The cross sectional area of the median nerve should be measured at the point of maximum enlargement. This usually occurs just proximal to the flexor retinaculum, a position located slightly cranial to the pisiform.

Two methods are used to measure the cross sectional area of the median nerve: the direct method, using the ellipse tool on the ultrasound machine, or the indirect method, by means of the ellipse formula [(maximum anteroposterior diameter) × (maximum transverse diameter) × ()]. There is a high degree of correlation between the two methods. Transducer positioning potentially affects measurement of the median nerve cross sectional area.

Practice Tip

The transducer should always be perpendicular to the nerve, including when the nerve courses obliquely from superficial to deep.

Tilting the transducer over the nerve or slightly changing wrist position can achieve optimal transducer orientation.

There is no consensus in the literature regarding the most appropriate median nerve size for establishing the diagnosis of CTS.

Key Point

An increased cross sectional area of more than 10 mm² at the proximal carpal tunnel (scaphoid–pisiform level) is generally accepted to be diagnostic of CTS.

Ranges between 9 mm² and 15 mm² have been proposed.

Practice Tip

Comparing cross sectional area measurements of the median nerve at the level of the carpal tunnel and pronator quadratus has been proposed to eliminate interindividual and internerve variability. A difference in the cross sectional area of 2 mm² or greater yielded the greatest sensitivity and specificity for the diagnosis of CTS.

Occasionally, the notch sign may occur at the distal edge of the retinaculum (inverted notch sign). In these cases, the nerve is flattened at the distal tunnel and no shape change is seen at the proximal tunnel. An inverted notch sign indicates distal compression, which should be highlighted in the report so that appropriate distal or palmar electrophysiological testing is performed.

Distal flattening of the nerve is evaluated with the flattening ratio, which is calculated at the distal tunnel (hamate level) by dividing the transverse diameter of the nerve by its anteroposterior diameter. A flattening ratio of more than 3 has been proposed as an additional finding of CTS.

Appearance

Practice Tip

The median nerve becomes uniformly hypoechoic with loss of the normal fascicular pattern (in CTS) due to overcrowding of oedematous nerve fascicles or fibrosis in chronic cases (Fig. 13.2).

Care must be taken to keep the transducer perpendicular to the nerve axis when evaluating the neural echotexture to avoid anisotropy and incorrect diagnosis of neuritis.

There is usually increased blood flow in the longitudinal perineural plexus and intraneural vessels with Doppler imaging. This is due to disturbances in the intraneural microvasculature and hyperaemia of inflammatory neuritis.

Restricted motion of the compressed median nerve beneath the flexor retinaculum during flexion and extension of the fingers may be seen, but this is a subjective finding.

Flexor Retinaculum Changes

Volar bowing of the flexor retinaculum is seen secondary to increased pressure within the carpal tunnel (Fig. 13.3). This is assessed at the distal end of the carpal tunnel (hamate–trapezium level). A line joining the hook of the hamate and the tubercle of the trapezium is made. The largest distance from this line to the retinaculum reflects the degree of volar bulging. A value of more than 4 mm is considered significant.

Figure 13.3 CTS. Transverse image shows hypoechoic fluid and synovitis (*) in the flexor digitorum tendon sheaths that separate the flexor tendons (T). There is flattening of the MN and volar bowing of the flexor retinaculum (arrows).

Carpal Tunnel Content

The most common cause of CTS is tenosynovitis of the flexor tendons. Ultrasound demonstrates hypoechoic fluid and synovitis in the tendon sheaths which separate the flexor tendons (Fig. 13.3). Doppler signal depends on the activity of the synovitis.

Practice Tip

When scanning for tenosynovitis in the carpal tunnel, dynamic scanning in the transverse plane during finger flexion and extension helps differentiate the tendons from echogenic synovitis.

Scanning both proximal and distal to the carpal tunnel is important to avoid false negatives as the synovial fluid accumulates outside the carpal tunnel where there is less resistance to sheath distension.

Space occupying lesions within the carpal tunnel are seen well with ultrasound. Ganglia are the commonest and appear as well-defined anechoic masses with no internal vascularity. They usually arise in the deep portion of the tunnel from the wrist joint and displace the median nerve and flexor tendons anteriorly against the flexor retinaculum. Solid tumours include lipomas (Fig. 13.4), neural tumours and giant cell tumours of the tendon sheath. A persistent median artery has been described as a cause of CTS. This is an accessory artery that arises from the ulnar artery in the proximal forearm and may accompany the median nerve through the carpal tunnel. Doppler imaging is useful for identifying it. Acute CTS may be secondary to acute thrombosis of this artery. Following penetrating injury, posttraumatic neuroma, which may affect only a portion of the median nerve, may occur (Fig. 13.5).