While the lateral decubitus view often can identify loculations, they can usually be ascertained by a standard erect view.3–
The fundamental question is, “Is it necessary to know whether the fluid is loculated prior to performing a thoracentesis?” Although many loculated effusions resolve without drainage,4
the presence of loculations correlates with the development of a complicated pleural effusion or empyema.11–13
The drainage of loculated effusions is sometimes necessary even in the face of the results of pleural fluid chemistry testing that do not suggest the need for tube thoracostomy.12
However, since loculations occur more frequently with larger effusions,12
it is not clear that the presence of loculations provides any more prognostic information than does the semiquantitative estimate of fluid volume based on chest radiographs obtained with the patient in the erect position.4
Even if the presence of loculations provides prognostic information, a thoracentesis would ordinarily be performed before deciding on the need for more aggressive therapy. If the thoracentesis is successful in removing all of the radiographically visible fluid, or if the fluid clearly shows the need for tube thoracostomy, the lateral decubitus view will yield no clinically useful information. In the case of an effusion that cannot be completely removed by thoracentesis, the presence of loculations can be presumed, and the subsequent management will be directed by the fluid characteristics. In some cases, the performance of tube thoracostomy may be difficult due to the presence of multiple and/or small loculations, and a CT scan of the chest will yield much more useful information than will radiograph made with a lateral decubitus view. Support for this line of thought (although not for omitting the lateral decubitus view) can be inferred from a published treatment algorithm14
that recommended the performance of a radiograph with the lateral decubitus view and then thoracentesis, whether or not the fluid is loculated. Interestingly, when one queries house officers about why they ordered a lateral decubitus radiograph, the invariable answer is to “see whether the fluid is loculated, because loculated fluid cannot be tapped without radiographic guidance.” I could find no support in the literature for this idea but have encountered it at several institutions. If a large pleural effusion is identifiable by the standard erect view chest radiographs and a physical examination, there should be little risk in performing a “blind” thoracentesis, whether the fluid is loculated or not. In the specific setting of multiloculated empyema, radiographic guidance by either CT scan or ultrasound can be invaluable in guiding the placement of small-bore drainage tubes.