SAEVA Congress 2018 Proceedings | 12-15 February 2018 | ATKV Goudini Spa
DESENSITISING THE EYE FOR EXAMINATION AND SURGERY
There are many combinations of intravenous sedation currently being advocated but in most instances alpha-2 agonists seem to be preferred . This article is not going to deal with that component of the sedation [ see references ] but we will discuss the periorbital nerve blocks that are useful for standing ophthalmic procedures .
Innervation of the eyelids and cornea :
The eyelids are innervated by three of the cranial nerves : oculomotor ( III ), trigeminal ( v ) and facial [ VII ] as well as the sympathetic system . The action of closing the eyelids involves the auriculopalpebral nerve , a branch of the facial nerve to the orbicularis oculi muscles . Sensory innervation to the eyelids is from the ophthalmic and maxillary branches of the trigeminal nerve . The ophthalmic branch subdivides into the frontal , lacrimal and nasociliary nerves . In the horse the frontal nerve passes through the supraorbital foramen as the supraorbital nerve supplying the upper eyelid . The lacrimal nerve , which receives contributions from the zygomatic branch of the maxillary nerve , supplies the temporal region of the upper and lower eyelids . The nasociliary nerve continues forward , becomes the infratrochlear nerve . It supplies the nasal aspect of the upper and lower eyelids , the third eyelid , caruncle , nasolacrimal duct and portions of the conjunctiva . The maxillary nerve branches into the zygomatic nerve which supplies the lower eyelid .
The entire innervation of the cornea is through the long ciliary branches of the ophthalmic division of the trigeminal nerve . These branches are located primarily in the anterior third of the stroma and epithelium . The deeper layers of the stroma are innervated with nerves that are more sensitive to pressure than to pain stimuli .
TOPICAL ANAESTHETICS .
Topical anaesthetic eye drops [ Novesin Wander benoxinate , Covostet tetracaine , Minims Amethocaine amethocaine ] are readily absorbed by the cornea , conjunctiva and mucous membrane of the nasolacrimal duct . Indications for their use include corneal and conjunctival cytology , tonometry , conjunctival biopsies , flushing of the nasolacrimal duct , manipulation of the conjunctiva and third eyelid for example , and to look for and remove foreign bodies . Analgesia is induced within 10-30 seconds after topical administration . The duration of effect after a single application is 10-15 minutes , however more prolonged effects will be produced with reapplication at 30 second intervals . The only common adverse effect of topical anaesthetics is local irritation and epithelial toxicity . Local irritation may last up to 20 seconds . Epithelial toxicity is characterised by superficial punctate lesions , which are more pronounced with prolonged use . They can markedly interfere with mitosis and cellular migration of the epithelium and they remove the protective precorneal tear film . It is because
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