How to evaluate the maxillary sinus with ultrasound for the presence of fluid
IMBUS of the maxillary sinus is a basic exam, but it exemplifies improved diagnostic accuracy, reduced cost, and improved patient satisfaction.
Traditional outpatient diagnosis of acute rhinosinusitis frequently results in overuse of antibiotics. IMBUS of the maxillary sinus is specific for clinically important fluid, but may miss subtle abnormalities that are usually clinically unimportant. The ethmoid and frontal sinuses are challenging to image and are rarely abnormal in isolation. Beyond initial experience, the frontal sinus only needs to be examined when the maxillary sinus is normal and the patient has complaints of forehead discomfort. The absence of maxillary sinus fluid is a strong reason to avoid antibiotics in most patients. A negative ultrasound can also be reassuring to patients. However, a patient with fever, severe symptoms suggestive of sinusitis, but no maxillary fluid may need empiric antibiotics or a CT scan to look for isolated ethmoid, frontal, or sphenoid sinusitis. A few published studies, and the ANGMA clinic experience, indicate that ultrasound can reduce antibiotic use for sinusitis. Only about ¼ of the ANGMA clinic patients with the possibility of sinusitis have fluid in the maxillary sinus. The presence of fluid does NOT differentiate between viral and bacterial disease, so the final antibiotic decision requires integration of ultrasound with the rest of the patient’s clinical findings.
A linear probe is used with a musculoskeletal exam type. The patient sits facing the examiner and the probe indicator is examiner left for the transverse views and cephalad for the parasagittal views. The depth is set to 6 cm. On the patient’s right side, a top and bottom grip is used for the transverse view, anchoring the little finger on the nose. The transverse view is begun at the bottom of the nose, rocking laterally a modest amount so the probe cord moves toward the right ear. The maxillary sinus does not extend laterally beyond the corner of the eye. It is important that the probe is perpendicular to the front wall of the sinus so the hyperechoic front wall is clear. Proper orientation will have the front wall mostly horizontal on the screen. The parasagittal view is just lateral to the nose, rocking the probe slightly down so the probe cord moves toward the chest. Fluid always appears first at the caudal aspect of the sinus. Below are the probe positions with CT correlates.
Below is a combination image showing the transverse views of a fully opacified right maxillary sinus with the CT scan on the left and the IMBUS on the right. The “V” or “U” shaped back wall is the distinctive appearance in the transverse view. In our experience, the back wall is 2.5–3.75 cm from the front wall in adults, depending on body size.
Sinus retention cysts are occasionally seen and are distinguished from free fluid in the sinus by noting that the echogenic back wall is not U/V shaped in transverse and is much shallower; in this case only 1.5 cm from the front wall.
Below is the parasagittal view of an abnormal maxillary sinus, which is not as distinctive as the transverse view, because a straight echogenic line for the back wall is seen. However, the length of the back wall line corresponds to how high up the sinus the fluid extends. Note that the back wall is not parallel to the front wall of the sinus, helping differentiate it from reverberation artifact in an air-filled sinus. Experienced examiners often perform the parasagittal view only when the transverse view is equivocal or positive.
For the left maxillary sinus, the transverse hand position on the probe changes to a medial/lateral grip with the little finger anchoring on the cheek.
Below are transverse views of normal left maxillary sinuses. Only the front wall with minimal reverberation artifact (A-lines) may be seen (upper left), but reverberation artifact may be more prominent (upper right, lower left). A mirror image with a single A-line (bottom right) can look like marked thickening of the mucosa or the posterior wall of the sinus, but it is not deep enough (1.4 cm from the front sinus wall) to be the posterior wall of the sinus.
Here is the spectrum of transverse maxillary sinus IMBUS findings with normal on the left and strongly positive on the right. The middle two images are weakly positive results seen with smaller amounts of fluid. The back wall is fainter and only part of the wall may show. This may lead to different treatment decisions than strongly positive results, depending on the rest of the clinical information.
Below is one of the few ANGMA clinic examples of a positive frontal sinus. Use a transverse probe position right at the eyebrow level. The frontal sinus does not extend laterally much beyond the mid-eye. Keep the probe horizontal, or the orbit can cause confusion. In normals there is an apparent thick front wall, but this is probably the front wall and then a mirror image with an A-line. A positive frontal back wall from fluid is shallower and curved, but not U/V shaped. In this example, the back wall was 1.5 cm from the front wall.