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is too fast or the volume to large. However, signs of discomfort could be absent in horses previously sedated with an alpha‐2 adrenergic agonist. Ataxia related to the spinal effects of alpha‐2 adrenergic agonists and local anesthetic drugs appears within 10–25 minutes after the injection.

       Treatment

      If ataxia occurs but the horse is still standing, it can be supported with a tail‐tie until the hind limbs regain the strength. Ataxia caused by an alpha‐2 adrenergic agonist agent may be reversed with an alpha‐2 adrenergic antagonist such as yohimbine; however, the analgesic effects will also be reversed [74]. If the horse becomes recumbent, general anesthesia may be necessary to continue surgery or to avoid injury of the horse and personnel if it is very agitated or distressed [81].

       Expected outcome

      Outcome is generally good if the horse does not undergo any major injury, especially if it becomes recumbent.

       Infection inside the spinal canal

       Definition

      Introduction of bacteria into the spinal canal during the performance of an epidural injection may cause an infection leading to epidural abscess formation, osteomyelitis of the vertebrae or diskospondylitis.

       Risk factors

       Contamination of the solution that is injected

       Lack of aseptic/sterile technique

       Advancement of the needle through infected tissue before entering the spinal canal

       Immunodeficiency

       Indwelling epidural catheters

       Pathogenesis

      Introduction of pathogens inside the spinal canal during epidural injection may cause spinal abscess formation, osteomyelitis of the vertebrae or diskospondylitis. If the infection erodes through the dura matter this may cause septic meningitis. If the osteomyelitis is extensive this could cause vertebral fracture. Severe infection may cause sepsis and the death of the patient.

      The author could not find any report of spinal infections related to epidural injections in horses. There is one case report of an iatrogenic spinal abscess following a cerebrospinal fluid tap in a horse [82]. Both bacterial and fungal spinal infections have been reported in the human literature secondary to epidural injections, most commonly related to steroid injections.

      In a retrospective study looking at the outcome of indwelling epidural catheters in an equine hospital, only 3 out of 43 horses developed local inflammation or signs of increased sensitivity associated with the catheter site, but none of these horses developed any other clinical sign and bacterial culture of the catheter did not yield any growth [87]. A similar retrospective study in dogs did not report any spinal infection, only a low incidence of inflammation and infection at the catheter entry site that resolved after catheter removal without treatment [88].

       Prevention

      Hair clipping, surgical preparation of the skin and strict sterile technique using sterile needles and sterile solutions should minimize the occurrence of this complication. A sterile adhesive surgical drape should be placed over the entry site of indwelling epidural catheters. The epidural injection should not be performed if there is any sign of infection in the skin or underlying tissues where the needle needs to penetrate.

       Diagnosis

      Clinical signs will depend on the localization of the infection and its extension. The most common symptoms associated with an infection in the spinal canal reported in humans are pain of the affected vertebral region and malaise. Horses with vertebral abscesses may present with pain, heat, swelling and crepitus over the affected area, and with signs of bacteremia (i.e. fever, anorexia, depression). Neurologic deficits may also be present, depending on the degree and level of spinal cord compression. The final diagnosis is reached with imaging modalities and cerebrospinal fluid tap and culture.

       Treatment

      For vertebral abscesses, the treatment consists of prolonged antimicrobial therapy, ideally based on results of culture and antibiogram of the pathogens involved. In humans, treatment of these infections consists of surgery (debridement and/or laminectomy) and antibiotic/antifungal treatment. Access to the infected area may be difficult in horses due to the large epaxial musculature. There is a case report of a successful dorsal laminectomy performed in a colt with an infection of the spinous process of the axis [89].

       Expected outcome

      In humans, the outcome of spinal infections following epidural injections was excellent with early therapy (surgery ± antimicrobials), although there are few cases of deaths related to this complication [83].

      Pruritus

       Definition

      Pruritus or itch is a subjective and irritating sensation that causes an urge to scratch. It can be more unpleasant than pain itself [90].

       Risk factors

       Administration of neuraxial (epidural and intrathecal) opioids

       Higher doses

       Pregnancy

       Co‐administration of epinephrine (adrenaline) with epidural opioids (controversial)

       Pathogenesis

      The administration of neuraxial (epidural and intrathecal) opioids has been shown to induce pruritus in several species, including horses, sheep, dogs, rats and humans. This side effect appears to be dose‐dependent. The exact mechanism/s of opioid‐induced pruritus is unclear, but it is likely that more than one mechanism is involved, including the mu and kappa opioid receptors, modulation of serotonergic pathways, prostaglandins release [91] or the activation of 5‐HT3 receptors [92]. In horses, the few reported cases of pruritus were associated with epidural administration of morphine (preservative‐free) through an epidural catheter alone [93, 94] or in combination with detomidine [95]; therefore, it could not be totally ruled out that pruritus was caused by local irritation due to the catheter itself or by detomidine or its preservative.

      In humans, parturients are the most susceptible to this complication, which may be due to the interaction of estrogen with opioid receptors [91].

      Co‐administration of epinephrine (adrenaline) may increase the severity or duration of the pruritus as it decreases the vascular uptake of the opioid from the epidural space, increasing its concentration, although the evidence is conflicting [91].

      In horses, there are only 3 case reports (4 horses) of pruritus associated with epidural administration of morphine through an epidural catheter [93–95]. The incidence of this complication in horses is unknown but in a retrospective study on the outcomes of epidural catheterization in 43 cases, no horses developed pruritus [87]. In humans, the reported incidence of pruritus after neuraxial opioids is between 30% and 100% [91].

       Prevention

      The lowest effective dose of neuraxial opioid should be used. In humans, there is no totally effective preventive treatment, although ondansetron (a 5‐HT3 receptor antagonist),

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