Anesthesia and analgesia should be provided to ferrets that are undergoing potentially painful surgical procedures. The epidural route of administration for anesthetic or analgesic drugs can be used. This column outlines the relevant ferret anatomy, indications and contraindications and technique of epidural administration of anesthesia and analgesia in ferrets.
When undergoing painful surgical procedures, ferrets should be provided with anesthesia and analgesia. Neuraxial techniques are useful in these cases, as supported by an increasing evidence base1,2. Epidural drugs can be selected to provide anesthesia, analgesia or both. There are several advantages of including an epidural drug in a multimodal anesthetic protocol: it can reduce the amount of other anesthetic drugs necessary, improve postoperative pain control and reduce postoperative recovery time1,2,3,4,5; its effects have a longer duration; and it avoids some of the side effects associated with the same drugs when administered by other routes1,2,3.
Ferrets have five to seven (commonly six) lumbar vertebrae and three fused sacral vertebrae4,5,6. Injections are commonly administered via the lumbosacral inter-space, which is easily located in ferrets, as they typically have prominent lumbar spinous processes. To reach the spinal cord, epidurally injected drugs must cross three membranes. The tough outer cover, or dura mater, forms a protective sac along the length of the spinal cord and continues through the lumbosacral space as a cover for the large nerve roots of the cauda equina. The arachnoid mater lies close to the dura and encloses the cerebrospinal fluid (CSF; the subarachnoid or intrathecal space). Directly covering the spinal cord is the thin and vascularized pia mater. The spinal cord in ferrets most often ends cranial to the lumbosacral space, making subdural (intrathecal) injection less feasible in the ferret than in other small animals (cat, rabbit)4.
Indications and contraindications
Epidural anesthesia and analgesia is most useful in procedures involving the abdomen, spine, pelvis, hind legs, tail and perineum3,4. Some drugs (morphine) may penetrate far enough cranially to provide added analgesia for thoracic and forelimb procedures. Epidural injection is contraindicated in cases with coagulopathy, sepsis, hypovolemia, skin infection and anatomic distortion (pelvic or sacral fractures)1,2.
Ferrets should be completely anesthetized (using inhalant or intravenous agents) before epidural injections are given to prevent them from moving during the procedure. All ferrets undergoing anesthesia should have a thorough physical and clinical evaluation beforehand. An intravenous catheter should be placed, and isotonic fluids should be administered at a rate of 10–20 ml per kg body weight per h to prevent hypovolemia. Additional fluid boluses are sometimes needed. Hair should be clipped from an area roughly 4 cm × 5 cm, large enough to allow observation of the bony landmarks for injection and to maintain sterility. The skin should be aseptically prepared using routine protocols and the procedure carried out using aseptic techniques.
Epidural injection is most easily done with the ferret in sternal recumbency with the hind limbs flexed cranially under the abdomen to fully open the lumbosacral space (Fig. 1). The craniodorsal wings of the ileum can be visualized and palpated between the thumb and middle finger of the cranial hand, and the spinous processes and lumbosacral space can be palpated with the index finger. The needle should be inserted caudal to the last lumbar vertebra, in the center of the lumbosacral space. The needle must be kept on the midline as approximated by the location of the spinous processes.
A 22-gauge, 1.5-in spinal needle should be inserted perpendicular to the skin (Fig. 2). Alternatively, a 25-gauge hypodermic needle may be used as the short 0.625-in tip rarely passes the epidural space4,5. The spinal needle is generally preferred, as its stylet prevents the creation of a skin plug that could obstruct CSF flow or act as a nidus for infection and inflammation3,4.
Penetration of the intervertebral ligaments may not produce the familiar 'popping' sensation in ferrets, especially when a sharp hypodermic needle is used4,5. If the needle is correctly placed, it should drop smoothly through the space until it encounters bone on the ventral floor of the spinal canal. Correct needle placement should be confirmed. In cases where bone is encountered without entering the epidural space, the needle should be slightly withdrawn and then redirected along the midline1,3,4.
Once the stylet is removed, the needle hub should be closely observed for blood or CSF. To confirm correct epidural placement, a 1-ml syringe containing 0.2 ml of sterile saline and 0.2 ml of air should be attached to the hub and gentle suction should be applied. No CSF or blood should be present in the needle (Fig. 3). Injection should offer no resistance, and the air space above the saline inside the syringe should not compress on injection but should move smoothly with the fluid interface.
Once placement of the needle in the epidural space is confirmed, the anesthetic solution should be slowly injected. Rapid injection may cause an increase in intracranial pressure, an extended cranial migration of the anesthetic block or uneven drug distribution within the epidural space3.
Vital signs should be continuously monitored during the procedure.
Common epidural drugs
Many anesthetic drugs are used in dogs and cats, but few are reported in ferrets4,5,7. The drugs used should be preservative-free to prevent neurotoxic or allergic responses3. There is a synergism between local analgesics and opioids; local anesthetics provide regional anesthesia, extend the duration of opioid analgesia and minimize the side effects of each individual drug1,3.
Bupivacaine (0.25%; 0.5 mg per kg body weight) can be administered with morphine (0.1 mg per kg body weight) epidurally, allowing for onset of analgesia within 15–30 min and duration of 8–24 h (refs. 4, 5, 7). Obese animals should be dosed in accordance with their lean body weight. Opioid and local anesthetic dose should be reduced by 50% if intrathecal injection is suspected or if the animal is pregnant1,3.
Effective local anesthesia will result in sensory, motor and autonomic blockade3. The opioids act at presynaptic sites in the dorsal horn to prevent the release of excitatory neurotransmitters (substance P) and on postsynaptic receptors to hyperpolarize afferent projection nerve fibers, thus reducing nociceptive transmission with little effect on motor function3.
Side effects of epidural drugs
Epidural injection of local anesthetic drugs can result in self-limiting paresis or paralysis requiring supportive medical care until the effect of the block diminishes3. Rarely, persistent sensory block has resulted in self-mutilation. Local anesthetic blockade of sympathetic tone can lead to vasodilation and relative hypovolemia, which are usually responsive to intravenous fluids or vasopressors1.
Rare side effects of epidural opioids include pruritus, urine retention, vomiting, nausea and respiratory depression3. In one study of epidural morphine in ferrets, none of the 40 animals used showed any side effects5.
Clinicians experienced with the epidural technique in other animals should not be deterred by the relatively small size of the ferret. Clinical experience clearly demonstrates the ease, efficacy and safety of its implementation in this species. Ferrets undergoing many surgical procedures can greatly benefit from the preemptive epidural analgesia and improved postoperative outcome.
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Eshar, D., Wilson, J. Epidural anesthesia and analgesia in ferrets. Lab Anim 39, 339–340 (2010). https://doi.org/10.1038/laban1110-339