Diagnostic perineural and joint blocks

Question 1

What is the mechanism of action of local anesthetics

Answer 1

-Pain travels to the CNS via thinly myelinated A delta and nonmyelinated C fibers
-Inhibit excitation of nerve endings by reversibly binding to and inactivating sodium channels
-Nerve blocks act by preventing the increase in membrane permeability to the sodium ions that occur when nerves are stimulated. Prevent rapid influx of sodium ions thereby inhibiting depolarization of the membrane
-Onset of nerve fiber desensitization: Sensory>Sympathetic>Motor

Question 2

What pharmacological properties affect how a LA works

Answer 2

-PKa, lipid solubility and protein binding all affect duration
-Lipid solubility also affects potency, ability to cross membrane, and onset
-Speed of onset is determined by the pKa ( the pH at which the ionized and unionized forms are present in equal amounts
-low the pKa the greater the non-ionized form in solution and the faster the speed of onset

Question 3

What are the positive and negative effects of adrenaline added to LAs

Answer 3

Positive effects: increased duration, decreased local anesthetic plasma concentration, vasoconstriction/less bleeding
Negative effects: tissue ischemia and necrosis, white hair formation, dehiscence (due to reduced blood supply)

Question 4

What types of LAs can be used

Answer 4

-Topical anesthetic: splash block on mucosa
-Local anesthesia: ring block, direct infiltration, line block
-Regional anesthesia: perineural anesthesia (nerve blocks), intrasynovial anesthesia (articular, bursal, thecal), IVRA, epidural anesthesia

Question 5

Local anesthesia techniques

Answer 5

Ring block: anesthetic injected subcutaneously in a ring above area to be blocked. Structures distal to block are anesthetized (pastern ring block, hindlimb low 6 point)
Direct infiltration: anesthetic injected directly at site to be blocked, bony prominences (splint bones), insertion of ligaments or tendons
Line block: anesthetic injected in a line cranial/dorsal to area to be blocked, lacerations, surgical incisions

Question 6

What are common sites of perineural anesthesia

Answer 6

-Palmar/plantar digital nerve
-abaxial sesamoid
- low 4 point
-Nerve block injected directly over nerve, anesthetizes at and distal to site

Question 6

What are common sites of perineural anesthesia

Answer 6

-Palmar/plantar digital nerve
-abaxial sesamoid
- low 4 point
-Nerve block injected directly over nerve, anesthetizes at and distal to site

Question 7

What types of intrasynovial anesthesia/blocks are there

Answer 7

-Joint blocks: anesthetic injected directly into the joint. Anesthetizes joint capsule, synovial membrane, does not anesthetize structures outside the joint efficiently, subchondral bone/SL branches. Common sites: distal interphalangeal (coffin) joint, fetlock joint, carpal joints (intercarpal and radiocarpal), distal tarsal joint (TMT and DIT), stifle joints (femoralorotibial and femoropatellar)
-Bursal blocks: common sites- navicular bursa and bicipital bursa
-Thecal blocks: common sites- digital flexor tendon sheath

Question 8

What are some important principles of diagnostic anesthesia

Answer 8

-Nerve blocks: structures distal to block are anesthetized, start dismally and work proximally, always check block (test skin sensation)
-Intrasynovial blocks: must be sterile, localized to particular structure, soft tissue injuries within DFTS may only partially block, subchondral bone pain not always blocked

Question 9

What equipment is needed for diagnostic anesthesia

Answer 9

-Sterile disposable needles: perineural (smallest gauge possible- 23 to 25 g), intra-synovial: big enough for synovial fluid (20g)
-Sterile non-lure lock syringes
-Local anesthetic solutions
-Gloves: exam, sterile
-Scrub: chlorhexidine(.05%), povidine iodine (.2%), alcohol 70%

Question 10

What should you look for in diagnostic anesthetic

Answer 10

-Improvement in lameness: check block has taken(skin sensation), sound, improved but still lame on original limb, >50%improvement with intrasynovial block considered a positive response, switch to another limb
-No improvement: keep working proximally, check if correct limb, wrong drug, mechanical lameness, other diagnostic (nuclear scintigraphy)

Question 11

Side effects of diagnostic anesthetic

Answer 11

-Soft tissue swelling: needle trauma, hematoma, cellulitis, small gauge needle breakage
-Synovitis: non-septic flare, septic
-Toxicity: lidocaine- 10-12 mg/kg, max dose in 500kg horse is 250mls of 2% lidocaine
-Skin sloughing: adrenaline containing solutions

Question 12

What nerve blocks do we need to know how to do for final year

Answer 12

-Palmar digital nerve block
-Abaxial sesamoid
-low 4 point (low volar)

Question 13

What position, nerves, and location are relevant for the palmar digital nerve block

Answer 13

-Position: standing or foot elevated, facing back, 1 hand holding limb, facing forward with foot between your knees
-Nerves blocked: medial palmar/plantar digital nerve, lateral palmar/plantar digital nerve
-Location: palmaromedial and palmarolateral pastern region, 1cm above collateral cartilage, palpate VAN, nerve palmaroaxial to vein and artery

Question 14

What regions are blocked in the palmar digital nerve block

Answer 14

-Heel region, palmar skin, navicular bursa, distal DDFT, palmar coffin joint, palmar processes of the pedal bone, collateral cartilage, bars/frog/entire sole, laminar corium, digital cushion
-Desensitizes: coffin joint

Question 15

What regions, nerves, and location are affected by the abaxial nerve block

Answer 15

-Position: standing or foot elevated, facing back with 1 hand holding limb, facing forward with foot between your knees
-Nerve blocks: medial palmar/ plantar digital nerve, lateral palmar/plantar digital nerve
-Location: distal border of proximal sesamoid bones, palpate VAN, nerve axial to vein and artery

Question 16

What regions are blocked in an abaxial nerve block

Answer 16

-Foot, P3, P2 and most of P1, coffin joint, pastern joint, part of the DDFT and digital flexor tendon sheath, distal sesamoidean ligaments, extensor branch of the suspensory ligament, extensor tendon insertion
-May also block: proximal sesamoid bones, fetlock joint

Question 17

What nerves are blocked in the low 4 and 6 point nerve blocks

Answer 17

Low 4 point forelimb: medial and lateral palmar nerves, medial and lateral palmar metacarpal nerve

Low 6 point hindlimb: medial and lateral plantar nerves, medial and lateral plantar metatarsal nerves, medial and lateral dorsal metatarsal nerves

Question 18

What locations are the palmar metacarpal, the palmar, and the dorsal metatarsal nerves

Answer 18

-Palmar metacarpal: just distal to button of splint bone, caution of palmar fetlock joint pouch
-Palmar: between DDFT and suspensory ligament, 3-5 cm proximal to button of splint, caution of DFTS
-Dorsal metatarsal: either side of extensor tendon or subcut dorsal ring at level of plantar metatarsal nerves

Question 19

What regions are blocked by the low 4 point nerve block

Answer 19

Regions: fetlock joint, proximal sesamoid bones, flexor tendons distal to block, digital flexor tendon sheath, entire distal limb

Question 20

What joint blocks do we need to know

Answer 20

-Distal interphalangeal (coffin)
-Metacarpophalangeal (only one well do in final year

Question 21

What is the position and location of the distal interphalangeal joint block

Answer 21

-Position: standing
-Location: 1)dorsal pouch: 1 cm dorsal to coronary band, 45 degree angle distal. 2) dorsal pouch: just above coronary band, midline, needle directed horizontally. 3)palmar pouch: abaxial, just above collateral cartilage, needle directed 45 degree angle dorsally or axially

Question 22

What structures are blocked with a distal interphalangeal joint block

Answer 22

-Coffin joint, navicular bursa (note that LA can diffuse from DIP joint to navicular bursa in 100% of horses in 15 minutes)
-May also block insertion of DDFT and sole

Question 23

What is the position and location of fetlock joint block

Answer 23

-Position: standing or limb elevated
-Location: a) proximopalmar: 2 cm distal to splint button, proximal to collateral sesamoidean ligaments, needle directed 45 degrees palmaroproximal to palmarodistal. B) dorsal: abaxial to CDE tendon. C) distopalmar: depression proximal to proximal palmar process of PI. D) palmar: between dorsal sesamoid and palmar distal MC3, flex fetlock

Question 23

What is the position and location of fetlock joint block

Answer 23

-Position: standing or limb elevated
-Location: a) proximopalmar: 2 cm distal to splint button, proximal to collateral sesamoidean ligaments, needle directed 45 degrees palmaroproximal to palmarodistal. B) dorsal: abaxial to CDE tendon. C) distopalmar: depression proximal to proximal palmar process of PI. D) palmar: between dorsal sesamoid and palmar distal MC3, flex fetlock

Question 24

What structures are blocked in a fetlock joint block

Answer 24

-Fetlock joint
-May also block proximal sesamoid bones, suspensory branches, digital flexor tendon sheath

Question 25

What joints are included in a carpal joint block

Answer 25

-Radiocarpal joint (proximal), middle carpal joint, and carpometacaparl joint (distal)

Question 26

What is the position and location of the tarsometatarsal joint block

Answer 26

-Position: standing
-Location: just proximal to lateral splint bone at level of palpable depression, betweenhead of splint and 4th tarsal bone, about 45 degree angle craniomedially and distally

Question 27

What structures are blocked in a tarsometatarsal joint block

Answer 27

-TMT joint, may also block DIT joint (76% of cases due to diffusion), origin of suspensory ligament, proximal MT3

Question 28

What is the position and location of the Distal intertarsal/ centrodistal tarsal joint block

Answer 28

-Position: standing
-Location: medial access, medial aspect of hock, insert needle horizontally between fused 1 and 2 tarsal bones and 3 and central tarsal bones, small depression just distal to cunean tendon

Question 29

What structures are blocked in a centrodistal joint block

Answer 29

-DIT joint, may also block TMT joint in 68% of cases,TC in 32% of cases

Question 30

What is the position and location of the tarsocrural/tibiotarsal joint block

Answer 30

-Position: standing
-Location: 1) dorsomedial pouch medial to saphenous vein. 2) dorsolateral pouch. 3) plantar pouches

Question 31

What structures are blocked by a tarsocrural joint block

Answer 31

-Tarsocrural joint, TCCQ/PIT joint

Question 32

Where, how, and why would you do caudal epidural anesthesia

Answer 32

-Where: first intercoccygeal space
-How:20 gauge needle perpendicular to skin on midline, bevel directed cranially, use hanging drop technique and make sure no resistance
-Why: for standing surgery of the rectum, anus, perineum, tail, vulva, vagina, penis, and in guitar region, or for relief of straining post operative or during dystocia

Question 33

What drugs can you use in a caudal epidural

Answer 33

-Local anesthetics: 2% lidocaine, mepivicaine (lasts for 90-120 minutes, mild ataxia)
-Alpha 2 adrenergics: 2% xylazine (lasts 150 minutes)
-Combo of A2A and LAs (can last up to 330 minutes)
-Dilute with saline to less than 10 mls in adult horse to avoid severe ataxia or recumbency

Question 34

When would you do epidural catheterization

Answer 34

-For chronic hindlimb pain, hindlimb laminitis, severe cellulitis