The fluoroscopic sniff test, also known as diaphragm fluoroscopy, is a quick and easy real time fluoroscopic assessment of diaphragmatic motor function (excursion). It is used most often to confirm absence of muscular contraction of the diaphragm during expiration in patients with phrenic nerve palsy or breathing difficulties following stroke. Chest radiograph demonstrating a newly elevated hemidiaphragm often precedes a sniff test.
With the patient either standing (preferred) or supine, frontal fluoroscopy of the diaphragm is performed first with the patient at rest and breathing quietly through an open mouth. Then the patient is asked to take a few quick short breaths in with a closed mouth ('sniffs') causing rapid inspiration. Occasionally repeating this in the lateral projection is required to evaluate the posterior hemidiaphragms.
It is best to ask the patient to practice sniffing before the study.
In normal diaphragmatic motion:
- the diaphragm contracts during inspiration: moves downwards
- the diaphragm relaxes during expiration: moves upwards
- both hemidiaphragms move together
- in healthy patients 1-2.5 cm of excursion is normal in quiet breathing 2
- 3.6-9.2 cm of excursion is normal in deep breathing 2
- up to 9 cm can be seen in young or athletic individuals in deep inspiration 2
- excursion in women is slightly less than men 2
In abnormal diaphragmatic motion:
- the affected hemidiaphragm does not move downwards during inspiration
- paradoxical motion can occur
Absence of diaphragmatic movement confirms phrenic nerve palsy in the appropriate clinical setting. A mass anyway along the course of the phrenic nerve requires exclusion, usually with neck and chest CT. A hilar mass due to lung cancer is the most common finding on CT and a classic exam case.
Normal diaphragmatic excursion can also be impaired in patients with:
- previous diaphragmatic trauma or surgery
- neuromuscular disorders
- previous stroke
- 1. Nason LK, Walker CM, McNeeley MF et-al. Imaging of the diaphragm: anatomy and function. Radiographics. 2012;32 (2): E51-70. doi:10.1148/rg.322115127 - Pubmed citation
- 2. Boussuges A, Gole Y, Blanc P. Diaphragmatic motion studied by m-mode ultrasonography: methods, reproducibility, and normal values. Chest. 2009;135 (2): 391-400. doi:10.1378/chest.08-1541 - Pubmed citation