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The sniff test is a fluoroscopic exam used to evaluate whether the diaphragm (the muscle that controls breathing) moves in the proper direction during various maneuvers including normal breathing and while rapidly inhaling. During normal inspiration and expiration, the diaphragm moves down and up respectively. Ideally, both the right and left sides of the diaphragm move in the proper direction simultaneously. The radiologist will observe this motion during the test. A maneuver that is also done during the exam is to have the patient "sniff" or quickly breathe in through the nose. This will exaggerate the difference in a paralyzed or abnormal hemi-diaphragm, which will move paradoxically in the wrong direction. An injury or abnormality resulting in paralysis of the phrenic nerve (the nerve responsible for diaphragmatic motion) can be detected in this manner.
The sniff test is a painless procedure that requires no preparation and takes about 15 minutes once the patient is in the examination room.
