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), propofol and non‐steroidal anti‐inflammatory drugs administered prophylactically decreased the incidence of neuraxial opioid‐induced pruritus [91].
Diagnosis
Pruritus following intercoccygeal epidural administration of morphine in horses typically occurs around the tail and gluteal areas [93–95]. Reported clinical signs include self‐excoriation due to rubbing of these caudal areas with walls, focal alopecia, biting of the flanks and even rolling on the back in apparent attempts to scratch the rump area.
Treatment
In the equine case reports, treatment consisted of removal of the epidural catheter and continued administration of phenylbutazone (which had been already initiated to treat the painful condition of the horse), and pruritus subsided over the following hours. In humans, treatment of stablished pruritus consists of the use of an opioid antagonist (naloxone, nalbuphine), propofol or ondansetron, but none of them is totally effective [91]. In cats, pruritus has been successfully treated with dexmedetomidine and ondansetron [96] or naloxone [97].
Expected outcome
Pruritus is very uncomfortable to the patient and can lead to self‐trauma, but outcome is good if no complications occur in the traumatized skin regions (e.g. infection).
Retrobulbar Blocks
Brainstem anesthesia
Definition
Brainstem anesthesia results from spread of the local anesthetic from the retrobulbar space directly to the brain.
Risk factors
Long‐beveled needles
Long needles
High volume of local anesthetic
Technique
Pathogenesis
Inadvertent puncture of the dural optic nerve sheath and injection of the local anesthetic into the subarachnoid space causes migration to the brain, resulting in anesthesia of the brainstem.
In humans, the incidence of central nervous system complications presumed to be caused by spread of the local anesthetic to the brainstem was estimated at 0.27% [98]. In horses, there are no case reports of this complication in the literature, but in a cadaveric study of ultrasound‐guided retrobulbar blocks inserting the needle into the orbital fossa dorsal and caudal to the eye, injection of contrast media into the optic nerve sheath could be confirmed in 1 in 40 cases [99]. The authors of this paper acknowledge that ultrasound visualization of the optic nerve in this case was not possible. In other animal species, brainstem anesthesia was suspected following a retrobulbar block in a cat under general anesthesia [100].
Prevention
Using short‐beveled (spinal needle) and shorter needles decreases the chance of puncturing the optic nerve sheath. Using the lowest effective volume (recommended 8–10 ml for a standard size horse) reduces the caudal spread toward the brain in case of puncture.
Of the three techniques described in horses to perform a retrobulbar block: four‐point block; modified Peterson; and injection into the orbital fossa above the dorsal orbital rim and zygomatic arch, the latter has been suggested to decrease the risk of optic nerve penetration. Ultrasound‐guidance during placement of the needle within the retrobulbar muscle cone may increase not only the effectiveness but also the safety of this block if the optic nerve is visualized and avoided [99]. Resistance during injection of the local anesthetic may be due to intraneural injection, in which case it should be immediately stopped and needle repositioned.
Diagnosis
Symptoms first appear within 2 minutes of injection. Signs reported in humans include confusion, shivering, seizures, paralysis, loss of consciousness, apnea, hypotension, bradycardia, and nausea/vomiting [101]. Respiratory arrest is the most common sign in humans and can last up to 30–60 min [98]. Neurological signs tend to resolve in 1–2 hours post‐injection, although in some cases they may last for up to 12 hours [98]. In a cat that had suspected brainstem anesthesia following retrobulbar block during general anesthesia, the observed signs included apnea, tachycardia and hypertension, within 5 min of injection, and delayed recovery [100]. Upon recovery from anesthesia, the cat presented tremors and nystagmus, lasting 20 minutes, and absent menace response, mydriasis and lack of dazzle and pupillary light reflex lasting 3 hours.
Treatment
There is no specific treatment. In the event of respiratory arrest of a horse during general anesthesia following a retrobulbar block, intermittent positive pressure ventilation should be instituted. The horse should not be recovered from anesthesia at least until spontaneous ventilation has been resumed. In recovery, the horse should be heavily sedated and anticonvulsive treatment readily available. Sling recovery may be considered. If brainstem anesthesia occurs with the horse standing, this could pose a risk to the personnel as the horse may collapse and/or seizure. Symptomatic treatment should be instituted (e.g. induction of general anesthesia, tracheal intubation, positive pressure ventilation, anticonvulsive treatment).
Expected outcome
In humans, the possibility of death because of this complication is rare (0.13%) [98]. The cat of the case report with suspected brainstem anesthesia made a full recovery with no neurological consequences [100]. In horses, it is unknown what the outcome would be as there are no reports in the literature, but due to the size and temperament of
