Sniff test for diaphragmatic paralysis is done in the radiology department using real time X-ray or fluoroscopy to look at how the diaphragms move. It’s a quick, easy and noninvasive way to look at the function of the diaphragm muscles. It is often ordered after a chest X-ray shows an elevated diaphragm.
The phrenic nerve controls the diaphragms and this test is a way to see if the diaphragm is paralyzed due to the nerve being injured or pinched anywhere along it’s course from the neck to the diaphragm. Common causes of injury to the nerve can be a tumor or mass pressing on the nerve, trauma causing injury, or an inflammatory process causing injury.
I explain the test to the patients and have them practice a sniff maneuver, which is quick breaths with a closed mouth. I then observe the movement of the diaphragms under X-ray during inspiration and expiration. I make sure that both diaphragms are moving up and down together. During inspiration, the diaphragm moves down and up during expiration or when you breathe out. I then have patients do a sniff maneuver and observe the diaphragms.
When there is a paralyzed diaphragm or phrenic nerve palsy, there will be limited or no movement of the diaphragm. The diaphragm will not move down during inspiration. In some cases, the diaphragm will move up during inspiration which is called paradoxical motion. Sometimes diaphragm motion can also be limited after surgery or trauma to the diaphragm, diseases that involve muscles and nerves, and strokes.
If the diaphragm is found to be paralyzed, then it’s necessary to image along the course of the phrenic nerve to exclude a mass. Since the nerve travels from the neck and through the chest, CT scan of the neck and chest may be needed. Your doctor will use your history and presentation to determine the need for any more testing.