Diagnostic analgesia, commonly referred to as nerve or joint “blocking,” refers to numbing an area using an analgesic (pain killer), such as lidocaine, for diagnostic purposes.
Diagnostic analgesia is a crucial component of lameness examinations, as numbing certain areas of the horse’s limbs can help identify where exactly pain is coming from.
The diagnostic analgesia process begins at the hoof, then moves up the limb. Between each nerve or joint block, the veterinarian evaluates the horse for lameness. If the lameness resolves, then the source of pain is likely between the last block applied and the previous one.
Once an area of pain is identified, diagnostic imaging, such as X-rays or ultrasound, is used to investigate the underlying cause of pain.
The most common blocks used in equine diagnostics are nerve blocks of the lower limb, such as the PD block, abaxial sesamoid block, and low four-point block. However, veterinarians can block structures all the way up to the hip or shoulder joint if necessary.
Uses of Diagnostic Analgesia
Veterinarians use diagnostic analgesia primarily during lameness examinations, where they use a systematic approach of applying analgesics to determine the location of lameness. [1] By starting at the hoof and working upwards, the veterinarian can localize the lameness to a particular structure or region. [1]
Types
There are two main types of diagnostic analgesia: perineural (around a nerve) and intraarticular (within a joint). [1]
Perineural analgesia (“nerve blocks“) involves applying an analgesic agent, like lidocaine, around a nerve bundle to numb it. [1] Doing so prevents pain sensation in the entire limb below the level of the nerve block. [1] If the lameness resolves, this indicates the painful area is somewhere between the previous nerve block and the current nerve block. [1]
Intraarticular analgesia (“joint blocks“) allows the veterinarian to test for pain within a specific joint. [1] Some horses do not respond completely to intraarticular analgesia, particularly if the soft tissues surrounding the joint are also painful. [1] A combination of perineural and intraarticular analgesia may be necessary to resolve lameness in these cases. [1]
Common analgesics used for diagnostic analgesia include: [1][2]
- Lidocaine
- Bupivacaine
- Mepivacaine
- Ropivacaine
Different analgesics have different efficacy periods. [3] The most common analgesic, mepivacaine, provides numbing for around 60-90 minutes. [3]
Procedure
Veterinarians prepare the injection site by cleaning the area thoroughly. [1] For perineural analgesia, wiping down the limb with alcohol is usually sufficient, as the needle does not enter critical structures. [1]
For intraarticular analgesia, the risks associated with infection are much higher, as an infected joint can have catastrophic consequences for the horse. [1] Veterinarians typically perform a complete surgical scrub to reduce the amount of bacteria on the skin surface. [1]
After preparing the injection site, the veterinarian inserts a needle and injects the analgesic. [1] Typically, veterinarians use between 1-5 mL of analgesic per injection site. [1]
The analgesic takes 5-10 minutes to take effect. [1] For perineural analgesia, the veterinarian can test the efficacy of the nerve block by poking the skin below the injection site with a pen or hoof pick. [1] If the horse reacts to the poke, then the block was not effective. [1] For intraarticular analgesia, it is difficult to test whether the block was effective. [1]
Once the block is ready, the veterinarian evaluates the horse at the walk or trot to assess lameness, comparing movement to the initial assessment from before the analgesic was applied. [1]
If the lameness resolves or improves, then it is likely that the affected area is below the last nerve block performed, or within the joint that was blocked. [1] By working their way up the limb systematically, veterinarians use this process to identify the affected area causing lameness. [1]
Diagnostic Imaging
After identifying the area causing lameness, veterinarians proceed to diagnostic imaging to visualize the cause of the horse’s pain. [4]
Imaging may include any or a combination of: [4]
- X-rays
- Ultrasound
- MRI
- CT scanning
- Nuclear scintigraphy (“bone scan”)
Interpretation
In most cases, a positive result (more than 75% improvement in lameness) provides accurate direction for the veterinarian to investigate further using diagnostic imaging. [3] However, false positives and false negatives can occur during the diagnostic process. [5]
A false positive is an improvement in lameness even though the cause of lameness was not numbed by the analgesic. [5] Veterinarians may then interpret this finding as a successful result, and pursue diagnostic imaging in the wrong place. [5]
Causes of false positives include: [5]
- Horses that “warm up” out of lameness
- Diffusion of the analgesic up the limb, numbing additional structures
- Subtle forms of lameness that may not be visible consistently
A false negative is no improvement in lameness, even when the veterinarian blocked the affected area. [5] False negatives typically result in the veterinarian continuing blocks up the limb, which can alter their interpretation of the affected area. [5]
Causes of false negatives include: [5]
- Injecting the analgesic into a blood vessel
- Misplacement or misdirection of the needle resulting in injection of analgesic in the wrong location
- Poor diffusion of analgesic into the affected area
- Inadequate time between administration of the analgesic and evaluation of lameness
Side Effects
The most common side effect of diagnostic analgesia in horses is swelling at the injection site, likely due to the needle damaging small vessels and forming a bruise. [1]
Other potential side effects include: [1]
- Cellulitis
- Joint infections
- Joint flare, a non-infectious condition caused by the analgesic irritating the joint capsule
Side effects after diagnostic analgesia are rare. [1]
Lower Limb Blocks
The lower limbs of the horse, below the knee and the hock, are anatomically similar. [1] Therefore, the nerve and joint blocks for these regions are also similar and can be considered together. [1]