Characterisation of hemidiaphragm dysfunction using dynamic chest radiography: a pilot study

Discussion

DCR is a rapid, well-tolerated and new technique to assess diaphragm motion. In a short period of time it produces clear, visually obvious images of the diaphragm in motion. Unlike the smaller images of fluoroscopy, it has a wide, high-resolution field of view that encompasses the entire thoracic cavity, allowing concurrent diagnostic imaging and obviating the need for a supplementary chest radiograph. Clear differences in diaphragm motion are demonstrated between healthy and abnormal hemidiaphragms using DCR that were consistent with the appearances visualised by the reporting radiographer. DCR produces novel metrics on diaphragm and chest wall motion, which may be of further research interest. In those cases in which ultrasound or fluoroscopy had also been carried out, DCR matched the sniff test findings.

Due to the limited cases where concurrent comparator techniques were carried out (three cases with traditional fluoroscopy and three with ultrasound), this work cannot reliably comment on the specificity of DCR in the diagnosis of diaphragm paralysis, but may hold several potential advantages over traditional fluoroscopy and ultrasound in the assessment of diaphragm dysfunction. Furthermore, although chest wall and abdominal musculature can influence diaphragm motion by altering intrathoracic pressure, such changes will not be unilateral and therefore paradoxical motion of one hemidiaphragm, especially during a forced respiratory manoeuvre, is suggestive of its denervation. We postulate that the significant difference in speed during sniff manoeuvre between the afflicted and unafflicted hemidiaphragms seen here represent a reduction in muscle strength on the afflicted side.

Traditionally, fluoroscopy is used to visualise the diaphragm during respiration: it is relatively quick to perform and is easily interpretable. However, it requires radiologist supervision, a dedicated fluoroscopy system, and may confer a significant radiation dose (upwards of 0.3 mSv): in order to reduce exposure, the field of view is necessarily small, excluding much of the lung parenchyma visible on a plain chest radiograph. Furthermore, the images acquired are not digitised, limiting their value.

M-mode ultrasound is portable and utilises a voluntary sniff test to provide results without ionising radiation, as well as measuring diaphragm thickness and thickening ratio. However, it requires skill and regular practice; this necessary expertise may not be readily available in some units. The values generated automatically by DCR might in future address the problems of reproducibility seen in ultrasound [8]. Whereas visualisation of the diaphragm by ultrasound may be obscured by the descending hemidiaphragm, this is also not the case with DCR. DCR has a much shorter acquisition time than ultrasound or fluoroscopy, with visual results available to the ordering clinician immediately after acquisition, at a lower radiation dose than fluoroscopy. Dynamic magnetic resonance imaging (MRI) has been employed as a tool to analyse diaphragm function [9, 10], but is not in routine clinical use due to its high cost and limited availability. The small footprint of DCR allows installation in a standard radiography room, rather than a separate fluoroscopy or MRI suite.

This pilot work has some limitations. Only six cases were compared against an established reference technique of diaphragm motion assessment such as ultrasound or fluoroscopy, and no cases made comparison to phrenic nerve stimulation studies or phrenic nerve electromyography. There is a lack of normative data in the form of healthy comparators, which may be a focus for future studies in this area.

DCR has been used to assess average diaphragm excursion during tidal and deep breathing in healthy volunteers [7, 11], and abnormal diaphragm dynamics in people with severe COPD [12], and pulmonary perfusion [13, 14]. Similar correlations between BMI and diaphragm motion, as shown in this work, have been demonstrated by Hata et al. [5] in healthy volunteers.

To our knowledge, this is the first time DCR has been used in the investigation of hemidiaphragm dysfunction, and there remains large potential for further work in characterisation of the abnormal diaphragm, for example following surgical plication. No single imaging or spirometric technique can accurately diagnose diaphragm paralysis, and each has its own advantages and disadvantages [3]. DCR, which combines the visual ease of fluoroscopy with the ability to provide quantifiable measures of hemidiaphragm and chest wall movement, may be of use as an adjunct technique. These metrics may also be of use in disease monitoring and outcome prediction, for example after thoracic surgery and other scenarios where forced respiratory manoeuvres for lung volume measurement are problematic. However, the effort-dependent nature of any voluntary technique to assess diaphragm motion [3] may still prove problematic. Here, DCR proved quick and straightforward to perform. It produces easily interpretable metrics on diaphragm and chest wall motion that warrant further study alongside established diagnostic tools such as ultrasound and traditional fluoroscopy. Larger studies are warranted based on these initial results.