Epidural anaesthesia

When can epidural anaesthesia be useful?

Epidural anaesthesia is useful to provide analgesia as part of a multimodal analgesic regime and facilitate various (painful) interventions, e.g. obstetric manipulations, perineal or vulval surgery, intraoperative analgesia and post-operative pain management.

How does epidural anaesthesia work?

The epidural space lies outside the dura mater (the outermost membrane of the spinal cord), between the spinal cord/nerves and the vertebrae. It is filled with fat and a few venous plexuses. Drugs administered into the space, depending on their lipid and blood solubility, will diffuse through the space to eventually be redistributed around the body. If drugs pass near to a spinal nerve, they can diffuse into that nerve to exert an (e.g. analgesic) action.

Where do I perform the injection?

Two locations are possible: the first (and second) intercoccygeal space, and the lumbosacral junction. In the horse, the spinal cord ends around L6–S1, so the former location is most commonly employed; it is the easiest site to access and is inherently safer (for the horse) as it completely avoids the risk of entering the subarachnoid space (true spinal injection). This talk will therefore focus on epidural anaesthesia in the first intercoccygeal space.

How do I perform an epidural in the intercoccygeal space?

1. Make sure that it is safe to stand at the horse’s tail, e.g. use stocks or back the horse up to a stable door or bales of shavings. The author often sedates horses to perform epidural injections, preferring romifidine at 40 µg/kg bwt, as it results in less ataxia, and it is helpful if the horse stands ‘square’.
2. Palpate the first intercoccygeal joint by lifting and ‘pumping’ the tail. It is the first joint in the tail where both vertebrae can be felt to move slightly as the tail is pumped, and is normally two to four finger-widths above the top of the middle tail hairs. The space can be difficult to find in fat animals so try this out on thinner animals first.
3. Clip and aseptically prepare the area.
4. Inject local anaesthetic subcutaneously and along the proposed needle path in the dorsal midline (2 mL of 2% mepivacaine, 25 gauge 16 mm needle). It is useful to use the needle to then scour/mark the skin to mark the injection site as the subcutaneous local anaesthetic can subsequently make palpating the intercoccygeal space more difficult.
5. Two approaches to injection can be taken (Fig 1): the needle is advanced perpendicular to the skin surface (labelled A); or the needle is advanced approximately 30 degrees from horizontal, in a cranioventral direction (can require horizontal or craniodorsal orientation in some horses) (labelled B). The techniques have subtle differences but the author’s preference is for approach B. With the horse standing square, insert a spinal needle (19 or 20 gauge 60 or 90 mm), bevel facing forward. Once the spinal needle is through the skin, withdraw the stylet and fill the needle hub with saline. Advance the spinal needle until either: [...]