Carpal Tunnel Syndrome
KEY FACTS
Terminology
Imaging
- • More swollen median nerve means more likely possibility of carpal tunnel syndrome (CTS)
- • Measure nerve cross-sectional area (CSA) at forearm, immediately proximal to tunnel, tunnel inlet, tunnel outlet, & distal to tunnel outlet (CSAc)
- • 2 main measurement methods
- • Largest CSA
- • Relative increase in nerve CSA
- • Additional US signs of CTS have reduced discriminatory power if used in isolation, though may improve discrimination when used with CSA measurements
Intraneural hyperemia
Retinacular bowing
Flattening ratio: Width:depth ratio > 3 of median nerve ↑ with CTS
Homogeneously hypoechoic with loss of normal fascicular pattern
If either CSA proximal to or distal to carpal tunnel is > 14 mm² or palmar volar bowing of flexor retinaculum at outlet > 1 mm, sensitivity, specificity & accuracy are 100%, 84%, and 93%, respectively
deep to the flexor retinaculum 
& superficial to the flexor tendons. The nerve usually divides just beyond the tunnel outlet.
lies immediately below the flexor retinaculum 
& superficial to the flexor digitorum tendons, which are surrounded by the ulnar bursa 
& the flexor pollicis longus, which is surrounded by the radial bursa 
. The extrinsic ligaments 
lie above the carpal bones.
. A nerve cross-sectional area (CSA) > 14 mm² is a positive predictor for CTS. Note also the loss of neural fascicular structure, which is an additional sign of CTS.
IMAGING
General Features
Ultrasonographic Findings
- • Increase in nerve caliber is, by far, most reliable US criterion of CTS
Swelling of median nerve either immediately proximal to, within, or immediately distal to carpal tunnel
- – Proximal: Proximal to TCL
- – Inlet: Between scaphoid tubercle and pisiform
- – Outlet: Between trapezium tubercle and hook of hamate
- – Distal to carpal tunnel: Distal to TCL
- – More swollen median nerve means more likely possibility of CTS
- – Rigid criteria not useful in clinical practice as median nerve compression is continuum rather than “all or none” phenomenon
Measure CSA by tracing around nerve perimeter
2 main measurement methods based on median nerve CSA, either of which can be used
- – Largest CSA of nerve
Median nerve CSA measured at 4 points: Immediately proximal to tunnel inlet, at tunnel inlet, at tunnel outlet, & distal to carpal tunnel
Optimal cutoff point: Median nerve CSA ≥ 14 mm²
Median nerve CSA ≥ 14 mm² at proximal or distal to carpal tunnel: Strong positive predictor of CTS (sensitivity/specificity/accuracy > 88%)
- – Relative increase in nerve CSA
Median nerve CSA measured at proximal 1/3 of pronator quadratus muscle level (CSAf) & immediately proximal to tunnel inlet, at tunnel inlet, & at tunnel outlet (CSAc)
Difference between pronator quadratus level measurement (CSAf) & any carpal tunnel measurement (CSAc) should not be > 4 mm²
Difference between CSAc & CSAf (ΔCSA) of > 4 mm² is strong positive predictor of CTS
- – Largest CSA of nerve
- • Additional US signs of CTS
- • Other US features of CTS may ↑ diagnostic accuracy when combined with CSA measurements, though, in isolation, each has lower discriminatory power
- • Secondary CTS mainly due to mass effect within carpal tunnel
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