Maxillary nerve block within the equine pterygopalatine fossa with different volumes: practicability, efficacy and side-effects

Summary : The maxillary nerve block within the pterygopalatine fossa is a commonly used technique in equine dental surgery. The objective of the present study was to determine the risks of the maxillary nerve block within the pterygopalatine fossa using two different doses of lidocaine and to evaluate methods for testing adequate analgesia of teeth and gingiva. In a preliminary experiment ten horses were trained to tolerate a mouth gag without sedation. A mechanical and thermal stimulation was applied on the palatal gingiva and the occlusal surface. Thermal as well as mechanical stimulation resulted in a reproducible defense reaction in all horses. The main study was designed as a prospective, randomised, blinded, experimental trial. Ten clinically healthy horses were used in a randomised cross-over design. Horses were sedated with 0.02–0.03mg/kg detomidine iv. A maxillary nerve block was performed on one side of the head with the previously described “Extraperiorbital-Fat-Body-Insertion” (EFBI) technique using either 2ml/100 kg BW lidocaine (group low) or 4ml/100 kg BW lidocaine (group high). A Schirmer tear test was performed before and after nerve block, on both sides of the head (blocked and non-blocked side). Analgesia of the teeth and the surrounding gingiva was assessed by applying mechanical and thermal stimuli on both sides of the head (blocked and non-blocked side) 10 and 25 minutes after lidocaine injection. Heart rate, respiratory rate, sweating, and change of the pupil size were recorded before and in 5 minute intervals after lidocaine injection for 35 minutes. No side effects were observed in 16 out of 20 experiments. Four horses showed blepharospasm, miosis, swollen palpebrae, superficial corneal ulcer, or uveitis. Symptoms disappeared completely after symptomatic treatment. Lacrimation was reduced after nerve block, even though no statistically significant differences were found for lacrimation rate before and after nerve block. Analgesia of teeth and gingiva was not assessable with thermal and mechanical stimuli in the sedated horse. The used dosage of lidocaine did not influence any of the recorded clinical parameters. Although the pterygopalatine fossa houses delicate anatomical structures, the EFBI-technique appears to be a safe method for blocking the maxillary nerve. It is recommended to apply lubricating ointments to keep the cornea moist and therefore avoid negative side effects on the eye.