Blind versus ultrasound-guided maxillary nerve block in donkeys

To describe the 'blind' and ultrasound-guided approaches to block the maxillary nerve in donkeys. To compare the success and complication rates between the 'blind' and ultrasound-guided techniques based on staining of nerves and other structures in cadavers and assessing level of analgesia in live animals.

Prospective anatomical and experimental study.

Eighteen cadaver heads and nine adult live donkeys.

Phase 1: the anatomical characteristics of the maxillary nerve and its related structures were investigated within the pterygopalatine fossa in five cadavers. Phase 2: 0.1 mL of methylene blue dye was injected blindly and via ultrasound guidance in 13 cadavers to stain the left and right maxillary nerves, respectively. Nerve staining and dye spreading were evaluated through cadaver dissection. Phase 3: the former procedures were applied in nine live donkeys using lidocaine hydrochloride 2% and the onset of analgesia was verified through needle pricking at the naris.

Ultrasound-guided deposition of methylene blue dye in cadavers and lidocaine injection in live animals were successful in all instances (accuracy = 100%) without inadvertent vascular penetration. Using the 'blind' technique, misdirection and intravascular deposition of dye were reported in four cadavers (accuracy = 69.2%) and neurovascular trauma was observed in live donkeys (five cases). Loss of cutaneous sensation in the ipsilateral naris was earlier in the ultrasound-guided approach (10.9 ± 1.8 minutes) than in the 'blind' technique (27.8 ± 3.2 minutes; p < 0.001).

An ultrasound-guided maxillary nerve blockade proved very practical and can be used to block the maxillary nerve with a high degree of accuracy while avoiding vascular penetration. Further studies are mandatory to validate its analgesic effectiveness in clinical situations.