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E Q U I N E D E N T I S T RY — A N I N T R O D U C T I O N
Nerve blocks of the equine head
Introduction
Local and regional anaesthesia of the equine head in combination with sedation and analgesia provides the
equine practitioner with a number of benefits, it is relatively simple to perform with minimal time, expense and
equipment required. The removal or reduction of painful stimuli associated with procedures in the head region
results in the following. Patients are easier to examine and perform procedures on resulting in improved working
conditions and outcomes. Regional anaesthesia can provide diagnostic information often allowing better
prognostic information for owners. The use and application of regional anaesthesia can reduce dose rates for
systemic sedatives and an increased number of procedures can be undertaken in the standing patient thereby
potentially reducing the costs and risks involved with general anaesthesia and patients that do require a general
anaesthetic can be maintained at a lighter plane of anaesthesia. 1,2,3 If effective preoperative analgesia is achieved
through the use of these techniques then post operative pain can be reduced.
Nerve supply to the head region
A number of nerves supply innervation to the head region of the horse, the nerves of interest for this article are
the trigeminal nerve (CNV) and its branches and the Facial nerve(CNVII). CN V is a sensory nerve supplying
innervation to the teeth, gingiva, and lips while CNVII is predominantly a motor nerve with some sensory
function supplying the muscles of facial expression.
Trigeminal nerve
The major sensory innervation of the head is the trigeminal nerve (CN V) which has three main branches:
ophthalmic, maxillary, and mandibular. The maxillary branch enters the caudal maxilla ventral to the orbit via
the maxillary foramen and runs thru the maxilla giving off branches that supply the maxillary cheek teeth. The
nerve exits at the infraorbital foramen which lies just rostral and dorsal to the facial crest.
The mandibular branch runs medially along the mandible branching into smaller nerves including the inferior
alveolar nerve which innervates the mandibular cheek teeth. The nerve exits the mandible at the mental foramen
on the rosrolateral aspect of the mandible in the diastema.
Local anaesthetic agents
Local anaesthetic agents block nerve conduction in all types of neurons, including pain, sensory, motor,
proprioceptive, and sympathetic nerve fibres. The minimum concentration of local anaesthetic necessary to block
conduction is higher for motor nerve fibres than for sensory fibres, so sensory anaesthesia can occur without
muscle blockade. Several agents are commonly used these include lignocaine and mepivicaine . Each with there
own duration of action. Local anaesthetics can be administered at specific sites (local block), near nerves
(regional block), topically, and by infusion. Local anaesthetics produce analgesic, antiarrhythmic, antishock,
central nervous system depressant, anesthetic sparring, mild antiinflammatory, and gastrointestinal promotility
effects. Symptoms of toxicity in horses include bradycardia, heart block, hypotension, delirium, seizures, and
respiratory and cardiac arrest. These effects are exaggerated in hypoxemic and acidotic horses. These can occur
as a result of intravascular injection or entry into the bloodstream through excessive perfusion. Local tissue
sloughing can occur in thin skinned breeds such as arabs and thoroughbreds.