Dysfunction of the Diaphragm

Imaging as a Diagnostic Tool

Nadir Kharma

Disclosures

Curr Opin Pulm Med. 2013;19(4):394-398. 

In This Article

Imaging Modalities in Diaphragmatic Dysfunction

There are several imaging tools to aid with the diagnosis of diaphragmatic weakness or paralysis.

Chest Radiographs

This is often the initial step in assessing diaphragmatic shape, position and contour. Chest radiograph will also help in evaluating the lungs for any other parenchymal abnormalities that may explain shortness of breath. The dome of the right hemidiaphragm overlaps anteriorly with the fifth rib and posteriorly with the tenth rib, and the dome of the left hemidiaphragm is one interspace lower than the right.

With bilateral diaphragmatic paralysis, bilateral, smooth elevation of the hemidiaphragms and small lung volumes are usually seen, and the costophrenic and costovertebral sulci are deep and narrow. The lateral view confirms a smooth contour and elevated diaphragmatic position. Plate-like atelectasis may also be present, usually at the lung base.[19]

The differential diagnosis of bilateral diaphragmatic elevation on the plain chest radiograph includes poor inspiratory film, obesity with decreased chest wall compliance, subpulmonic effusions, subdiaphragmatic process such as ascites, ileus and organomegaly, and pleural adhesions.

In unilateral diaphragmatic paralysis, the sensitivity of plain chest radiograph is as high as 90%, whereas its specificity is, however, low (44%).[20]

Fluoroscopy and Sniff Test

Fluoroscopy is a form of functional imaging. It is usually utilized after diaphragmatic elevation is detected on chest radiograph. The examination is simple to perform and the results are easy to interpret.

Rationale and Technique of Sniff Test

The patient is usually instructed to practice deep breathing and sniffing. Sniffing is performed with short, sharp inspiratory effort through the nostrils. Following that and while the patient's chest is in the anterior position, the technician will record two or three quiet breaths and then two or three deep breaths and finally two or three sniffs. Then, the sequence of quiet breaths, deep breaths and sniffs is repeated with the patient in a lateral position. This will ensure an adequate view of the anterior and posterior parts of the hemidiaphragms. In addition, the anterior chest wall and sternal motion can be appreciated in the lateral position.[5]

Fluoroscopy in Normal Individuals

On quiet and deep inspiration, both hemidiaphragms will descend or move downwards as the anterior chest wall moves upward. Normal excursion is at least one rib interspace in adults. The diaphragmatic excursion may be asymmetric with a slight delay or lag on one side, usually the right.[21] In the lateral projection, the excursion of the posterior part of the hemidiaphragm may be greater than that of the anterior part, especially on the right.

On sniffing, both hemidiaphragms move caudally. The anterior chest wall may not initially move upward as much as on slow deep inspiration. In addition, some asymmetry of excursion may be noticed with some lag on one side. Again, the motion of the posterior aspect of the hemidiaphragm, especially the right one, may be more exaggerated than that of the anterior part.[5]

Pathologic Findings in Fluoroscopy

In weakness of one or both hemidiaphragms, excursion is reduced or delayed on quiet and deep inspiration. If weakness is more severe, motion may be paradoxical on deep breathing and even on quiet breathing. The same paradoxical pattern will be seen with sniffing.

With paralysis of one hemidiaphragm, downward movement is absent and paradoxical motion on quiet and deep inspiration can be seen. On sniffing, there is usually paradoxical motion.

In paralysis of both hemidiaphragms, the two may move paradoxically together if anterior chest wall motion is noticeable. Witnessing any caudal motion of hemidiaphragm on quiet or deep inspiration suggests weakness and not paralysis.[5]

Utility of Fluoroscopy

Although fluoroscopy sniff test can be positive in over 90% of unilateral diaphragmatic weakness,[21] the test is not as useful in diagnosing bilateral diaphragmatic paralysis. False-positive results of the sniff test can be observed in 6% of patients without diaphragmatic paralysis.[21,22]

Patients with bilateral diaphragmatic paralysis may also have a normal test result. This is attributed to the active contraction of the abdominal muscles during expiration, followed by abrupt relaxation of the abdominal muscles at the onset of inspiration, resulting in downward motion of the paralyzed diaphragm. Potentially, this may be falsely interpreted as active diaphragmatic contraction.[23]

Ultrasonography

This modality is more portable than fluoroscopy and it has the benefit of being free of ionizing radiation. Ultrasound is also noninvasive. These advantages make diaphragmatic ultrasound more attractive especially for young adults or mechanically ventilated patients.

Ultrasonography shares a similar concept with the fluoroscopy sniff test. Normal hemidiaphragms will move caudally with inspiration. There is similar excursion of the two hemidiaphragms, although the motion of the left hemidiaphragm may be slightly greater than that of the right hemidiaphragm. A paralyzed hemidiaphragm will have no caudal movement with quiet or deep breathing and may have paradoxical movement.

At ultrasound, the diaphragm appears as a thick echogenic line. M-mode ultrasound can be used to measure the direction of diaphragmatic motion and magnitude of excursion.[24–26] Ultrasound can also be used to measure changes in the thickness of the diaphragm during inspiration. Thickening of the diaphragm with inspiration is consistent with diaphragmatic shortening.[27] It follows that lack of thickening with inspiration is suggestive of diaphragmatic paralysis.[28] Ultrasonography can be used to diagnose both unilateral and bilateral diaphragmatic dysfunction.[28,29] In addition, ultrasonography can be used to assess recovery from diaphragmatic weakness. Again, this is done by measurement of diaphragmatic thickness.[30]

However, ultrasonography has few limitations. It is operator dependent and requires significant expertise, as the field of view is often small and may suffer from intervening lung or bowel air. These organs can obscure diaphragmatic movement, making assessment of diaphragm function difficult.

Computed Tomography

Computed tomography (CT) scanning of the thorax is usually not performed routinely to evaluate diaphragmatic function. However, CT may be helpful to assess subdiaphragmatic processes contributing to diaphragmatic dysfunction or elevation seen on a plain chest radiograph. These processes include subdiaphragmatic abscess, ascites, organomegaly or ileus. In addition, CT may help in further evaluation of the lung parenchyma and pleura in cases of suspected obstructive disease, subpulmonic effusion or pleural thickening.[5]

MRI

Another potentially promising tool that is not yet in routine clinical practice is dynamic MRI of the chest. This modality has the advantage of obtaining sequential images that can be arranged to look like a movie. This reveals dynamic changes in the diaphragm during inspiration that may be more useful than still MRI.[31] The use of dynamic MRI in the evaluation of diaphragmatic function was reported in the literature. Quantitative evaluation with MRI may allow study of the excursion, synchronicity and velocity of diaphragmatic motion.[31–33] MRI has the advantage of being free of ionizing radiation. However, the utility of this mode of imaging is limited by cost considerations. In addition, it is not as portable as other imaging tools such as ultrasonography. Special expertise in interpreting the images is required and may not be available in many centres. Whether MRI yields more superior or clinically relevant information that may alter the management of patients is uncertain.

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