The significance of ultrasonographic carpal tunnel outlet measurements in the diagnosis of carpal tunnel syndrome

Abstract

). The sponsor had no direct involvement in the collection, analysis and interpretation of data and in the preparation of the manuscript. 2 HIGHLIGHTS Median nerve enlargement in CTS is significantly greater at the tunnel outlet than at the inlet  It is postulated that pressure progressively increases from proximal to distal within the tunnel  The addition of outlet measurements increases diagnostic sensitivity and accuracy of CTS 3 ABSTRACT Objective: A retrospective study to investigate the utility of ultrasonographic carpal tunnel outlet measurements in the diagnosis of carpal tunnel syndrome (CTS).Methods: 118 hands of 87 patients with electrophysiologically confirmed CTS and 44 control hands of 23 subjects were assessed. Cross-sectional areas (CSA) of the median nerve were measured at the tunnel inlet, outlet, and forearm. Longitudinal diameters (LAPD) were measured at the inlet, proximal tunnel, distal tunnel, and outlet.Results: CSA at the outlet (median: 18 mm 2 ) and its palm-to-forearm-ratio (median: 2.7) were significantly larger than CSA at the inlet (median: 15 mm 2 ) and its wrist-to-forearm-ratio (median: 2.2) (p <0.001). 27% of the hands showed enlargement only at the outlet versus 13% only at the inlet.LAPD jump was significantly greater, suggesting relief of higher pressure, at the outlet / distal tunnel versus inlet / proximal tunnel (p <0.001). Conclusion:Median nerve enlargement in CTS is greater at the tunnel outlet than at the inlet. We postulate that this is explained by the progressive increase of pressure within the tunnel from proximal to distal.Significance: The addition of outlet measurements to inlet measurements increased sensitivity and accuracy of the ultrasonographic diagnosis of CTS by 15% and 10%, respectively.

Use of Multiple Ultrasonographic Parameters in Confirmation of Carpal Tunnel Syndrome

“…Interestingly, the outlet cross‐sectional area has also shown good diagnostic characteristics as a single parameter, which was recently reported in several studies . Also, we observed that almost half of all symptomatic wrists (64 of 135) had an outlet cross‐sectional area that was at least slightly larger than the inlet cross‐sectional area (and equal to or larger than its cutoff value), whereas 15% of them (7.4% of all symptomatic wrists) had an inlet cross‐sectional that was smaller than or at the cutoff value (10 mm 2 ).…”

“…Also, we observed that almost half of all symptomatic wrists (64 of 135) had an outlet cross‐sectional area that was at least slightly larger than the inlet cross‐sectional area (and equal to or larger than its cutoff value), whereas 15% of them (7.4% of all symptomatic wrists) had an inlet cross‐sectional that was smaller than or at the cutoff value (10 mm 2 ). Few studies have previously reported similar findings …”

“…Few studies have previously reported similar findings. 25,[31][32][33] The definitive pathophysiologic or pathoanatomic background and clinical implications of these two, possibly distinct, US types of CTS are still unclear and should be explored further. One explanation is that the site (pre-or post-tunnel) of the largest median nerve enlargement is dependent on the exact location of compression inside the tunnel.…”

Carpal tunnel syndrome in advanced age: A sonographic and electrodiagnostic study

“…Since the first description by Buchberger et al, 5,6 a number of sonographic abnormalities have been reported in individuals with clinical CTS, including: increase in median nerve cross-sectional area (CSA) at the level of the pisiform bone (carpal tunnel inlet) and hamate; increase in nerve CSA at the level of the pisiform bone compared with the area at the distal radius (swelling ratio) or forearm (wrist-to-forearm ratio); increase in the flattening ratio at the hook of the hamate; palmar bowing of the flexor retinaculum; isolated enlargement of the median nerve in the distal carpal tunnel; and, more recently, changes in nerve echogenicity and vascularity. [7][8][9][10][11] The most consistent finding in the literature, however, is an increase in the median nerve CSA at the level of the pisiform bone. 3,7 There is supporting level A evidence that median nerve CSA at the wrist is reliable and can be offered "as an accurate diagnostic test for CTS."…”

“…Nerve ultrasound is increasingly being adopted into clinical neurophysiology practice for evaluation of CTS. Since the first description by Buchberger et al ., a number of sonographic abnormalities have been reported in individuals with clinical CTS, including: increase in median nerve cross‐sectional area (CSA) at the level of the pisiform bone (carpal tunnel inlet) and hamate; increase in nerve CSA at the level of the pisiform bone compared with the area at the distal radius (swelling ratio) or forearm (wrist‐to‐forearm ratio); increase in the flattening ratio at the hook of the hamate; palmar bowing of the flexor retinaculum; isolated enlargement of the median nerve in the distal carpal tunnel; and, more recently, changes in nerve echogenicity and vascularity …”