An overview of local anesthetics in over-the-counter products

Over-the-counter (OTC) local anesthetics have historically been used to alleviate pain in several common conditions including toothache and sore throat. With a rise in chronic conditions and an aging population, there has been an increase in associated chronic pain-related disorders. Individuals with chronic pain often seek OTC treatments for quick and accessible pain relief. There are several common OTC local anesthetics, including benzocaine, lidocaine, and dibucaine, which are readily available to patients in several formulations. In order to appropriately advise patients on the use of local anesthetics, it is important to understand their key characteristics, including the mechanism of action, clinical properties, pharmacokinetics, clinical applications, and adverse reactions, which may occur.

Patch-test reactions to topical anesthetics: retrospective analysis of cross-sectional data, 2001 to 2004

BACKGROUND

Allergy to topical anesthetics is not uncommon. The cross-reactivity among topical anesthetics and the screening value of benzocaine alone are not well understood.

OBJECTIVES

The goals for this study were: (1) to evaluate the frequency and pattern of allergic patch-test reactions to topical anesthetics, using North American Contact Dermatitis Group (NACDG) data, and (2) to compare these results to allergen frequencies from other published studies.

METHODS

The NACDG patch-tested 10,061 patients between 2001 and 2004. In this analysis patients were included who had positive patch-test reactions to one or more of the following: benzocaine, lidocaine, dibucaine, tetracaine, and prilocaine.

RESULTS

Of patch-tested patients, 344 (3.4%) had an allergic reaction to at least one anesthetic. Of those, 320 (93.0%) had an allergic reaction to only one topical anesthetic. Overall, reactions to benzocaine (50.0%, 172 of 344) were most prevalent, followed by reactions to dibucaine (27.9%, 96 of 344); however, reactions to dibucaine were significantly more frequent in Canada than in the United States (relative risk [RR], 2.31; 95% confidence interval [CI], 1.67-3.20; p < .0001). Of patients reacting to more than one anesthetic, most (79%, 19 of 24) reacted to both an amide and an ester.

CONCLUSIONS

Of the topical anesthetics tested, benzocaine was the most frequent allergen overall. Over 50% of allergic reactions to topical anesthetics in this study would have been missed had benzocaine been used as a single screening agent. Cross-reactivity patterns were not consistent with structural groups.

[Glucose attenuating local anesthetic-induced hemolysis]

BACKGROUND

The effect of glucose on local anesthetic-induced neural damage has not been fully studied. We examined the effect of glucose on hemolysis induced by local anesthetics.

METHODS

The mean EC50 values (the local anesthetic level that causes destruction of half of the red blood cells in vitro) of lidocaine HCl, tetracaine HCl and dibucaine HCl were determined with 0% and 7.5% glucose contained in Krebs solution at pH 6.4.

RESULTS

The mean EC50 values of lidocaine HCl, tetracaine HCl, and dibucaine HCl in 0%-glucose Krebs solution were 6.51%, 0.45%, 0.17%, respectively, which increased significantly to 7.05%, 0.64% and 0.23%, in 7.5% glucose Krebs solution at pH 6.4.

CONCLUSIONS

Glucose may have a protective role in local anesthetic-induced neural damage.

[Cauda equina syndrome following intrathecal dibucaine]

We report three cases of cauda equina syndrome following spinal anesthesia with dibucaine. In two cases, the lumbar puncture was repeated and additional doses of dibucaine were administered to obtain adequate sensory blockade. In the last case, spinal anesthesia worked well with single injection of dibucaine. In all cases patients complained of varying degrees of bladder and bowel dysfunction, perineal sensory loss and lower extremity motor weakness on the next day, and the diagnosis of cauda equina syndrome was made. With only one case, the symptom disappeared four months later, but the rest of the patients suffered from sensory disturbance and defecation for more than four months after the surgery. One possible cause is a direct neurotoxic effect of high concentration dibucaine due to its maldistribution within the subarachnoid space. We have to consider the neurotoxicity and dose of the local anesthetic for obtaining a safer method and for preventing this complication.

Spinal anesthesia hypotension in elective cesarean section in parturients wearing extra-strong compression stockings

We have routinely applied an extra-strong graduated compression stocking to cesarean section patients to reduce the incidence of spinal anesthesia hypotension. Because bupivacaine has recently become available in Japan, we compared the incidence of spinal hypotension using either 2.0 ml of hyperbaric 0.3% dibucaine or 0.5% bupivacaine. There were 98 full-term parturients wearing the stocking who received 2.0 ml injection of dibucaine or bupivacaine for elective cesarean section. When systolic blood pressure decreased to 90-100 mm Hg or to less than 70% of the pre-anesthesia value, ephedrine was injected intravenously. There was no significant difference in systolic blood pressure or heart rate during spinal anesthesia between the dibucaine and bupivacaine groups. Although the demographic data and various data related to anesthesia or surgery were similar in the groups, the fluid volume and the dose and incidence of ephedrine injection during anesthesia showed significant differences: the mean dose was 3.6 and 1.5 mg and the incidence was 41% and 19% in the dibucaine and bupivacaine groups, respectively. Spinal anesthesia using bupivacaine results in a lower incidence of spinal hypotension compared with dibucaine and, in combination with fitting the extra-strong stockings onto both legs, is clinically useful for cesarean sections.

[Anaphylactoid reaction to dibucaine during spinal anesthesia]

A 71-year-old man with a history of allergic rhinitis for 6 years received spinal anesthesia using 2 ml of 0.3% dibucaine for transurethral prostatectomy. Two months previously he had undergone prostate biopsy and cystoscopy under spinal anesthesia with isobaric bupivacaine uneventfully. Forty five minutes after injection of dibucaine he complained of itching in the periorbital area, and developed tremor and muscle rigidity followed by loss of consciousness. Soon after, his blood pressure decreased to 40 mmHg, and erythema appeared over his body. Symptoms were relieved by epinephrine, hydrocortisone and antihistamine agents, but ten minutes after the treatment he again developed hypotension and erythema. Continuous infusion of epinephrine was needed for complete relief of symptoms. An intradermal test with 0.3% dibucaine carried out 6 days after surgery demonstrated a 12 x 8 mm wheal with flare. Although anaphylactic reaction to an amide local anesthetic has been reported to be quite rare, this is the 7th case report of anaphylactic reaction to dibucaine used for spinal anesthesia in Japan.

[Spinal anesthesia with bupivacaine for a patient with a history of severe leg pain after intrathecal dibucaine]

An 87-year-old man was scheduled for the 11th transurethral bladder tumor resection (TURBT). He had a history of non-active syphilis for 21 years, diabetes mellitus for 7 years, and severe emphysema. Preoperative physical examination of the lower extremities, revealed loss of knee-jerk reflex, and loss of vibratory and proprioceptive perception. Four years previously, he underwent TURBT twice under spinal anesthesia with dibucaine, which caused severe leg pain during anesthesia. Therefore, subsequent TURBTs (eight times) were performed under general anesthesia with tracheal intubation, which frequently caused postoperative respiratory distress. Recently, bupivacaine, less neurotoxic than dibucaine, was on the market in Japan for use in spinal anesthesia. Therefore we planned spinal anesthesia using 0.5% bupivacaine, 2.0 ml. This time, he did not complain of leg pain during anesthesia, and postoperative conditions were satisfactory. We can conclude that bupivacaine is very useful for spinal anesthesia especially in patients with a history of leg pain by spinal anesthesia with dibucaine.

[Dibucaine for spinal anesthesia is a probable risk for cauda equina syndrome]

A 64-year-old man was scheduled for transure thral resection of the prostate. The patient's medical history showed borderline diabetic state and two uncomplicated surgeries under spinal anesthesia. Spinal anesthesia was performed at the L 3/4 interspace using hyperbaric 0.24% dibucaine 2.2 ml, which was followed by general anesthesia because the anesthesia level had spread only to the lower left side of the body. On the next day, he complained of difficulty of defecation and urination combined with hypesthesia around the anus, which was diagnosed as cauda equina syndrome. The symptoms had not changed for three weeks. Then, there was a gradual recovery but slight hypesthesia remained even four months after the surgery. Speculation of this clinical etiology suggests that high concentration of dibucaine, having maldistributed inside the intrathecal space, affected cauda equina, which resulted in irreversible nerve damage. There were other risk factors for cauda equina syndrome in this patient such as lithotomy position, history of frequent spinal anesthesia, diabetes and advanced age. None of these are contraindication for spinal anesthesia. Many elderly patients particularly undergoing urological surgeries are likely to have such risk factors. Therefore at least dibucaine should be avoided for spinal anesthesia because of its high neurotoxicity compared with other local anesthetics.

[A comparative study of membrane destructive action of dibucaine HCl and bupivacaine HCl]

Membrane destructive action of dibucaine HCl and bupivacaine HCl was examined in vitro using red blood cells. The EC50 value (local anesthetic concentration needed to destroy half of red blood cells in vitro) of each local anesthetic was determined at pH 5.0, 6.0 and 7.0 to obtain a pH-EC50 curve. The pH change of each commercially available local anesthetic solution (0.3% dibucaine HCl and 0.5% bupivacaine HCl) was also determined with the dilution by cerebrospinal fluid to yield a pH-concentration curve. The EC50 values of dibucaine HCl were 0.17%, 0.18% and 0.13% at pH 5.0, 6.0 and 7.0, respectively. The EC50 values of bupivacaine HCl were 1.36% and 1.36% at pH 5.0 and 6.0, respectively. The EC50 value of bupivacaine HCl at pH 7.0 could not be measured for the low solubility in water at pH 7.0. The pH-EC50 curve of dibucaine HCl intersected the pH-concentration curve, although the pH-EC50 curve of bupivacaine located in the right side (higher concentration) for the pH-concentration curve of bupivacaine HCl, which implies clinical level of dibucaine HCl may produce membrane destruction action and low possibility of membrane destruction with clinical level of bupivacaine HCl.

Allergic contact dermatitis to cinchocaine

A case of allergic contact dermatitis to cinchocaine is presented to highlight the confusion regarding the classification of, and cross-reactivity between, topical anaesthetic agents and the inadequacy of the use of benzocaine as a single screening agent for topical anaesthetic contact allergy. Our patient presented with a 10-day history of acute perianal dermatitis. Allergic contact dermatitis to Proctosedyl ointment, which was applied for 3 weeks prior, was suspected. On patch testing, he reacted to cinchocaine, a constituent of Proctosedyl ointment, and to the actual product, but had no reaction to benzocaine or lignocaine.

Blood concentrations of tetracaine and its metabolite following spinal anesthesia

Blood concentrations of tetracaine and its metabolite, p-butylaminobenzoic acid, were measured after spinal anesthesia with tetracaine which had been administered to patients under going orthopedic surgery. Tetracaine, an ester anesthetic, was given to 10 patients, the dose was 8-14mg, and blood samples were collected 1, 2 and 6h after the injection of tetracaine. We used gas chromatography/mass spectrometry for purposes of analysis. Tetracaine was not detected in any blood sample, but the metabolite was detected in each sample with the mean concentrations of 126.5, 97.9 and 43.3ng/ml at 1, 2 and 6h, respectively. This data will be useful in determination of the cause of death after spinal anesthesia with tetracaine.

[Severe lightning pain during spinal anesthesia in a patient with diabetic neuropathy]

A 71-year-old woman with diabetic neuropathy who had undergone amputation of the right lower leg for diabetic gangrene 4 years previously, experienced severe lightning pain in both legs during spinal anesthesia. She was scheduled for skin grafting for a burn ulcer on her left foot. Her preoperative physical examination revealed hypesthesia in both legs due to diabetic neuropathy. Spinal anesthesia was performed with a combined spinal-epidural needle at the L 4-5 interspace using 2.0 ml of 0.3% hyperbaric dibucaine in the left lateral position. The region of hypesthesia was spread below Th 4. Ten minutes later, she complained of severe lightning pain in both legs and midazolam 1 mg was administered intravenously against agitation. The severe lightning pain diminished after the administration of pentazocine 7.5 mg intravenously in the recovery room. There was no worsening of neurological findings 5 hours later when the effect of spinal anesthesia disappeared. This clinical picture seems to be different from that of reported cases of phantom limb pain during spinal anesthesia in which severe lightning pain occurred in both legs. This case suggests that patients with diabetic neuropathy might develop severe lightning pain during spinal anesthesia using dibucaine.

Allergic contact dermatitis in patients with anogenital complaints

OBJECTIVE

Allergic contact dermatitis is a common anogenital disease.

STUDY DESIGN

The results of patch testing performed on 1,008 patients evaluated for allergic anogenital contact dermatitis from 21 dermatologic departments in the Information Network of Departments of Dermatology (IVDK) from 1992 to 1997 were analyzed.

RESULTS

A final diagnosis of allergic contact dermatitis was made in 351 patients (34.8%). Irritant contact dermatitis was diagnosed in 230 patients (22.8%) and other, nonspecified forms of perianal eczema in 228 patients (22.6%). The remaining cases were due to other distinct dermatologic diseases. Analyzing patch test data from this large group of patients showed that the allergen spectrum resembled that of all patients (54,000) tested from 1992 to 1997. However, dibucaine HCl ranked fourth among contact allergens in the study population. Positive reactions to (chloro-)-methyl-isothiazolinone and to benzocaine were observed more frequently among patients with anogenital complaints as compared to the total IVDK population (3.7% vs. 2.5% and 2.7% vs. 1.5%, respectively). In general, active agents of topical medications and popular remedies, preservatives and ointment bases appeared to cause allergic reactions most frequently.

CONCLUSION

Patients with chronic anogenital diseases seem to have a high risk of becoming sensitized. For patch testing we recommend the standard series, dibucaine HCl, propolis, bufexamac and other ingredients from topical preparations according to the patient's history.

Thermal destabilization of transmembrane proteins by local anaesthetics

Local anaesthetics, in addition to anaesthesia, induce the synthesis of heat shock proteins (HSPs), sensitize cells to hyperthermia, and increase the aggregation of nuclear proteins during heat shock. Anaesthetics are membrane active agents, and anaesthesia appears to be due to altered ion channel activity; however, the direct effect of heat shock is protein denaturation. These observations suggest that local anaesthetics may sensitize cells to hyperthermia by interacting with and destabilizing membrane proteins such that protein denaturation is increased. It is shown, using differential scanning calorimetry (DSC), that the local anaesthetics procaine, lidocaine, tetracaine and dibucaine destabilize the transmembrane domains of the Ca2+ -ATPase of sarcoplasmic reticulum and the band III anion transporter of red blood cells. The transmembrane domain of the Ca2+ -ATPase has a transition temperature (Tm) of denaturation of 61 degrees C which is decreased, for example, to 53 degrees C by 15 mM lidocaine. The degree of destabilization (deltaTm) by each anaesthetic is proportional to the lipid to water partition coefficient, and the increased sensitization by anaesthetics with larger partition coefficients and at higher pH suggests that the uncharged forms of the anaesthetics are responsible for destabilization. A Hill analysis of deltaTm for the Ca2+ -ATPase as a function of the concentration of anaesthetic in water gives dissociation constants (Kd) on the order of 10(-4) M, if binding occurs directly from the aqueous phase. This demonstrates moderate affinity binding. However, dissociation constants of 1-3 M are obtained, if binding occurs through the lipid phase, which demonstrates low affinity binding. Thus, the interaction of local anaesthetics with the Ca2+ -ATPase may be moderately specific or non-specific depending on the mechanism of interaction. The observation that local anaesthetics also destabilize the transmembrane domain of the band III protein of erythrocytes suggests that destabilization of transmembrane proteins is a general property of anaesthetics, which is at least in part a mechanism of sensitization to hyperthermia.

[Severe lightning limb pain induced by spinal anesthesia]

We report a case in which spinal anesthesia induced a severe lightning limb pain. A 71-year-old man presented for prostate biopsy. Preanesthetic examinations revealed slight hypesthesia in the L 5-S 1 dermatomal segments in the right leg. The patient reported that he had received "local anesthetic" in the lumbar spine 16 years previously because of severe lumbago, and that his hyposthesia had originated from the "local anesthetic". Unfortunately we had no way to know the anesthetic technique performed 16 years ago. The spinal anesthesia was uneventfully introduced with a 25 G Quincke needle at the L 3-4 interspace using 2.0 ml 0.3% hyperbaric dibucaine in the left lateral positions. As soon as the patient was put into the supine position, he started to complain about severe lightning pain in the region of his hyposthesic segments. Severe lightning pain completely diminished 4 hours later when the effect of spinal anesthesia disappeared.

[Clinical study of patients with positive reactions in patch tests with local anesthetics]

Local anesthetics have been contained in various OTC drugs and the number of cases of allergic contact dermatitis due to local anesthetics has increased recently. Thirty-five hundreds and fourteen cases suspected of suffering from drug eruption or contact dermatitis were the subjects of this study during the 8 years from January, 1988 to December, 1995. One hundred and thirty-eight cases, suspected of having allergic contact dermatitis were patch tested with topical medicaments including local anesthetics and 70 (50.7%) of them showed positive. In 49 of the 60 cases who reacted positively to anesthetics extensive patch tests with the individual ingredients of the products were carried out. The local anesthetics causing positive reactions were as follows (in order of frequency): dibucaine hydrochloride (35 cases: 71.4%), ethyl aminobenzoate (12 cases: 24.5%), lidocaine hydrochloride (2 cases: 4.1%), and procaine hydrochloride (1 case: 2.0%). The number of patients displaying allergies to local anesthetics has increased, especially cases caused by dibucaine hydrochloride. And the total positive reaction rate to dibucaine hydrochloride in the patch tests was 3.7%, which was as high that to the well-known strong sensitizer PPDA (3.9%) and neomycin (2.9%). Forty-three cases sensitive to local anesthetics were also studied for cross-sensitivity.

Chemical, pharmaceutical and clinical standardization of the TRUE Test caine mix

The development of the caine mix patch of the TRUE TestTM involved extensive formulation work to obtain optimal release, stability and clinical function. Hydroxypropyl cellulose, ordinarily used in the TRUE TestTM, induced crystallization and inadequate release of benzocaine. Addition of the crystal poison glucose caused rapid degradation of the caines. Polyvinylpyrrolidone (PVP) proved to be a vehicle which incorporated the caines in a stable preparation on the patch. Rapid release of the caines from this vehicle when applied on membranes was confirmed by in vivo patch test studies.

Comparative efficacy and tolerability of two ointment and suppository preparations ('Uniroid' and 'Proctosedyl') in the treatment of second degree haemorrhoids in general practice

A multi-centre general practice, open study was carried out in 89 patients with second degree haemorrhoids to compare the efficacy and tolerability of two antibiotic-corticosteroid combinations ('Uniroid' and 'Proctosedyl') in ointment and suppository formulations. Patients were allocated at random into 4 groups and received treatment with one of the trial preparations for 1, 2 or 3 weeks, as required, with weekly assessments of response. There were no significant differences between the various groups at the start of treatment. Significant improvement occurred in all groups during treatment. Both suppository and ointment formulations were broadly comparable and control of symptoms was achieved from Week 2 onwards, building up to levels in excess of 90% after 3 weeks of therapy. With regard to the symptoms of pain and itching, suppositories gave marginally greater relief in the early stages of treatment, while both ointment and suppositories were associated with similar reduction in bleeding from haemorrhoids. Whereas both suppository formulations were about equal in reducing anal discharge, 'Uniroid' ointment was clinically superior to 'Proctosedyl' ointment in controlling this symptom over the 3-week trial period. No unwanted effects were experienced attributable to treatment. No statistically significant differences between the two ointment and the two suppository formulations were identified in this study and all four preparations were found to be efficacious in the majority of patients studied.

Benzocaine--an unsatisfactory indicator of topical local anaesthetic sensitization for the U.K

Three thousand consecutive patients undergoing patch tests were tested with a mix of benzocaine, amethocaine hydrochloride and cinchocaine hydrochloride. Eighty-four subjects (2.8%) were found to be allergic to this mix. Forty of these were patch tested with the constituents of the mix: 21 (52.5%) patients were sensitive to amethocaine hydrochloride or cinchocaine hydrochloride or both, but not to benzocaine. We recommend that a non-irritant 'caine mix' containing benzocaine, cinchocaine and amethocaine should be included in the standard patch test series for the U.K.

A clinical double-blind study of dibucaine and tetracaine in spinal anesthesia

The effects of 0.25% dibucaine and 0.5% tetracaine used for the production of spinal anesthesia were compared in 30 healthy surgical patients. Fifteen patients were assigned to each of the two agents using a randomized, observer and patient-blinded method. Success rate, times of onset and regression of analgesia, intensity of motor blockade, and cardiovascular effects were assessed and the results were analyzed statistically. There were no differences in success rate, in latency, or in duration of action between the two spinal anesthetic agents. Tetracaine was found to be associated with a significantly greater decrease in arterial pressure and more complete motor blockade.

[Peripheral paralysis or "bulbar threat"? (Further comment on the pathogenesis of respiratory arrest and vascular collapse during cerebrospinal anesthesia)]

The study of 9776 cases of spinal anesthesia carried out in the clinic and the results of the experiments on dogs have lead to the conclusion that respiratory arrest and drop of arterial pressure during spinal anesthesia result from the block of the peripheral conductors rather than from paralysis of bulbar centres. The artificial pulmonary ventilation constitutes the only effective means in combating respiratory depression in spinal anesthesia.