Clinical Neurophysiology xxx (2015) xxx–xxx

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Editorial

Diaphragm ultrasound in amyotrophic lateral sclerosis and other neuromuscular disorders See Article, pages xxx–xxx

Respiratory dysfunction may accompany neuromuscular disorders, caused by combinations of inspiratory muscle weakness, inspiratory muscle fatigue and chest wall abnormalities such as kyphoscoliosis (Roussos and Macklem, 1982). Respiratory muscle weakness is particularly important in amyotrophic lateral sclerosis (ALS) and respiratory failure is a common cause of death in this patient group (Kiernan et al., 2011). Monitoring for the development of respiratory muscle weakness in ALS is important, as it has prognostic implications and interventions such as non-invasive ventilation may improve survival and quality of life (Vrijsen et al., 2013). The best method of measuring and monitoring respiratory dysfunction in ALS is still debated, and there are issues with many respiratory function tests particularly when there is significant bulbar weakness (Simon et al., 2014b). Neurophysiological assessment of the diaphragm in ALS poses some difficulties. Diaphragm compound muscle action potential (CMAP) amplitude as recorded with phrenic nerve conduction studies (NCS) appears to correlate with other indicators of respiratory dysfunction in ALS (Evangelista et al., 1995). However, phrenic NCS need to be carefully performed as electrode location and respiration may affect the amplitude of the diaphragmatic response (Bolton, 1993). Similarly, while abnormalities on needle EMG of the diaphragm in ALS may be useful in identifying sub-clinical respiratory muscle involvement early in the course of disease (Stewart et al., 2001), diaphragm EMG may provoke fear of complications, particularly pneumothorax, and as such tends to be avoided by clinicians. In such a setting, the development of diaphragm ultrasound may provide a useful tool to supplement clinical, functional and neurophysiological studies of respiratory muscle weakness. And it comes with the benefit of being painless and risk-free. From a practical perspective, diaphragm ultrasound is commonly undertaken with the ultrasound transducer placed in the 9th intercostal space in the anterior axillary line. The diaphragm can then be identified as a hypoechoic band between the rib shadows and under the intercostal muscles, sandwiched between the hyperechoic peritoneum and pleura (Fig. 1). In normal individuals, the diaphragm will thicken with inspiration, and it may be seen to reflect off the chest wall in full inspiration as the descending lung displaces it (Supplementary Video).

In terms of other potential benefits, diaphragm ultrasound may also give an indication of the possible underlying pathological processes contributing to diaphragmatic dysfunction (Boon et al., 2014; Sarwal et al., 2013). For example, identification of a hemidiaphragm of normal thickness but without change in thickness during inspiration and expiration is suggestive of acute phrenic nerve palsy (Fig. 1). Bilateral diaphragm atrophy with minimal thickening with inspiration suggests a chronic neuromuscular (either neurogenic or myopathic) cause of diaphragm weakness, including ALS (Fig. 1). Detecting abnormalities on ultrasound may not obviate the need for further investigations such as neurophysiological studies, but ultrasound may also provide information to help guide these studies. Specifically, ultrasound may guide the electromyographer by measuring the depth of the diaphragm below the chest wall and ascertaining diaphragmatic thickness. This may enable an estimate of the required depth of EMG needle insertion, hence minimising the risk of pneumothorax. Ultrasound visualisation of diaphragm twitch improves the diagnostic accuracy of phrenic NCS (Johnson et al., 2014). Separately, ultrasound has also been utilised for the detection of muscle denervation and fasciculations (Arts et al., 2011; Simon, 2014; Simon et al., 2014a), although there have been few studies of diaphragm ultrasound in ALS (Hiwatani et al., 2013; Pinto et al., 2015). From a technical perspective, it is accepted that the mechanics of inspiration are relatively complex, involving the diaphragm, the external intercostal muscles and accessory muscles of respiration. Furthermore, the diaphragm should also be considered as two muscles (De Troyer et al., 1981). Contraction of the costal portion produces expansion of the rib cage and displacement of the abdominal contents. Contraction of the crural portion, which does not have a direct action on the rib cage, results in displacement of the abdominal contents. It is in that context that the study by Pinto and colleagues in this issue of Clinical Neurophysiology (Pinto et al., 2015) makes a substantial contribution to the understanding of sonographic diaphragm changes in ALS. Diaphragm ultrasound evaluates changes in the costal portion of the diaphragm during inspiration, while most respiratory function tests performed in the office (such as forced vital capacity (FVC) and forced expired volume in one second (FEV1)) are of expiratory forces and flows.

http://dx.doi.org/10.1016/j.clinph.2015.04.066 1388-2457/Ó 2015 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.

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Editorial / Clinical Neurophysiology xxx (2015) xxx–xxx

Fig. 1. Diaphragm ultrasound thickness changes at the point of full expiration and inspiration. In a normal subject (A) the diaphragm (arrow head) thickens on inspiration. In a patient with acute phrenic neuropathy (B), diaphragm thickness remains normal but there is minimal thickening with inspiration. In a patient with ALS and respiratory insufficiency (C), the diaphragm thickness becomes reduced and there is little diaphragm thickening with inspiration. ICM – intercostal muscles.

Sniff nasal inspiratory pressure (SNIP) perhaps most closely reflects diaphragm function in isolation, and it was noted that change in diaphragm thickness between inspiration and expiration was related to SNIP in regression analyses. However, it was interesting to note that diaphragm thickening was greater with voluntary inspiration rather than SNIP or maximal voluntary inspiration, perhaps reflecting greater recruitment of auxiliary muscles in these latter non-physiological manoeuvres. It was encouraging that diaphragm thickness on ultrasound correlated with diaphragm CMAP amplitude on phrenic nerve studies. As such, reduction in diaphragm thickness and thickening with inspiration are likely to directly reflect inspiratory muscle function in ALS. However, despite the Pinto study, the utility of diaphragm ultrasound as an assessment of diaphragm function in ALS has not been fully resolved. A reliable measure of inspiratory muscle weakness in patients with severe bulbar dysfunction is needed. In the study by Pinto and colleagues (Pinto et al., 2015), diaphragm ultrasound measurements did not correlate with pulmonary

function tests in the bulbar-onset subgroup, and patients with severe bulbar weakness were excluded from the study. From the data presented it is difficult to ascertain whether diaphragm ultrasound represents an accurate assessment of inspiratory muscle strength in this population given that the reference measurements of pulmonary function tests in bulbar-onset ALS may be unreliable (Simon et al., 2014b), and that reduced central respiratory drive may be responsible for a component of hypoventilation in this group (Hadjikoutis and Wiles, 2001). Further, the time course of respiratory muscle involvement, which was not reported in the study by Pinto and colleagues (Pinto et al., 2015), may yet prove to be an important determinant of diaphragm thickness, similar to muscle atrophy, and hence change in diaphragm thickness, may occur relatively late in the course of denervation. Finally, the diagnostic accuracy of diaphragm ultrasound for the detection of respiratory muscle weakness has not yet been determined. For this to be accurately ascertained, an appropriate reference standard would be required, such as a combination of oesophageal

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manometry and diaphragm EMG, although these studies are necessarily invasive and cumbersome. Despite these issues, it seems likely that diaphragm ultrasound will become an indispensible component of the diagnostic armamentarium of the neuromuscular physician. Further studies are encouraged to develop this useful non-invasive technique. Acknowledgements Dr Simon gratefully acknowledges funding from the National Health and Medical Research Council of Australia and the Motor Neurone Disease Research Institute of Australia (grant #1039520). This work was supported by funding to Forefront, a collaborative research group dedicated to the study of motor neurone disease, from the National Health and Medical research Council of Australia Program Grant (#1037746). Conflict of interest: None of the authors have potential conflicts of interest to be disclosed. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.clinph.2015.04. 066. References Arts IM, Overeem S, Pillen S, Schelhaas HJ, Zwarts MJ. Muscle changes in amyotrophic lateral sclerosis: a longitudinal ultrasonography study. Clin Neurophysiol 2011;122:623–8. Bolton CF. AAEM minimonograph #40: clinical neurophysiology of the respiratory system. Muscle Nerve 1993;16:809–18. Boon AJ, Sekiguchi H, Harper CJ, Strommen JA, Ghahfarokhi LS, Watson JC, et al. Sensitivity and specificity of diagnostic ultrasound in the diagnosis of phrenic neuropathy. Neurology 2014;83:1264–70. De Troyer A, Sampson M, Sigrist S, Macklem PT. The diaphragm: two muscles. Science 1981;213:237–8. Evangelista T, Carvalho M, Pinto A, Luis Mde L. Phrenic nerve conduction in amyotrophic lateral sclerosis. J Neurol Sci 1995;129(Suppl.):35–7. Hadjikoutis S, Wiles CM. Respiratory complications related to bulbar dysfunction in motor neuron disease. Acta Neurol Scand 2001;103:207–13.

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Hiwatani Y, Sakata M, Miwa H. Ultrasonography of the diaphragm in amyotrophic lateral sclerosis: clinical significance in assessment of respiratory functions. Amyotroph Lateral Scler Frontotemporal Degener 2013;14:127–31. Johnson NE, Utz M, Patrick E, Rheinwald N, Downs M, Dilek N, et al. Visualization of the diaphragm muscle with ultrasound improves diagnostic accuracy of phrenic nerve conduction studies. Muscle Nerve 2014;49:669–75. Kiernan MC, Vucic S, Cheah BC, Turner MR, Eisen A, Hardiman O, et al. Amyotrophic lateral sclerosis. Lancet 2011;377:942–55. Pinto S, Alves P, Pimentel B, Swash M, de Carvalho M. Ultrasound for assessment of diaphragm in ALS. Clin Neurophysiol 2015 (This Issue). Roussos C, Macklem PT. The respiratory muscles. NEJM 1982;307:786–97. Sarwal A, Walker FO, Cartwright MS. Neuromuscular ultrasound for evaluation of the diaphragm. Muscle Nerve 2013;47:319–29. Simon NG. Dynamic muscle ultrasound – Another extension of the clinical examination. Clin Neurophysiol 2014. http://dx.doi.org/10.1016/ j.clinph.2014.10.153. Simon NG, Ralph JW, Lomen-Hoerth C, Poncelet AN, Vucic S, Kiernan MC, et al. Quantitative ultrasound of denervated hand muscles. Muscle Nerve 2014. http://dx.doi.org/10.1002/mus.24519. Simon NG, Turner MR, Vucic S, Al-Chalabi A, Shefner J, Lomen-Hoerth C, et al. Quantifying disease progression in amyotrophic lateral sclerosis. Ann Neurol 2014b;76:643–57. Stewart H, Eisen A, Road J, Mezei M, Weber M. Electromyography of respiratory muscles in amyotrophic lateral sclerosis. J Neurol Sci 2001;191:67–73. Vrijsen B, Testelmans D, Belge C, Robberecht W, Van Damme P, Buyse B. Noninvasive ventilation in amyotrophic lateral sclerosis. Amyotroph Lateral Scler Frontotemporal Degener 2013;14:85–95.



Neil G. Simon Prince of Wales Clinical School, University of New South Wales, Australia Brain and Mind Research Institute, Sydney Medical School, The University of Sydney, Australia ⇑ Address: Suite 4101, 834 Pittwater Rd, Dee Why, NSW 2099, Australia. Tel.: +61 2 9982 2270; fax: +61 2 9981 7880. E-mail address: [email protected] Matthew C. Kiernan Brain and Mind Research Institute, Sydney Medical School, The University of Sydney, Australia Available online xxxx

Diaphragm ultrasound in amyotrophic lateral sclerosis and other neuromuscular disorders.

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