Sniff test

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
Share article

Article information

rID: 35785
System: Chest
Synonyms or Alternate Spellings:
  • Diaphragm fluoroscopy

Support Radiopaedia and see fewer ads

Cases and figures

  • Drag
    Case 1: normal sniff test
    Drag here to reorder.
  • Drag
    Case 2: abnormal sniff test
    Drag here to reorder.
  • Drag
    Case 3: abnormal sniff test
    Drag here to reorder.
  • Drag
    This sniff test s...
    Case 4: abnormal sniff test
    Drag here to reorder.
  • Updating… Please wait.

    Alert accept

    Error Unable to process the form. Check for errors and try again.

    Alert accept Thank you for updating your details.