Carpal Tunnel Syndrome: Accurate Diagnosis with Ultrasound

Dr. Segnini
Dec 17, 2025
3 min read

Updated: Dec 22, 2025

Dr. Segnini Carpal tunnel syndrome ultrasound

When the nerve is compressed

Carpal Tunnel Syndrome (CTS) is a very common compression neuropathy in which the median nerve becomes trapped as it passes through the carpal tunnel, a rigid channel in the wrist. Classic symptoms include tingling, numbness, and pain in the thumb, index, middle, and part of the ring finger, which often worsen at night. Although the clinical presentation is suggestive, ultrasound has emerged as a first-line diagnostic tool, complementary to electromyography (EMG).

Why is ultrasound ideal for carpal tunnel syndrome?

Ultrasound overcomes several limitations of other methods:

Direct visualization: Allows you to see the median nerve and its surrounding structures in real time, not just infer their function.
Dynamics: Compression can be assessed during wrist movements (flexion/extension).
Accessible and fast: No long waiting lists, convenient for the patient.
Identifies structural causes: Detects abnormalities that could cause compression, which EMG does not see.
Guiding treatments: Essential for accurate and safe perineural injections.

Key Ultrasound Findings: What the Radiologist Looks For

Ultrasound diagnosis is based on morphological changes of the median nerve, comparing it to itself in non-compressed areas.

1. Main Sign: Thickening and Flattening of the Median Nerve

"Notary's Sign" or " Flattening Ratio": The nerve, which is normally oval, flattens inside the carpal tunnel (at the level of the pisiform bone) due to pressure. Then, just before entering the tunnel (at the level of the pronator teres), it thickens, forming a characteristic bulge. This difference in shape is very specific.
Cross-Sectional Area (CSA) Measurement: The most important and objective parameter . The area of the nerve (in mm²) is measured in a cross-section. An area > 10-12 mm² (cut-off points vary slightly) is highly suggestive of carpal tunnel syndrome (CTS). The measurement is reproducible and quantifiable.

2. Changes in the Internal Structure of the Nerve

Fascicular Pattern : A healthy nerve appears as a "bundle of fibers" ( dark hypoechoic fascicles surrounded by bright hyperechoic connective tissue ). In carpal tunnel syndrome (CTS), this pattern is obliterated or becomes homogeneously gray ( hypoechoic ) due to edema and inflammation.
Hypervascularization on Doppler : In active and inflammatory cases, increased blood flow may be observed within or around the compressed nerve.

3. Identification of Structural Causes (Unique Advantage of Ultrasound)

Ultrasound can answer the question : why is the nerve being compressed?

Flexor tenosynovitis : Thickening and inflammation of the sheaths surrounding the tendons, reducing the space.
Ganglion or Synovial Cyst: Cystic mass that compresses the nerve.
Anatomical Variants: Anomalous muscles or arteries within the tunnel.
Osteoarthritis or Malunited Fractures: That reduce the size of the canal.

Exploration Protocol: Step by Step

Position: Patient seated, hand resting on the table.
Cross-section: The scan is performed from the distal forearm to the palm , identifying the median nerve (round/oval, superficial to the flexor tendons). Measurements of the area are taken at key points: before, at the entrance, and inside the carpal tunnel.
Longitudinal Section: To evaluate continuity, change in caliber and fascicular pattern .
Dynamic Maneuvers: The patient is asked to flex and extend their fingers or wrist to see if the tendons dynamically "trap" the nerve.
Comparison: Both wrists are always evaluated for comparison, even if only one is symptomatic.

Ultrasound vs. Electromyography (EMG): Are They Rivals?

No, they are complementary . Each one answers a different question:

EMG (Electromyography): Evaluates the electrical function of the nerve. It measures conduction velocity and muscle activity. It is the functional "gold standard," but it is surgery-dependent and may be normal in mild compressions.
Ultrasound: Evaluates structure and morphology . It is excellent for mild-to-moderate cases, for finding anatomical causes, and for post-treatment follow-up.

The ideal approach: Ultrasound as the first step due to its accessibility and anatomical visualization. If doubt persists or quantification of axonal damage is needed , EMG is requested.

Conclusion: More than an Add-on, an Essential Tool

Ultrasound has revolutionized the diagnosis of carpal tunnel syndrome (CTS) by providing a direct, objective, and dynamic anatomical visualization of median nerve compression. Its ability to measure thickening, identify structural causes, and guide interventions makes it a natural extension of the clinical examination. For the patient, this means a faster, less invasive diagnosis and a more precise treatment plan , whether through postural changes, injections, or surgery.