eMedicine Specialties > Radiology > Musculoskeletal
Carpal Tunnel Syndrome: Imaging
Updated: Apr 16, 2009
Radiography
Findings
Plain radiographs are useful for evaluating the wrist and carpal bones for trauma and fractures (especially the hook of the hamate and the tubercle of the trapezium), severe osteoarthritis, and other arthropathies.
Degree of Confidence
Plain films are of limited use in diagnosing or evaluating carpal tunnel syndrome. Plain films do not effectively visualize the small soft-tissue structures of the carpal tunnel, many of which can be involved in the syndrome. A carpal tunnel view of the wrist on plain films provides only a very rough idea of the cross-sectional area of the carpal tunnel.
Computed Tomography
Findings
CT is useful for its ability to display and evaluate the cross-sectional volume of the carpal tunnel and for detecting subtle calcification in the tendons within the canal. CT also provides an excellent tool for evaluating the carpal bones through multiplanar and 3-dimensional reconstructions.
Degree of Confidence
CT is limited in its ability to visualize the median nerve and tendons of the carpal tunnel well enough to allow a definitive differential diagnosis to be rendered. Therefore, other methods of visualizing the soft tissues of the carpal tunnel are preferable.
Magnetic Resonance Imaging
Carpal tunnel syndrome. Normal findings on an axial spin-echo T1 MRI of the carpal tunnel showing the intermediate signal intensity of the median nerve (arrow).
Carpal tunnel syndrome. Normal findings of isointense-to-hypointense appearance of the median nerve on fast spin-echo T2-weighted MRI (arrow). Note the fairly well-defined nerve fascicles within the median nerve sheath.
Carpal tunnel syndrome. Axial fast spin-echo T2-weighted MRI with fat saturation. Note the increased T2-weighted signal within the median nerve (arrow). A slightly increased cross sectional area of the nerve is noted but the nerve architecture is preserved, consistent with early or mild inflammation.
Carpal tunnel syndrome. Fast spin-echo T2-weighted MRI illustrates more pronounced increased signal within the median nerve (arrow). Note the small amount of fluid within the carpal tunnel, a secondary sign of inflammation. Slightly less optimal fat saturation is noted than on other images, which is a common occurrence.
Carpal tunnel syndrome. Axial fast spin-echo T2-weighted MRI with greater increase in signal and loss of definition within the nerve (arrow). Inflammatory change is noted within the carpal tunnel, adjacent to the flexor digitorum superficialis tendons. The appearance is consistent with pronounced inflammatory change within the carpal tunnel.
Findings
- In patients with flexor tenosynovitis, axial MRI demonstrates bowing of the flexor retinaculum.
- Inflamed synovium and tendon sheaths demonstrate low signal intensity on T1-weighted images and increased signal intensity on T2-weighted, T2*-weighted, and short tau inversion recovery (STIR) sequences.
- Regardless of the etiology of carpal tunnel syndrome, changes in the median nerve are similar and include the following:
- Diffuse swelling or segmental enlargement of the median nerve may be demonstrated (usually seen best at the level of the pisiform).
- The median nerve may flatten (usually demonstrated best at the level of the hamate).
- Palmar bowing of the flexor retinaculum may be noted (usually demonstrated best at the level of the hamate).
- Increased T2-weighted signal intensity within the median nerve occurs, which is demonstrated best on axial fast spin-echo (FSE) T2-weighted images (see Images 5 through 9). If FSE signal sequences are not available, axial gradient-recalled echo (GRE) or inversion recovery (IR) sequences also are sensitive to the increased edema in the median nerve that accompanies carpal tunnel syndrome.
- MRI also is useful in detecting and characterizing space-occupying lesions, such as neuromas, ganglion cysts, lipomas, and hemangiomas.
- Enlargement or swelling of the median nerve proximal to the carpal tunnel, termed a pseudoneuroma, has been documented using MRI.
- Flow-sensitive sequences or dynamic contrast-enhanced MRI can detect a circulatory disturbance causing carpal tunnel syndrome, which is a cause separate from deformation or compression of the median nerve.
- One of two abnormal patterns of median nerve enhancement is usually demonstrated: marked enhancement of the nerve (attributed to hypervascular edema) or noticeable lack of enhancement (attributed to nerve ischemia).
- As with the symptoms of carpal tunnel syndrome, MRI findings in the syndrome may vary with wrist position: flexion or extension of the wrist during the scan can alter the visualization of the median nerve from marked enhancement to complete lack of enhancement, presumably because of mechanical obstruction of blood flow to the nerve. These actions are associated with exacerbation of clinical symptoms.
- Attempted surgical therapy for carpal tunnel syndrome may result in incomplete release of the flexor retinaculum. This can be detected by a residual increase in T2 signal of the median nerve within the carpal tunnel and by direct visualization of the still-connected fibers of the retinaculum.
- Transverse carpal ligament release from the hook of the hamate can cause the contents of the carpal canal and/or the flexor tendons to demonstrate a volar convexity caused by the loss of the normal roof support of the flexor retinaculum.
- In addition to incomplete release of the flexor retinaculum, postoperative MRI changes in failed carpal tunnel surgery include excessive fat within the carpal tunnel, neuromas, scarring, and persistent neuritis. A normal postoperative finding is widening of the fat stripe posterior to the flexor digitorum profundus tendons.
- MRI studies following carpal tunnel release may demonstrate an increase in carpal tunnel volume of up to 24%, often accompanied by a change in shape from oval to circular, resulting in increased anteroposterior and mediolateral diameters.
Degree of Confidence
Early detection of the subtle changes of carpal tunnel syndrome requires soft tissue discrimination not possible using standard radiographs or CT. The ability to evaluate the cross sectional morphologic and signal characteristics of the medial nerve and adjacent structures makes MRI invaluable in characterizing both normal anatomy and abnormal pathology in the carpal tunnel.
Significant differences often are present in patients with carpal tunnel syndrome, despite the subjective flattening of the median nerve at the lateral and distal carpal row. Flattening ratios have been used to document statistically significant flattening of the median nerve at the level of the hamate. The median nerve may display enlargement or dilation at the level of the pisiform and concomitant compression and flattening at the level of the hook of the hamate.
False Positives/Negatives
Since carpal tunnel involvement is bilateral in as many as 50% of patients, comparison with the contralateral wrist can be misleading. Alterations in the median nerve signal intensity may represent edema or demyelination within neural fibers and thus are somewhat nonspecific. Both T1 and T2 signal intensities may be decreased when fibrosis of the median nerve is present. Evaluation of swelling is accomplished by comparing the cross-sectional area of the median nerve at the level of the pisiform and hamate to the cross-sectional area of the median nerve at the level of the distal radius.
Ultrasonography
Findings
The development of high-resolution ultrasound (US) transducers (7-15 MHz) has allowed evaluation of normal and abnormal US appearances of the median nerve and adjacent tendons. High-resolution US allows noninvasive imaging of the carpal tunnel and its contents. It has several advantages over MRI, including being relatively fast and inexpensive and allowing additional dynamic and blood flow imaging with relatively little additional time.
On transverse US scans, the normal median nerve is elliptical and flattens progressively as it courses distally. Median nerve compression is revealed on US by the classic triad of nerve flattening in the distal tunnel, nerve swelling at the level of the distal radius (less frequently in the proximal tunnel), and palmar bowing of the flexor retinaculum.
Since the shape of the nerve varies as it passes through the tunnel, indexes have been introduced to better quantify abnormal findings; a nerve cross-sectional area greater than 9 mm2 at the level of the proximal tunnel is reported to be the best criterion for the diagnosis.
Degree of Confidence
A good correlation has been demonstrated between the measured US area of the median nerve and the degree of findings of electromyography or functional outcome after surgery. Reduced transverse sliding of the nerve beneath the retinaculum during flexion and extension of the index finger also may be seen, but this sign is harder to quantify and may be too subjective. Only a few studies have compared US and MRI in evaluating carpal tunnel syndrome, but these demonstrated that US is capable of producing results similar to MRI. MRI has been shown to be superior to US in identification of subtle cases and MRI demonstrates better sensitivity than color and power Doppler US in showing changes caused by nerve edema and blood perfusion abnormalities.
More on Carpal Tunnel Syndrome |
| Overview: Carpal Tunnel Syndrome |
 Imaging: Carpal Tunnel Syndrome |
| Follow-up: Carpal Tunnel Syndrome |
| Multimedia: Carpal Tunnel Syndrome |
| References |
| Further Reading |
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Further Reading
Related eMedicine topics
Carpal Tunnel Syndrome (Emergency Medicine)
Carpal Tunnel Syndrome (Physical Medicine and Rehabilitation)
Carpal Tunnel Syndrome (Orthopedic Surgery)
Clinical guidelines
AAOS Clinical Practice Guidelines (CPG)
Guideline on the Treatment of Carpal Tunnel Syndrome
Guideline on the Diagnosis of Carpal Tunnel Syndrome
Carpal Tunnel Syndrome: Diagnosis and Treatment Trial
Post-Operative Mobilization for Carpal Tunnel Syndrome
Effect of Lumbrical Stretching on Carpal Tunnel Syndrome
Treatment Efficacy of OMT for Carpal Tunnel Syndrome
Treatment of Carpal Tunnel Syndrome With Dynamic Splinting
Steroid Injection Treatment of Carpal Tunnel Syndrome
Corticosteroid Injection as a Predictor of Outcome in Carpal Tunnel Release
Keywords
carpal tunnel syndrome, median nerve injury, wrist injury, repetitive stress syndrome, carpus, carpal bone, flexor retinaculum, traverse carpal ligament, scaphoid tubercle, trapezium, flexor digitorum, flexor tendons, flexor pollicis longus, flexor tenosynovitis
