Diagnostic Ultrasound in Carpal Tunnel Syndrome: A Helpful Additional Tool
Carpal tunnel syndrome(CTS) is one of the most common pain conditions seen in clinical practice. Information on this subject is easily accessible and available in different formats. There have been some excellent reports published in this journal that covered the epidemiology, clinical, anatomic, and pathologic features of this condition.1
Despite all this, we do not have one diagnostic test that can accurately and reliably detect the presence and/or absence of CTS. Our current testing capabilities with electromyography (EMG) rely on the severity of the disease: meaning a certain, as yet unidentified, threshold amount of physiological dysfunction must be present for the test to be able to detect disease. Those persons who have the disease—but have yet to reach this threshold—will test false negative.2 Although the EMG/nerve conductive velocity study (NCS) is a valuable test in the management and identification of CTS, it should not be the sole determinant of whether a patient has the condition or not.3
This report will focus on a different way to characterize CTS beyond the electrophysiologic profile of the median nerve, into the anatomical domain of ultrasound (US) imagery. We will discuss the benefits and the limitations of musculoskeletal US imaging for the condition of CTS, and how an anatomical perspective can indeed complement physiological dysfunction data in developing a diagnostic profile for CTS cases.
Ultrasonography of the Median Nerve
In our particular area of Southeastern Michigan, we service a large population of manufacturing-based employees whose job requirements mandate continuous hand/wrist motion. Patients are required by insurance to have objective evidence of CTS before being approved for nonsurgical or surgical treatment. This can take the form of a positive clinical examination, including Phalen's and Tinel's signs; corresponding symptom reports; and EMG/NCS corroboration. Using US to aid in the diagnosis of CTS provides another perspective on how we can more completely define the condition. Diagnostic US gives us an anatomical reference point for characterizing the presence and clinical features of CTS. What we have learned in our practice is that occupationally induced CTS is a reactive condition that responds quickly to offending movements, and symptoms can manifest within minutes of beginning activity in those susceptible individuals. Those symptoms correlate well with median nerve swelling, which can be detected using US. Here we will focus on those important anatomical features of median nerve pathology as they relate to both direct and indirect irritation, compression and/or kinking of the nerve, and the visual manifestations of these different forces on the median nerve as it traverses the carpal tunnel.
Signs and Symptoms
The classic signs of CTS have been well described: a dull and achy discomfort in the hand within the distribution of the median nerve, along with weakness and paresthesia in the form of tingling or numbness, or both (Figure 1). The pathophysiology of CTS also has been well described: median nerve mechanical compression leading to a cascade of microvascular changes starting with reduced epineural blood flow that occurs with mild nerve compression. Continued compression can lead to edema (inflammation) in both epineurium and endoneurium; while persistent nerve compression, of 8 hours or greater, has been shown to block axonal transport.4 If compression is not relieved, more protein leaks out into the tissues, which become more edematous, and the vicious cycle continues. Mechanical nerve compression leads to anatomical and physiological nerve changes, with the anatomical changes presumably manifesting as a result of the physiological events. It is these morphological changes (nerve swelling, nerve compression) that can be visualized using diagnostic US.5
A schematic of the carpal tunnel region is depicted in Figure 2. It's important to note that the tunnel is not enclosed at either end (inlet or outlet). The carpal tunnel holds the four tendons of flexor digitorum superficialis, the four tendons of flexor digitorum profundus, the flexor pollicis longus, and the median nerve, all traversing the tunnel, with tendons contained by their flexor retinaculum that extends across the palmar surface of the wrist. The four flexor tendons are all enclosed within a sheath and the median nerve traverses adjacent to these flexor tendons. The position of the median nerve makes it vulnerable to compression forces either due to wrist positioning (hyperflexion) or adjacent flexor swelling (tenosynovitis). Both conditions will predispose the median nerve to being compressed between tendons and carpal ligament.6
Distinct Sonographic Appearance
The median nerve at the wrist is a relatively simple structure to scan since it lies directly under the transverse carpal ligament and can be found and visualized rather easily. Nerves have a distinct sonographic appearance or echo-signature by virtue of their echo-texture appearing as a combination of hyperechoic bands separated by hypoechoic (fascicular) lines when viewed in long (longitudinal) axis. In short or transverse axis, the nerve appears speckled and has alternatively been described as having a “starry night” appearance.
There have been a number of indicators or sonographic biomarkers suggestive of CTS when scanning a symptomatic wrist/hand. Because the area of the median nerve throughout its course generally does not change, a quick comparative study (eg, measurement of median nerve distal to the elbow and before the carpal tunnel inlet) can be informative. Some of the more common pathological markers of disease include the use of ratios, such as inlet nerve size over outlet nerve size; and cut points, such as >15 mm2 cross-sectional nerve area, being positive for CTS; percent change in nerve circumference in the pre-inlet region; visual criteria, such as nerve deformation (crimping), flattening, and/or hypertrophy; median nerve mobility while flexing/extending the wrist; reduced fascicular discrimination on short-axis view; and the often described sonographic triad of palmar or ventral bowing combined with nerve flattening in the distal tunnel and nerve swelling at the distal radius.7
An accurate differential diagnosis, we believe, has more to do with the collective findings of history, symptoms, clinical examination, and diagnostic testing (imaging). It should be understood that no one singular test or measure for CTS is universally reliable enough to stand on its own without further investigation and corroboration. We have identified peripheral nerves that appear to be hypertrophied and even fibrotic in appearance (hyperechoic), yet the patient has no symptoms—and vice versa.