A 54-year-old man comes to the office due to shortness of breath at night. He has also felt weak but has had no dyspnea on exertion or leg swelling. His wife adds that the patient has had occasional cough while eating and that his speech is slightly slurred. He has no chronic medical conditions and is a lifelong nonsmoker. Vital signs are within normal limits. BMI is 25 kg/m2. On physical examination, the soft palate and uvula are completely visualized. The tongue is mildly atrophic with visible fasciculations. The lungs are clear on auscultation and heart sounds are normal. The abdomen is flat and moves outward during expiration. There is no extremity edema. Chest x-ray shows elevated hemidiaphragms but no parenchymal opacities. Compared to a healthy individual's tests, this patient's pulmonary function testing is most likely to reveal which of the following findings?
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This patient with shortness of breath at night has features concerning for amyotrophic lateral sclerosis (ALS). Dysphagia (eg, coughing when eating), dysarthria, and tongue atrophy with fasciculations are manifestations of bulbar-onset ALS. Diaphragmatic involvement (ie, diaphragmatic weakness) can lead to paradoxical breathing movement (ie, expansion of the abdomen on expiration rather than inspiration) and elevation of the diaphragm on chest x-ray. This results in poor inspiratory strength, shallow breathing, and hypoventilation. Patients often report orthopnea (eg, shortness of breath while in bed) because when they are in a recumbent position, gravity is not assisting with lung expansion.
Like in all causes of respiratory muscle weakness (eg, Guillain-Barré syndrome), pulmonary function studies in ALS demonstrate a restrictive pattern due to an extrinsic limitation of lung expansion. This includes a reduction in total lung capacity, forced vital capacity (FVC) (the amount of air forcibility exhaled after maximal inspiration), and forced expiratory volume in 1 second (FEV1), resulting in a normal or increased FEV1/FVC ratio. Maximal inspiratory pressure (reflecting diaphragmatic strength) and maximal expiratory pressure (reflecting abdominal strength) are also reduced.
However, because the pulmonary parenchyma and alveoli are unaffected in ALS, the diffusion capacity of the lungs for carbon monoxide (DLCO), a measurement of alveolar gas exchange, remains normal. This contrasts with intrinsic pulmonary causes of restriction (eg, pulmonary fibrosis), which limit diffusion of gas across the alveoli, reducing the DLCO.
(Choices A, C, and E) A decrease in DLCO suggests impaired gas exchange due to intrinsic lung pathology. This patient's restrictive lung disease is due to an extrapulmonary cause: chest wall and diaphragm weakness due to ALS.
(Choice D) Patients with chest wall and diaphragm weakness have lower vital capacity due to an inability to fully expand the lungs. Even if forced expiration is minimally impaired, which is unlikely in this patient with weakened respiratory musculature, this results in decreased forced vital capacity.
Educational objective:
Amyotrophic lateral sclerosis often causes weakness of the muscles of respiration, resulting in a restrictive pattern on spirometry (ie, reduced vital capacity, forced expiratory volume in 1 second [FEV1], forced vital capacity [FVC] with a normal or increased FEV1/FVC ratio), with reduced maximal inspiratory and expiratory pressure. The diffusion capacity for carbon monoxide, a measurement of alveolar gas exchange, is preserved.