Rebecca O’Bryan, MD
Marcus Pai, MD, PhD
Stonsaovapak C, Nimithpornchai S, Kimura J, Piravej K. Physiological localization by sensory and motor inching studies and structural abnormalities detected by ultrasonographic changes in carpal tunnel syndrome. Arch Phys Med Rehabil
. 2022;103(3):494-504.e1. doi:10.1016/j.apmr.2021.08.001
This is a cross-sectional study that investigated carpal tunnel syndrome (CTS) to determine correlation of electrophysiologically localized entrapment site using inching study with anatomic entrapment site identified by ultrasound (US). A total of 15 hands without and 40 hands (ages 20 to 65) with mild to moderate CTS were studied.
In all 40 patients with CTS, localized latency prolongation was identified in a 1 cm segment, most commonly 2-3 cm distal to the distal wrist crease, for both sensory and motor nerve conduction study. US revealed a narrowing of the median nerve cross sectional area (CSA), at 1-2 cm distal to the distal wrist crease. Recording from the second lumbrical was employed to allow straight motor inching. Sensory and motor localization of latency prolongation was highly correlated. US identified anatomic entrapment point in “most cases.”
The conclusion was made that electrophysiological focal conduction delay occurs 1-2 cm more distally compared with the structural CSA narrowing detection by US.
While inching in the diagnosis of CTS is not routinely employed in most EDX labs, this is a very interesting approach, and the use of the second lumbrical eliminates the need to attempt a curve to the atrial premature beat. Localizing US vs EDX localization was not approached at the individual patient level, which in my opinion might have provided more convincing data supporting this approach. However, the overall finding of EDX prolongation slightly distal to US localized narrowing anatomically is logical, and could be investigated at the level of each patient for better determination as to this theory’s viability.