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.