-response curves did not differ significantly between the vertex and anterior positions. Using paired TMS, we also compared intracortical inhibition/facilitation (ICI/ICF) curves. In comparison to the vertex, the MEP elicited from the anterior position was not inhibited at short interstimulus intervals (1-5 ms) and was more facilitated at long interstimulus intervals (9-20 ms). The patterns of response were identical for the costal and crural diaphragms. We conclude that the two coil positions represent discrete areas that are likely to be the PMC and SMA, with the latter wielding a more excitatory effect on the diaphragm.

Corticospinal control of respiratory muscles in chronic obstructive pulmonary disease

Patients with chronic obstructive pulmonary disease (COPD) face an increased respiratory load and in consequence have an elevated respiratory drive. We used transcranial magnetic stimulation (TMS) to investigate associated changes in corticospinal excitability both at rest and during voluntary facilitation at different levels of inspiratory effort. Diaphragm and abdominal motor thresholds were significantly lower in COPD than healthy controls, but the quadriceps response was the same. In patients there was a significant increase in diaphragm response from rest during 20% inspiratory efforts but no further increase with greater efforts. In controls there was a further stepwise increase at 40% and 60% of inspiratory effort. The cortical silent period was significantly shorter in COPD. Using paired stimulation to study intracortical inhibitory and excitatory circuits we found significantly less excitability of intracortical facilitatory circuits in patients at long (>7 ms) interstimulus intervals. These results suggest that there is a ceiling effect in motor control output to the respiratory muscles of patients with COPD.

Facilitation of the diaphragm response to transcranial magnetic stimulation by increases in human respiratory drive

The human respiratory neural drive has an automatic component (bulbospinal pathway) and a volitional component (corticospinal pathway). The aim of this study was to assess the effects of a hypercapnia-induced increase in the automatic respiratory drive on the function of the diaphragmatic corticospinal pathway as independently as possible of any other influence. Thirteen healthy volunteers breathed room air and then 5 and 7% hyperoxic CO2. Cervical (cms) and transcranial (tms) magnetic stimulations were performed during early inspiration and expiration. Transdiaphragmatic pressure (Pdi) and surface electromyogram of the diaphragm (DiEMG) and of the abductor pollicis brevis (apbEMG) were recorded in response to cms and tms. During inspiration, Pdi,cms was unaffected by CO2, but Pdi,tms increased significantly with 7% CO2. During expiration, Pdi,cms was significantly reduced by CO2, whereas Pdi,tms was preserved. DiEMG,tms latencies decreased significantly during early inspiration and expiration (air vs. 5% CO2 and air vs. 7% CO2). DiEMG,tms amplitude increased significantly in response to early expiration-tms (air vs. 5% CO2 and air vs. 7% CO2) but not in response to early inspiration-tms. DiEMG,cms latencies and amplitudes were not affected by CO2 whereas 7% CO2 significantly increased the apbEMG,cms latency. The apbEMG,tms vs. apbEMG,cms latency difference was unaffected by CO2. In conclusion, increasing the automatic drive to breathe facilitates the response of the diaphragm to tms, during both inspiration and expiration. This could allow the corticospinal drive to breathe to keep the capacity to modulate respiration in conditions under which the automatic respiratory control is stimulated.

[Therapy of day time fatigue in patients with multiple sclerosis]

Fatigue is the most common symptom of multiple sclerosis. 75%-90% of patients with multiple sclerosis report having fatigue, and 50%-60% describe it as the worst symptom of their disease. Fatigue is significantly associated with reduced quality of life and is also a major reason for unemployment, especially for patients with otherwise minor disability. The mechanisms underlying abnormal levels of fatigue in multiple sclerosis are poorly understood. To date, drug treatment has been only partially successful in alleviating fatigue, and effects vary widely from patient to patient. Amantadine and modafinil showed to be effective in the treatment of fatigue in some studies. Non-pharmacological management of fatigue in multiple sclerosis includes inpatient rehabilitation and endurance training. There is also evidence, that pulsing electromagnetic fields may improve fatigue associated with multiple sclerosis. This paper summarizes the recent literature on pathophysiology, diagnosis and therapy of the most common symptom of multiple sclerosis.

Effect of voluntary facilitation on the diaphragmatic response to transcranial magnetic stimulation

We assessed recruitment curves of the surface diaphragm motor-evoked potential (MEP) after transcranial magnetic stimulation during relaxation and at three different levels of facilitation (20, 40, and 60% of maximal inspiratory esophageal pressure) in 10 healthy subjects (six young and four elderly). MEP amplitude recruitment curves varied between individuals during relaxation and at each level of facilitation. Amplitude recruitment curves during relaxation were reproducible in individual subjects. Inspiratory maneuvers caused a decrease in motor threshold and latency and an increase in MEP amplitude, positively correlated to the intensity of facilitation. These changes were similar in young and elderly subjects. The best fit for MEP amplitude recruitment curves for each condition was obtained with a Boltzmann model. The performance of repeated submaximal inspiratory maneuvers did not affect the amplitude recruitment curves of the relaxed diaphragm. We conclude that the recruitment curve of the diaphragm with transcranial magnetic stimulation is repeatable and changes consistently with facilitation and will, therefore, be a robust experimental tool for the investigation of supraspinal pathways to the diaphragm.

Validation of surface recordings of the diaphragm response to transcranial magnetic stimulation in humans

The integrity of the central efferent motor pathways to the diaphragm can be assessed by using transcranial magnetic stimulation to measure the latency of the corresponding motor evoked potentials with surface electrodes. Because transcranial magnetic stimulation does not activate the diaphragm alone, signal contamination is a potential problem. To evaluate this issue, surface diaphragmatic motor-evoked potential latencies were compared with latencies recorded from diaphragm needle in 9 healthy volunteers. Surface latencies of muscles likely to contaminate the diaphragm signals (serratus anterior, pectoralis major, and tranversus abdominis) were also recorded. The latencies in response to nonfocal transcranial stimulation from surface electrodes were not significantly different from the needle ones (17 +/- 1.3 vs. 17.2 +/- 1.1 ms, respectively) but were significantly different from the latencies of the other muscles. In two cases, signal contamination appeared likely (serratus anterior in 1 case, abdominal muscles in 1 case). It is possible to reliably measure the latency of the diaphragm response to transcranial magnetic stimulation with adequately positioned surface electrodes.

Are the neurophysiological techniques useful for the diagnosis of diaphragmatic impairment in multiple sclerosis (MS)?

OBJECTIVE

To characterize cortico-diaphragmatic pathway involvement in multiple sclerosis (MS) by means of transcranial magnetic stimulation (TMS), and verify its clinical impact.

METHODS

TMS from diaphragm (Dia), and abductor digiti minimi (AbdV degrees ) was performed in 26 MS patients. Phrenic nerve (PN) conduction study was also performed. Expanded disability status scale (EDSS) and fatigue descriptive scale (FDS) were measured. Forced vital capacity (FVC), forced expiratory volume at the first second (FEV1), peak expiratory flow (PEF) were tested: the predicted percentage value (% pred) was considered.

RESULTS

Cortical motor evoked potential (Cx-MEP) latency and central motor conduction time (CMCT) were prolonged, respectively, in 31 and 23% of patients from Dia, in 76 and 79% from AbdV degrees. PN-compound motor action potential (CMAP) was normal. EDSS correlated to Cx-MEP from AbdV degrees (P<0.01), and PN-CMAP amplitude (P<0.05), FEV1 % pred (P<0.01), PEF % pred (P<0.01). PN-CMAP amplitude correlated to FVC % pred P=0.05, FEV1 % pred P<0.01, PEF % pred P<0.01. Fatigue was related to AbdV degrees Cx-MEP and CMCT (P<0.05 and P<0.01).

CONCLUSIONS

Cortico-diaphragmatic pathway is impaired only in a minority of MS patients. Lack of correlation between TMS findings from Dia and respiratory tests argues against its routinary use to detect subclinical respiratory alterations. Fatigue seems to be related to the motor impairment rather than to respiratory distress.

Diaphragm compound muscle action potential measured with magnetic stimulation and chest wall surface electrodes

To seek a method to reliably measure phrenic nerve conduction time (PNCT) with magnetic stimulation we investigated two stimulus sites, placing the magnetic coil at the cricoid cartilage (high position) or close to the clavicle (low position). We also compared compound muscle action potential (CMAP) recorded from three different sites: in the sixth to eighth intercostal spaces in the anterior axillary line (Ant-a); in the 8th intercostal space close to the midclavicular line; and with one electrode at the lower sternum and the other at the costal margin. Fourteen normal subjects were studied. The PNCT measured by magnetic stimulation in the high position recorded from (Ant-a) was 7.6+/-0.6 on the left side and 8.4+/-0.7 on the right. The PNCT recorded from all three sites become much shorter when the magnetic coil was moved from the high to the low position. Our results show that PNCT can be accurately measured with magnetic stimulation when care is taken to avoid coactivation of the brachial plexus.

Idiopathic bilateral diaphragmatic paralysis

A 41-year-old man complained of subacute onset of dyspnea and pain in the neck and chest. He was diagnosed with bilateral diaphragmatic paralysis, based on clinical inspection of the breathing pattern and transdiaphragmatic pressure recording, and was trained to use a portable bi-level positive airway pressure apparatus (BiPAP). Needle electromyography showed profuse fibrillation potentials and positive waves in the diaphragm, more abundant on the right than left side, and no response to phrenic nerve stimulation. Other muscles were not involved. Follow-up examinations, performed at 9 and 12 months after onset of paralysis, demonstrated a slow but progressive improvement of the patient's respiratory function, together with the appearance of reinnervation potentials in the diaphragm, and polyphasic, long-latency responses to phrenic nerve stimulation. The subacute onset of the paralysis associated with local pain, and its subsequent recovery, suggest bilateral proximal lesions in the phrenic nerves. In the absence of traumatic or metabolic causes, these findings suggest that the phrenic nerve can be a target in idiopathic neuritis.

Validation of improved recording site to measure phrenic conduction from surface electrodes in humans

Phrenic nerve stimulation, electrical (ES) or from cervical magnetic stimulation (CMS), allows one to assess the diaphragm contractile properties and the conduction time of the phrenic nerve (PNCT) through recording of an electromyographic response, traditionally by using surface electrodes. Because of the coactivation of extradiaphragmatic muscles, signal contamination can jeopardize the determination of surface PNCTs. To address this, we compared PNCTs with ES and CMS from surface and needle diaphragm electrodes in five subjects (10 phrenic nerves). At a modified recording site, lower and more anterior than usual (lowest accessible intercostal space, costochondral junction) with electrodes 2 cm apart, surface and needle PNCTs were similar (CMS: 6.0 +/- 0.25 ms surface vs. 6.2 +/- 0.13 ms needle, not significant). Electrodes recording the activity of the most likely sources of signal contamination, i.e., the serratus anterior and pectoralis major, showed distinct responses from that of the diaphragm, their earlier occurrence strongly arguing against contamination. With ES and CMS, apparently uncontaminated signals could be consistently recorded from surface electrodes.

Magnetic stimulation of the central and peripheral nervous systems

Since 1985, when the technique of transcranial magnetic stimulation (TMS) was first developed, a wide range of applications in healthy and diseased subjects has been described. Comprehension of the physiological basis of motor control and cortical function has been improved. Modifications of the basic technique of measuring central motor conduction time (CMCT) have included measurement of the cortical silent period, paired stimulation in a conditioning test paradigm, repetitive transcranial magnetic stimulation (rTMS), and peristimulus time histograms (PSTH). These methods allow dissection of central motor excitatory versus inhibitory interplay on the cortical motor neuron and its presynaptic connections at the spinal cord, and have proven to be powerful investigational techniques. TMS can be used to assess upper and lower motor neuron dysfunction, monitor the effects of many pharmacological agents, predict stroke outcome, document the plasticity of the motor system, and assess its maturation and the effects of aging, as well as perform intraoperative monitoring. The recent use of rTMS in the treatment of depression and movement disorders is novel, and opens the way for other potential therapeutic applications.

Bilateral phrenic paralysis in a patient with systemic lupus erythematosus

Respiratory manifestations of systemic lupus erythematosus (SLE) are frequent. They include respiratory muscle abnormalities, which have been implicated in the pathogenesis of the "shrinking lung syndrome" (SLS). We report the case of a patient with this syndrome, in whom diaphragmatic paralysis due to demyelinating phrenic lesions was diagnosed at the same time as SLE. Follow-up studies showed a favorable clinical and diaphragmatic outcome with corticosteroid therapy, but little change in spirometry. It is concluded that severe diaphragm palsy is possibly due to phrenic nerve lesions in SLE, and that the link between diaphragm dysfunction and the SLS is probably not a straightforward one.

Expiratory muscle activation by functional magnetic stimulation of thoracic and lumbar spinal nerves

OBJECTIVE

This study was conducted to stimulate respiratory muscles by functional magnetic stimulation (FMS) of the spinal nerves (T1-L5) to obtain maximum expiratory function.

DESIGN

A prospective before and after trial.

SETTING

Functional Magnetic Stimulation Laboratory, Spinal Cord Injury Service, VA Palo Alto Health Care System, Palo Alto, CA.

PARTICIPANTS

Twelve normal able-bodied subjects.

INTERVENTION

A commercially available magnetic stimulator with a round magnetic coil (MC) was used. Respiratory muscle activation was achieved by placing the MC at each spinous process ranging from T1 to L5 vertebral levels.

MAIN OUTCOME MEASURE

The planned major outcome was to determine the optimal MC placement for producing maximal expiratory pressure (MEP) and expiratory reserve volume (ERV) by FMS. These measurements were compared with the subjects' voluntary maximal efforts. A profile with varying stimulation intensities was also obtained in select individuals for determining the highest expiratory pressure.

RESULTS

Stimulation at the T9 spinal level resulted in the highest mean MEP and ERV. Stimulation between T8 and L5 produced similar MEP and ERV as obtained from the T9 MC placement. The mean maximum MEP and ERV produced by FMS were 76.8 +/- 6.4 cm H2O (7.52 +/- 0.62 kPa) and 1.28 +/- 0.15 L, which were 67% and 79% of the subjects' voluntary maximal efforts, respectively. A stimulation intensity of 80% resulted in the highest expiratory pressure.

CONCLUSION

FMS of lower thoracic and lumbar regions produced significant expiratory pressures and volumes. FMS of the expiratory muscles may prove to be a valuable technique for restoring cough in patients with spinal cord injury or other neurologic diseases, and in critical care or perioperative settings.

Neuralgic amyotrophy with phrenic nerve involvement

Phrenic nerve involvement is a rare feature in patients with neuralgic amyotrophy (Parsonage-Turner syndrome). We report four patients who initially presented with severe dyspnea in the absence of lung disease. All patients had a history of infectious disease or surgery and of pain of sudden onset in the shoulder region. Weakness of the proximal arm was observed in only one. Radiographic and pulmonary function studies, phrenic nerve conduction studies, and needle electromyogram (EMG) of the diaphragm documented diaphragmatic paralysis which was unilateral in one patient, bilateral in two patients, and recurrent on alternating sides in another one. Follow-up studies remained abnormal for up to 4 years. Neuralgic amyotrophy with phrenic nerve involvement should be considered in patients presenting with severe, unexplained dyspnea of sudden onset.

Clinical and electrophysiological findings in critical illness polyneuropathy

Sixty two patients with critical illness polyneuropathy (CIP) were studied prospectively to determine the clinical and electrophysiological profile, to assess the prognostic value of respiratory electrophysiology in determining the duration of ventilation and to analyze the role of neuromuscular blocking agents (NMBA) and steroids. Limb motor and sensory nerve conductions, bilateral phrenic nerve onset latencies, bilateral diaphragmatic compound muscle action potentials (CMAP), unilateral diaphragmatic needle electromyography (EMG), limb muscle EMG, time on the ventilator, time in the intensive care unit (ICU), dosage of NMBA and steroids were analyzed in 62 patients. The diagnosis of CIP was made by clinical criteria, electrophysiological criteria and exclusion of any other condition suspicious of an axonal neuropathy. The results of phrenic nerve conduction studies and diaphragmatic EMG were compared to normal mean values in 25 healthy subjects. The most common finding in our study were reduced CMAPs and abnormal spontaneous activity in muscle, occuring in 100%. Forty per cent had reduced CMAPs but normal sensory nerve action potentials (SNAP). These patients had normal CK-levels and normal findings, unspecific changes, type 2 fibre atrophy or denervation atrophy on muscle biopsy. Seventy seven per cent of patients had abnormal diaphragmatic CMAPs and spontaneous activity in the diaphragm indicating denervation of the diaphragm is common in CIP. There was no statistically significant relationship to the dosage of NMBA and steroids, and the respiratory electrophysiological studies, duration of ventilation and stay in the ICU.

Electromyography of the diaphragm in neuromuscular disease

We compared the diaphragmatic electromyographic (EMG) recordings from 32 patients with known neuromuscular disease and respiratory symptoms (23 neuropathies, 9 myopathies) to recordings from 23 normal subjects. Turns analysis of 219-ms sections, or epochs, of the EMG demonstrated a significant overlap between diagnostic groups, although some epochs from neuromuscular patients were significantly different from normal. Empirical rules were derived to infer neuropathic and myopathic involvement of the diaphragmatic EMG.

Functional magnetic stimulation for restoring cough in patients with tetraplegia

OBJECTIVE

To evaluate the usefulness of functional magnetic stimulation (FMS) as a noninvasive method for assisting cough in patients with tetraplegia.

DESIGN

A prospective before-after trial.

SETTING

The functional magnetic stimulation laboratory of a spinal cord injury (SCI) service.

PARTICIPANTS

Thirteen male SCI patients, with injury levels between C4 and C7.

INTERVENTION

A commercially available magnetic stimulator with a round magnetic coil (MC) was used. Expiratory muscle activation was achieved by placing the MC along the lower thoracic spine.

MAIN OUTCOME MEASURE

The planned major outcome measures were the maximal expired pressure (MEP), expiratory reserve volume (ERV), and forced expiratory flow rate (FEF) by FMS compared with voluntary maximal efforts. Another outcome was the optimal MC placement and stimulation intensity that would result in highest expired pressure.

RESULTS

The mean (+/-SEM) MEP, ERV, and FEF generated by FMS were 66.40 +/- 6.69 cmH2O, .77 +/- .14 L, and 5.28 +/- .42 L/sec, respectively. They were 118%, 169%, and 110% of voluntary maximum efforts. MC placement at the T10 to T11 spinous process and stimulation intensity at 80% produced the highest MEP and FEF.

CONCLUSION

FMS of the expiratory muscles produced significant expired pressures, volumes, and flow rates when compared with voluntary maximum efforts; therefore, FMS can be used as an effective method to restore cough in tetraplegic patients.

Functional magnetic stimulation of expiratory muscles: a noninvasive and new method for restoring cough

The purpose of this study was to assess the effectiveness of functional magnetic stimulation (FMS) for producing expiratory function in normal human subjects. Twelve able-bodied normal subjects were recruited for this study. FMS of the expiratory muscles was performed by using a magnetic stimulator and placing the magnetic coil along the lower thoracic spine. Results showed that peak expired pressure, volume, and flow rate generated by FMS at the end of normal inspiration (102.5 +/- 13.62 cmH2O, 1.6 +/- 0.16 liters, and 4.8 +/- 0.35 l/s, respectively) were comparable to their voluntary maximal levels (P > 0.1). The optimal coil placement was between T7 and T11, and the optimal stimulation parameters were a frequency of 25 Hz and 70-80% of maximal intensity. We conclude that 1) FMS of the lower thoracic nerves in normal subjects resulted in a significant expiratory function comparable to their voluntary maximum; 2) FMS was noninvasive and was well tolerated by all subjects; and 3) FMS may be useful to produce cough in patients in critical care or perioperative settings, or in patients with neurological disorders.

Electrophysiological respiratory studies in the critical care unit

Respiratory electrophysiological studies are useful in the investigation and monitoring of respiratory failure. Phrenic nerve conduction studies and needle electromyography of the diaphragm are invaluable in establishing the diagnosis, determining the severity, and following the progression of peripheral respiratory muscle dysfunction. In addition to these established methods, repetitive phrenic nerve stimulation is of diagnostic value in patients with neuromuscular transmission defects and dyspnea. The diagnosis of impaired central respiratory drive can often be accomplished by the newly-developed techniques of transcortical magnetic stimulation of the motor cortex with recording of the diaphragm and phrenic nerve somatosensory evoked potentials. These studies are of particular value in critically ill patients where both the central and peripheral lesions may impair respiration.

Evaluation of early motor and sensory evoked potentials in cervical spinal cord injury

To determine the efficacy of motor evoked potentials (MEP) and sensory evoked potentials (SEP) in the assessment of severe cervical injury, 17 subjects with severe cervical injury were studied. During the 1st week post-injury and post-surgical treatment, all subjects were submitted to electromyogram (EMG) recordings, dermatomal somatosensory evoked potentials (D.SEP), posterior tibial nerve somatosensory evoked potentials (PTN.SEP), MEP and bilateral cervical electrical stimulations with recording of the diaphragm. For the D.SEP, the latencies of the N9 and N20 responses and the conduction time (N9-N20) were measured in the upper limbs; the latencies of the P40 and P60 responses were measured in the lower limbs. MEP were recorded from distal upper and lower limb muscles following transcranial electrical stimulation of the cortex. (Magnetic stimulation was not indicated because of implanted metallic material in the cervical skull of many patients.) A SEP and MEP grading system was used to improve the assessment of different root neurological levels. In patients with incomplete lesions PTN.SEP, D.SEP and MEP responses could be recorded in territories that were clinically deficient. Patients with complete lesions and absent SEP and MEP responses had a poor outcome. A good correlation was found between the severity of the spinal cord injury and SEP grading. For MEP, the presence or absence of intercostal responses (C4) to cervical and cortical stimulation was the best prognostic indicator. The combined electrophysiological exploration of MEP and SEP proved to be a useful tool for monitoring patients with severe spinal cord injury.

Comparison of magnetic and electrical phrenic nerve stimulation in assessment of phrenic nerve conduction time

Cervical magnetic stimulation (CMS), a nonvolitional test of diaphragm function, is an easy means for measuring the latency of the diaphragm motor response to phrenic nerve stimulation, namely, phrenic nerve conduction time (PNCT). In this application, CMS has some practical advantages over electrical stimulation of the phrenic nerve in the neck (ES). Although normal ES-PNCTs have been consistently reported between 7 and 8 ms, data are less homogeneous for CMS-PNCTs, with some reports suggesting lower values. This study systematically compares ES- and CMS-PNCTs for the same subjects. Surface recordings of diaphragmatic electromyographic activity were obtained for seven healthy volunteers during ES and CMS of varying intensities. On average, ES-PNCTs amounted to 6.41 +/- 0.84 ms and were little influenced by stimulation intensity. With CMS, PNCTs were significantly lower (average difference 1.05 ms), showing a marked increase as CMS intensity lessened. ES and CMS values became comparable for a CMS intensity 65% of the maximal possible intensity of 2.5 Tesla. These findings may be the result of phrenic nerve depolarization occurring more distally than expected with CMS, which may have clinical implications regarding the diagnosis and follow-up of phrenic nerve lesions.