Sniff test

Last revised by Liz Silverstone on 11 Jun 2024

The fluoroscopic sniff test is a useful addition to diaphragm fluoroscopy and is used to evaluate diaphragmatic contraction and excursion in patients with suspected phrenic nerve palsy or paralysis, breathing difficulties following stroke or recent elevation of a hemidiaphragm on chest radiograph.

US can also demonstrate diaphragmatic motion and is portable (see case 5).

Technique

The following technique is suggested:

  1. ask the patient to practice sniffing before the study

  2. with the patient standing, perform frontal fluoroscopy of the diaphragm at rest, breathing quietly through an open mouth

  3. ask the patient to take a few quick short breaths in with a closed mouth ('sniffs') causing rapid inspiration

  4. repeating (3) in the lateral and/or oblique projection is necessary to evaluate the posterior hemidiaphragms

  5. supine examination during respiration helps to avoid false negative studies due to compensatory contraction of the abdominal wall muscles, see figure 1

Findings

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

Interpretation

Absence of appropriate diaphragmatic movement can confirm phrenic nerve palsy or diaphragmatic paralysis in the appropriate clinical setting. Conditions affecting the cervical spine, phrenic nerve itself or neuromuscular transmission require further evaluation. 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

  • cardiac surgery (2-20%), cold cardioplegia and phrenic frostbite

  • neuromuscular disorders

  • previous stroke

Practical points

  • the diaphragm is more inferior in the erect position and abdominal wall muscle contraction can raise the diaphragm in expiration, followed by passive descent in inspiration, thereby mimicking normal diaphragmatic motion in a patient with diaphragmatic paralysis (supine evaluation minimizes this effect, see figure 1) 4

  • posterior diaphragmatic excursion is usually greater than anterior diaphragmatic excursion 4

  • there is considerable normal variation, with overlap between normal and abnormal

  • some normal people exhibit paradoxical motion, most commonly limited to the anterior diaphragm 6

  • unbalanced use of the upper rib cage and neck muscles may cause paradoxical motion of the diaphragm 7

  • oblique fluoroscopy allows simultaneous evaluation of both hemidiaphragms and facilitates identification of left and right

  • bilateral paralysis is harder to recognize and elevation of the ribs during inspiration may give the illusion of diaphragmatic descent

  • CT correlation is helpful and can demonstrate atrophy of the diaphragmatic crus and a high or narrow posterior costophrenic recess in diaphragmatic paralysis 3

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