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target lymph nodes are identified, the EBUS probe is withdrawn, and TBNA is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.
