title: Ultrasound-guided perineural injection of the tibial nerve in the horse versus a blind technique
Creators: name: Bellitto, Nicholas Alberto
Publisher: name: University of Glasgow
Year: 2024

description: Background: Proficiency in performing tibial perineural analgesia is an essential skill for clinicians carrying out lameness examinations in horses, allowing accurate localisation of the source of pain. Blind tibial perineural analgesia, however, often fails to provide reliable and prompt onset of analgesia despite the superficial location of the tibial nerve. The most common causes of failure include erroneous subcutaneous injection without penetration of the superficial crural fascia, erroneous intramuscular injection of the lateral deep digital flexor muscle or intravascular injection of the caudal root of the saphenous and caudal femoral veins. To overcome these difficulties ultrasound (US)-guided techniques for tibial perineural analgesia have recently been described and evaluated in cadaver studies but data supporting the use of US-guided tibial perineural analgesia clinically remains.

Objective: To compare US-guided and blind tibial perineural analgesia techniques in lameness investigation.

Materials and Methods: This study describes a randomised, prospective clinical trial. All cases were horses presented for lameness investigation which required tibial perineural analgesia. The cases were randomly assigned to US-guided or blind tibial perineural analgesia. Perineural analgesia was performed at the caudomedial aspect of the distal crus, about 10 cm proximal to the tuber calcanei between the common calcaneal tendon and the lateral deep digital flexor muscle. Injections were performed with the limb bearing weight and using mepivacaine hydrochloride 2% (w/v). Blind tibial perineural injections were performed after the nerve had been palpated with the limb in a non-weightbearing position. US-guided injections were performed using an 8-12 MHz linear transducer which was placed in a transverse orientation; the needle was inserted caudal to the nerve and redirected during injection to allow distribution of the anaesthetic agent around the nerve (single skin penetration). Onset of tibial perineural analgesia was assessed by testing loss of skin sensation at the medial and lateral heel bulbs, which were selected as autonomous zones (dermatomes) of the tibial nerve and following a review of the literature. Loss of skin sensation was assessed by measuring the mechanical nociceptive threshold (MNT) of each skin location using a hand-held algometer with a 1 mm diameter pin. A MNT value of 25 Newton (N) would indicate complete loss of skin sensation (MNT values for this specific pin had been previously validated). Skin sensation was assessed, prior to injection and then at four intervals postinjection (10–15, 20–25, 30–35 and 40–45 minutes). At each recording, 3 measurements were performed for each skin location and the mean value was used for analysis. The time taken to perform each injection technique and any adverse reactions were recorded (e.g. horse that snatched the limb away or kicked out). Summary statistics were performed to examine differences between groups. The frequency of skin desensitisation was compared between groups using a Fisher's exact test and the length of time taken to perform injections was compared using a Mann–Whitney U test.

Results: Sixteen US-guided and 11 blind injections were included in the study. All cases undergoing US-guided injection lost skin sensation, whereas this occurred in only one case receiving the blind injection. The US-guided group had a significantly higher probability of skin sensation loss (p &lt; 0.001), although the injection technique took significantly longer to complete compared to the blind group (p &lt; 0.001). No adverse reactions were noted with either perineural injection technique.

Conclusions: The US-guided technique described here resulted in a significantly higher percentage of cases with tibial nerve analgesia compared to cases undergoing the blind technique. No differences in patient tolerance and operator safety were observed between the injection techniques. The US-guided technique was straightforward to perform and resulted in complete tibial nerve analgesia within 30-35 minutes in all patients. The findings of this study suggest that the US-guided technique, therefore, should be used instead of the blind technique during lameness investigation when possible.