In healthy individuals, mucociliary clearance and cough mechanisms are effective and efficient in defending against secretion encumbrance, but these mechanisms may become ineffective if the systems malfunction and/or in the presence of excessive bronchial secretions.

Individuals with weak or impaired inspiratory and/or expiratory muscles, with or without glottis closure issues (bulbar insufficiency, tracheostomy), will have decreased peak cough flow (PCF). Weakness of the inspiratory muscles leads to a progressive decrease in vital capacity (VC). Reduced ability to cough leads to secretion retention, predisposing to progessive respiratory morbity. Severe bulbar dysfunction and glottic dysfunction most commonly occurs in patients with amyotrophic lateral sclerosis (ALS), spinal muscle atrophy (SMA) type 1, and other rarer neuromuscular disorders (NMD). 

Inability to close the glottis and vocal cords results in complete loss of the ability to cough and swallow. Difficulty swallowing liquids may result in pooling of saliva and mucus in the pharynx. Alterations in alveolar ventilation, atelectasis, mucus plugging, and recurrent respiratory tract infections (RTI's), as a consequence of an ineffective cough; together with severe bulbar dysfunction, are the main causes of morbility and mortality in patients with NMD.