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Case Report  |   December 2009
Carpal Tunnel Syndrome: Ultrasonographic Imaging and Pathologic Mechanisms of Median Nerve Compression
Author Notes
  • Address correspondence to Benjamin M. Sucher, DO, EMG Labs of Arizona Arthritis and Rheumatology Associates, 10599 N Tatum Blvd, Suite F-150, Paradise Valley, AZ 85253-1053. E-mail: drsucher@msn.com 
Article Information
Imaging / Neuromusculoskeletal Disorders
Case Report   |   December 2009
Carpal Tunnel Syndrome: Ultrasonographic Imaging and Pathologic Mechanisms of Median Nerve Compression
The Journal of the American Osteopathic Association, December 2009, Vol. 109, 641-647. doi:10.7556/jaoa.2009.109.12.641
The Journal of the American Osteopathic Association, December 2009, Vol. 109, 641-647. doi:10.7556/jaoa.2009.109.12.641
Abstract

Median nerve compression is a well-known cause of carpal tunnel syndrome (CTS). Yet, reasons why the most common idiopathic form of CTS develops in certain individuals are not well understood. To further understand the compressive mechanisms at work in CTS development, the authors used ultrasonographic imaging of the median nerve to evaluate 2 patients with CTS. Findings were compared to those of 2 control subjects who did not have CTS. In the patients who had CTS, the transverse carpal ligament was pulled taut by thenar muscle contraction as the flexor tendons tightened, compressing the median nerve between the ligament and tendons. No such compression was observed with the control subjects. Thus, a pathologic mechanism of median nerve compression was confirmed in the patients with CTS. Demonstration of such pathologic mechanisms during prehensile hand movement may improve understanding of how to treat patients with CTS and prevent nerve injury.

Carpal tunnel syndrome (CTS) is generally considered to be caused by median nerve compression at the wrist.1 The carpal tunnel is a narrow and relatively unyielding space that readily entraps the median nerve between the tough fibrous transverse carpal ligament ventrally and carpal bones dorsally. Because the median nerve is the softest structure within the carpal tunnel, it usually sustains injury first. Repetitive finger activity is believed to contribute to CTS—either because pressure from the flexor tendons irritates the adjacent median nerve or because inflammatory swelling of the tendon sheaths increases compartment pressure within the carpal tunnel.1,2 
Previous studies have measured intracarpal pressure in patients with CTS and in subjects without CTS, demonstrating higher pressures in those with CTS.1,2 However, no known previous studies have directly imaged the median nerve during activity to identify direct mechanical compression. High-resolution diagnostic ultrasonography is now available to readily observe the response to hand maneuvers that might challenge the median nerve. 
The present case report demonstrates how the median nerve responds to prehensile hand activity in patients with CTS and in individuals without CTS, providing improved understanding of the pathologic mechanisms responsible for nerve compression. 
Reports of Cases
Carpal Tunnel Syndrome Case 1
The patient in CTS case 1 was a man aged 24 years with a several-week history of pain, numbness, tingling, and weakness in his right hand, primarily involving the first three digits. Physical examination revealed positive results in Tinel's test and Phalen's test over the right carpal tunnel, as well as palpatory restriction over the right carpal tunnel. 
Electrodiagnostic (EDX) studies confirmed median neuropathy at the right wrist. The median distal motor latency was 6.0 milliseconds, and the median distal sensory latency to the index finger was 5.3 milliseconds. Results of comparative radial and ulnar studies were normal, with ulnar distal motor latency at 3.0 milliseconds and ulnar distal sensory latency at 3.7 milliseconds and with all response amplitudes normal. Motor distances were 8 cm, and sensory distances were 14 cm. Needle electromyographic examination yielded normal results except for increased membrane irritability within the right thenar muscles—consistent with mild denervation. These findings were compatible with a diagnosis of moderate-to-severe CTS. 
Immediately after EDX, the median nerve in the patient's right wrist was imaged transversely with high-resolution ultrasonography (13 MHz M-Turbo system; SonoSite Inc, Bothell, Washington) to measure the cross-sectional area in the proximal carpal tunnel at the level of the pisiform (Figure 1). Images revealed median nerve enlargement, with a cross-sectional measurement of 14 mm2 (compared with the normal upper limit of 11 mm2).3,4 
The median nerve was then imaged longitudinally and transversely at the mid-distal tunnel in a neutral relaxed position. Imaging was repeated during dynamic stress testing (DST), which involves a sustained isometric contraction of the thumb tip against the tips of the second and third digits using a hard rubber ball for resistance (Figure 2). Longitudinal imaging (Figure 3A) and transverse imaging (Figure 3B) in the neutral, prestress position showed normal results. However, longitudinal imaging during DST showed obvious flattening of the median nerve, with a segment of the nerve compressed in the mid-distal carpal tunnel (Figure 4A). Similar findings were noted in the transverse plane, with median nerve elongation and flattening during DST (Figure 4B-4C). 
Figure 1.
Transverse ultrasonographic image of the right wrist of a 24-year-old man with carpal tunnel syndrome (case 1). Image is at the proximal carpal tunnel at the level of the pisiform, showing the cross-sectional area measurement of the median nerve (outlined with white dots to the left of the “A”) at 14 mm2 (0.14 cm2).
Figure 1.
Transverse ultrasonographic image of the right wrist of a 24-year-old man with carpal tunnel syndrome (case 1). Image is at the proximal carpal tunnel at the level of the pisiform, showing the cross-sectional area measurement of the median nerve (outlined with white dots to the left of the “A”) at 14 mm2 (0.14 cm2).
Figure 2.
A hard rubber ball is used to provide resistance to thumb and digit flexion in dynamic stress testing, allowing thenar muscle fixation during isometric contraction to challenge, or stress, the median nerve in the carpal tunnel. The relatively unyielding ball maintains separation of the thumb from the fingertips, preventing the thenar mass from moving medially and pushing the ultrasonography transducer off the nerve.
Figure 2.
A hard rubber ball is used to provide resistance to thumb and digit flexion in dynamic stress testing, allowing thenar muscle fixation during isometric contraction to challenge, or stress, the median nerve in the carpal tunnel. The relatively unyielding ball maintains separation of the thumb from the fingertips, preventing the thenar mass from moving medially and pushing the ultrasonography transducer off the nerve.
These findings indicated that as the thenar muscle mass contracted and tightened the transverse carpal ligament, a portion of the muscle bulged dorsally into the carpal tunnel (Figure 4B-4C). Simultaneously, the contracting flexor muscles tightened the flexor tendons ventrally, adding to the compressive effect by creating a more rigid and unyielding tunnel floor—essentially “sandwiching” the median nerve between the transverse carpal ligament and the flexor tendons. 
Carpal Tunnel Syndrome Case 2
The patient in CTS case 2 was a woman aged 56 years with a 4-month history of numbness and tingling in her right upper extremity. Physical examination was unremarkable except for revealing positive results in Tinel's test and Phalen's test over the right carpal tunnel, as well as palpatory restriction over the right carpal tunnel. 
Electrodiagnostic studies confirmed median neuropathy at the right wrist. The median distal motor latency was 4.0 milliseconds (with 2.7 milliseconds to the ulnar distal motor latency), and the median distal sensory latency to the thumb, at 10 cm, was 3.2 milliseconds. The radial distal sensory latency to the thumb was 2.5 milliseconds (at 10 cm), and all response amplitudes were normal. Results of the needle electromyographic examination were normal. 
As with the patient in case 1, ultrasonographic imaging was obtained with the patient in case 2 after EDX. This imaging revealed an enlarged right median nerve, with a cross-sectional measurement of 16 mm2, at the level of the pisiform. Longitudinal imaging (Figure 5) revealed right median nerve compression during DST. This compression was documented with prestress and stress diameter measurements, showing that the median nerve diameter decreased from a prestress measurement of 0.26 cm to a measurement of 0.19 cm during stress—representing a 27% compressive narrowing. 
Control Cases
The subject in control case 1 was a woman aged 39 years with symptoms of pain, numbness, tingling, and weakness in her left upper limb. Electrodiagnostic studies yielded a positive result for CTS on the left side—though EDX on the right side showed normal results. Ultrasonographic imaging revealed an enlarged left median nerve diameter of 18 mm2 and a normal right median nerve diameter of 8 mm2. Longitudinal imaging of the right wrist during DST (Figure 6A-6B) revealed no nerve compression, with the right median nerve diameter increasing from a prestress measurement of 0.24 cm to a stress measurement of 0.25 cm—representing a 4.2% increase. 
Figure 3.
Prestress ultrasonographic images of the right wrist of a 24-year-old man with carpal tunnel syndrome (case 1). In the longitudinal view (A), the median nerve is the darker, hypoechoic linear structure traversing from the left side of the image, almost reaching the right side (arrow a). The brighter, hyperechoic linear structure with a fibrillar pattern just below the nerve is the flexor tendon (arrow b). In the transverse image (B), the median nerve is the ellipsoid darker structure in the central upper left (arrow c), just below the brighter linear transverse carpal ligament (arrow d).
Figure 3.
Prestress ultrasonographic images of the right wrist of a 24-year-old man with carpal tunnel syndrome (case 1). In the longitudinal view (A), the median nerve is the darker, hypoechoic linear structure traversing from the left side of the image, almost reaching the right side (arrow a). The brighter, hyperechoic linear structure with a fibrillar pattern just below the nerve is the flexor tendon (arrow b). In the transverse image (B), the median nerve is the ellipsoid darker structure in the central upper left (arrow c), just below the brighter linear transverse carpal ligament (arrow d).
The subject in control case 2 was a woman aged 40 years with symptoms of numbness and tingling in her right upper limb. However, EDX showed normal results. Ultrasonographic imaging also revealed normal results, with a right median nerve size of 10 mm2 at the pisiform. Imaging during DST (Figure 6C-6D) revealed no nerve compression, with the right median nerve diameter increasing from a prestress measurement of 0.21 cm to a stress measurement of 0.23 cm—representing a 9.5% increase. 
Discussion
Static ultrasonographic imaging to measure median nerve enlargement at the level of the pisiform has been described as a pathologic finding in CTS.3,4 Other findings, such as median nerve flattening or “notching” in the distal carpal tunnel, have also been observed in CTS—though less consistently than proximal nerve enlargement.3,4 
Some previous studies have used ultrasonographic imaging to identify a decrease in nerve “sliding” within the carpal tunnel during passive index finger motion.5 For example, Nakamichi and Tachibana5 observed that in normal control subjects, the median nerve slides transversely to a position in the carpal tunnel that is “freer” (ie, has reduced pressure), but in subjects with CTS, the median nerve has restricted motion (ie, decreased sliding) and increased exposure to compression. In another study,6 active contraction with fingertip loading was used to demonstrate an increase in pressure within the carpal tunnel during index finger pinch gripping. However, none of these previous studies challenged or imaged the median nerve directly during active muscle contraction and tendon tightening to observe nerve compression dynamically within the carpal tunnel in patients with CTS. 
Prehensile hand movement requires fixation of the primary thumb movers (ie, abductor pollicis brevis and opponens pollicis) at their base of attachment, where they anchor to the transverse carpal ligament.7,8 Such fixation allows muscle contraction to pull the thumb toward the other digits for controlled grasping functions. A solid immobile base of attachment prepares the muscles to freely move the thumb. 
When more powerful and sustained grasping or pinching functions are required, such as firmly holding a tool or pencil (the most common form of prehension7), the thumb tip becomes immobile and the anchor becomes the mobile segment. As a result, thenar muscle contraction pulls the transverse carpal ligament taut—instead of moving the thumb—and the muscle bulges dorsally into the carpal tunnel. At the same time, the flexor tendons to the thumb and digits are pulled taut, and the tendons “bowstring” toward the underside of the transverse carpal ligament, “sandwiching” the median nerve between the tendons and transverse carpal ligament (Figure 4B-4C). Ultrasonographic images also demonstrate apparent flattening or compression of the median nerve during DST maneuvers, as seen in Figure 3A, Figure 4A, and Figure 5A-5B. 
Figure 4.
Stress ultrasonographic images of the right wrist of a 24-year-old man with carpal tunnel syndrome (case 1). In the longitudinal image (A), the small upward-pointing arrow (a) indicates the region of median nerve compression, seen as a depression or flattening along the median nerve to the left of the white arrow and beneath the downward-pointing red arrows (b). In the transverse image (B), the median nerve is elongated and flattened (arrows c) as the transverse carpal ligament bulges dorsally (arrow d). The thenar muscle mass (dark wedge-shaped area on left) contracts and pulls the ligament to the left (arrow e), which flattens the portion of the ligament directly above the median nerve. Note that a small dark area located centrally above the ligament in Figure 3B is slightly larger in Figure 4B and has moved more dorsally as it pushes the edge of the ligament further into the carpal tunnel and against the median nerve (arrow d). In the transverse image of maximum stress (C), additional median nerve flattening is seen as the patient increases the intensity of thumb and digit flexion contraction. The edge of the thenar muscle and the transverse carpal ligament can be seen intruding further dorsally into the carpal tunnel against the ventral surface of the median nerve.
Figure 4.
Stress ultrasonographic images of the right wrist of a 24-year-old man with carpal tunnel syndrome (case 1). In the longitudinal image (A), the small upward-pointing arrow (a) indicates the region of median nerve compression, seen as a depression or flattening along the median nerve to the left of the white arrow and beneath the downward-pointing red arrows (b). In the transverse image (B), the median nerve is elongated and flattened (arrows c) as the transverse carpal ligament bulges dorsally (arrow d). The thenar muscle mass (dark wedge-shaped area on left) contracts and pulls the ligament to the left (arrow e), which flattens the portion of the ligament directly above the median nerve. Note that a small dark area located centrally above the ligament in Figure 3B is slightly larger in Figure 4B and has moved more dorsally as it pushes the edge of the ligament further into the carpal tunnel and against the median nerve (arrow d). In the transverse image of maximum stress (C), additional median nerve flattening is seen as the patient increases the intensity of thumb and digit flexion contraction. The edge of the thenar muscle and the transverse carpal ligament can be seen intruding further dorsally into the carpal tunnel against the ventral surface of the median nerve.
A classic “squeeze play” appears to be at work in this mechanism, with the roof (ie, transverse carpal ligament) of the carpal tunnel tightening and lowering and the floor (ie, flexor tendons) of the carpal tunnel tightening and rising—thereby compressing the median nerve. 
In dynamic studies using digital video recording of several patients with CTS, the author demonstrated that the median nerve is actively compressed during pinch activity. This compression is particularly obvious with transverse ultrasonographic imaging in the mid-tunnel region. However, maintaining such imaging can be challenging because the thenar muscle contracts and pushes the ultrasonography transducer off the nerve. 
Figure 5.
Ultrasonographic images of the right wrist of a 56-year-old woman with carpal tunnel syndrome (case 2). Longitudinal images of the median nerve at prestress (A) and during stress (B) depict nerve diameter measurements between the “A” markers. The measurements show the initial nerve diameter of 0.26 cm decreasing to 0.19 cm during stress compression. Slight indentation or flattening can be seen along the upper (ie, ventral) surface of the nerve as the thenar muscle bulges downward (ie, dorsally).
Figure 5.
Ultrasonographic images of the right wrist of a 56-year-old woman with carpal tunnel syndrome (case 2). Longitudinal images of the median nerve at prestress (A) and during stress (B) depict nerve diameter measurements between the “A” markers. The measurements show the initial nerve diameter of 0.26 cm decreasing to 0.19 cm during stress compression. Slight indentation or flattening can be seen along the upper (ie, ventral) surface of the nerve as the thenar muscle bulges downward (ie, dorsally).
It is interesting to note that in individuals without CTS, the median nerve simply “slides” out of the way, avoiding compression and often allowing the nerve to enlarge (Figure 6). This observation supports the theories of Hunter9 and Phalen10 regarding fibrous fixation of the median nerve in patients with CTS. Thus, fibrosis within the carpal tunnel of patients with CTS may prevent the median nerve from sliding out of harms way during routine hand activity. In fact, this author previously suggested that manipulation and stretching may be successful in alleviating symptoms of CTS by breaking up adhesions or fibrous fixations.8 
Implications of understanding the pathologic mechanisms leading to development of CTS may include improved treatment of patients as well as potential prevention of median nerve injury. Reasons why CTS develops in some individuals who perform repetitive or vigorous hand activities but not in others are not clearly understood. Some researchers have observed contractile cells in the transverse carpal ligaments of patients with CTS, suggesting that the ligaments in these patients were in a constant state of contraction.11 This author previously observed relative mechanical restriction over the carpal tunnel in patients with CTS, as measured by quantitative palpation—a finding that could correlate with tightness in the transverse carpal ligament.8 It is unknown if these patients had preexisting abnormalities that contributed to the development of CTS or if the specific patterns of movement in these patients created the abnormalities that subsequently led to median nerve injury. 
However, repetitive or sustained contractions of the thenar muscles, combined with possible contraction of myofibroblasts within the transverse carpal ligament, causes relatively increased tightness in the transverse carpal ligament, leading to further foreshortening and pressure on the median nerve. In addition, perpetual contraction of the thenar muscles contributes to their hypertrophy, leading these muscles to protrude into the carpal tunnel during activity.8,11 
These observations suggest a multifactorial causation in CTS, including increased intracarpal pressure2,5; decreased median nerve mobility (from fibrous fixation)5,10; median nerve deformation (ie, compression, stretching, traction)9; increased stiffness of the synovium and flexor retinaculum (ie, transverse carpal ligament)5; relative thenar muscle hypertrophy or increased thenar muscle mass with intrusion into the carpal tunnel; and flexor tendon thickening and tightening during activity. The latter two processes would substantially contribute to compression by tightening and lowering the transverse carpal ligament at the same time that the floor (ie, flexor tendons) is tightened and raised during prehensile activity (ie, thenar flexion and opposition to the first two digits). 
Figure 6.
Ultrasonographic images of the right wrists of two control subjects without carpal tunnel syndrome. Longitudinal images of the first control subject's median nerve at prestress (A) and during stress (B) depict nerve diameter measurements between the “A” markers. These measurements show the initial nerve diameter of 0.24 cm increasing to 0.25 cm during stress—revealing a lack of compression. Longitudinal images of the second control subject's median nerve at prestress (C) and during stress (D) also depict nerve diameter measurements between the “A” markers. These measurements show the initial nerve diameter of 0.21 cm increasing to 0.23 cm during stress—revealing a lack of compression.
Figure 6.
Ultrasonographic images of the right wrists of two control subjects without carpal tunnel syndrome. Longitudinal images of the first control subject's median nerve at prestress (A) and during stress (B) depict nerve diameter measurements between the “A” markers. These measurements show the initial nerve diameter of 0.24 cm increasing to 0.25 cm during stress—revealing a lack of compression. Longitudinal images of the second control subject's median nerve at prestress (C) and during stress (D) also depict nerve diameter measurements between the “A” markers. These measurements show the initial nerve diameter of 0.21 cm increasing to 0.23 cm during stress—revealing a lack of compression.
  
Dynamic Stress Test for Carpal Tunnel Syndrome
Using ultrasonographic imaging, Dr Sucher found median nerve compression in patients with carpal tunnel syndrome.
This “squeeze play” action may be supportive evidence suggesting that most cases of CTS are not idiopathic, as previously claimed.1 In fact, CTS may simply be a self-defensive mechanism in which excessive activity of the hand causes thenar muscle movement and flexor tendon movement to compress the nerve supplying the muscle that generates the activity—leading to weakness and atrophy that cause the compression to “back-off,” allowing the nerve to recover. 
Previous studies8,12-14 demonstrated that osteopathic manipulative treatment (OMT) of the wrist led to increase in size of the carpal tunnel and alleviation of CTS symptoms as nerve conductions improved. A primary maneuver used for OMT in patients with CTS is a myofascial technique that involves thenar muscle abduction and extension, which applies traction to the transverse carpal ligament, most likely elongating the ligament and increasing space within the carpal tunnel.12 A secondary effect of this maneuver, not previously elucidated, probably involves elongation of the thenar muscles, releasing focal “mounding” and prominence that could intrude into the carpal tunnel during active contraction. Another primary maneuver used for OMT in patients with CTS is a myofascial technique that involves wrist and digit hyperextension to stretch and elongate the flexor tendons.12 
The present case study using high-resolution diagnostic ultrasonography suggests that the mechanism of carpal tunnel release with OMT impacts several of the suspected factors causing CTS. Stretching of the transverse carpal ligament reduces tension in that structure and leads to increased space within the carpal tunnel, decreasing pressure on the median nerve. At the same time, release of thenar muscle tightness leads to decreased muscle intrusion into the carpal tunnel. Elongation of the flexor tendons should decrease thickening and tightening on the other side of the carpal tunnel and also decrease pressure on the median nerve. 
These effects of OMT could prepare a normal carpal tunnel for improved activity tolerance, thereby making OMT a valuable component of a program of CTS prevention. 
Conclusion
Diagnostic ultrasonographic imaging of the carpal tunnel adds a new dimension to understanding the pathologic mechanisms involved in the development of CTS. It is now possible to directly image median nerve compression during prehensile hand activity—heretofore unconfirmed as a contributory cause of nerve injury. In addition, observation of thenar muscle intrusion into the carpal tunnel indicates that this intrusion may be a factor previously unsuspected in CTS causation. 
The results of the present study regarding etiologic mechanisms of CTS add evidence to support previous findings8,12-14 that suggested effectiveness of aggressive OMT and stretching approaches to CTS management. Furthermore, the new findings indicate that application of ultrasonography during DST can open a window to prevention of CTS. 
 The author has no relevant financial relationships or conflicts of interest to disclose.
 
I give special thanks to Christine Quinn, RDMS, and Lois Ferguson, RT, RDMS, RVT, of Sonosite Inc in Bothell, Washington, for generous use of the M-Turbo system to acquire the ultrasonographic images for CTS case 1 of this report. 
Rosenbaum RB, Ochoa JL. Carpal Tunnel Syndrome and Other Disorders of the Median Nerve. 2nd ed. Amsterdam, The Netherlands: Butterworth-Heinemann; 2002:67,106.
Rojviroj S, Sirichativapee W, Kowsuwon W, Wongwiwattananon J, Tamnanthong N, Jeeravipoolvarn P. Pressures in the carpal tunnel. A comparison between patients with carpal tunnel syndrome and normal subjects. J Bone Joint Surg Br. 1990;72:516-518. http://www.jbjs.org.uk/cgi/reprint/72-B/3/516. Accessed November 12, 2009.
Beekman R, Visser LH. Sonography in the diagnosis of carpal tunnel syndrome: a critical review of the literature [review]. Muscle Nerve. 2003;27:26-33.
Bodner G. Nerve compression syndromes. In: Peer S, Bodner G, eds. High-Resolution Sonography of the Peripheral Nervous System. 2nd ed. Berlin, Germany: Springer-Verlag;2008 : 71-122.
Nakamichi K, Tachibana S. Restricted motion of the median nerve in carpal tunnel syndrome. J Hand Surg Br. 1995;20:460-464.
Keir PJ, Bach JM, Rempel DM. Fingertip loading and carpal tunnel pressure: differences between a pinching and a pressing task. J Orthop Res. 1998;16:112-115.
Kapandji IA. The Physiology of the Joints, Volume One, The Upper Limb. Edinburgh, Scotland: Churchill Livingstone;1979 .
Sucher BM. Palpatory diagnosis and manipulative management of carpal tunnel syndrome [review]. J Am Osteopath Assoc. 1994;94:647-663. http://www.jaoa.org/cgi/reprint/94/8/647. Accessed November 12, 2009.
Hunter JM. Recurrent carpal tunnel syndrome, epineural fibrous fixation, and traction neuropathy [review]. Hand Clin. 1991;7:491-504.
Phalen GS. Reflections on 21 years' experience with the carpal-tunnel syndrome. JAMA. 1970;212:1365-1367.
Allampallam K, Chakraborty J, Bose KK, Robinson J. Explant culture, immunofluorescence and electron-microscopic study of flexor retinaculum in carpal tunnel syndrome. J Occup Environ Med. 1996;38:264-271.
Sucher BM. Myofascial release of carpal tunnel syndrome. J Am Osteopath Assoc. 1993;93:92-101.
Sucher BM. Myofascial manipulative release of carpal tunnel syndrome: documentation with magnetic resonance imaging. J Am Osteopath Assoc. 1993;93:1273-1278.
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Figure 1.
Transverse ultrasonographic image of the right wrist of a 24-year-old man with carpal tunnel syndrome (case 1). Image is at the proximal carpal tunnel at the level of the pisiform, showing the cross-sectional area measurement of the median nerve (outlined with white dots to the left of the “A”) at 14 mm2 (0.14 cm2).
Figure 1.
Transverse ultrasonographic image of the right wrist of a 24-year-old man with carpal tunnel syndrome (case 1). Image is at the proximal carpal tunnel at the level of the pisiform, showing the cross-sectional area measurement of the median nerve (outlined with white dots to the left of the “A”) at 14 mm2 (0.14 cm2).
Figure 2.
A hard rubber ball is used to provide resistance to thumb and digit flexion in dynamic stress testing, allowing thenar muscle fixation during isometric contraction to challenge, or stress, the median nerve in the carpal tunnel. The relatively unyielding ball maintains separation of the thumb from the fingertips, preventing the thenar mass from moving medially and pushing the ultrasonography transducer off the nerve.
Figure 2.
A hard rubber ball is used to provide resistance to thumb and digit flexion in dynamic stress testing, allowing thenar muscle fixation during isometric contraction to challenge, or stress, the median nerve in the carpal tunnel. The relatively unyielding ball maintains separation of the thumb from the fingertips, preventing the thenar mass from moving medially and pushing the ultrasonography transducer off the nerve.
Figure 3.
Prestress ultrasonographic images of the right wrist of a 24-year-old man with carpal tunnel syndrome (case 1). In the longitudinal view (A), the median nerve is the darker, hypoechoic linear structure traversing from the left side of the image, almost reaching the right side (arrow a). The brighter, hyperechoic linear structure with a fibrillar pattern just below the nerve is the flexor tendon (arrow b). In the transverse image (B), the median nerve is the ellipsoid darker structure in the central upper left (arrow c), just below the brighter linear transverse carpal ligament (arrow d).
Figure 3.
Prestress ultrasonographic images of the right wrist of a 24-year-old man with carpal tunnel syndrome (case 1). In the longitudinal view (A), the median nerve is the darker, hypoechoic linear structure traversing from the left side of the image, almost reaching the right side (arrow a). The brighter, hyperechoic linear structure with a fibrillar pattern just below the nerve is the flexor tendon (arrow b). In the transverse image (B), the median nerve is the ellipsoid darker structure in the central upper left (arrow c), just below the brighter linear transverse carpal ligament (arrow d).
Figure 4.
Stress ultrasonographic images of the right wrist of a 24-year-old man with carpal tunnel syndrome (case 1). In the longitudinal image (A), the small upward-pointing arrow (a) indicates the region of median nerve compression, seen as a depression or flattening along the median nerve to the left of the white arrow and beneath the downward-pointing red arrows (b). In the transverse image (B), the median nerve is elongated and flattened (arrows c) as the transverse carpal ligament bulges dorsally (arrow d). The thenar muscle mass (dark wedge-shaped area on left) contracts and pulls the ligament to the left (arrow e), which flattens the portion of the ligament directly above the median nerve. Note that a small dark area located centrally above the ligament in Figure 3B is slightly larger in Figure 4B and has moved more dorsally as it pushes the edge of the ligament further into the carpal tunnel and against the median nerve (arrow d). In the transverse image of maximum stress (C), additional median nerve flattening is seen as the patient increases the intensity of thumb and digit flexion contraction. The edge of the thenar muscle and the transverse carpal ligament can be seen intruding further dorsally into the carpal tunnel against the ventral surface of the median nerve.
Figure 4.
Stress ultrasonographic images of the right wrist of a 24-year-old man with carpal tunnel syndrome (case 1). In the longitudinal image (A), the small upward-pointing arrow (a) indicates the region of median nerve compression, seen as a depression or flattening along the median nerve to the left of the white arrow and beneath the downward-pointing red arrows (b). In the transverse image (B), the median nerve is elongated and flattened (arrows c) as the transverse carpal ligament bulges dorsally (arrow d). The thenar muscle mass (dark wedge-shaped area on left) contracts and pulls the ligament to the left (arrow e), which flattens the portion of the ligament directly above the median nerve. Note that a small dark area located centrally above the ligament in Figure 3B is slightly larger in Figure 4B and has moved more dorsally as it pushes the edge of the ligament further into the carpal tunnel and against the median nerve (arrow d). In the transverse image of maximum stress (C), additional median nerve flattening is seen as the patient increases the intensity of thumb and digit flexion contraction. The edge of the thenar muscle and the transverse carpal ligament can be seen intruding further dorsally into the carpal tunnel against the ventral surface of the median nerve.
Figure 5.
Ultrasonographic images of the right wrist of a 56-year-old woman with carpal tunnel syndrome (case 2). Longitudinal images of the median nerve at prestress (A) and during stress (B) depict nerve diameter measurements between the “A” markers. The measurements show the initial nerve diameter of 0.26 cm decreasing to 0.19 cm during stress compression. Slight indentation or flattening can be seen along the upper (ie, ventral) surface of the nerve as the thenar muscle bulges downward (ie, dorsally).
Figure 5.
Ultrasonographic images of the right wrist of a 56-year-old woman with carpal tunnel syndrome (case 2). Longitudinal images of the median nerve at prestress (A) and during stress (B) depict nerve diameter measurements between the “A” markers. The measurements show the initial nerve diameter of 0.26 cm decreasing to 0.19 cm during stress compression. Slight indentation or flattening can be seen along the upper (ie, ventral) surface of the nerve as the thenar muscle bulges downward (ie, dorsally).
Figure 6.
Ultrasonographic images of the right wrists of two control subjects without carpal tunnel syndrome. Longitudinal images of the first control subject's median nerve at prestress (A) and during stress (B) depict nerve diameter measurements between the “A” markers. These measurements show the initial nerve diameter of 0.24 cm increasing to 0.25 cm during stress—revealing a lack of compression. Longitudinal images of the second control subject's median nerve at prestress (C) and during stress (D) also depict nerve diameter measurements between the “A” markers. These measurements show the initial nerve diameter of 0.21 cm increasing to 0.23 cm during stress—revealing a lack of compression.
Figure 6.
Ultrasonographic images of the right wrists of two control subjects without carpal tunnel syndrome. Longitudinal images of the first control subject's median nerve at prestress (A) and during stress (B) depict nerve diameter measurements between the “A” markers. These measurements show the initial nerve diameter of 0.24 cm increasing to 0.25 cm during stress—revealing a lack of compression. Longitudinal images of the second control subject's median nerve at prestress (C) and during stress (D) also depict nerve diameter measurements between the “A” markers. These measurements show the initial nerve diameter of 0.21 cm increasing to 0.23 cm during stress—revealing a lack of compression.
  
Dynamic Stress Test for Carpal Tunnel Syndrome
Using ultrasonographic imaging, Dr Sucher found median nerve compression in patients with carpal tunnel syndrome.