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False Negative, Negative Inspiratory Tests in an ALS Patient

M. Ross, M. Burge, B. Miller, J. Dalrymple, L. Wesselius
Faculty Mentor: Christelle Sabatier

Pulmonary function can be monitored with various diagnostic tests which evaluate how much air a patient can exhale during a forced breath, known as the forced vital capacity, and the maximal pressure a patient creates during inhalation through either the mouth or nose, known as inspiratory pressure tests. The forced vital capacity (FVC) with supine challenge and the maximal inspiratory pressure (MIP) tests are commonly used to evaluate the diaphragm function in ALS patients. The sniff nasal inspiratory pressure (SNIP) is another negative inspiratory test that can be helpful for patients with disease onset in their cranial muscles. Generally, these tests yield declining values as the diaphragm becomes weak. However, a 67-year-old man with limb onset ALS presented conflicting results regarding his diaphragm strength when evaluated with all pulmonary function tests. The patient reported no difficulty breathing at rest but showed difficulty breathing when lying flat. With serial pulmonary function tests the patient showed a decrease in FVC, however, the corresponding pulmonary function tests reported normal to high breathing function. When evaluated further, studies showed no nerve conductions to the diaphragm or any diaphragm movement. This case study showed that some ALS patients could generate excellent negative inspiratory pressures at a time when the diaphragm is severely weak. We speculate that enhanced function of accessory breathing muscles likely accounts for this discrepancy. This finding suggests caution must be used when interpreting pulmonary function tests in ALS patients and a single measurement of inspiratory function cannot be relied on to accurately estimate diaphragm function.