Validation of the accuracy of needle placement as used in diagnostic local analgesia of the maxillary nerve for investigation of trigeminally mediated headshaking in horses.

Diagnostic local anaesthesia of the maxillary nerve is a valuable aid in the diagnosis of trigeminally mediated headshaking in horses. Our objective is to validate the accuracy of needle placement in this procedure and to identify any correlation between accuracy of the technique and operator experience. Using a small volume of contrast medium, the procedure was performed bilaterally on 30 horse cadaver heads by three groups with different levels of experience with the technique. The location of deposition was then identified using computed tomography (CT). Contrast medium was deposited around the target site in 53.3% (32/60) of injections. An experienced operator succeeded in deposition around the target area significantly (p<0.05) more often (80%, 16/20) than did the less and non-experienced performers (40%, 16/40). A negative response to diagnostic local anaesthesia of the maxillary nerve does not disprove facial dysaesthesia as the cause of headshaking in that horse as a false negative response could arise due to failure to deposit local anaesthetic around the target area. Increased experience in performing the procedure decreases the probability of false negative results.

Peripheral nerve stimulation under ultrasonographic control to determine the needle-to-nerve relationship.

OBJECTIVE: To determine the needle-to-nerve distances during electrical nerve location in dogs at different currents and pulse duration using a peripheral nerve stimulator (PNS) under ultrasound control (US), and the minimal electrical thresholds (MET) necessary to obtain a motor response (MR) after achieving needle-to-nerve contact. STUDY DESIGN: Prospective in vivo experimental trial in a clinical setting ANIMALS: Thirty dogs, scheduled for locoregional anaesthesia of the sciatic nerve. METHODS: Needle-to-nerve distance was measured ultrasonographically after obtaining the MR of sciatic nerve with 2, 1 and 0.5 mA and pulse duration 0.1 ms (NS0.1). Thereafter the needle was placed in contact with the nerve and MET was determined. The procedure was repeated with 0.3 ms (NS0.3). Finally the needle was reintroduced to contact the sciatic nerve guided only by US, thus MET-US was determined. Data were analysed using Kruskal-Wallis or Mann-Whitney tests. RESULTS: Needle-to-nerve distances were greater when MR was obtained with 2 mA than with 1 and 0.5 mA at 0.1 and 0.3 ms. No significant differences were observed between the needle-to-nerve distances using 0.1 or 0.3 ms. The MET [median (range)] was 0.4 (0.18-1.3) mA in NS0.1, 0.32 (0.12-0.8) mA in NS0.3; while MET-US was 0.7 (0.32-1.5) mA. When the needle contacted the nerve, the MR achieved with currents below 0.3 mA was obtained in 17.2, 40 and 0% of cases using NS0.1, NS0.3 and US respectively. CONCLUSIONS AND CLINICAL RELEVANCE: The electrical current necessary to obtain a MR decreased as the needle moved towards the nerve. However when the needle tip contacted the nerve, an MR with low current intensity could not be obtained. Thus the absence of motor response at currents below 0.3 mA cannot rule out needle-epineurium contact. When ultrasound is combined with PNS, it is more important to assess the correct needle position than searching for an MR at low currents.