Multimodal analgesia is a huge component of any surgery administered before, during and after procedures. A common element of this multimodal approach is the use of loco-regional blocks such as epidurals. While epidurals provide good analgesia by blocking nociceptive responses during many procedures on the hindlimb, they are not recommended with such conditions as lumbosacral abnormalities, skin infections in the area where the epidural would be administered, uncorrected hypovolemia or bleeding disorders. Because of these conditions, as well as the desire to minimize potential complications from epidurals, there are now a number of new analgesic techniques. One such technique is local blockage of the femoral and sciatic nerves.
The femoral nerve arises from L4-L6 vertebrae, and the sciatic nerve arises from L6-S2 vertebrae. In general, the femoral nerve provides innervation to the medial aspect of the hindlimb distally to just below the stifle. The sciatic nerve, along with its branches, provides innervation to the remainder of the hindlimb (Figure 1).
To perform a femoral or sciatic nerve block, a nerve locator (Figure 2) with insulated needles is recommended. Insulated needles, in particular, are helpful because they connect to the nerve locator and have an injection port. A current of 1 to 1.5 mA can be used to help find the nerve, and then the current is reduced to 0.5 mA to help further find the exact location. A myoclonus response should still be noted at 0.5 mA but tends to disappear at 0.2 mA.
Femoral nerve block
Before you begin, all skin sites should be aseptically prepared. There are two approaches to femoral nerve blocks—a medial and a pre-iliac. For the medial approach, palpate the femoral artery and find the femoral nerve just cranial to the artery. A corresponding myoclonus response should be noted at 0.5 mA. Aspirate anesthetic to ensure that the needle is not placed in an artery or vein and inject it next to the nerve (Figure 3).
Injecting saline to test before injecting the anesthetic isn’t required, but for a very small patient in which only a small volume of anesthetic is used, then saline would be acceptable.
The pre-iliac approach to the lumbar plexus is thought to be easier, provide more complete nerve blockage and avoids the femoral artery entirely. Make an imaginary line starting at the transverse process of L6 and perpendicular to the spine. Make a second imaginary line at the most cranial prominence of the wing of the ilium and parallel to the spine. The intersection of these two lines is the injection site. Direct the needle 35 degrees to 45 degrees caudally (Figure 4).
Sciatic nerve block
The most common approach for this block is the gluteal approach. To begin, draw an imaginary line connecting the iliac crest to the ischial tuberosity and draw two short, perpendicular lines across it to divide the first line into thirds (Figure 5).
Draw another long line from the iliac crest and parallel to the spine. Then draw one more line, starting from the short perpendicular line, dividing the first line into the second and third thirds. This line will travel dorsally to intersect the second long line (from the iliac crest and parallel to the spine) perpendicularly. The injection point is in the middle of the last (third) line drawn. Find a myoclonus response at 0.5 mA first, then aspirate prior to injecting.
Other nerves in the hindlimb
Some have suggested that blockage of the lateral femorocutaneous nerve, which emerges at L3-L4 vertebrae, is needed. This nerve innervates the lateral aspect of the stifle; however, in many cases, no difference is noted whether or not this nerve is blocked. Furthermore, the obturator nerve emerging from L4-L5 vertebrae is only a motor nerve and therefore can be ignored.
Bupivacaine at a concentration of 0.5 percent using 0.1-0.15 ml/kg is sufficient if the nerve is localized. Alternatively, the maximal dose of 1 mg/kg can be drawn up and split into two different injection sites. This can be done for all of the nerve blocks described here.
By taking advantage of blocking these two nerves, you can achieve the desired amount of analgesia for your patients—without the undesirable effects you may encounter with epidurals.