The “sniff test,” formally known as diaphragm fluoroscopy or dynamic diaphragm ultrasound, is a specialized medical imaging procedure used to assess the function of the diaphragm. This large, dome-shaped muscle is the primary engine of respiration, responsible for drawing air into the lungs. The test is a dynamic diagnostic tool that captures the diaphragm’s movement in real-time as the patient performs a specific breathing maneuver. By visualizing the diaphragm’s excursion and direction, physicians determine if the muscle is working correctly or if it is weakened or paralyzed.
Identifying the Need for a Sniff Test
A physician typically orders a diaphragmatic sniff test when a patient presents with unexplained or persistent breathing difficulties. The most common symptom prompting this evaluation is dyspnea, or shortness of breath, that cannot be fully explained by other causes like asthma or heart failure. Difficulties breathing that are pronounced when lying down, a condition called orthopnea, strongly suggest diaphragmatic dysfunction because abdominal contents shift and push against the weakened muscle.
The test is also indicated when a standard chest X-ray shows an elevated hemidiaphragm, suggesting a problem with the muscle’s resting position. Suspected phrenic nerve injury, which controls the diaphragm, necessitates this test. Damage to this nerve, often resulting from trauma or cardiothoracic surgery, can lead to unilateral or bilateral diaphragm paralysis. Evaluating the diaphragm’s mechanical response helps determine the extent of its weakness or complete paralysis.
The Diagnostic Procedure Step-by-Step
The sniff test is performed using real-time imaging technology, most commonly fluoroscopy, which uses a continuous stream of X-rays, or dynamic ultrasound. For a fluoroscopic assessment, the patient is usually positioned upright, either sitting or standing. The imaging equipment is then positioned to capture the entire movement of both the left and right sides of the diaphragm.
The technologist or radiologist will first observe the diaphragm during quiet, normal breathing to establish a baseline of movement. The patient is then instructed to perform a series of specific maneuvers, including taking deep breaths. The most critical part involves the “sniff” maneuver, where the patient is asked to inhale forcefully and rapidly through the nose with the mouth closed. This sharp, quick inhalation is designed to activate the diaphragm and exaggerate its movement, making any dysfunction more apparent.
During the maneuver, the real-time images are recorded, often as a video loop known as cine-fluoroscopy. The imaging specialist visually tracks the diaphragm’s excursion, which is the distance it travels, and the speed and direction of its motion. If ultrasound is used, a transducer is placed on the chest or abdomen, and M-mode imaging is employed, which displays the movement of a single point on the diaphragm over time. This technique allows for the precise measurement and quantification of the diaphragm’s displacement.
Understanding the Test Results
Interpreting the sniff test relies on observing the direction of diaphragmatic movement during rapid inspiration. A healthy diaphragm contracts and moves downward (caudally) into the abdomen during the sniff, increasing the volume of the chest cavity. The normal excursion distance is significant during this forced maneuver, often reaching several centimeters. Both hemidiaphragms should move downward simultaneously and symmetrically.
The key finding indicative of a problem is “paradoxical motion.” This occurs when the weakened or paralyzed hemidiaphragm moves upward (cranially) into the chest cavity during the sniff, instead of moving down. This upward movement is a passive displacement, where the sudden increase in abdominal pressure from the descending healthy diaphragm pushes the non-functional side up. Paradoxical upward motion greater than two centimeters is highly suggestive of paralysis.
A reduced or absent downward motion, without the full paradoxical upward movement, suggests significant diaphragmatic weakness rather than complete paralysis. The degree of paradoxical movement observed relates directly to the severity of the dysfunction. A positive sniff test confirms diaphragmatic dysfunction, guiding treatment decisions such as surgery like diaphragm plication to stabilize the muscle.