A 65-year-old man with a known diagnosis of amyotrophic lateral sclerosis (ALS) presents with complaints of morning headaches, unrefreshing sleep, and excessive daytime sleepiness. His wife notes that he seems to be breathing shallowly during sleep. His dysarthria and limb weakness have progressed slowly. On examination, his oxygen saturation is 95% on room air while seated.
Which of the following is the most appropriate next step in the evaluation of this patient?
- Measure forced vital capacity (FVC). (correct answer)
- Obtain a chest radiograph.
- Prescribe modafinil for daytime sleepiness.
- Initiate empiric nocturnal oxygen therapy.
Explanation: The patient's symptoms of morning headaches, poor sleep, and daytime somnolence are classic signs of nocturnal hypoventilation due to diaphragmatic weakness in ALS. This is an early sign of respiratory failure. The most appropriate next step is to perform pulmonary function tests, specifically measuring forced vital capacity (FVC) or sniff nasal inspiratory pressure (SNIP), to quantify the degree of respiratory muscle weakness. An FVC below 80% predicted would be concerning, and an FVC below 50% is an indication to initiate noninvasive ventilation (NIV). A chest radiograph is not indicated as infection is not suspected. Prescribing a stimulant like modafinil would be treating a symptom without addressing the underlying life-threatening cause. Initiating oxygen alone can suppress the hypoxic drive to breathe and worsen hypercapnia.