SAEVA Congress 2018 Proceedings | 12-15 February 2018 | ATKV Goudini Spa
the junction of the supraorbital process and the orbital rim along the ventral orbital rim . The nerve may be palpated in this position .
Lacrimal nerve block : This blocks sensory innervation [ Lacrimal nerve , CN V ] of the temporal ( lateral ) canthus and the lateral 20 % of the upper eyelid . The lacrimal nerve is reached by directing the needle medially along the dorsal rim of the orbit just medial of the lateral canthus . Three millilitres anaesthetic is injected just under the rim of the orbit .
Infratrochlear nerve block : This blocks sensory innovation [ Ophthalmic branch of CN V ] to the medial canthus and associated upper and lower eyelids , third eyelid , caruncle and part of the conjunctiva . The landmark is a small notch or irregularity of the upper rim of the orbit near the medial canthus . The anaesthetic is injected slightly rostral to this notch .
RETROBULBAR ANAESTHESIA
Retrobulbar nerve blocks are performed for surgical procedures ( enucleations , corneal surgery ) in sedated large animal patients where general anaesthesia is not desirable , and as an adjunct to general anaesthesia , especially in horses . It facilitates surgical procedures in horses by proptosing the globe and allowing a lighter plane of general anaesthesia without ocular movement . When performed correctly this will block the following nerves : Optic ( II ), causing temporary blindness , Oculomotor ( III ), Trochlear ( IV ), Ophthalmic and Maxillary branches of the Trigeminal ( V ) and Abducent ( VI ).
It will result in anaesthesia of the globe and eyelids , mydriasis and transient vision loss . Potential complications include penetration of the optic nerve or posterior globe , retrobulbar haemorrhage or intrathecal injection of anaesthetic which may result in death . It also has the potential to increase intraocular pressure and should not be used in patients with deep or perforating corneal ulcers . There are basically two techniques used in horses :
1 . Four point block : 5-10ml of Lignocaine is injected into each quadrant of the muscle cone . The dorsal and ventral quadrants may be injected by placing the needle through the lid or directly through the conjunctiva . The medial and lateral quadrants are injected through the conjunctiva . Failure to inject into the muscle cone may cause the drug to migrate under the conjunctiva . One can also use a curved 20G spinal needle . The three and nine O ’ clock positions should be avoided as the main blood supply to the globe is in this location .
2 . Alternatively a retrobulbar block can be done . Shave and prep the skin over the supraorbital fossa just posterior to the posterior aspect of the dorsal orbital rim . Insert a 9cm , 22 gauge spinal needle through the skin directing the needle towards the base of the orbital cone . A slight deviation of the globe is felt just prior to penetrating of the extraocular muscle cone . A popping sensation is experienced as the needle penetrates the orbital cone . Ten to 15ml of Lignocaine is injected . This 4-point block is easier to perform than the retrobulbar nerve block , but will only block the extraocular muscles and will cause more tissue
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