The median nerve travels into the carpal tunnel from its deep location between the flexor digitorum superficialis and flexor digitorum profundus. The median nerve takes a shallow path over approximately 2 cm to reach its more superficial location at the level of the radial metaphysis. As the median nerve enters the carpal tunnel, it gives rise to the recurrent thenar motor branch. This has a variable origin and course. It is important to note the anatomy to avoid complications from carpal tunnel release. The recurrent thenar motor branch (RTMB) can arise proximal to the transverse carpal ligament, penetrate the ligament, or arise distal to the ligament. The recurrent thenar motor branch can also arise from the radial aspect or the ulnar aspect of the nerve.
The median nerve at the level of the carpal tunnel should measure between 6 to 10 mm² in a cross-sectional area. There are varying validated measurements used as a cutoff for size in the diagnosis of carpal tunnel syndrome. Between 10 mm² and 14 mm² cross-sectional areas is considered diagnostic of carpal tunnel syndrome in the presence of symptoms. This has a sensitivity of 47% and a specificity of 89% or higher(4,9).
Many other authors will consider a cutoff of 10 mm² cross-sectional area as abnormal. During the exam, the sonographer should also take note of a positive sonographic Tinel sign. If the patient has reproduction of symptoms, and the symptoms are in the distribution of the median nerve involving the thumb, pointer, and middle finger, then that is a positive sign.
Additional signs include tram track loss (fig 8) and loss of normal fascicular architecture. The nerve at any portion along its course should have a tram track sign on longitudinal imaging and normal honeycomb or fascicular architecture on transverse imaging.
The nerve can be indented at the carpal tunnel by the transverse carpal ligament, which is also a significant sign. The nerve under the ligament will be narrowed.
Once in the palm, distal to the lunate, and proximal to the palmar arch, the median nerve branches into the digital nerves; this can easily be identified on ultrasound anatomy.
Approximately 10% of patients will have a bifid median nerve depending on the study population. The ulnar branch is typically smaller than the radial branch, in this author’s experience. When measuring these nerves, the operator measures the cross-sectional area (CSA) individually and then adds them together for the total CSA. The presence of a bifid nerve may or may not increase the risk of developing carpal tunnel syndrome (CTS), as there are conflicting small studies showing both outcomes, however, it is not diagnostic of CTS (5)
Patients may also have trifid median nerves or even more branches as an anatomical variant. This is much rarer than the bifid median nerve, which occurs nearly on a daily or weekly basis if scanning multiple wrists a week. Again the operator will measure each individual branch’s cross-sectional area and the total cross-sectional area will be the final reported cross-sectional area.
The persistent median artery will also frequently be encountered (fig. 9) and may be located within the nerve, between bifid median nerve bundles, or alongside a normal singular median nerve. The artery can be large and approximate the size of the normal ulnar or radial artery or be barely visible, showing pulsation on grayscale but no detectible flow. Care needs to be taken not to mistake a persistent median artery within the nerve bundle as an increased doppler signal associated with median neuropathy.
There are also cases whereby a ganglion cyst, synovitis, or a mass can contribute to carpal tunnel syndrome. Synovitis of the flexor compartment would most commonly be seen with an inflammatory arthropathy such as rheumatoid arthritis. At least one study has shown that there is no greater incidence of asymptomatic median nerve enlargement in rheumatoid arthritis, confirming the validity of CSA measurement to diagnose carpal tunnel syndrome in this patient population. In other words, rheumatoid arthritis does not cause enlargement of the median nerve (6).
Additional normal variant muscles may also occur in the carpal tunnel that can cause compression (7). The distal most muscle, other than the thenar musculature, is the pronator quadratus muscle which is deep into the carpal tunnel. If an additional muscle is seen within the flexor compartment, this is an accessory muscle. These muscles have been named; however, it is most important that they are identified. Also, comparing the contralateral side can help in this regard when identifying an additional accessory muscle.