[Amyotrophic lateral sclerosis (ALS): evaluation of respiratory function]

Area of Science:

• Neurology
• Pulmonology
• Respiratory Medicine

Background:

• Respiratory involvement is a constant and rapidly progressing feature of Amyotrophic Lateral Sclerosis (ALS), often leading to respiratory failure.
• Subtle respiratory symptoms and signs of muscle weakness require close clinical evaluation, as dyspnea and orthopnea are typically late findings.
• Nocturnal respiratory events, including obstructive sleep apnea syndrome and hypoventilation, are indicated by daytime hypersomnolence and morning headaches.

Framework:

• Clinical evaluation should include assessments for accessory muscle recruitment, supine abdominal paradox, and airway encumbrance.
• Vital capacity (VC) is a standard test, but spirometry has limitations in detecting moderate inspiratory muscle weakness; supine VC can improve diaphragmatic assessment.
• Peak expiratory flow during cough (cough PEF) assesses cough efficiency, with values below 160-270 L/min suggesting impaired airway clearance.

Implementation:

• Arterial blood gases are recommended at initial evaluation and for clinical signs, lung function decline, or sleep desaturations.
• Specific respiratory muscle strength tests, such as Maximal Inspiratory Pressure (PImax) and Sniff Nasal Inspiratory Pressure (SNIP), are sensitive and prognostic.
• Maximal Expiratory Pressure (PEmax) below 45 cm H2O may suggest poor cough efficiency, though correlation with cough PEF can be variable.

Implications:

• Nocturnal oxymetry is valuable for screening sleep apneas and hypoventilation, though criteria for significant desaturations are debated.
• Suspected obstructive sleep apnea syndrome warrants confirmation via polysomnography.
• Comprehensive respiratory assessment is crucial for managing ALS patients and predicting prognosis.