Ultrasonographic anatomy of the long thoracic nerve: A reappraisal using high frequency (24-MHz) probe

The purpose of this study was to analyze the potential of ultrasound with a high frequency probe (24-MHz) in the assessment of the long thoracic nerve (LTN) and describe ultrasonographic landmarks that can be used for standardization.

Ultrasonography analysis of the LTN was done on 2 LTNs in a cadaver specimen and then on 30 LTNs in 15 healthy volunteers (12 men, 3 women; mean age, 28.8±3.8 [SD] years; age range: 24-39 years) by two independent radiologists (R1 and R2) using a 24-MHz probe. Interrater agreement was assessed using Kappa test (K) and intraclass correlation coefficient (ICC).

In the cadaver, dissection confirmed that the India ink was injected near the LTN in the middle scalene muscle. In volunteers, visibility of the LTN above the clavicle was highly reproducible for the branches arising from C5 (R1: 87% [26/30]; R2: 90% [27/30]; K=0.83) and from C6 (R1: 100% [30/30]; R2: 97% [29/30]; K=0.94). Where the nerve emerged from the middle scalene muscle, the mean diameter was 0.85±0.24 (SD) mm (range: 0.4-1.6mm) for R1 and 0.9±0.23 (SD) mm (range: 0.4-1.7mm) for R2 (ICC: 0.96; 95% CI: 0.92-0.98%). Along the thoracic wall, where LTN run along the lateral thoracic artery, the mean diameter was 0.83±0.19 (SD) mm (range: 0.5-1.27mm) for R1 and 0.89±0.21 (SD) mm (range: 0.6-1.2mm) for R2 (ICC: 0.86; 95% CI: 0.72-0.93%).

The LTN can be analyzed with ultrasound using high-frequency probe by using the C5 and C6 roots, the middle scalene muscle above the clavicle and the lateral thoracic artery on the chest wall as landmarks.