Diagnostic Anesthesia for Lameness of the Forelimb

What is the most commonly used anesthetic for nerve blocks?

What is its mechanism of action?

Mepivacaine

Reversibly block sodium voltage-gated channels

What is the major difference between lidocaine and mepivacaine?

Both have quick onset, but lidocaine lasts 1-2 hours and mepivacaine lasts 2-3 hours

What anesthetic has an intermediate onset of action and 3-6 hour duration?

Bupivacaine

What allows mepivacaine to be a good option for anesthetic in terms of how it interacts with the tissue?

pKa is close to pH of the tissues

What nerve are blocked with the palmar digital nerve block?

Lateral and medial palmar digital nerves

What are the landmarks for the palmar digital nerve block?

Either side of the deep digital flexor tendon just proximal to the collateral cartilages (palpable neurovascular bundle)

What does the palmar digital nerve block? (4)

1. DIJ

2. Navicular

3. Heel region of sole

4. Toe region of sole

What TWO important things does a PDNB NOT block?

1. Dorsal 20% of hoof (dorsal laminae; extensor process)

2. Dorsal coronary band

3. Dorsal pastern (up to distal P1)

Name at least THREE conditions that may respond to a PDNB.

1. Navicular Syndrome

2. Heel Pain Syndrome

3. Wing fractures of P3

4. Sub-solar abscess

5. Pedal osteitis

Blocking a horse out in a left forelimb lameness to a PDNB that causes it to become more lame in the right forelimb would put what condition at the top of your differential list?

Navicular

What nerves are involved in the abaxial sesamoid nerve block?

Lateral and medial palmar digital nerves

What are the landmarks for the abaxial sesamoid nerve block?

Lateral/abaxial to proximal sesamoids

What can be partially or completely desensitized in some horses with an abaxial sesamoid nerve block? (Hint: A joint but not the next one up)

Metacarpophalangeal joint

What is a major disadvantage to abaxial sesamoid nerve blocks in terms of misdiagnosis due to block?

Can accidentally block out a portion of the fetlock

What structures does the abaxial sesamoid nerve block?

Pastern joint and down (+/- proximal sesamoid bones and palmar fetlock)

What THREE major conditions might respond to an abaxial sesamoid nerve block?

1. Laminitis

2. Ring bone

3. Soft tissue injuries of pastern

What nerves are blocked by the mid-pastern ring block?

What makes it different than a PDNB?

Lateral and medial palmar digital nerves

Blocks dorsal regions of foot by additionally blocking the dorsal branches

What structures are blocked by the mid-pastern ring block?

Pastern and below

True or False: The same conditions that respond to an abaxial sesamoid nerve block will be the same in a mid-pastern ring block.

True!

If the mid-pastern and abaxial sesamoid both block the same structures, why is the mid-pastern performed more commonly, especially in the sport horse?

Eliminate the risk of accidentally blocking out the fetlock, where sport horses are prone to injury (you can substitute this block for the abaxial sesamoid)

What nerves are blocked in a low four-point block?

Medial and lateral palmar nerves

Palmar metacarpal nerves

What are the landmarks for the medial and lateral palmar nerves in the low four-point?

Above tendon sheath between suspensory and DDFT

What are the landmarks for the palmar metacarpal nerves in the low four-point?

Base of the splint bone at button

Aside of the structures already blocked by an abaxial sesamoid, what is blocked by the low four-point block? (3)

1. Fetlock joint

2. Distal digital tendon sheath

3. Insertion branches of suspensory ligament

What TWO major conditions should respond to a low four-point block?

1. Fetlock conditions

2. Tendon sheath conditions

The ramus communicans is higher on the side of the ------------ n and lower on the side of the --------- n.

Medial palmar

Lateral palmar

In a low four-point nerve block, the needles should be placed (proximal/distal) to the ramus communicans.

Why?

Distal

Avoid leaving communication to other region of the nerve

What is a major contraindication/concern to the low four-point block?

Poking the digital flexor tendon sheath inadvertently

True or False: Both the low four-point and high four-point involve the medial and lateral palmar nerves alongside the medial and lateral palmar metacarpal nerves.

True!

Where on the limb is the high four-point performed?

Proximal metacarpus

Why can the nerves involved not be felt when performing a high four-point?

Covered by fascia

What is the landmark of the medial and lateral palmar nerves in the high four-point nerve block?

Weight bearing dorsal to DDFT at proximal metacarpus

What is the landmark of the medial and lateral palmar metacarpal nerves in the high four-point nerve block?

Carpus flexed, either head of the splint bones axially

What is the major disadvantage to a high four-point block?

Inadvertent injection of carpometacarpal joint

The high four-point nerve block is used to block the entire distal limb, with the exception of -----------, which is not usually obtained.

Proximal portion of cannon bone (MCIII)

What THREE major conditions may respond to a high four-point block?

1. Suspensory desmitis

2. Inferior check ligament desmitis

3. Splint exostosis

4. Proximal sesamoid disease (if not blocked by low four-point)

What nerve block is considered an alternative to the high four-point block?

Lateral palmar nerve block

The lateral palmar nerve gives rise to the ------ and ---- nerves and one or more branches that supply innervation to the proximal portion of the ---------.

Medial palmar metacarpal n

Lateral palmar metacarpal n

Suspensory ligament

Name the TWO approaches to the lateral palmar nerve block and their landmarks.

Lateral approach/Wheat Block: distal aspect of accessory carpal

Medial approach: medial aspect of accessory carpal bone on the distal third

Which lateral palmar nerve block approach is more advantageous because you cannot accidentally block the carpal sheath?

What is the one downside?

Medial approach

Can be painful

What does the lateral palmar nerve block desensitize?

Palmar subcarpal region

True or False: You should only perform a lateral palmar nerve block after doing a high four-point block.

False! You should only perform a lateral palmar nerve block after doing a LOW four-point block.

What condition may respond to a lateral palmar nerve block?

Proximal suspensory disease

True or False: Median and ulnar nerve blocks are usually performed together to anesthetize all structures distal to the level of injection.

True!

Why would the ulnar nerve sometimes need to be blocked separately?

Determine if remaining proximal palmar metacarpal pain that failed to go away after high four-point

What is the landmark for an ulnar nerve block?

10cm proximal to accessory carpal bone in groove separating ulnaris lateralis and flexor carpi ulnaris mm

What is the landmark for a median nerve block?

Caudomedial border of the radius and ventral border of superficial pecs

True or False: If a horse still has a true lameness after median and ulnar nerve blocks, the lameness might be in the elbow, shoulder, or even the facets.

True!

True or False: The median and ulnar nerve, when blocked, desensitize the entire proximal forelimb to the hoof except some regions of the skin.

True!

What is the major disadvantage/thing to be mindful of if you block the median and ulnar nerves?

Forelimb weakness/limb instability (fall/trip risk due to extensor function loss)

What is the median and ulnar nerve block primarily used for?

Determine if pain is proximal or distal to the injection site

What approach of the distal interphalangeal (coffin) joint is the easiest?

What structure must it pass through?

Dorsal parallel approach

Common digital extensor tendon

What is the landmark of the dorsal parallel approach?

Immediately proximal to coronary band

What approach of the distal interphalangeal (coffin) joint avoids the sensitive coronary band?

Dorsal inclined approach

What might be a reason to choose a dorsal inclined approach over a dorsal approach to a DIJ block?

Avoid highly innervated coronary band and accommodate diverse foot shapes

What approach of the distal interphalangeal (coffin) joint is less painful and risks accidental navicular bursa injection?

Lateral approach

What is the landmark of the lateral approach to the DIJ?

Immediately proximal to lateral cartilage

How many mLs of anesthetic should be put in the coffin joint?

Hint: NOT the standard 1.5 ml as in perineural blocks

5-6 mLs

What THREE structures are blocked with a distal interphalangeal joint block?

1. DIJ

2. Navicular apparatus

3. Toe region of sole

What THREE major conditions should respond to the DIJ block?

1. Coffin joint arthritis

2. Coffin joint fractures

3. Navicular syndrome

What is the most common approach to blocking the navicular (podotrochlear) bursa?

What structure must be punctured for this approach?

Distal palmar approach

DDFT

True or False: After a distal palmar approach to the navicular bursa block, DDFT trauma is often seen.

False! DDFT trauma has NOT been a clinical issue with this block.

What approach to the navicular bursa block avoids puncture of the DDFT but requires ultrasound assistance?

Lateral approach

True or False: The navicular bursa is about a finger width below the coronary band.

True!

What is the landmark for the distal palmar approach of the navicular bursa block?

Peak of the heel bulbs

What TWO things are blocked with a navicular bursa/podotrochlear block?

1. Navicular apparatus

2. Dorsal margin of sole

What TWO major conditions should respond to a navicular bursa block?

1. Septic navicular bursitis

2. Severe navicular syndrome

What are the FOUR approaches to the PIJ block?

1. Dorsal

2. Dorsolateral

3. Palmar proximal

4. Palmar lateral

What FOUR conditions respond to a PIJ block?

1. Osteoarthritis (ringbone)

2. Chip fractures

3. Palmar/plantar eminence fracture

4. Septic pastern

What is the Auburn-preferred technique to blocking the digital flexor tendon sheath by aiming toward the central intersesamoidean region?

Palmar/plantar axial sesamoidean

True or False: The palmar/plantar axial sesamoidean approach to the DFTS is very accurate, even when the DFTS is not distended with synovial fluid.

True!

What is a major disadvantage of the palmar/plantar axial sesamoidean approach to the DFTS?

Penetrates flexor tendons (not clinically significant)

What is the landmark for the palmar/plantar approach to the DFTS?

Just off abaxial sesamoid under proximal sesamoid

What approach to the DFTS involves a needle inserted to the lateral proximal sesamoid bone into a depression created by the base of the sesamoid and the lateral border of the superficial flexor tendon?

Basilar sesamoidean approach

What TWO things are blocked with a DFTS block?

1. Flexor tendon sheath

2. DDFT distal to DFTS into navicular bursa and P3 insertion

What is the major disadvantage of a DFTS block?

How can you ensure this hasn't happened when doing this block?

Leakage that desensitizes lateral aspect of distal limb

Test skin sensation at lateral heel bulb to ensure can feel

What SIX conditions may respond to DFTS block?

1. Septic tendon sheath

2. DDFT/SDFT tendonitis

3. Distal sesamoidean lig desmitis

4. Palmar annual lig desmitis

5. Proximal/distal digital lig desmitis

6. Insertion desmopathy of DDFT to P3

What approach to the fetlock joint block involves a weight-bearing limb with a needle inserted under the lateral edge of the common digital extensor tendon at the palpable joint space?

Dorsal approach

What structure in the fetlock joint is palpable the limb slightly flexed in the gap between the sesamoid and cannon bone?

Collateral sesamoidean ligament

What vascular area of the fetlock pouch usually result sin blood that contaminates the sample if hit?

Proximal palmar/plantar pouch

What FOUR structures are blocked with a fetlock joint block?

1. Fetlock joint

2. Articular part of proximal sesamoids

3. Distal cannon (MCIII)

4. Proximal P1

What FOUR conditions may respond to a fetlock joint block?1

1. Dorsal P1 chip fractures

2. Subchondral cysts of distal cannon

3. Proximal sesamoid fractures

4. Capsulitis/synovitis of fetlock joint

it's gonna be okay you got it homie

lol shut up horse boy