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Synovial hypertrophy is swelling of the synovial lining, but the definition does not state whether there is hyperemia.
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Synovitis is the term for synovial hypertrophy and hyperemia as demonstrated by Doppler activity.
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A way to differentiate synovial hypertrophy from fluid, which may have the same sonographic appearance, is to apply pressure. Fluid is displaceable, and synovial hypertrophy is not.
Synovitis is a key feature of arthritis and important in the diagnosis of any arthritic condition. Inflamed synovial lining can result in synovial hypertrophy, increased perfusion, and excess synovial fluid.
Definitions
Differentiating synovial hypertrophy from a fluid collection in a joint may sometimes be difficult. Key features that may help are part of the definitions for synovitis and fluid provided by the Outcome Measures in Rheumatoid Arthritis Clinical Trials (OMERACT) group, with the name recently broadened to Outcome Measures in Rheumatology to reflect expanded initiatives.
OMERACT proposed an ultrasound definition for synovitis : an abnormal hypoechoic (relative to subdermal fat, but it sometimes may be isoechoic or hyperechoic) intra-articular tissue that is nondisplaceable and poorly compressible and that may exhibit a Doppler signal. Fluid is an abnormal hypoechoic (relative to subdermal fat, but it sometimes may be isoechoic or hyperechoic) intra-articular material that is displaceable and compressible and that does not exhibit a Doppler signal ( Fig. 6-1 ).
The definition for synovitis is very broad, confirming that synovial hypertrophy is an abnormal intra-articular tissue that is nondisplaceable and poorly compressible. In the OMERACT definition, Doppler activity is not necessary to diagnose synovitis, taking into account that not all ultrasound equipment has a sensitive enough Doppler system to differentiate between inactive disease and active disease.
It is important to maintain correct terminology. Synovial hypertrophy is swelling of the synovial lining, but the definition does not state whether there is hyperemia. Synovitis is the term for synovial hypertrophy and hyperemia as demonstrated by Doppler activity. Ultrasound can be used to detect and diagnose synovitis and synovial hypertrophy. Studies have even shown that ultrasound is superior to clinical assessment of joint swelling and tenderness.
Synovial Hypertrophy
Under typical conditions, the normal synovial lining may not be visible or is barely visible ( Fig. 6-2 ). When synovial hypertrophy is present, it covers the joint, often altering the normal contour of the joint capsule ( Fig. 6-3 ). Normally, the joint capsule follows joint anatomy, but it may bulge outward when there is synovial hypertrophy, and it may even change from a concave to a convex shape ( Fig. 6-4 ). The synovial thickness has a huge span, ranging from very minimal hypertrophy to severe hypertrophy with villi, debris, and fluid ( Fig. 6-5 ). The ultrasound appearance of the synovium correlates with arthroscopic findings.
The ultrasound appearance of synovial hypertrophy varies from almost anechoic to hyperechoic. The appearance is also related to the gain setting and the choice of the gray color map in gray-scale ultrasound. The higher the gain, the more hyperechoic it appears, and the lower the gain setting, the more hypoechoic or anechoic the synovial hypertrophy may appear. Similar results may be seen when choosing different gray color maps ( Fig. 6-6 ).
During the ultrasound examination, it may be difficult to determine whether the synovial lining is thickened enough to be called abnormal or it is within the normal range. Schmidt and colleagues made a contribution to the determination of normality. In a study of 102 normal participants, a normality range was proposed for joints, cartilage, and tendons. This range may be used for guidance, but it is not exact. There are substantial gender and individual differences.
Synovitis
Doppler ultrasound can be used to visualize a perfusion in the inflamed synovium. It aids the diagnosis of inflammatory activity in a joint (i.e., synovitis) that can be associated with the abnormal production of fluid.
Power Doppler and color Doppler are used, and in both cases, color information is superimposed on the gray-scale image. When the transducer is held still and the patient is immobile, the only thing moving in the image plane is blood. When the ultrasound beams are reflected by objects in motion (i.e., red blood cells), they change frequency, and the difference between the emitted frequency and the received frequency is called the Doppler shift ( Fig. 6-7 ). That information is transformed by the ultrasound machine to color information displayed as the color pixels in the image. The Doppler data provide information about perfusion ( Fig. 6-8 ). With Doppler ultrasound, it is possible to see various degrees of perfusion, and as with synovial hypertrophy, it may be of interest to grade the degree of perfusion as a way of measuring changes in perfusion during treatment.
All joints may show Doppler activity when inflamed, but the hip joint rarely shows Doppler activity because of the deep location of the joint. The Doppler image is more sensitive to attenuation due to depth than the gray-scale image. The shoulder also shows less Doppler activity compared with similar superficial joints. It may be related to the positioning of the shoulder during examination, because the investigated structures often can be seen only in extension (meaning strain on tendons and capsule), which may compromise the perfusion. Studies have shown that joints should be examined in the neutral position because full flexion or extension appears to minimize the Doppler activity.
If the ultrasound unit has a very sensitive Doppler system, it may be possible to see flow even in normal joints—most frequently in the wrist joint, seldom in the metacarpophalangeal joints, and almost never in the proximal interphalangeal joints. The amount of perfusion seen is very minimal compared with that seen in arthritis patients, but it shows that Doppler activity does not always equal inflammation.
Morphology of Synovitis
The appearance of synovitis may be very different on B-mode ultrasound, changing from a hypoechoic to isoechoic pattern and progressing to a hyperechoic pattern. It usually is not possible to determine the nature of an intra-articular mass, and even though fluid is displaceable, the only way to make sure is to perform a joint puncture. Infectious arthritis may look like any other type of arthritis.
Crystal arthropathies may have echo-rich synovial fluid and deposits on the cartilage or even have intracartilage deposits, but these features are not always present, and the appearance of synovium in this case does not always ensure a correct diagnosis. Puncture is necessary.
Ultrasound Evaluation of Synovitis
A way to differentiate synovial hypertrophy from fluid, which may have the same sonographic appearance, is to apply pressure. Fluid is displaceable, and synovial hypertrophy is not. It is possible to apply pressure to the synovial hypertrophy to compress it, but this does not displace it.
The amount of pressure is not as important when working with B-mode ultrasound alone as when applying color or power Doppler. Applying too much pressure may remove all signs of flow as the pressure compresses the vessels, or it may only eliminate part of the flow ( Fig. 6-9 ). It is therefore important to apply only a minimal amount of pressure. Some ways to ensure minimal pressure come with practice, but using generous amounts of gel and keeping a small film of gel visible in the top of the image during examination may help. Gel pads also may be used.
Doppler Modality
When choosing a Doppler modality in a rheumatologic setting, the issue is the sensitivity for flow. The theoretical advantage of power Doppler has disappeared with the newer high-end machines, in which color Doppler tends to be more sensitive than power Doppler ( Fig. 6-10 ). A satisfactory explanation for this has not been put forward. In less-expensive equipment or in older equipment, power Doppler has the highest sensitivity. The choice between color and power Doppler depends on the equipment.
