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Science News: Role of Ultrasonography in Severe Distal Median Nerve Neuropathy

Submitted by Rocio (Carolina) Garcia Santibanez, MD
Edited by Lisa Williams, MD

V Iyer. Role of Ultrasonography in Severe Distal Median Nerve Neuropathy. J Clin Neurophysiol. 2019 Jul;36(4):312-315.

Electrodiagnostic (EDX) localization of a median mononeuropathy in the distal forearm may be challenging in severe cases with absent evoked median sensory responses and absent motor responses to the APB and lumbricals; abnormal needle electromyography limited to the APB may be suggestive of a localized injury at the carpal tunnel, however, rarely the lesion may be more proximal. This study looked at the use of ultrasound in 46 patients with clinical and electrophysiologic evidence of severe carpal tunnel syndrome (CTS). Patients with peripheral neuropathy and a history of carpal tunnel surgery were excluded. Ultrasound imaging of the median nerve was performed with an 8-18 MHz probe. The cross-sectional area (CSA) of the median nerve was measured using a long axis view along the carpal tunnel measuring from the inlet to the outlet (maximum and minimum diameter Dmax/Dmin). The short-axis views were measured at the wrist (CSAw) and the forearm (CSAf) using the distal wrist crease and 10cm proximal to the distal wrist crease as land marks. The ultrasonic criteria used for diagnosing median nerve entrapment were CSAw >13mm2, CSAw/CSAf >1.4, and Dmax/Dmin 1.3. In 42 out of the 46 patients, the ultrasound localized the pathology at the wrist. In the other 4 the abnormality was proximal to the wrist (subluxated lunate, neuroma, schwannoma and hamartoma).

Comments: In severe cases of CTS, with nonlocalizable EDX studies (absent evoked sensory responses and absent motor responses), the addition of ultrasonic imaging may be a useful adjunctive tool in confirming entrapment of the median nerve at the carpal tunnel and also detecting pathology proximally that may mimic CTS in the forearm. Although CTS is more common than most alternative diagnoses, the treatment is significantly different for other potential diagnoses and ultrasound may be useful in sorting this out.

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