Effects of Inspiratory Muscle Training on Exertional Breathlessness in Patients With Unilateral Diaphragm Paralysis

1. RATIONALE AND BACKGROUND Diaphragmatic paralysis is an uncommon, yet underdiagnosed cause of dyspnea. Although the diaphragm performs most of the work, normal ventilation also requires the simultaneous contraction of other respiratory muscles (ie, scalene, parasternal portion of the internal and external intercostal muscles, sternocleidomastoid, trapezius). An increased effort in the struggle to breathe may fatigue the accessory muscles and lead to ventilatory failure. It is thus far not clear whether specific training of the respiratory muscle can improve or restore diaphragm function or accessory respiratory muscle function in these patients.

Depending on the etiology of the diaphragmatic paralysis, the prognosis of unilateral disease usually is good. Patients with phrenic injuries may recover fully or partially. At times, patients may also spontaneously recover from idiopathic disease. Patients who do not recover from unilateral diaphragmatic dysfunction generally lead relatively normal lives. Many patients however keep reporting symptoms of dyspnea that interfere with performance of daily activities. In this group, dyspnea typically develops with exertion, leading to increased ventilatory demands. Dyspnea can also be elicited by changes in posture (e.g. lying supine, bending forward). Depending on the activity profile of the subjects this might have an impact on quality of life. The investigators will therefore evaluate symptoms and recruitment of respiratory muscles of these patients during exertion (i.e. while performing an exercise test on a cycle ergometer).

The morbidity of the unilateral paralysis is mainly based on the underlying pulmonary functional status and the etiology of the paralysis. Most of the patients that are referred to the University Hospital Leuven report symptoms of dyspnea during exertion or with changes in posture. Diaphragmatic paralysis is more likely to affect the left hemidiaphragm. Unilateral diaphragmatic paralysis is characterized by abnormalities of pulmonary and respiratory muscle function. Patients develop restrictive ventilatory impairment, and the vital capacity and total lung capacity frequently are below 70% of the predicted normal values. Lung capacity is reduced further when the patient assumes the supine position.

In contrast to bilateral disease, physicians can usually diagnose unilateral paralysis with only radiographic studies. Because accessory muscle contraction may create the appearance of diaphragmatic movement, fluoroscopy studies may be misleading the physician when diagnosing bilateral diaphragmatic paralysis. In fluoroscopic sniff testing, paradoxical elevation of the paralyzed diaphragm is observed with inspiration and confirms diaphragmatic paralysis. However, the sniff test is not very specific; 6% of normal persons exhibit paradoxical motion on fluoroscopy. Computerized tomography may be indicated in certain patients to evaluate for potential causes of diaphragmatic paralysis that are due to mediastinal pathology and malignancy. MRI may be indicated in certain patients to determine the presence of pathologic conditions involving the spinal column or nerve roots that are causing diaphragmatic paralysis.

M-mode ultrasonography is a relatively simple and accurate test for diagnosing paralysis of the diaphragm in the adult population and it can be performed at the bedside. The paralyzed side shows no active caudal movement of the diaphragm with inspiration and abnormal paradoxical movement (ie, cranial movement on inspiration), particularly with the sniff test. B-mode ultrasonography of diaphragm thickness in the zone of apposition of the diaphragm to the rib cage can also provide a sensitive and specific noninvasive assessment of diaphragmatic paralysis. Less than 20% thickening of the diaphragm muscle during inspiration is diagnostic of diaphragmatic paralysis. Ultrasonography can also be used to serially monitor patients with diaphragmatic paralysis for recovery.

Phrenic nerve conduction studies can be carried out with placement of an oesophageal electrode to record diaphragmatic contractions and stimulation of the nerve in the neck either with surface stimulation or with a monopolar needle electrode at the level of the cricoid cartilage. Alternatively diaphragmatic electromyography may be carried out. The two tests are complementary. Electromyography may reveal a neuropathic versus myopathic pattern, depending on the etiology. This can be accomplished by stimulation of the phrenic nerve at the neck. Phrenic nerve stimulation can be done with electrical (surface or needle electrodes) and magnetic stimulation. Operator expertise is an important factor in testing. Technical issues with electromyography include proper electrode placement to avoid "cross-talk" from adjacent muscles and variable results due to variable subcutaneous fat among individuals. Using intraesophageal EMG electrodes is an alternative technique to avoid these issues. A method of unilateral magnetic stimulation of the phrenic nerve with concurrent assessment of diaphragm EMG and transdiaphragmatic pressures has also been described.

Measuring the vital capacity in the upright and supine positions is the most important part of the pulmonary function test. Diaphragmatic paralysis reduces the measured compliance of the lungs and a restrictive pattern can develop. Normally, vital capacity in recumbency decreases by 10%. In contrast, patients with unilateral diaphragmatic paralysis show a 15-30% decrease in vital capacity in supine position. This decrease is from cephalad displacement of abdominal contents.Pulmonary function test results, however, are not always consistent and do not always correlate with the severity of dyspnea from diaphragmatic paralysis.

Patients with diaphragmatic dysfunction and paralysis have a decrease in maximal inspiratory pressures (Pi,max). These patients cannot generate high negative inspiratory pressures. Therefore, the Pi,max in these patients is often reduced relative to normal values (Pi,max <70% predicted). Important to note is that decreased maximal pressures are the hallmark of diaphragmatic paralysis.

The transdiaphragmatic pressure is the criterion standard for diagnosis. Normal transdiaphragmatic pressure is approximately 148 cm water in men and 122 cm water in women. Unilateral diaphragmatic paralysis is associated with a maximal transdiaphragmatic pressure of greater than 70 cm water, and thus does not significantly effect transdiaphragmatic pressure generation during normal ventilatory behaviors, but can compromise higher-force, nonventilatory, behaviors like coughing or sneezing and also exercise breathing. Twitch transdiaphragmatic pressures after bilateral magnetic phrenic nerve stimulation are frequently used as a non-volitional assessment to confirm the presence of diaphragm weakness.

The functional limitations in individuals with respiratory disorders are not exclusively related to dyspnea. For example, balance impairments and resulting risk of falling are increasingly recognized issues in patients with COPD. Regarding the fact that postural balance is especially impaired in COPD patients with inspiratory muscle weakness, it can be hypothesized that diaphragm dysfunction plays an important role in the observed balance impairments. This is supported by the fact that the diaphragm is not only the principal inspiratory muscle, but also plays an important role in stabilizing the trunk during balance tasks. However, it remains unknown whether patients with diaphragm paresis show balance impairments, and whether these can be attributed to a dysfunction of the diaphragm. Furthermore, Kaufman et al. who followed up 180 patients with diaphragmatic paralysis, suggested that also low back pain might be associated with this disorder due to the fact that the postural function of the diaphragm is often disturbed in patients with low back pain. Also, individuals with low back pain show an increased diaphragm fatigability. However, neither the relation between diaphragmatic paralysis and postural control, neither between diaphragmatic paralysis and low back pain has been investigated yet.

Treatment options for UDP are limited. Diaphragmatic plication via mini thoracotomy is sometimes considered in the University Hospital Leuven if severe symptoms persist for longer than 12 months after initial diagnosis. Preliminary data indicate that daily inspiratory muscle strength and endurance training can lead to increased nondiaphragmatic inspiratory muscle recruitment and help those with symptoms from diaphragmatic paralysis. Randomized controlled trials comparing intervention groups with improvements achieved by natural recovery in the first months after diagnosis are however so far lacking. In addition there is a lack of comparable data between healthy subjects and patients.

The objective of the current study is therefore to investigate the effects of daily inspiratory muscle training in the first 6 months following diagnosis of unilateral diaphragmatic paralysis. A second objective is to compare (changes in) respiratory muscle function and activation patterns between healthy control subjects and patients. The investigators hypothesize that respiratory muscle training in symptomatic patients with UDP (in comparison with a control group) will reduce symptoms of exertional dyspnea (primary outcome) and will improve respiratory muscle function (at rest and during exercise) and pulmonary function (sitting and supine). Secondly, we hypothesize that the (changes in) recruiting patterns of diaphragmatic and non-diaphragmatic muscles will differ between healthy control subjects and patients both at baseline and in response to the intervention.

1. SPECIFIC OBJECTIVES
2. Investigate effects of IMT on intensity and quality of dyspnea perception in symptomatic patients with unilateral diaphragm paralysis during exercise
3. To identify and distinguish mechanisms responsible for dyspnea reduction after the intervention by exploring inter-relationships between the post-intervention change in dyspnea and changes in relevant measures of respiratory muscle function at rest and during exercise.
4. To compare changes in respiratory muscle recruitment patterns and respiratory neural drive to the diaphragm, as well as scalene, sternocleidomastoid and parasternal intercostal muscles in response to 6 months of inspiratory muscle strength- or endurance training during exercise.
5. To compare differences in (changes in) respiratory muscle recruitment patterns and respiratory neural drive to the diaphragm, as well as scalene, sternocleidomastoid and parasternal intercostal muscles, and postural control between healthy controls and patients.