Q: How the "sniff" test works for unilateral hemidiaphragm paralysis?


Answer: "Sniff" test, now known for almost 50 years,  is performed in radiology department under fluoroscope, while the patient sniffs forcefully. The sniff test shows paradoxical elevation of the paralyzed hemidiaphragm with inspiration compared with the rapid descent of the normal hemidiaphragm. Sniff test is positive in more than 90% of patients.  Once patient is comfortable with normal breathing, the patient is asked to take a few quick short breaths in with a closed mouth causing rapid inspiration. This maneuver is repeated in the lateral projection  to evaluate the posterior hemidiaphragms. The affected hemidiaphragm does not move downwards during inspiration or paradoxical motion may occur.


Reference: 

Alexander C. Diaphragm movements and the diagnosis of diaphragmatic paralysis. Clin Radiol 1966; 17:79.