Topical Anesthesia
A number of local anesthetic formulations are available for topical
anesthesia ( Table 14-9
),
with lidocaine, dibucaine, tetracaine, and benzocaine being the drugs most commonly
used. In general, these preparations provide effective, but relatively short durations
of analgesia when applied to mucous membranes or abraded skin. In addition, lidocaine
and tetracaine sprays have been used for endotracheal anesthesia before intubation.
The topical anesthetic formulation EMLA, which is a eutectic mixture of 2.5% lidocaine
base and 2.5% prilocaine base, is widely used for cutaneous analgesia through intact
skin.[88]
Clinical studies have demonstrated that
this preparation can decrease the pain associated with the percutaneous insertion
of intravenous needles and cannulas.[89]
TABLE 14-9 -- Various preparations for topical anesthesia
|
Anesthetic Ingredient |
Concentration (%) |
Pharmaceutical Application Form |
Intended Area of Use |
|
Benzocaine |
1–5 |
Cream |
Skin and mucous membrane |
|
20 |
Ointment |
Skin and mucous membrane |
|
20 |
Aerosol |
Skin and mucous membrane |
|
Cocaine |
4 |
Solution |
Ear, nose, throat |
|
Dibucaine |
0.25–1 |
Cream |
Skin |
|
0.25–1 |
Ointment |
Skin |
|
0.25–1 |
Aerosol |
Skin |
|
0.25 |
Solution |
Ear |
|
2.5 |
Suppositories |
Rectum |
|
Lidocaine |
2–4 |
Solution |
Oropharynx, tracheobronchial tree, nose |
|
2 |
Jelly |
Urethra |
|
2.5–5 |
Ointment |
Skin, mucous membrane, rectum |
|
2 |
Viscous |
Oropharynx |
|
10 |
Suppositories |
Rectum |
|
10 |
Aerosol |
Gingival mucosa |
|
Tetracaine |
0.5–1 |
Ointment |
Skin, rectum, mucous membrane |
|
0.5–1 |
Cream |
Skin, rectum, mucous membrane |
|
0.25–1 |
Solution |
Nose, tracheobronchial tree |
|
EMLA |
Lidocaine 2.5 |
Cream |
Intact skin |
|
Prilocaine 2.5 |
|
|
|
TAC |
Tetracaine 0.5 |
Solution |
Cut skin |
|
Epinephrine 1:200,000 |
|
|
|
Cocaine 11.8 |
|
|
|
LET |
Lidocaine 4 |
Solution |
Cut skin |
|
Epinephrine 1:200,000 |
|
|
|
Tetracaine 0.5 |
|
|
|
EMLA, eutectic mixture of lidocaine and prilocaine; LET, lidocaine-epinephrine-tetracaine;
TAC, tetracaine-epinephrine-cocaine. |
|
Modified from Covino B, Vassallo H: Local Anesthetics:
Mechanisms of Action and Clinical Use. Orlando, FL, Grune & Stratton, 1976. |
In addition, EMLA has been successfully used for cutaneous anesthesia in skin-grafting
procedures.[90]
This preparation must be applied
under an occlusive bandage for 45 to 60 minutes to obtain effective cutaneous anesthesia;
longer application times increase the depth and reliability of skin analgesia. EMLA
appears to be quite safe in neonates, and methemoglobinemia is exceedingly uncommon
after the application of prilocaine. EMLA is more effective for newborn circumcision
than placebo is, but less effective than dorsal penile nerve block.[91]
Several alternative topical local anesthetic formulations are also in use, including
tetracaine gel[92]
and liposomal lidocaine. Physical
methods to accelerate local anesthetic transit across skin are also under study,
including iontophoresis, local heating, and electroporation.
Topical anesthesia through cut skin is commonly used in pediatric
emergency departments for liquid application into lacerations that require suturing.
Historically, such anesthesia has been provided by a mixture of tetracaine, epinephrine
(adrenaline), and cocaine, known as TAC. TAC is usually supplied as 0.5% tetracaine,
1:2,000 epinephrine, and 10% to 11.8% cocaine, although studies suggest that more
dilute concentrations may be almost equally effective and less likely to cause toxicity.
The generally recommended safe maximum dose is 3 to 4 mL for adults and 0.05 mL/kg
for children. TAC is ineffective through intact skin; in contrast, it can be absorbed
rapidly from mucosal surfaces and lead to toxic reactions. There is a report of
a fatal reaction after application to a nasal laceration, with presumed dripping
into the mouth and rapid mucosal absorption.[93]
[94]
Because of concern regarding cocaine toxicity and the potential
for diversion and abuse, several groups have investigated alternative cocaine-free
topical preparations. Tetracaine-phenylephrine and lidocaine-epinephrine-tetracaine
(LET) preparations are as effective as TAC.[95]
Over the past 5 years, non-cocaine-containing formulations, especially LET, have
largely supplanted TAC. Similarly, cocaine has in the past been widely administered
by otolaryngologists as a solution or aerosol into the nasal passages because it
provides both mucosal analgesia and vasoconstriction. In recent years, cocaine has
been increasingly replaced for nasal application by combined use of an adrenergic
agonist (oxymetazoline or phenylephrine)
and a local anesthetic such as 2% to 4% lidocaine; more dilute solutions are recommended
for infants and children. Systemic absorption of phenylephrine can cause severe
hypertension and reflex bradycardia; oxymetazoline is associated with much less systemic
effect and has a considerably wider margin of safety.
