Ultrasound Imaging of the Sural Nerve: Ultrasound Anatomy and Rationale for Investigation

Abstract

Background Damage to the sural nerve (SuN) may arise from surgical stripping or thermal ablation of the small saphenous vein (SSV). Objective This study aims to demonstrate that visualisation of the SuN and its point of contact with the SSV (‘risk point’) using ultrasound imaging can be achieved in routine clinical practice. Type of study This is a cohort study. Patients Fifteen normal subjects and five patients with chronic venous insufficiency (CVI) (two with a dilated, incompetent SSV). Method The SuN was identified using high-resolution ultrasound imaging using 14- and 18-MHz probes. Two manoeuvres were found to improve visualisation: (1) the contrast of the nerve was increased compared with the other tissues by varying the angle of insonation; and (2) the transducer was moved up and down the limb for a short distance during transverse imaging of the calf. The muscles and other soft tissues appeared ‘out of focus’, whereas the SuN retained both shape and echogenicity. Once the nerve has been identified, proceeding proximally, the point of separation of the two components is often detectable. It is then possible to follow the two different nerves observing the medial sural cutaneous nerve (MSCN) inside the ‘triangle’ of connective tissue below the SSV joining the tibial nerve and the lateral sural cutaneous nerve (LCSN) joining the common peroneal nerve, which runs inside a tiny fascial duplication. The extent of nerves, which were identified, was recorded in each limb as well as their anatomical distribution. Results The SuN and the point at which it might be at risk were identified on ultrasound images in 39 of 40 limbs (97%) studied. In transverse section, it was readily identified within the saphenous compartment. It lies in close proximity to the SSV only in the distal third of the limb, where the two components of the nerve: MSCN, a branch of the tibial nerve; and LSCN, a branch of the common peroneal nerve join together. The relationship between the SuN and the SSV is very variable, with the nerve running separately or in close contact with the vein for variable distances, in many different combinations. Conclusions The SuN and ‘risk point’ can be identified by ultrasonography (US). We propose that this technique could be used to prevent damage to the SuN during surgical or thermal ablation of the SSV and during Achilles tendon surgery.