anesthetics (e.g. lidocaine, mepivacaine) due to a metabolite that is produced during ester hydrolysis named p‐aminobenzoic acid (PABA). However, some preservatives used in formulations of amide‐type local anesthetics, such as methylparaben or sodium metabisulfite, are metabolized to PABA [66] and therefore they could also cause allergic reactions.
There are reports of horses’ deaths due to acute anaphylactic reactions to some drugs (e.g. trimethoprim sulphadiazine, phenylbutazone, water‐soluble benzylpenicillin salts), but to the best of the author's knowledge, the occurrence of allergic reactions induced by local anesthetics in horses has not been reported.
Prevention
Skin testing is used in humans with suspected hypersensitivity to a local anesthetic that require loco‐regional anesthesia, again to determine whether they are truly allergic and to which drug/s. However, these tests are rarely positive [67]. Another option is to do an in vitro leukocyte migration test, but this test has a high rate of false positives and false negatives [68]. If these tests are not possible, an antihistamine drug (e.g. diphenhydramine, chlorphenamine) could be administered before the suspect drug or alternative drugs/therapies may be considered (e.g. general anesthesia). Cross‐reactivity between ester‐type local anesthetics occurs in humans, but is not common between amides or between esters and amides [67].
Diagnosis
The clinical signs of an allergic reaction may be mild, normally including urticaria with presence of wheals and/or facial edema, which may be self‐limiting and resolve without any specific treatment [69]. However, an allergic reaction could also be severe and life‐threatening with acute bronchoconstriction, hypotension, arrhythmias, laryngospasm and pulmonary edema, which if not treated promptly, will lead to the death of the horse. When allergic reactions develop during general anesthesia, the apparent severity of clinical signs may be attenuated; however, anesthesia does not preclude the development of these reactions [70].
When these reactions occur, especial attention should be paid to the cardio‐respiratory status of the horse and if a life‐threatening reaction is developing, treatment should be instituted immediately. If an allergic reaction occurs during general anesthesia, even if it is apparently mild, the recovery from anesthesia should be closely monitored for signs of respiratory obstruction, as laryngospasm could occur after extubation [69].
Treatment
Treatment might not be necessary for mild reactions. If the urticarial reaction is very significant, a glucocorticoid ± an antihistamine drug may be administered. If the reaction is severe causing bronchoconstriction, laryngospasm, pulmonary edema and/or cardiovascular collapse, immediate tracheal intubation, oxygen supplementation and administration of epinephrine (adrenaline) are lifesaving. In these instances, the administration of fluids and glucocorticoids (e.g. methylprednisolone) may also be necessary. In humans, a systematic review of the literature failed to find any evidence supporting or refuting the usefulness of glucocorticoids for the treatment of anaphylaxis [71].
Expected outcome
When reactions are mild the outcome is generally good even without any treatment [69]. Acute anaphylactic reactions may lead to the death of the horse if not treated promptly.
Complications Related to Specific Loco‐Regional Blocks
Epidural Analgesia
Ataxia/recumbency
Definition
During or shortly after an epidural injection, the horse may start showing signs of instability of the pelvic limbs and if severe, this may lead to collapse (recumbency).
Risk factors
Factors that increase the risk of ataxia include:
High doses and/or volumes of drugs
Additive effect of epidural drugs when used in combination (e.g. alpha‐2 adrenergic agonist + local anesthetic)
Additive effect with systemically administered drugs
Epidural catheters placed too rostral within the canal
Pregnancy
Obesity
Deteriorated clinical condition of the horse (weakness, exhaustion)
Excessive speed of injection, irrespective of the drug used
Pathogenesis
Sedation and ataxia following epidural administration of different drugs is common in the horse, especially following alpha‐2 adrenergic agonist drugs, due to their systemic absorption and central nervous system effects [72]. Detomidine causes more systemic effects (sedation, head drop, ataxia) than xylazine when administered epidurally, due to its increased lipophilicity and systemic absorption [73]. There are several studies reporting ataxia following epidural administration of alpha‐2 adrenergic agonists in horses, especially detomidine [74]. Recumbency has been reported following the administration of 50 microg/kg epidural detomidine in a horse [75].
Local anesthetic drugs cause non‐specific blockade of nerve transmission, therefore causing motor as well as sensory blockade. Ataxia of the hind limbs and potentially recumbency may occur when high doses of a local anesthetic are administered into the sacrococcygeal or intercoccygeal epidural space due to excessive cranial migration (blockade of the caudal lumbar and cranial sacral spinal roots) [76].
Epidural ketamine can also cause ataxia, although this is usually mild [77].
Epidurally‐administered opioid drugs (e.g. morphine, methadone, etc.) do not produce motor blockade and therefore do not cause ataxia [78].
Some cases of ataxia/recumbency may be related to a local compressive effect from the injected volume of solution, and not to the spinal effects of the drug, due to fast administration of a high volume [79]. A total epidural volume of 20 mL administered at a rate of 1 mL every 10 seconds was painful in a study in horses, and the authors concluded that this was possibly due to compression of sacral and lumbar spinal roots [78].
Prevention
The most commonly recommended volume of epidurally administered drugs in the literature, irrespective of the drug, is 10–15 mL per horse (for an average size horse weighing 500 kg), to avoid excessive cranial migration and hind limb ataxia. However, higher volumes (20–30 mLs) of diluted drugs that do not cause motor deficits (i.e. opioids) have been administered without causing ataxia.
In pregnant mares or obese horses, the total volume of epidural solutions should be reduced as the cranial spread is increased.
When a local anesthetic and an alpha‐2 adrenergic agonist are combined, the dose of the local anesthetic should be reduced. Less lipophilic alpha‐2 adrenergic agonist drugs with less systemic absorption are preferred by the author (e.g. xylazine instead of detomidine) as they cause less systemic effects and have a longer duration of spinally‐mediated
