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

Volitional inspiration is mediated by two independent output channels in the primary motor cortex

The diaphragm is a multifunctional muscle that mediates both autonomic and volitional inspiration. It is critically involved in vocalization, postural stability, and expulsive core-trunk functions, such as coughing, hiccups, and vomiting. In macaque monkeys, we used retrograde transneuronal transport of rabies virus injected into the left hemidiaphragm to identify cortical neurons that have multisynaptic connections with phrenic motoneurons. Our research demonstrates that representation of the diaphragm in the primary motor cortex (M1) is split into two spatially separate and independent sites. No cortico-cortical connections are known to exist between these two sites. One site is located dorsal to the arm representation within the central sulcus and the second site is lateral to the arm. The dual representation of the diaphragm warrants a revision to the somatotopic map of M1. The dorsal diaphragm representation overlaps with trunk and axial musculature. It is ideally situated to coordinate with these muscles during volitional inspiration and in producing intra-abdominal pressure gradients. The lateral site overlaps the origin of M1 projections to a laryngeal muscle, the cricothyroid. This observation suggests that the coordinated control of laryngeal muscles and the diaphragm during vocalization may be achieved, in part, by co-localization of their representations in M1. The neural organization of the two diaphragm sites underlies a new perspective for interpreting functional imaging studies of respiration and/or vocalization. Furthermore, our results provide novel evidence supporting the concept that overlapping output channels within M1 are a prerequisite for the formation of muscle synergies underlying fine motor control.

Novel role for non-invasive neuromodulation techniques in central respiratory dysfunction

Respiration is a crucial steady-state function of human life. Central nervous system injury can damage the central respiratory pattern generator (CRPG) or interrupt its outflow, leading to central respiratory paralysis and dysfunction, which can endanger the patient's life. At present, there is no effective means to reverse this process. Commonly used non-invasive neuromodulation techniques include repetitive transcranial magnetic stimulation (rTMS), transcranial direct current stimulation (tDCS) and so forth, which have been widely applied in nervous system diseases and their various secondary symptoms, but rarely in respiratory function. Clinical and animal studies have confirmed that TMS is also suitable for investigating the excitability and plasticity of ascending corticospinal respiratory pathways. In addition, although rTMS and tDCS differ in their respective mechanisms, both can regulate respiratory networks in healthy individuals and in diseased states. In this review, we provide an overview of the physiology of respiration, the use of TMS to assess the excitability of corticophrenic pathways in healthy individuals and in central respiratory disorders, followed by an overview of the animal and clinical studies of rTMS, tDCS and so forth in regulating respiratory circuits and the possible mechanisms behind them. It was found that the supplementary motor area (SMA) and the phrenic motor neuron (PMN) may be key regulatory areas. Finally, the challenges and future research directions of neuroregulation in respiratory function are proposed. Through understanding how neuromodulation affects the respiratory neural circuit non-invasively, we can further explore the therapeutic potential of this neuromodulation strategy, so as to promote the recovery of respiratory function after central nervous system diseases or injury.

Neurophysiological Correlates in Patients with Syringomyelia and Chiari Malformation: The Cortico-Diaphragmatic Involvement

Purpose. Brainstem syndromes have frequently been reported in Chiari syndrome and in syringobulbia; previous studies have shown that determining the central motor conduction time (CMCT) along the circuit of the phrenic nerve makes the assessment of the voluntary control of the respiratory pathway possible. In our study, we evaluated the transcranial magnetic stimulation (TMS) of the phrenic nerve in patients affected by Chiari syndrome and/or syringomyelia (Syr) with the aim of identifying subclinical neurophysiological alterations. Methods. One hundred patients (75 females; average age: 51 ± 13.08 SD; range: 18–76) affected by Chiari syndrome and/or Syr without dyspnea were selected. The magnetic stimulation of the second motor neuron correlating with the phrenic nerve was performed using cervical magnetic stimulation (C5-MEP); the cortical MEP after magnetic stimulation (Cz-MEP) was recorded by magnetic stimulation of the motor cortex (areas corresponding to the diaphragm). The CMCT was calculated. The differences between the patients and controls were calculated (Student’s t test). Results. In 13% of the patients, the Cz-MEP were absent bilaterally, and the CMCT was not evaluable. In all these cases, bulbar/cervical Syr was present at MRI; in 10 of them, this was associated with CM1. A bilateral response was obtained in all the other patients (87%), and the CMCTs were normal. All the patients with alterations/absence of Cz-MEP presented bulbar/cervical Syr at MRI. The C5-MEP latency was prolonged or absent in 48%; of these, 84% presented bulbar/cervical Syr associated with CM1 at MRI. The C5-MEP latency values were significantly higher in the group of patients. Conclusions. Neurophysiological alterations of the diaphragmatic pathway were recorded in a group of Chiari syndrome and, particularly, in bulbar/cervical Syr. Future studies with larger cohorts of patients are needed to further assess the specific role of the TMS of the phrenic nerve in CM/Syr patients.

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.

The Relationship between Selected Demographic Factors and Speech Organ Dysfunction in Sporadic ALS Patients

Background and objectives: Speech disorders are observed in 30% of newly diagnosed sporadic amyotrophic lateral sclerosis (ALS) patients. Characterized by a dynamic course, dysfunction of articulation has not so far been well understood. The aim of this study was to analyze the influence of demographic factors (sex, age, duration of the disease) and concomitant diseases (degenerative spine disease, depression, hypertension, hypothyroidism, hyperthyroidism, and allergy) on the functioning of speech organs in ALS patients. Materials and Methods: The study group consisted of 65 patients with sporadic ALS. Patients were examined for articulatory functions by means of the Frenchay Dysarthria Assessment (FDA). Results: 68% of the study sample had spinal disorders. Logistic regression analysis showed that a decline in the functioning of lips, soft palate, length of phonation, and voice loudness was more common among men. Patients diagnosed with degenerative spine disease more often suffered from respiratory disorders, while younger patients (<60 years of age) significantly more often had the impairment of the sentence and spontaneous speech functions. Conclusions: The male gender in patients with ALS is associated with an increased risk of deterioration of the phonation length function. Patients under 60 years of age are associated with more often pronouncing sentences disorders and spontaneous speech disorders.

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.

Thoracoabdominal asynchrony and paradoxical motion in middle stage amyotrophic lateral sclerosis

AIM

To assess thoracoabdominal asynchrony (TAA) and the presence of paradoxical motion in middle stage amyotrophic lateral sclerosis (ALS) and its relationships with chest wall tidal volume (V_T,CW), breathing pattern and cough peak flow (CPF).

METHODS

Phase angle (θ) between upper (RCp) and lower ribcage (RCa) and abdomen (AB), as well as percentage of inspiratory time for the lower ribcage (IP_RCa) and abdomen (IP_AB) moving in opposite directions were quantified using optoelectronic plethysmography in 12 ALS patients during quiet breathing and coughing. Paradoxical motion of the compartments was based on threshold values of θ and IP, obtained in twelve age and sex matched healthy persons.

RESULTS

During quiet breathing, significantly higher RCa and AB θ (p < .05), IP_RCa (p = 0.001) and IP_AB (p < 0.05) were observed in ALS patients as compared to controls. In ALS patients, correlations between RCa and AB θ with forced vital capacity (FVC) (r=-0.773, p < 0.01), vital capacity (r=-0.663, p < 0.05) and inspiratory capacity (IC) (r=-0.754, p < 0.01), as well as between RCp and RCa θ with FVC (r=-0.608, p < 0.05) and CPF (r=-0.601, p < 0.05) were found. During coughing, correlations between RCp and AB θ with CPF (r=-0.590, p < 0.05), IC (r=-0.748, p < 0.01) and V_T,CW (r=-0.608, p < 0.05), as well as between RCa and AB θ with CPF (r=-0.670, p < 0.05), IC (r=-0.713, p < 0.05) and peak expiratory flow (r=-0.727, p < 0.05) were also observed in ALS patients. ALS patients with paradoxical motion presented lower vital capacity and FVC_%pred (p < 0.05) compared to those without paradoxical motion.

CONCLUSIONS

Middle stage ALS patients exhibit TAA and paradoxical motion during quiet spontaneous breathing and coughing. In addition, diaphragmatic weakness (i.e. decrease in excursion of the RCa and AB compartments) was observed earlier in the lower ribcage rather than the abdominal compartment in this population.

Respiratory measures in amyotrophic lateral sclerosis

OBJECTIVE

Amyotrophic lateral sclerosis (ALS) is a progressive neuromuscular disease that causes skeletal muscle weakness, including muscles involved with respiration. Death often results from respiratory failure within 3-5 years. Monitoring respiratory status is therefore critical to ALS management, as respiratory/pulmonary function tests (PFTs) are used to make decisions including when to initiate noninvasive ventilation. Understanding the different respiratory and PFTs as they relate to disease progression and survival may help determine which tests are most suitable.

METHODS

This review describes the tests used to assess respiratory muscle and pulmonary function in patients with ALS and the correlations between different respiratory measures and clinical outcomes measures.

RESULTS

The most commonly used measurement, forced vital capacity (VC), has been shown to correlate with clinical milestones including survival, but also requires good motor coordination and facial strength to form a tight seal around a mouthpiece. Other tests such as slow VC, sniff inspiratory pressure, or transdiaphragmatic pressure with magnetic stimulation are also associated with distinct advantages and disadvantages.

CONCLUSIONS

Therefore, how and when to use different tests remains unclear. Understanding how each test relates to disease progression and survival may help determine which is best suited for specific clinical decisions.

Breathing: Motor Control of Diaphragm Muscle

Breathing occurs without thought but is controlled by a complex neural network with a final output of phrenic motor neurons activating diaphragm muscle fibers (i.e., motor units). This review considers diaphragm motor unit organization and how they are controlled during breathing as well as during expulsive behaviors.

Usefulness of phrenic latency and forced vital capacity in patients with ALS with latent respiratory dysfunction

OBJECTIVES

The pulmonary function test (PFT) is a non-invasive and easily available technique to assess respiratory function in patients with amyotrophic lateral sclerosis (ALS); however, patients with dyspnea sometimes show normal PFT findings. Herein, we investigated whether phrenic nerve conduction study (NCS) and PFT are useful to evaluate respiratory function of patients with ALS with normal value ranges in the PFT.

METHODS

We prospectively enrolled 34 patients with definite or probable ALS, who showed FVC (%) ⩾80 of predicted and 78 healthy subjects. PFT and phrenic NCS were performed with the measurement of forced vital capacity (FVC, %), forced expiratory volumes in 1s (FEV1, %), FEV1/FCV ratio (%), and phrenic compound muscle action potential amplitude, and latency.

RESULTS

Compared to healthy controls, ALS patients showed delayed phrenic nerve latency and the decrease of FVC (%) (p=0.006 and p<0.0001, respectively). ROC curve analysis demonstrated that phrenic latency (AUC=0.7655) and FVC (%) (AUC=0.8239) discriminated ALS patients from healthy subjects.

CONCLUSION

We demonstrated that ALS patients had early respiratory dysfunction, despite normal PFT findings.

SIGNIFICANCE

Phrenic latency and FVC (%) can be helpful to discriminate ALS patients with latent respiratory dysfunction from healthy subjects.

Transcranial magnetic stimulation and amyotrophic lateral sclerosis: pathophysiological insights

Amyotrophic lateral sclerosis (ALS) is a rapidly progressive neurodegenerative disorder of the motor neurons in the motor cortex, brainstem and spinal cord. A combination of upper and lower motor neuron dysfunction comprises the clinical ALS phenotype. Although the ALS phenotype was first observed by Charcot over 100 years ago, the site of ALS onset and the pathophysiological mechanisms underlying the development of motor neuron degeneration remain to be elucidated. Transcranial magnetic stimulation (TMS) enables non-invasive assessment of the functional integrity of the motor cortex and its corticomotoneuronal projections. To date, TMS studies have established motor cortical and corticospinal dysfunction in ALS, with cortical hyperexcitability being an early feature in sporadic forms of ALS and preceding the clinical onset of familial ALS. Taken together, a central origin of ALS is supported by TMS studies, with an anterograde transsynaptic mechanism implicated in ALS pathogenesis. Of further relevance, TMS techniques reliably distinguish ALS from mimic disorders, despite a compatible peripheral disease burden, thereby suggesting a potential diagnostic utility of TMS in ALS. This review will focus on the mechanisms underlying the generation of TMS measures used in assessment of cortical excitability, the contribution of TMS in enhancing the understanding of ALS pathophysiology and the potential diagnostic utility of TMS techniques in ALS.

Ventilatory control in ALS

Amyotrophic lateral sclerosis (ALS) is a fatal, progressive neurodegenerative disease. ALS selectively causes degeneration in upper and lower (spinal) motor neurons, leading to muscle weakness, paralysis and death by ventilatory failure. Although ventilatory failure is generally the cause of death in ALS, little is known concerning the impact of this disorder on respiratory motor neurons, the consequences of respiratory motor neuron cell death, or the ability of the respiratory control system to "fight back" via mechanisms of compensatory respiratory plasticity. Here we review known effects of ALS on breathing, including possible effects on rhythm generation, respiratory motor neurons, and their target organs: the respiratory muscles. We consider evidence for spontaneous compensatory plasticity, preserving breathing well into disease progression despite dramatic loss of spinal respiratory motor neurons. Finally, we review current and potential therapeutic approaches directed toward preserving the capacity to breathe in ALS patients.

Pulmonary function tests and diaphragmatic compound muscle action potential in patients with sporadic amyotrophic lateral sclerosis

BACKGROUND

Respiratory failure is the primary cause of death in patients with amyotrophic lateral sclerosis (ALS). Diaphragmatic compound muscle action potentials (DCMAP) are valid parameters to assess the respiratory muscle innervation.

AIM

In this study we propose to establish evidence of pulmonary dysfunction in patients with ALS and its relation to DCMAP parameters among patients with sporadic ALS.

MATERIALS AND METHODS

Twenty nine patients (M-20, F-9) diagnosed to have sporadic ALS by El. Escorial criteria, without symptoms of pulmonary dysfunction, and able to perform the PFT satisfactorily, were studied. Thirty controls (M-20, F-10) were selected from patient's relatives. Forced vital capacity (FVC), forced expiratory volume in one second (FEV(1)), peak expiratory flow rate (PEFR) and maximum voluntary ventilation (MVV) were measured by spirometry. Maximum expiratory pressure (MEP) was measured by digital peak pressure monitor. Right phrenic nerve conductions (DCMAP) were performed and the latencies and amplitude of diaphragmatic com-pound action potential (DCMAP) was recorded in controls and ALS patients.

RESULTS

The mean age of patients was 51.41 +/- 10.72 years (37-82) and control was 53.57 +/- 8.85 years (30-68). None of the patients had symptoms or clinical evidence of respiratory dysfunction. The FVC, FEV1, PEFR, MVV, MIP and MEP were significantly (P < 0.001) reduced in ALS. The mean DCMAP amplitude was reduced among patients (610 +/- 506.231 muv) as compared to controls (1303.33 +/- 584.56, P < 0.001) and mean latency was increased in patients (9.73 +/- 2.57 ms) compared to controls (7.69 +/- 0.87, P = 0.001). There was significant negative correlation between PFTs and latencies of DCMAP. Amplitude of DCMAP did not correlate with PFTs.

CONCLUSION

There is significant negative correlation between DCMAP latencies and PFTs suggesting early loss of myelinated fibres and diaphragmatic dysfunction. DCMAP latencies may be a good indicator of early respiratory muscle involvement and also of disease progression in ALS.