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• If significant resistance is encountered, draw back and redirect slightly (as the perineural sheath may have been penetrated or the needle may be abutting the bony orbit)
2. Splash block of the orbit
• Useful with ocular or orbital neoplasia due to risk of a RBB seeding the tumor to other tissues
• After the globe and orbital tissue have been removed and the orbit has been well flushed, local anesthetic is deposited into the orbit and left in-situ for 3-5 minutes
• The surgeon can begin wound closure during this time
• The authors recommend not to subsequently flush the orbit after introduction of local anesthetic to avoid dilution and possible alteration of the splash block effects and duration
Clinical tips
Aspiration before injection of local anesthetic will help decrease the risk of intravascular injection. Assessing the resistance to injection will also decrease the chance of perineural injection. The pressure generated by injection into the optic nerve sheath or intrascleral injection is three to four times that produced by injection into the retrobulbar adipose tissue (i.e. 135 vs. 35 mmHg) (Wang et al. 1989).
The inferior-temporal palpebral block appears to yield consistent distribution of injectate and is clinically applicable. No significant difference in intraocular pressure between treated and non-treated eyes was observed nor were any complications associated with retrobulbar injection found (Accola et al. 2006). Use of a retrobulbar needle will mitigate the need to bend a spinal needle prior to performing a retrobulbar block.
Advantages
If bupivacaine or ropivacaine is used, post-operative analgesia will extend for approximately 4-6 hours. Use
of a RBB reduces ocular movement during surgery and will enable the patient to be comfortably maintained under general anesthesia with less inhalational drug concentrations. A RBB will decrease the need for non- depolarizing muscle relaxants and intermittent positive pressure ventilation and further reduce the potential complications associated with these drugs (Hazra et al. 2008). The inferior-temporal palpebral technique appears to have minimal morbidity associated with its use.
Complications
Failure to direct the needle into the appropriate site will result in inadequate anesthesia and analgesia of the area. Other complications that may occur with a RBB include the following:
1. Inadvertent penetration of the globe - This may lead to catastrophic consequences for the eye, however is
An Urban Experience
unlikely to occur with careful technique, and if the eye is slated to be removed, likely the only relevant problem is potential damage to globe histopathology.
2. Intravascular injection - Failure to aspirate prior to injection may result in local anesthetic injection into the ophthalmic artery
3. Intrathecal injection -If the optic nerve is directly injected, local anesthetic may come into contact with the subarachnoid space due to its close association with the meninges. Careful attention to the pressure required to inject local anesthetic is warranted and if the veterinarian encounters resistance, the needle should be redirected.
4. Retrobulbarhemorrhage - due to puncture of the vessels around the globe.
5. Optic nerve damage / extraocular muscle myopathy / other neuropathy - due to penetration of the perineural sheath with the needle or toxic effects of the local anesthetic agent.
6. Proptosis and/or subsequent exposure keratitis - more common when large volumes of local anesthetic are used. Brachycephalic breeds may be predisposed due to shallow orbits.
Infiltration anesthesia
Indications
Infiltration of the eyelids can be used as an adjunct to any eyelid surgery or enucleation surgery. Ophthalmic surgical procedures that may benefit from infiltration anesthesia include canthoplasty, entropion/ectropion repair, wedge resection, laceration repair, extensive eyelid reconstructive procedures (e.g. lip to lid, modified Kuhnt-Szymanowski procedure), enucleation surgery as an adjunct to a retrobulbar block (Accola et al. 2006).
It may also help to decrease post-surgical entropion by slight eversion of the eyelids. Smoother recovery from anesthesia may be seen due to improved post-operative analgesia.
Local anesthetics usages
Longer acting local anesthetics such as bupivacaine or ropivacaine are recommended for optimal post-operative pain control. Mepivacaine and lidocaine have shorter durations of action and are of limited use in this clinical setting. Epinephrine can be added to the local anesthetic to increase duration of action but this must be weighed against the possible decrease in blood flow.
Equipment
• 3 – 5 mL syringe
• 25 – 27 guage needle • Local anesthetic
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