What is epidural anesthesia?
The epidural is a neuroaxial technique that provides preemptive analgesia by injecting drug into the epidural space surrounding the spine (Fig 1).
Epidurals can be used for abdominal surgeries, perineal surgeries, and orthopedic procedures involving the pelvic limb or spine. Some opioids can also travel cranially to provide supplemental analgesia for chest and thoracic limb procedures. Epidurals serve as an adjunct to systemic analgesia, and epidural analgesia also reduces the amount of anesthetic drug needed. Epidurals can also shorten recovery time (Sladky 2000).
Despite these many benefits, clinicians are often hesitant to perform epidurals in small mammals, however the technique is relatively safe, easy and effective.
Potential adverse effects
One possible adverse effect of epidural analgesia is transient paresis or paralysis. Additional rare side effects with epidural opioids can include pruritus, vomiting or nausea, respiratory depression, and urinary retention.
Epidural injections are contraindicated in patients with coagulopathy, sepsis, skin infection, hypovolemia, or when anatomy has been distorted by pelvic or sacral fractures.
To perform an epidural you will need:
- Pre-warmed surgical scrub
- An appropriately sized needle (Box 1)
Box 1. Recommended needle size for epidural injection in small mammals Species Needle size Ferret or rabbit 22-gauge 1.5-in spinal needle
24-gauge spinal needle
25-gauge hypodermic needle
Chinchilla or guinea pig 25-gauge hypodermic needle
24-gauge spinal needle
- An assistant to keep the needle stabilized while switching syringes
- Epidural drug: Select an agent to provide anesthesia, analgesia, or both. The drug selected should be preservative-free to prevent neurotoxic or allergic responses. Morphine is the most commonly used opioid because it is highly potent and long lasting (Box 2). Other opioid agents for use may include buprenorphine, oxymorphone, and hydromorphone.
Box 2. Epidural morphine dose in select small mammals Species Epidural morphine dose (mg/kg) Ferret 0.1 Rabbit 0.22
Performing an epidural injection
- To perform an epidural injection, the patient should be completely anesthetized.
- Place the patient in ventral recumbency with the hips flexed cranially to fully open the lumbosacral space. (Figs 2-3).
- Identify the cranial aspects of the wings of the ilium 3. (Fig 4).
- Visualize a transverse line connecting these two points. The lumbosacral junction sits very near this line on midline (Fig 5).
- Palpate the vertebral spinous processes and the lumbosacral space (Fig 6).
- Shave fur over the lumbosacral space. The area should be large enough to easily view bony landmarks and maintain sterility (Fig 7).
- Aseptically prepare the skin.
- Holding the needle perpendicularly with the bevel facing cranially, insert the needle exactly on midline caudal to the last lumbar vertebrae. The needle should drop smoothly through the lumbosacral space until it encounters bone on the floor of the spinal canal (Fig 8). The distinctive popping sensation felt in dogs is usually not appreciated in ferrets, particularly when a hypodermic needle is used, however the ferret’s tail will frequently twitch as the needle engages nerve tissue.
- To confirm correct needle placement: Attach an empty 1 ml syringe and slowly inject 0.1 ml of air OR Inject 0.2 ml of sterile saline combined with 0.2 ml of air. There will be a lack of resistance to this injection of air if the needle is within the epidural space. Also when the syringe is gently aspirated, no blood or cerebrospinal fluid should appear within the needle hub. If there is resistance to injection of air or if the skin bulges, advance the needle until it fully penetrates the ligamentum flavum, then check for resistance again.
- Once needle position is verified, replace the empty syringe with one containing the anesthetic agent. Slowly administer drug into the epidural space. Caution: Rapid injection can cause uneven drug distribution within the epidural space and even an increase in intracranial pressure.
- Be sure to empty the patient’s urinary bladder before it rouses from general anesthesia. Bladder and sphincter tone will hopefully return to normal before the bladder fills.
Did the epidural injection work?
A successful epidural is associated with loss of anal sphincter tone and loss of tail tone as well as a drop in systemic blood pressure. It can take 15-30 minutes for full analgesic effects to develop. Effects may persist anywhere from 8 to 24 hours.