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]

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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]

Plantar or Palmar Digital Block

The “PD block” numbs the palmar (in the front limb) or plantar (in the hind limb) aspect of the digital nerve. [1] The PD block numbs the portion of this nerve as it runs into the hoof. [1]

Lameness conditions that often resolve after a PD block include: [1]

In horses that respond positively to the PD block, the veterinarian may decide to perform intraarticular blocks to identify any sources of joint pain in that region. [1]

Locations include: [1]

  • Navicular bursa (space between the navicular bone and the flexor tendon)
  • Coffin joint
  • Pastern joint

Abaxial Sesamoid Block

If the horse does not respond to a PD block, the veterinarian proceeds to an abaxial sesamoid block. The abaxial sesamoid block numbs the palmar or plantar nerves as they pass next to the sesamoid bones. [1] This numbs feeling from the fetlock down. [1]

Conditions that can resolve after an abaxial sesamoid block include: [1]

  • Fractures of the coffin bone
  • Ringbone
  • Osteochondrosis dissecans of the pastern joint
  • Injuries to the sesamoidean ligaments on either side of the fetlock
  • Injury or inflammation of the digital flexor tendons below the fetlock

Intraarticular blocks within the area blocked by an abaxial sesamoid block include: [1]

  • Coffin joint
  • Pastern joint

Veterinarians can also block the digital flexor tendons by placing analgesic into the sheath that surrounds the tendons. [1]

Low Four-Point Block

The low four-point block is the next step after an abaxial sesamoid block, and blocks the palmar or plantar nerve as they pass next to the ends of the splint bones. [1] This nerve block places the needle very close to the fetlock joint capsule, so most veterinarians do a full surgical preparation for these blocks as a precaution. [1]

Conditions that block after a low four-point block include: [1]

  • Arthritis
  • Osteochondrosis dissecans
  • Fractures
  • Injury or inflammation within the flexor tendons

Veterinarians can also block the fetlock joint or the digital flexor tendon sheath at the level of the fetlock. [1]

In the hind limbs, many veterinarians perform a six-point block, to ensure that the lower aspect of the cannon bone is completely numbed. [1]

High Four-Point Block

The high four-point block numbs the plantar or palmar nerves higher up on the limb, near the heads of the splint bones. [1] Similar to the low four-point block, this block typically requires surgical preparation to minimize risk if the knee or hock’s joint capsule is punctured. [1]

Possible conditions that respond to a high four-point block include: [1]

  • Inflammation or injury of the suspensory ligament
  • Fracture of the cannon bone
  • Damage to the periosteum (connective tissue covering a bone) of the cannon bone
  • Inflammation or injury to the flexor tendons

After the high four-point block, most blocks continuing up the limbs are intraarticular blocks, as large nerves become relatively inaccessible due to increased muscle density. [1]

Upper Front Limb Blocks

The upper front limb blocks begin at the knee (carpus) and continue up to the shoulder joint, where the scapula meets the humerus. [1]

Knee Joint Blocks

The middle carpal joint exists between the two layers of cuboidal bones, the small bones that make up the knee joint. [1] Blocking this joint space typically blocks feeling in the entire knee joint, as there is a connection between the middle carpal joint and the carpometacarpal joint between the knee and the cannon bone. [1]

Conditions that block to a middle carpal joint block include arthritis and bone fractures. [1]

Some veterinarians perform a carpal sheath block, which numbs the tendons running past the knee. [1] Horses that respond to this block typically have inflammation or injury in these tendons. [1]

The antebrachiocarpal joint is the joint between the knee and the ulna and radius. [1] Blocking this joint can indicate arthritis or fractures of the bones making up this joint. [1]

Median and Ulnar Blocks

The median and ulnar nerves are accessible at the back of the knee, allowing for numbing of the entire lower limb and the knee. [1] This block can also numb the lower part of the ulna and radius. [1]

Locations of injuries and fractures that respond to a median and ulnar nerve block include: [1]

  • Soft tissue around the knee
  • End of the radius
  • Check ligaments: small ligaments between the deep digital flexor tendon and the bottom of the knee

Elbow and Shoulder Joint Blocks

Blocking the elbow and shoulder joints can indicate injury or arthritis within these joints. [1] A positive shoulder block can also indicate osteochondrosis of the shoulder joint. [1]

Veterinarians can also block the bicipital bursa, the space between the tendon of the biceps and the shoulder joint. [1] This block can help identify bursitis (inflammation within the bursa), tendon injury, or injury to the top of the humerus. [1]

Upper Hind Limb Blocks

The upper hind limb blocks begin at the level of the hock, starting with the tarsometatarsal joint. [1]

Hock Joint Blocks

The hock has three separate joint spaces that veterinarians can block. [1] The joints are: [1]

  • Tarsometatarsal joint (TMT), between the lower bones of the hock and the cannon bone
  • Centrodistal joint, between the two layers of cuboidal bones within the hock
  • Tarsocrural joint, between the upper portion of the hock and the tibia

Successful blocks of these joints can indicate arthritis or fracture of the bones making up each joint space. [1] A positive tarsocrural joint block can also indicate osteochondrosis of the hock joint. [1]

Fibular and Tibial Nerve Blocks

The deep fibular nerve and tibial nerve run alongside the tibia, and are accessible just above the hock. [1] Veterinarians most commonly use these nerve blocks in horses suspected of having hock pain of the lower hock joint, as this joint can be difficult to block. [1]

A positive response to these blocks can indicate: [1]

  • Arthritis
  • Osteochondrosis
  • Fractures

These block also point to injury or inflammation of the deep digital flexor tendon, talus (the large bone making up most of the hock), calcaneus (the bone forming the point of the hock), or gastrocnemius tendon (the main tendon at the back of the leg).

Stifle Joint Blocks

The stifle is a complex joint composed of the femur, the tibia, and the patella (kneecap). [1] There are three joint spaces in the stifle, which a veterinarian can block separately to better localize pain within the joint. [1]

The three joint spaces are: [1]

  • Medial femorotibial: between the femur and tibia on the inside of the leg
  • Lateral femorotibial: between the femur and tibia on the outside of the leg
  • Femoropatellar:  between the femur and the patella

Horses that respond to a stifle joint block may have: [1]

  • Inflammation of the joint capsule
  • Arthritis
  • Osteochondrosis dissecans
  • Fractures
  • Injury to the large ligaments within the stifle
  • Injury to a menisci, cartilage plates that support the knee

Hip Joint Blocks

The coxofemoral joint, or the hip joint, is the connection between the hind limb and the horse’s pelvis. [1] Veterinarians can block this joint using a needle that is up to 15 cm (6 inches) long. [1] This block can indicate inflammation of the joint capsule, arthritis, or fracture of the hip joint. [1]

Veterinarians can also block the trochanteric bursa, the space between the femur and the tendon of the gluteal muscle. [1] Successful blocks of the trochanteric bursa can indicate inflammation within the bursa, tendon inflammation, or inflammation of the femur. [1]

Summary

Diagnostic analgesia is the process of numbing an area with a painkiller to identify a source of pain. This procedure is most commonly used for lameness examinations in horses.

  • Veterinarians use a combination of nerve blocks and joint blocks during evaluation
  • Evaluation starts at the hoof and works upwards, with lameness exams between each step
  • After identifying a painful area, diagnostic imaging such as X-rays or ultrasound confirm the cause of pain
  • The most common blocks used in horses are the PD block, abaxial sesamoid block, and low four-point block
  • Veterinarians can block the limbs all the way up to the hip or shoulder joint if needed

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References

  1. Ross, M.W. and Dyson, S.J., Diagnosis and Management of Lameness in the Horse. Elsevier, St. Louis, UNITED STATES. 2010.
  2. Silva, G.B. et al. Duration and Efficacy of Different Local Anesthetics on the Palmar Digital Nerve Block in Horses. Journal of Equine Veterinary Science. 2015.
  3. Barker, W. Equine Distal Limb Diagnostic Anaesthesia: (1) Basic Principles and Perineural Techniques. In Practice. 2016.
  4. Barker, W. Equine Distal Limb Diagnostic Anaesthesia: (2) Intrasynovial Anaesthesia. In Practice.
  5. Schumacher, J. et al. Diagnostic Analgesia of the Equine Digit. Equine Veterinary Education. 2013.