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 inspiration in patients with phrenic nerve palsy or breathing difficulties following stroke. Chest radiograph demonstrating a newly elevated hemidiaphragm often precedes a sniff test.
In critically unwell patients who can not attend the fluoroscopy unit in the radiology department, bedside US assessment can be used to demonstrate appropriate diaphragmatic movement with normal respiration and when asked to sniff (see case 5).
Technique
The following technique is suggested:
ask the patient to practice sniffing before the study
with the patient either standing (preferred) or supine, perform frontal fluoroscopy of the diaphragm at rest, breathing quietly through an open mouth
ask the patient to take a few quick short breaths in with a closed mouth ('sniffs') causing rapid inspiration
occasionally, repeating (3) in the lateral projection is required to evaluate the posterior hemidiaphragms
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 diaphragmatic movement confirms phrenic nerve palsy in the appropriate clinical setting. A mass anywhere along the course of the phrenic nerve requires further workup, 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