Pedia News

ENDOBRONCHIAL ULTRASONOGRAPHY


ENDOBRONCHIAL ULTRASONOGRAPHY
Endobronchial ultrasonography (EBUS) has likely had the largest impact on the field of bronchoscopy since the advent of the flexible bronchoscope in 1967. Use of ultrasonography in the airways evolved from endoscopic ultrasonography (EUS). Transducers needed to be made small enough to pass into the airway (or through the working channel of the bronchoscope) without obstructing the airway and to also achieve “coupling” to the airway wall because air is a potent reflector of ultrasound waves.


The first studies investigating EBUS were with a 20-MHz radial probe transducer. This relatively high-frequency probe allows excellent visualization of the layers of the airway wall and has been shown to be more sensitive than chest computed tomography scanning for determining airway invasion versus compression by tumor. Radial-probe EBUS also significantly increased the yield of transbronchial needle aspiration (TBNA). Unfortunately, performing radial-probe EBUS-TBNA is not a real-time sampling technique. The EBUS probe is inserted through the working channel of the bronchoscope, the target lymph nodes are identified, the EBUS probe is withdrawn, and TBNA is performed in the standard fashion.
Radial probe EBUS has also been used to identify peripheral nodules. The diagnostic yield for bronchoscopic sampling of peripheral nodules smaller than 3 cm in size is generally quite poor (25%-70%). Radial probe EBUS has been shown to increase the yield of peripheral nodule sampling, especially when combined with electromagnetic navigation bronchoscopy, up to as high as 90%. The sonographic characteristics of peripheral EBUS have also been shown to correlate with pathologic findings.
More recently, a 7.5-MHz convex-probe EBUS bronchoscope has been developed. The major benefit of this bronchoscope is that it allows real-time visualization of the needle entering the lymph node. Color power Doppler can also be used to identify vascular structures.
Convex-probe EBUS-TBNA has become a technique of choice for the staging of lung cancer. Whereas EBUS-TBNA can reach almost all of the lymph node stations, other procedures such as mediastinoscopy and EUS fine-needle aspiration are more limited. The performance characteristics (sensitivity, specificity, positive and negative predictive values) are nearly equivalent for more invasive procedures such as mediastinoscopy. In many centers, EBUS-TBNA has replaced mediastinoscopy as the initial procedure for the evaluation of mediastinal and hilar lymphadenopathy. It is important to understand, however, that a nondiagnostic EBUS-TBNA procedure is not equivalent to a negative result. Because the false-negative rate for EBUS-TBNA can be as high as 14%, all nondiagnostic results from EBUS-TBNA require either appropriate surgical sampling or clinical follow-up.
EBUS-TBNA has also been shown to be extremely useful for the diagnosis of lymphoma and sarcoidosis.
There is a definite skill set that one needs to acquire before performing EBUS-TBNA. A thorough under-standing of extrabronchial anatomy, including the location of the various lymph node stations and blood vessels as well their relationship to each other and endo- bronchial anatomy is essential. One also needs to appreciate the technical differences of the bronchoscope itself. Unlike standard bronchoscopes that have a zerodegree view (i.e., looking straight ahead), the convexprobe EBUS bronchoscope has a 30-degree oblique view. This prevents visualization of the ultrasound probe; however, one needs to appreciate its presence so as to avoid injury to the vocal cords and distal airways. The needle system is also novel, and it is important to review its use with support staff before performing the procedure on a patient. The operator also needs to understand the “knobology” of the ultrasound processor and be able to adjust the depth, contrast, and gain at a minimum.