Introduction

Local anesthesia can be a useful adjunct to multimodal analgesic surgical planning and improve postoperative analgesia. Retrobulbar blocks are routinely used for enucleation or evisceration/prosthesis surgeries and may be used by some veterinary ophthalmologists for intraocular surgery (Accola et al. 2006; Myrna et al. 2010). Topical anesthesia is commonly used to facilitate ocular examination in the conscious small animal patient. Small animal patients typically require heavy sedation or general anesthesia to perform regional anesthesia safely for both the operator and the patient. Potential side effects should be considered prior to performing local anesthesia. The possible effects on intraocular pressure are of particular concern, as this may affect the overall outcome of surgery and treatment. Knowledge of regional anatomy and physiology is essential to perform these techniques safely and effectively. Equipment needs are minimal; needles and syringes routinely available in small animal practice are sufficient.

Indications for Use of Ophthalmic Local Anesthesia

Numerous ophthalmic procedures may be performed with adjunctive local anesthesia.

Examples include:

• Ophthalmic examination (application of topical anesthetic and rarely, an auriculopalpebral block unless examining a horse)
• Pre-operative analgesia
• Enucleation
• Evisceration with intrascleral prosthesis
• Corneal and/or conjunctival surgery
• Intraocular surgery where central rotation of the globe is desirable
• Immobilization of the eyelids to enable surgery while maintaining a light plane of anesthesia
• Postoperative analgesia
• As for pre-operative
• Infiltration anesthesia following eyelid surgery

Contraindications

As with performance of all loco-regional blocks, the underlying health of the target tissues should be taken into consideration. Large scale tissue disruption (i.e., infection) will affect onset and uptake of local anesthetic drugs such that there may be an increased chance of systemic toxicity. Central nervous system toxicity is possible with direct injection of local anesthetic into the perineural optic nerve sheath. Due to space limitations required of the lecturers, the author will focus on those blocks most commonly employed in small animal practice and readers are referred to the reference list for additional information.

Retrobulbar Block (RBB)

Regional anesthesia for enucleation, evisceration/prosthesis and/or intraocular surgery.

Equipment

• 1.5-inch 22-gauge spinal needle bent to approximately 20-degree angle, or a retrobulbar needle
• Maximum 2 mL local anesthetic (calculate maximum dose per patient and include other local blocks being performed)
• 5 mL syringe

Technique and Step by Step Procedure

There are now several RBB described in the small animal literature. The author is choosing to describe in detail those drops she has most clinical experience with.

1.  Inferior-temporal palpebral RBB

a.  The lower eyelid hair may be clipped at the injection site or a transconjunctival approach employed. Routine ophthalmic surgical preparation with dilute povidone-iodine solution is performed.

b.  A retrobulbar needle can be used or a 1.5-inch 22-gauge spinal needle can be bent to an approximate 20° angle

c.  Use the lateral canthus and the middle of the lower eyelid as landmarks. The needle should be positioned midway between those two points, along the inferior eyelid at the level of the orbital rim

d.  The needle is directed along the floor of the orbit and then redirected dorsally and towards the nose to reach the apex of the orbit

e.  A slight popping sensation may be detected (piercing of the orbital fascia)

f.  The needle should then be redirected slightly dorsally and nasally toward the orbital apex

g.  Aspiration prior to injection is warranted and strongly recommended

h.  Local anesthetic is injected into the orbit

i.  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

a.  Useful with ocular or orbital neoplasia due to risk of a RBB seeding the tumor to other tissues

b.  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

c.  The surgeon can begin wound closure during this time

d.  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 mm Hg) (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, postoperative 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 non­depolarizing 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 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.  Retrobulbar hemorrhage - 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 postoperative analgesia.

Local Anesthetics Usages

Longer acting local anesthetics such as bupivacaine or ropivacaine are recommended for optimal postoperative 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-gauge needle
• Local anesthetic

Technique

After surgery has been completed but before anesthesia is terminated local anesthetic can be infiltrated into the area using a small gauge needle and syringe. The needle should be inserted into the skin and subcutaneous tissues adjacent to the wound to infiltrate local anesthetic. It may be useful to insert the needle to the hub and inject local anesthetic as the needle is slowly withdrawn. Typically, 2–3 different injection sites will be necessary to adequately infiltrate an upper or lower eyelid in its entirety.

Clinical Tips

Using too large a syringe will increase the pressure required to inject local anesthetic into the subcutis and may result in inadequate levels of anesthetic distribution. Similarly, using too large needle gauge will result in a needle tract that leaks local anesthetic instead of enabling drug diffusion into the eyelids. Expect the eyelids to swell slightly with local anesthetic administration which will distort lid conformation. For this reason, I recommend infiltration of the tissues after any reconstructive surgery is planned (e.g., entropion/ectropion repair). By contrast, if local anesthetic with epinephrine is used, injection of the eyelid margin prior to their removal in an enucleation procedure will lessen the amount of bleeding encountered.

Advantages and Disadvantages

Advantages include improved analgesia in the postoperative period, smoother recovery, decreased need for opioid analgesia, decreased sedation, and faster recovery time. A surgical advantage of infiltrative anesthesia in eyelid surgery is its ‘space-occupying’ effect. This is particularly relevant in patients with a strong spastic component to their entropion which may be exacerbated from surgical manipulation of the eyelids and result in sutures rubbing on the cornea and possible ulceration during recovery from general anesthesia. Infiltration of local anesthetic to slightly evert the eyelids may negate the need to place additional tacking sutures (Giuliano 2008). Disadvantages of infiltration anesthesia include delayed wound healing and wound break down. In the author’s opinion, this is rarely encountered when used as an adjunct for ophthalmic surgery, as the volume of drug injected is small. As with all loco-regional anesthesia, care must be taken to ensure that toxic systemic doses are avoided. When operating on small dogs or cats, if a greater volume of drug is needed (e.g., when performing an enucleation surgery and a RBB, 4-point peritomy block, and regional eyelid infiltration are all planned), dilution of the appropriate local anesthetic dose is recommended using sterile saline.

Topical Anesthesia

Indications

Topical local anesthesia is routinely used in the ophthalmic examination to facilitate applanation tonometry, corneal and conjunctiva! scraping or biopsy, and foreign body removal. Topical anesthesia can also be used to supplement analgesia during corneal conjunctival surgery.

Drugs

Proparacaine/Proxymetacaine

In clinically normal dogs a single drop of 0.5% proparacaine produced corneal anesthesia for approximately 45 minutes with the maximal effects for 15 minutes. A second drop applied 1 minute after initial treatment resulted in an increase of anesthetic effects to 55 minutes with maximum effects for 25 minutes (Herring et al. 2005). Recovery from topical anesthesia is dose dependent, with lower doses having the shortest duration of action (Poise et al. 1978). An increase in duration of anesthesia may be seen in dogs and cats with decreased corneal sensitivity. Decreased corneal sensitivity has been associated with diabetes mellitus (Good et al. 2003), mesaticephalic and brachycephalic skull conformation in dogs and brachycephalic conformation in cats (Blocker, Van Der Woerdt 2001). Duration of anesthetic effects on the domestic shorthair feline cornea induced by a single topical application of 0.5% proparacaine ophthalmic solution is considerably shorter than the reported duration of corneal anesthesia in dogs (Binder, Herring 2006). One drop of 0.5% proparacaine produced maximal anesthesia for 5 minutes and lasted for approximately 25 minutes (Binder, Herring 2006).

Tetracaine

Tetracaine is not commonly used in veterinary medicine as it often causes marked conjunctival hyperemia, chemosis, and pain on application. Proparacaine is generally recommended as an alternative (Bartfield et al. 1994).

Other Topical Anesthetic Agents

Limited options have been available for treatment of ocular pain. Topical use of morphine has been shown to control pain associated with corneal wounds in dogs without systemic side effects or delayed corneal wound healing, but is a controlled drug (Stiles et al. 2003). Nalbuphine, a potent synthetic mixed opiate with some analgesic effects is a non-scheduled drug and might represent a useful alternative to topical morphine. A pilot study evaluating topical nalbuphine compared to oral tramadol in the treatment of corneal pain in dogs suggests that topical nalbuphine is not effective for treating corneal pain (Clark et al. 2011). Five percent diphenhydramine solution has been shown to have an anesthetic effect when administered topically to rabbits (Suffridge et al. 2009).

Advantages and Disadvantages

Certain topical anesthetics will decrease tear production and improve drug bioavailability from the conjunctival sac (Patton, Robinson 1975). Decreased lacrimation will affect results of Schirmer tear test values for at least 45 minutes and possibly longer after the application of 1 drop of 0.5% proparacaine. Schirmer tear tests aimed at evaluating both basal and stimulated tear production should, therefore, be performed prior to the application of topical anesthetic (Hamor et al. 2000). Repeat application of topical anesthetics is toxic to the corneal epithelium and should not be used therapeutically (Herse, Siu 1992; Judge et al. 1997; Nam et al. 2006; McGee, Fraunfelder 2007). Sample acquisition for culture from the corneoconjunctival area is recommended prior to instillation of local anesthetic due to their antimicrobial activity. Finally, storage of proparacaine at room temperature for more than 2 weeks results in a decrease in drug effect; therefore, refrigeration of opened bottles is recommended (Stiles et al. 2001).

Specific Aspects of Loco-Regional Anesthesia of the Eye in Cats

Intravenous administration of lidocaine is commonly considered to be particularly toxic in cats due to the potential of myocardial and CNS depression (Chadwick 1985; Lemke, Dawson 2000). However, numerous studies evaluating the toxic effects of lidocaine do not support the cat as being particularly sensitive to this drug (Pypendop, Ilkiw 2005). Cats are generally of lower body weight than most dogs, thus care must be taken to calculate the correct volume of local anesthetic drug that can be safely administered to the feline patient (O’Brien et al. 2010). In addition to careful dose calculation, as with dogs, aseptic technique should be used and syringes should be aspirated before local anesthetic injection to avoid inadvertent intravascular administration.

Acknowledgements

Special thanks to Doctors Karen Walsh and Ellison Bentley for their contributions to these lecture notes.

References

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2.  Bartfield JM, Holmes TJ, Raccio-Robak N. A comparison of proparacaine and tetracaine eye anesthetics. Acad Emerg Med. 1994;1(4):364–367.

3.  Chadwick HS. Toxicity and resuscitation in lidocaine- or bupivacaine­infused cats. Anesthesiology. 1985;63:385–390.

4.  Clark JS, Bentley E, Smith LJ. Evaluation of topical nalbuphine or oral tramadol as analgesics for corneal pain in dogs: a pilot study. Vet Ophthalmol. 2011;14:1–7.

5.  Giuliano EA. Regional anesthesia as an adjunct for eyelid surgery in dogs. Top Companion Anim Med. 2008;23:51–56.

6.  Herring IP, Bobofchak MA, Landry MP, et al. Duration of effect and effect of multiple doses of topical ophthalmic 0.5% proparacaine hydrochloride in clinically normal dogs. Am J Vet Res. 2005;66:77–80.

7.  Lemke KA, Dawson SO. Local and regional anesthesia. Vet Clin North Am Small Anim Pract. 2000;30:839–857.

8.  Myrna KE, Bentley E, Smith LJ. Effectiveness of injection of local anesthetic into the retrobulbar space for postoperative analgesia following eye enucleation in dogs. J Am Vet Med Assoc. 2010;237:174–177.

9.  Stiles J, Honda CN, Krohne SG, et al. Effect of topical administration of 1% morphine sulfate solution on signs of pain ano corneal wound healing in dogs. Am J Vet Res. 2003;64:813–818.

10.  Stiles J, Krohne S, Rankin A, et al. The efficacy of 0.5% proparacaine stored at room temperature. Vet Ophthalmol. 2001;4:205–207.