4
Guide to Local
Anesthetic Medications
Vivian Schiedler and Bryan S. Sires
Topical Ocular Anesthetics
Proparacaine or tetracaine drops can be applied to the ocular surface to help minimize the discomfort of subconjunctival injections. Facial prep solutions, bright lights, and repeated opening of eyes intraoperatively to adjust lid height may cause drying and ocular irritation. Treating both eyes helps increase patient comfort and cooperation. Tetracaine is the most potent topical agent1 and can be used directly on the tarsal plate during procedures such as ptosis repair in which anesthetic use is minimized to avoid levator muscle paralysis. Use of topical anesthetic drops should be limited in all patients to prevent iatrogenic corneal epitheliopathy. The postoperative analgesic regimen should never include topical ocular anesthetic agents.
Lidocaine
Lidocaine is the most commonly used local anesthetic. As an amide, it is hepatically metabolized and has a longer duration of action than ester anesthetics, which are locally metabolized by esterases. Lidocaine is available in 0.5, 1, 2, and 4% concentrations with or without epinephrine. The lowest concentration is preferable in infants and young children secondary to the risk of systemic toxicity. It has a rapid onset of action (30–60 seconds) but a relatively short duration (30–120 minutes).1 Its duration of action is shortened without epinephrine since it is the most potent vasodilator of all local anesthetics. The maximum recommended dose of lidocaine with epinephrine is 7 mg/kg, which is equivalent to 50 ml of a 1% lidocaine concentration for a 70-kg person.2 Without epinephrine, the maximum recommended lidocaine dose should be halved.
Bupivacaine
Bupivacaine is also an amide local anesthetic. It is four times as potent as lidocaine and has a longer onset of action (5 minutes) as well as a longer duration of action (120–180 minutes). 1 Therefore, it is commonly
15
16 V. Schiedler and B.S. Sires
mixed with lidocaine in order to provide continuous analgesia during longer procedures as well as some postoperative analgesia. The maximal recommended dose of bupivacaine with epinephrine is 2.5–3 mg/kg.2 This is equivalent to 35–40 ml of a 0.5% bupivacaine concentration for a 70-kg person.
Epinephrine
Local anesthetics cause vasodilation secondary to paralysis of vascular smooth muscle. This can be counteracted by epinephrine. This limits absorption of the local anesthetic into the bloodstream, thereby preventing systemic side effects. Vasoconstriction also increases the local anesthetic duration of action by slowing its removal. Due to its vasoconstrictive effect, epinephrine should be used cautiously in the penis, digits,3 and ears to avoid tissue necrosis.
Epinephrine should be used cautiously in young children and patients with cardiac conditions as it raises heart rate and blood pressure and can cause arrhythmias. The maximal recommended dose for cardiac patients is 0.2 mg in 40 ml of a 1 :200,000 dilution.4,5 The optimal concentration to prolong the duration of local anesthesia is 1 :200,000. Higher concentrations do not significantly enhance anesthetic duration and can increase the risk of side effects. It can be diluted to 1 :400,000 by using a 50 :50 mixture of lidocaine with a 1 :200,000 concentration of epinephrine and bupivacaine without epinephrine. Of note, the epinephrine concentration can be unpredictable in the premixed form and is usually much lower than labeled. For a more reliable 1 :100,000 dilution, one can add 5 ml of 8.4% sodium bicarbonate to a 50-ml bottle of 2% lidocaine and 0.55 ml of epinephrine 1 :1000 (John B. Holds, MD, personal communication).
EMLA
Topical anesthetic creams can increase patient tolerance of local anesthetic injections. EMLA cream is a mixture of 2.5% lidocaine and 2.5% proparacaine applied to intact skin with an overlying occlusive dressing to enhance absorption.6 For minor procedures, including venipuncture or anesthetic injection, EMLA should be applied 1 hour ahead of time. For split thickness skin graft harvesting or laser treatment, it should be applied for 2 hours. Presurgical planning with a prescription and careful instructions on the amount, site, and time of application are necessary for maximal efficacy. The upfront effort involved on behalf of the patient and the staff giving the instructions can outweigh the analgesic benefit of EMLA cream as compared to other quicker analgesic approaches.
Although systemic side effects are rare, bloodstream levels are directly related to area and duration of application and body weight. Local but transient side effects include skin blanching, edema, and erythema. Periocular use is contraindicated since contact with the ocular surface causes severe irritation and requires copious irrigation.
Chapter 4 Guide to Local Anesthetic Medications 17
Other Topical Anesthetics
A number of other topical skin anesthetics are available, including ElaMax, Topicaine, and topical tetracaine gel. These topical agents all have limitations, as well as a maximum surface area that can be treated if toxicity is to be avoided.
The editors have had particular success with Betacaine LA ointment, which is compounded from lidocaine, prilocaine, and phenylephrine and is available from Custom Scripts Pharmacy in Tampa, Florida. This compounded ointment has the advantage of being applied without occlusion and providing superior anesthesia in 10 to 20 minutes.
Bicarbonate
To reduce the pain of local anesthetic injection, bicarbonate 8.4% can be added to the mixture in a 1 :4 to 1 :10 ratio. By buffering the slightly acidic pH of the anesthetic (lidocaine pH is 6.4), pain is significantly reduced without affecting the onset or duration of action of the anesthetic.7,8
Benzyl Alcohol
An alternative to bicarbonate for pain reduction of local anesthetic injection is saline with 0.9% benzyl alcohol. It is a bacteriostatic agent with local anesthetic properties. 9 The authors use a 1 :1 :1 mixture of 2% lidocaine with 1 :100,000 epinephrine, 0.75% bupivacaine, and saline with 0.9% benzyl alcohol for direct infiltration of local periocular and facial anesthesia.
References
1.Benz JD. Injectable local anesthetics. AORN J 1992;55:274–284.
2.Tetzlaff JE. The pharmacology of local anesthetics. Anesthesiol Clin North Am 2000;18:217–233.
3.Krunic AL, Wang LC, Soltani K, Weitzul S, Taylor RS. Digital anesthesia with epinephrine: an old myth revisited. J Am Acad Dermatol 2004;51: 755–759.
4.Brown RS, Rhodus NL. Epinephrine and local anesthesia revisited. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2005;100(4):401–408.
5.New York Heart Association. Use of epinephrine in connection with procaine in dental procedures. J Am Dent Assoc 1955;50:108.
6.Lidocaine and prilocaine. Available at: http://www.rxlist.com/cgi/generic2/ emla.htm. Accessed November 19, 2005.
7.Davies RJ. Buffering the pain of local anaesthetics: a systematic review. Emerg Med 2003;15:81–88.
8.Parham SM, Pasieka JL. Effect of pH modification by bicarbonate on pain after subcutaneous lidocaine injection. Can J Surg 1996;39:31–35.
9.Yuen VH, Dolman PJ. Comparison of three modified lidocaine solutions for use in eyelid anesthesia. Ophth Plast Reconstr Surg 1999;15:143–147.
5
Motor Nerve Blocks in
Oculofacial Surgery
Vivian Schiedler and Bryan S. Sires
Facial akinesia is most useful for intraocular surgery to prevent forceful eyelid closure. It may also be useful in select patients undergoing cosmetic or functional procedures, such as those with hemifacial spasm or blepharospasm.
Facial Nerve Blocks
The Van Lint block prevents eyelid closure by anesthetizing only the distal branches of the facial nerve. The needle is inserted at the intersection between a line drawn parallel to the lateral orbital rim and a line drawn parallel to the inferior orbital rim. Approximately 2 ml of local anesthetic is injected along the lateral orbital rim to affect the temporal branches innervating the upper lid. Another 2 ml is injected along the inferior orbital rim to affect the zygomatic branches innervating the lower lid (Figures 5.1 and 5.2). Because buccal branches may overlap with zygomatic branches to innervate the medial lower eyelid from below, the van Lint approach may not fully block this area. Several advantages of the van Lint block include ease of application, anatomic selectivity of branches that innervate only the eyelids, and a lower risk of iatrogenic facial nerve injury than the more proximal blocks.1,2
The Atkinson block is more proximal than the van Lint. About 5 ml of local anesthetic is administered just inferior to the zygomatic arch and directed over the bone toward the top of the ear as well as in the opposite direction (Figures 5.3 and 5.4). It may not be entirely effective at producing full eyelid akinesia due to the multiple anastomoses between the temporal, zygomatic, and buccal branches of the facial nerve.2
The O’Brien block is the most proximal of the aforementioned facial nerve blocks. Approximately 5 ml of local anesthetic is infiltrated just anterior to the tragus of the ear over the condyloid process of the mandible at a depth of 1.0 cm (Figure 5.5). As with the Atkinson block, there is some degree of unpredictability in achieving full facial akinesia due to anatomic variability of the course of the facial nerve and its major branches.1
18
Chapter 5 Motor Nerve Blocks in Oculofacial Surgery 19
Figure 5.1. A Van Lint block involves two injection tracts. The superior eyelid contributes more to forceful eyelid closure than the inferior lid. Blocking the superior lid is accomplished by injecting local anesthetic along the superotemporal orbital rim to anesthetize the distal branches of the facial nerve that supply the superior eyelid.
Figure 5.2. The inferior eyelid is blocked in Van Lint fashion by injecting local anesthetic along the inferotemporal orbital rim.
20 V. Schiedler and B.S. Sires
Figure 5.3. The Atkinson block is a two-tract procedure. Approximately 2–3 ml of local anesthetic is injected along a linear tract over the zygomatic arch toward the top of the ear.
Figure 5.4. Another 2–3 ml of local anesthetic is injected in a linear tract over the zygomatic arch in the opposite direction as the first tract.