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9 Articles in Volume 13, Issue #3
Comprehensive Rehabilitation of the Cancer Pain Patient
Neuropathy in the Cancer Patient: Causes and Cures
The Basics of Breakthrough Pain: Transmucosal Fentanyl
The Use of Botulinum Toxin in Migraines: A Review
Complex Regional Pain Syndrome: Systemic Complications
Diagnostic Ultrasound in Carpal Tunnel Syndrome: A Helpful Additional Tool
The Homebound Adolescent Headache Patient
Editor's Memo: Neurosteroids—Gaining Ground In Pain Management Research
Ask the Expert: Monitoring Liver Function

Diagnostic Ultrasound in Carpal Tunnel Syndrome: A Helpful Additional Tool

Occupationally induced carpal tunnel syndrome 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 ultrasound.

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.

Case Example #1

The US scan (Figure 3) is of a 54-year-old woman who works on the assembly line at a parts plant, and is scheduled for CTS-release surgery in the very near future. This case was sent to us from the plant's medical department as a courtesy after finding out we needed verified CTS cases to scan for this report. This patient has already been “worked up” medically including a hand/wrist surgery consult and corroborating NCS exam. In the more typical course of events, however, the US exam would be ordered first, and the results, along with severity of symptoms, would usually determine what the next step will be (ie, physical therapy, splinting, injections). In this case, we have taken two measures: one just before the carpal tunnel inlet, and the other at the inlet to demonstrate the size differences between the two nerve sections. The 2-mm2 size differential indicates there is an inflammatory-mediated change within the nerve, which is visualized as swollen (larger and darker). This patient was symptomatic despite not meeting the generally accepted >15 mm2 cross-sectional area cut-off point for CTS conventionally measured in short-axis views,8 and in fact, many of our patients who go on to have CTS-release surgery do not meet that particular size threshold on ultrasonography.

Figure 4 depicts a split-screen short-axis view showing both right and left (normal) median nerve cross-sectional areas taken at the same point (note the bony landmarks) from the same patient as above. Not only is there a size differential on the symptomatic side (right side, 6 mm2) between successive point measures, but the side-to-side measures differ as well when we compare symptomatic versus asymptomatic views (left side, 4 mm2).

The US criteria of >15 mm2 is not always present, as already mentioned, at least in our symptomatic population. It has been our experience that relatively small changes in size between different points on the nerve area are, in and of themselves, indicators of swelling and/or flattening, and both empirically correlate well with nerve dysfunction. Without US, this patient would not have met the criteria for CTS treatment. In effect, we use US as a first-line screening test for several large employers in our area. Over the years, their respective medical teams have had opportunities to evaluate the cost effectiveness of this form of testing across various clinical conditions and have deemed the test to have “high utility” in helping to detect the presence, severity, and possible causes of CTS. When we detect tenosynovitis as the cause, we have had good success using physical therapy and non-steroidal anti-inflammatory drugs. In the case where nerve deformation is detected, we look at job station adjustments and positional splinting as primary interventions. If the US test detects a mass, such as a ganglion cyst, giant cell tumor of the tendon sheath, or schwannoma, surgical excision might be considered. The point is that US is an important tool in the decision-making process because it not only confirms the presence and severity of the condition, it allows continued monitoring and assessment of the status of the condition and the effectiveness of the interventions.

Case Example #2

The next case example can be seen in Figure 5. In this patient, a 60-year-old woman, there is bilateral involvement with both median nerves achieving (right side, 15 mm2) and exceeding (left side, 18 mm2) the generally accepted threshold cross-sectional area of 15 mm2. In this case, the patient had carpal tunnel release surgery on the left wrist (left side) several years ago and continues to have mild to moderate symptoms. The right wrist (right side) has not had surgery, but it is being contemplated since CTS is moderate to extreme in terms of symptom severity. The patient continues to work on an assembly line with restrictions and with symptoms appearing very soon into her activities. She was sent to physical therapy for treatment and eventually found the best intervention to be job rotation onto a different line that used larger muscle groups.

Although industries that are intensive in upper extremity motions will often use “threshold limit values” to describe hand activity levels, in terms of peak forces and level of activity, these exposure assessments should only be used as guidelines. These guidelines are somewhat useful in providing population-based exposure data, but do virtually nothing to identify susceptibility in a worker to develop CTS. Exposure data need to be adjusted for other factors such as vibration, temperature, posture, contact stress, and relative positions of the hand to wrist, forearm, and elbow.

Case Example #3

There are several key sonographic indicators suggesting that nerve involvement is the primary symptom generator other than a ≥15-mm2 degree enlargement of the nerve. Figure 6 is a US example of nerve kinking on the left-side view (arrow) using a long-axis approach to scanning the median nerve. This 58-year-old woman has had a corroborating NCS and displays classical symptoms of occupational CTS including numbness in the palmar aspect of digits 1 to 3, combined with hand weakness. This patient was referred for physical therapy treatment after US findings confirmed her complaints. The exam helped verify the condition and provided evidence for the case manager to justify approving treatment. This means the patient's complaints were validated and treatment was expedited, which typically ensures a better outcome. The median nerve in this case is seen as hypoechoic presumably from compression-related epineurial irritation–mediated inflammation. The healthy right-side wrist is shown adjacent to the involved side for better contrast. The shapes of the two median nerves are visibly different, with the left side showing some deformation at the carpal inlet as compared to the asymptomatic right side showing better continuity in size.

Discussion

The use of US as an adjunct in the diagnosis of CTS is a growing trend. It is not enough to have the most cutting-edge technology anymore without giving thought to the incremental value of adding a new test to the diagnostic paradigm. The advantages of US include low cost, convenience, and patient preference for this technology. Whether the CTS treatment is with a steroid injection, medical management, physical therapy, or even surgery on a target structure, serial US scans provide an element of consistent follow up that is difficult to achieve with other forms of testing.

The most cited limitations of US continue to be operator dependence and variations in image quality amongst devices sold around the country. As protocols become more familiar to an increasing number of practitioners, and training programs become more abundant to satisfy the growing interest, we will see the proliferation of musculoskeletal US applications grow. With this growth will come a better understanding of the advantages and limitations of the technology. Until that time, practitioners should use caution in how they interpret US findings in occupationally induced CTS.

Our rehabilitation centers in Michigan are in a unique position to be working with some large industrial clients who have very high rates of occupationally induced CTS, usually because of repetitive strain situations especially in factory-line workers. As a result, we have the opportunity to scan quite a few cases in any given year. Table 1 lists some of the observations we have documented over the years of scanning suspected CTS patients.9-14

Conclusion

Ultrasonographic assessment of median nerve status in CTS has evolved into a valuable adjunctive test, and some would argue a primary diagnostic tool ready to be a first-line screening test. Diagnostic US has not overcome the fundamental limitation of being operator dependent—meaning, the image produced relies rather heavily on the interaction between user and device. Training, experience, and technique all figure into the equation, but at the same time can be remedied rather simply—invest time in both the science and the art of this useful modality. As described in this report, US scans can be used to detect pathological changes in the median nerve in patients suspected of having CTS. However, other broader applications include serial testing to gauge tissue healing, as a biofeedback device for patients and practitioners, as the criterion test to validate orthopedic manual tests, to assess the utility of an intervention using pre/post scans, and finally as a teaching tool allowing anyone the appreciation of dynamic real-time anatomical understanding of the interrelationships between structures in the body.

From our perspective, 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. Our suspicion is that in these early stages of the disease, NCS may be falsely negative, but the condition is present nonetheless. More study is forthcoming, including pre- and post-work shift scans that will be able to demonstrate the highly plastic capabilities of the median nerve and how well these changes correlate with patient symptom reports. It is our belief that demonstrable morphologic changes can be seen in a susceptible patient in as little as an 8-hour shift, with the pre-shift status resembling a normal, healthy nerve; post-shift status causing sonographic markers such as nerve hypertrophy; and fluid-induced hypoechogenicity becoming more prominent. Whatever the eventual findings, it is clear that well-trained sonographers will play an important role in the early detection, diagnosis, and treatment of CTS.

Last updated on: May 25, 2017
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Non-surgical Decompression Treatment for Carpal Tunnel Syndrome
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