Abstract
The main reason for short survival in amyotrophic lateral sclerosis (ALS) is involvement of respiratory muscles. Severe compromise of diaphragmatic function due to marked loss of motor units causes poor inspiratory strength leading to symptomatic respiratory fatigue, and hypercapnia and hypoxemia, often firstly detected while sleeping supine. Weakness of expiratory muscles leads to cough weakness and poor bronchial clearance, increasing the risk of respiratory infection. Respiratory tests should therefore encompass inspiratory and expiratory function, and include measurements of blood gases during sleep. Non-volitional tests, such as phrenic nerve stimulation, are particularly convenient for investigating respiratory function in patients unable to perform standard respiratory function tests due to poor cooperation or facial weakness. However, SNIP is a sensitive test when patients with bulbar involvement are able to perform the necessary maneuvers. It is likely that central respiratory regulation is disturbed in some ALS patients, but its evaluation is more complex and not regularly implemented. Practical tests should incorporate tolerability, sensitivity, easy application for regular monitoring, and prognostic value. Impending respiratory failure can cause increased circulating inflammatory markers, but molecular assessment of respiratory distress requires further study. In future, home-monitoring of patients with accessible devices should be developed.
Highlights
Respiratory insufficiency (RI) in amyotrophic lateral sclerosis (ALS) usually emerges in the late stage of disease progression, it may sometimes be the presenting feature [1, 2]
Maximal inspiratory pressure (MIP) and nasal inspiratory pressure during a maximal sniff (SNIP) are inexpensive and noninvasive respiratory measures that access maximal inspiratory muscular strength, the first against a mouth occlusion and the second using a plug inserted in one nostril [50,51,52]
The efficiency of the expiratory muscles can be addressed by evaluating the peak expiratory flow (PEF) and the peak cough flow (PCF), and maximal expiratory pressure (MEP) evaluates the strength of these muscles
Summary
Respiratory insufficiency (RI) in ALS usually emerges in the late stage of disease progression, it may sometimes be the presenting feature [1, 2]. Forced vital capacity (FVC) is a non-invasive respiratory test that has long been used in ALS It assesses both the inspiratory and expiratory loops, requiring expiration done forcefully after a maximal inspiration, as opposed to slow vital capacity (SVC). SVC has been preferred in a number of recent trials, as it is very strongly correlated with FVC (and with other respiratory tests such as Maximal Inspiratory Pressure and Maximal Expiratory Pressure), as well as with ALSFRS-R [36] It is a predictor of progression, the need for positive pressure ventilation, and survival in ALS [37, 38]. Sleep studies have been investigated for a long time in ALS In this disorder, reduction of the rapid eye movement (REM) sleep stage is typically observed, in particular when the diaphragm is markedly affected and accessory respiratory muscles are weak [47]. There is a strong link between severity of respiratory function impairment, poor quality of sleep, and daytime somnolence, in ALS [49]
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