A medical sniff test, also known as chest or diaphragm fluoroscopy, is an imaging technique. It uses a continuous X-ray beam to create dynamic images, allowing healthcare professionals to observe the real-time movement of internal structures, primarily the diaphragm, during respiration. This test provides a visual assessment of how these structures function. It offers a detailed view of motion that static X-rays cannot capture.
Why a Sniff Test is Done
A sniff test evaluates the function of the diaphragm, a large, dome-shaped muscle at the base of the lungs that plays a central role in breathing. It helps diagnose conditions affecting the diaphragm’s movement, such as paralysis or weakness, often stemming from phrenic nerve issues. Patients with unexplained shortness of breath or dyspnea may undergo this test to determine if diaphragmatic dysfunction is the underlying cause. It also assesses diaphragm mobility before or after surgical procedures.
Steps for Performing the Sniff Test
Patients typically stand or sit in front of an X-ray camera, though they may lie on an inclined table if unable to stand. The medical professional positions the patient to ensure both sides of the diaphragm are visible for assessment. Patients may be asked to remove jewelry and change into a hospital gown to prevent interference with the imaging.
The test begins with instructions for the patient to breathe quietly and normally. The fluoroscopy unit captures real-time images of the diaphragm’s usual motion during quiet inspiration and expiration. Patients then take deep breaths, fully inhaling and exhaling, allowing observation of the diaphragm’s excursion during maximal effort.
The key part of the test is the “sniff” maneuver: a quick, forceful breath in through the nose with the mouth closed. This action activates and exaggerates the diaphragm’s movement, making any abnormalities more apparent. The radiologist continuously observes the diaphragm’s movement on a monitor throughout these patterns. The entire procedure typically takes 15 to 20 minutes.
Interpreting Sniff Test Findings
Medical professionals analyze the real-time fluoroscopic images to interpret sniff test findings, focusing on the diaphragm’s movement during various breathing patterns. Normally, during inspiration, the diaphragm contracts and moves downwards; during expiration, it relaxes and moves upwards. Both sides of the diaphragm, known as hemidiaphragms, should move together in the same direction. A normal report indicates the diaphragm is moving as expected.
An abnormal finding, particularly indicative of diaphragm paralysis or weakness, is called paradoxical motion. This occurs when a hemidiaphragm moves upward during inspiration instead of downward, or downward during expiration instead of upward. For example, if one side ascends while the other descends during a sniff, it strongly suggests paralysis on the elevated side. The absence of appropriate diaphragmatic movement or reduced excursion during quiet and deep breathing also points to dysfunction. These observations help clinicians confirm a diagnosis and determine the extent of diaphragmatic impairment.