Assessment of the motor pathway to the diaphragm using cortical and cervical magnetic stimulation in the decision-making process of phrenic pacing

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.

Phrenic nerve conduction study to diagnose unilateral diaphragmatic paralysis

BACKGROUND

Unilateral diaphragmatic paralysis (UDP) has major clinical and etiological implications and, therefore, is important to diagnose. Lung function tests and invasive transdiaphragmatic pressure (Pdi) measurements are widely used to this end but, contrary to phrenic nerve conduction study (NCS), they require volitional maneuvers and/or may be poorly tolerated by patients. The purpose of this study was to compare the diagnostic accuracy of Pdi and phrenic NCS for UDP.

METHODS

We retrospectively reviewed 28 patients with suspected UDP. The diagnosis established during a multidisciplinary meeting was the reference standard.

RESULTS

Phrenic NCS correlated well with Pdi (r = 0.82, P < .005), and the two tests showed good agreement (κ = 0.82, P < .005). Phrenic NCS and Pdi measurements both had 95% sensitivity, 87.5% specificity, 95% positive predictive, and 87.5% negative predictive values.

CONCLUSIONS

Both tests were highly sensitive and specific. Phrenic NCS measurement is a simple, reproducible, noninvasive method whose results correlate well with Pdi and provide insight into the UDP mechanism. In the most difficult cases, combining lung function tests, respiratory muscle assessments, and phrenic NCS can help to establish the diagnosis.

Diaphragm pacing: the state of the art

Diaphragm pacing (DP) is an orphan surgical procedure that may be proposed in strictly selected ventilator-dependent patients to get an active diaphragm contraction. The goal is to wean from mechanical ventilation (MV) and restore permanent efficient breathing. The two validated indications, despite the lack of randomised control trials, concern patients with high-level spinal cord injuries (SCI) and central hypoventilation syndromes (CHS). To date, two different techniques exist. The first, intrathoracic diaphragm pacing (IT-DP), based on a radiofrequency method, in which the electrodes are directly placed around the phrenic nerve. The second, intraperitoneal diaphragm pacing (IP-DP) uses intradiaphragmatic electrodes implanted through laparoscopy. In both techniques, the phrenic nerves must be intact and diaphragm reconditioning is always required after implantation. No perioperative mortality has been reported and ventilator-weaning rate is about 72% to 96% in both techniques. Improvement of quality of life, by restoring a more physiological breathing, has been almost constant in patients that could be weaned. Failure or delay in recovery of effective diaphragm contractions could be due to irreversible amyotrophy or chest wall damage. Recent works have evaluated the interest of IP-DP in amyotrophic lateral sclerosis (ALS). After some short series were reported in the literature, the only multicentric randomized study including 74 ALS patients was prematurely stopped because of excessive mortality in paced patients. Then, another trial analysed the place of IP-DP in peripheral diaphragm dysfunction but, given the multiple biases, the published results cannot validate that indication. Reviewing all available literature as in our experience, shows that DP is an effective method to wean selected patients dependent on ventilator and improve their daily life. Other potential indications will have to be evaluated by randomised control trials.

Respiratory neuromodulation in patients with neurological pathologies: for whom and how?

Implanted phrenic nerve stimulation is a technique restoring spontaneous breathing in patients with respiratory control failure, leading to being dependent on mechanical ventilation. This is the case for quadriplegic patients with a high spinal cord injury level and for patients with congenital central hypoventilation syndrome. The electrophysiological diaphragm explorations permits better patient selection, confirming on the one hand a definite issue with central respiratory command and on the other hand the integrity of diaphragmatic phrenic nerves. Today there are two different phrenic stimulation techniques: the quadripolar intrathoracic stimulation and the bipolar intradiaphragmatic stimulation. Both techniques allow patients to be weaned off their mechanical ventilator, improving dramatically their quality of life. In fact, one of the systems (phrenic intradiaphragmatic stimulation) was granted social security reimbursement in 2009, and now both are reimbursed. In the future, phrenic intradiaphragmatic stimulation may find its place in the intensive care unit, for patients needing it temporarily, for example, after certain surgeries with respiratory complications as well as diaphragmatic atrophies induced by prolonged mechanical ventilation.

[Diaphragmatic palsy and dysfunction: from physiology to surgery]

The clinical presentations of diaphragm dysfunctions vary according to etiologies and unilateral or bilateral diseases. Elevation of the hemidiaphragm from peripheral origins, the most frequent situation, requires a surgical treatment only in case of major functional impact. Complete morphological and functional analyses of the neuromuscular chain and respiratory tests allow the best selection of patients to be operated. The surgical procedure may be proposed only when the diaphragm dysfunction is permanent and irreversible. Diaphragm plication for eventration through a short lateral thoracotomy, or sometimes by videothoracoscopy, is the only procedure for retensioning the hemidiaphragm. This leads to a decompression of intrathoracic organs and a repositioning of abdominal organs without effect on the hemidiaphragm active contraction. Morbidity and mortality rates after diaphragm plication are very low, more due to the patient's general condition than to surgery itself. Functional improvements after retensioning for most patients with excellent long-term results validate this procedure for symptomatic patients. In case of bilateral diseases, very few bilateral diaphragm plications have been reported. Some patients with diaphragm paralyses from central origins become permanently dependent on mechanical ventilation whereas their lungs, muscles and nerves are intact. In patients selected by rigorous neuromuscular tests, a phrenic pacing may be proposed to wean them from respirator. Two main indications have been validated: high-level tetraplegia above C3 and congenital alveolar hypoventilation from central origin. After progressive reconditioning of the diaphragm muscles following phrenic pacing at thoracic level, more than 90% of patients can be weaned from respirator within a few weeks. This weaning improves the quality of life with more physiological breathing, restored olfaction, better sleep and better speech. The positive impact of diaphragm stimulation has also been evaluated in other degenerative neurological diseases, particularly the amyotrophic lateral sclerosis. For either central or peripheral diaphragm dysfunctions, a successful surgical treatment lies on a strict preoperative selection of patients.

Clinical advances in diaphragm pacing

Diaphragmatic pacing is a valuable tool that can significantly benefit certain patients with respiratory insufficiency provided they have an intact phrenic nerve and a functional diaphragm. Careful patient selection is critical to successful long-term results. The main populations that derive benefit from pacing include those with congenital or acquired central hypoventilation syndrome and more commonly those with a high cervical spinal cord injury, where the phrenic nerves remain intact. The pacing electrode of most phrenic nerve pacemakers is implanted directly on the phrenic nerve. A newer device relies on intramuscular implantation of the electrode on the diaphragm at the phrenic nerve motor point. Most patients can be successfully weaned from mechanical ventilation for a substantial time each day, if not completely. This has significant impact on quality of life and implications for healthcare costs. The potential exists for application of this technology to patients with other types of respiratory failure as investigative experience emerges. These include the chronic progressive disease, amyotrophic lateral sclerosis, or temporary scenarios in difficult-to-wean intensive care unit patients. This enabling technology should hold a place in the thoracic surgeon's armamentarium.

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.

Assessment of Diaphragm and External Intercostals Fatigue from Surface EMG using Cervical Magnetic Stimulation

This study was designed: (1) to test the reliability of surface electromyography (sEMG) recording of the diaphragm and external intercostals contractions response to cervical magnetic stimulation (CMS), (2) to examine the amount and the types of inspiratory muscle fatigue that developed after maximum voluntary ventilation (MVV) maneuvers.Ten male college students without physical disability (22.1±2.0 years old) participated in the study and each completed a control (quiet breathing) trial and a fatigue (MVV maneuvers) trial sequentially. In the quiet breathing trial, the subjects maintained quiet breathing for five minutes. The subjects performed five maximal static inspiratory efforts and received five CMS before and after the quiet breathing. In the MVV trial, subjects performed five maximal inspiratory efforts and received five CMS before, immediately after, and ten minutes after two sets of MVV maneuvers performed five minutes apart. Maximal inspiratory pressure (PImax), sEMG of diaphragm and external intercostals during maximal static inspiratory efforts and during CMS were recorded. In the quiet breathing trial, high intraclass correlation coefficients (ICC=0.95-0.99) were observed in all the variables. In the MVV trial, the PImax, the EMG amplitude and the median power frequency during maximal static inspiratory efforts significantly decreased in both the diaphragm and the external intercostals immediately after the MVV maneuvers (P0.05). It is concluded that the sEMG recordings of the diaphragm during maximal static inspiratory efforts and in response to CMS allow reproducible sequential assessment of diaphragm contractility. MVV maneuvers resulted in inspiratory muscles fatigue, possibly central fatigue.

Spontaneous recovery of diaphragmatic strength in unilateral diaphragmatic paralysis

The aim of the present study was to evaluate diaphragmatic strength in patients with unilateral diaphragmatic paralysis and to determine whether patients with recent diaphragm paralysis develop lower inspiratory pressure than patients with longstanding diaphragmatic paralysis. Twenty patients (16 men and 4 women, 62+/-12 years) and six control subjects were included (4 men and 2 women, 53+/-15 years) in the study. Esophageal pressure during sharp sniff (Pes,sniff), bilateral cervical phrenic nerve magnetic stimulation (Pes,cms) and unilateral phrenic nerve stimulation (Pes,ums) (in nine patients) were measured. Sixteen patients presented right diaphragmatic paralysis and four, left diaphragmatic paralysis. Pes,sniff was higher in control subjects than in patients with diaphragmatic paralysis (respectively 110+/-22 cmH2O and 82+/-24 cmH2O, P<0.05). There was no difference in Pes,cms between patients with diaphragmatic paralysis and control subjects (14+/-7 cmH2O vs. 16+/-4 cmH2O; ns). Pes,ums after stimulation of the affected phrenic nerve was less than 4 cmH2O, was 8+/-2 cmH2O after stimulation of the intact phrenic nerve and was correlated to Pes,cms (R=0.87, P<0.01). There was a positive correlation between Pes,cms, Pes,ums of the intact hemidiaphragm, Pes,sniff and the time from the onset of symptoms and the diaphragmatic explorations (respectively R=0.86, P<0.0001; R=0.72, P<0.05; R=0.48, P<0.05). In conclusion, diaphragmatic strength after unilateral diaphragmatic paralysis seems to improve with time.

Inspiratory muscle pacing in spinal cord injury: case report and clinical commentary

BACKGROUND/OBJECTIVE

A significant fraction of patients with cervical spinal cord injury suffer from respiratory muscle paralysis and dependence on chronic mechanical ventilation. In selected patients, diaphragm pacing (DP) through electrical stimulation of the phrenic nerves provides an alternative to mechanical ventilation with significant advantages in life quality.

METHODS

A case report of an individual who successfully underwent DP using intramuscular diaphragm electrodes. A brief review of the state of the art of DP including the clinical benefits of DP, patient selection and evaluation, description of equipment, methods of transition from mechanical ventilation to DP, potential complications and side effects, long-term outcome, and potential future developments in this field is included.

RESULTS

Several available DP systems are available, including conventional ones in which electrodes are positioned directly on the phrenic nerves through thoracotomy and less invasive systems in which electrodes are placed within the diaphragm through laparoscopy. For patients with only unilateral phrenic nerve function, a combined intercostal and unilateral diaphragm pacing system is under development.

CONCLUSIONS

In patients with ventilator-dependent tetraplegia, there are alternative methods of ventilatory support, which offer substantial benefits compared to mechanical ventilation.

Acute respiratory failure and sleep-disordered breathing in Arnold–Chiari malformation

We report on the case of a 32-year-old man who was admitted after an episode of acute respiratory failure. Clinical and laboratory investigations revealed nocturnal hypoventilation with predominately obstructive sleep apneas accompanied by lower cranial nerve palsies, cerebellar and mild pyramidal signs. Magnetic resonance imaging disclosed Arnold-Chiari type I malformation with syringomyelia. Transcranial magnetic stimulation demonstrated the integrity of the corticodiaphragmatic pathway and it was postulated that the respiratory disorder was mainly due to the severe and irreversible lower cranial nerve palsies. Two years after decompressive craniectomy, sleep disordered-breathing persisted despite no radiological evidence of brain stem compression. Nevertheless, non-invasive positive pressure ventilation (NIPPV) during sleep proved to be quite effective in the management of the patient's refractory respiratory insufficiency. In conclusion, Arnold-Chiari type I may rarely present with acute respiratory failure and sleep apneas. An electrophysiological investigation into the mechanism of the respiratory dysfunction is presented.

[Surgery for diaphragmatic palsy]

Diaphragmatic palsy leads to a permanent ascension of one or both hemi-diaphragms with highly variable functional impact. The underlying mechanisms can be divided into two main categories: neurological or muscular disorder leading to peripheral dysfunction; defective or non-transmitted central command causing central dysfunction. A complete morphological and functional work-up is required to determine the circumstances leading to diaphragmatic palsy and the uni- or bilateral nature of the paralysis. The entire phreno-diaphragmatic transmission chain from the cranium to the diaphragmatic muscle must be analyzed to search for a local cause. Function tests are used to examine central command and transmission, function of the phrenic nerve, and the capacity of the diaphragmatic muscle to generate sufficient pressure for efficacious ventilation. Once indirect causes of diaphragmatic ascension (independent of the phreno-diaphragmatic system) have been ruled out, surgery may be proposed for symptomatic, permanent and irreversible diaphragmatic paralysis. A tension procedure may be sufficient in the event of eventration with or without phrenic palsy. For well-selected patients with central paralysis due to supraspinal lesions with intact nerves and muscles, implantation of a phrenic pacemaker may be helpful to eliminate positive pressure mechanical ventilation and restore more physiological respiration.

Expiratory-Synchronized Sleep in a Quadriplegic Patient Using Inspiratory Neck Muscles To Breathe

In a patient with C3 quadriplegia causing complete diaphragm paralysis who developed inspiratory neck muscles (INM) hypertrophy to sustain ventilation, spontaneous breathing deeply altered sleep architecture, relegating sleep to the expiratory phase of the ventilatory cycle. A polysomnographic recording performed during mechanical ventilation (without INM activity), showed that sleep was abnormal but unaffected by the respiratory cycle. During spontaneous breathing, the polygraphic recordings showed expiratory microsleep episodes, with inspiratory arousals synchronous to bursts of INM activity. This case report illustrates the powerful adaptability of the respiratory and sleep control systems to maintain each vital function.

Disorders of the respiratory muscles

The act of breathing depends on coordinated activity of the respiratory muscles to generate subatmospheric pressure. This action is compromised by disease states affecting anatomical sites ranging from the cerebral cortex to the alveolar sac. Weakness of the respiratory muscles can dominate the clinical manifestations in the later stages of several primary neurologic and neuromuscular disorders in a manner unique to each disease state. Structural abnormalities of the thoracic cage, such as scoliosis or flail chest, interfere with the action of the respiratory muscles-again in a manner unique to each disease state. The hyperinflation that accompanies diseases of the airways interferes with the ability of the respiratory muscles to generate subatmospheric pressure and it increases the load on the respiratory muscles. Impaired respiratory muscle function is the most severe consequence of several newly described syndromes affecting critically ill patients. Research on the respiratory muscles embraces techniques of molecular biology, integrative physiology, and controlled clinical trials. A detailed understanding of disease states affecting the respiratory muscles is necessary for every physician who practices pulmonary medicine or critical care medicine.

Intracortical inhibition and facilitation of the response of the diaphragm to transcranial magnetic stimulation

Respiratory muscles respond to a subcortical automatic command and to a neocortical voluntary command. In diseases such as stroke or motor neurone disease, an abnormal diaphragmatic response to single transcranial magnetic stimuli can identify a central source for respiratory disorders, but this is not likely to be the case in disorders affecting intracortical inhibitory and facilitatory mechanisms. This study describes the response of the diaphragm to paired transcranial magnetic stimulation. Thirteen normal subjects were studied (age range, 22 to 43 years; 7 men; phrenic conduction, <6.8 msec; latency of diaphragmatic motor evoked potential, <20.5 msec). Motor evoked potentials in response to paired stimulation were obtained in eight subjects only, with the motor threshold in the remaining five subjects too high to absorb the loss of power inherent in the double-stimulation montage. Interstimulus intervals less than 5 msec resulted in a statistically significant inhibition (p < 0.01 for interstimulus intervals of 1 and 3 ms), whereas intervals longer than 6 msec were facilitatory (maximal, 15 msec). The diaphragmatic pattern matched that of the biceps brachii. The authors conclude that it is possible to study intracortical inhibition and facilitation of diaphragmatic control, although not in all subjects. Technical improvement should alleviate current limitations and make paired transcranial magnetic stimulation a tool to study respiratory muscle abnormalities in settings in which intracortical interactions are important, such as movement disorders.

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.

Respiratory aspects of neurological disease

Neurological disease may result in respiratory dysfunction; however the manifestations of respiratory dysfunction in such patients may be atypical because of wider effects of their underlying condition. In the present review we have considered separately acute neuromuscular respiratory disease (as well as aspects of respiratory muscle function relevant to intensive care), chronic neuromuscular respiratory disease, sleep related disorders, respiratory consequences of specific neurological diseases, and neurological features of respiratory disease. Approaches to specific clinical problems are discussed; in many instances this can be expedited by close cooperation with a respiratory physician. We suggest that management of respiratory dysfunction in neurological disease depends critically on three factors: firstly, knowledge of when respiratory dysfunction is likely to occur; secondly, maintaining a high index of clinical suspicion (specifically apparently vague symptoms should not be uncritically attributed to the underlying neurological condition); and, thirdly, the pursuing of appropriate investigations.

Diaphragm pacing for respiratory insufficiency

Diaphragm pacing (DP) by electrical stimulation of the phrenic nerve offers important advantages to a highly select group of patients with respiratory paralysis. The patient wears an external radiofrequency (RF) transmitter over an implanted receiver, and a stimulating current is induced without the need for any transcutaneous wires. We review the conditions and requirements of patients who may benefit most from DP. We outline the preoperative evaluation and procedures for surgical implantation. We discuss the risk of diaphragmatic fatigue posed by initiation of DP and the use of gradual conditioning to limit this problem. Other problems encountered by patients in the course of DP can be minimized by well-instructed home caregivers and by systematic medical follow-up. Although a few patients derive considerable benefit from DP, many patients with respiratory paralysis are better treated by less invasive means such as nasal bilevel positive airway pressure or intermittent positive pressure ventilation, which we also review.