Page 196 - Adams and Stashak's Lameness in Horses, 7th Edition

162 Chapter 2

of the median nerve plus the palmar branch of the ulnar
nerve. The lateral palmar nerve courses in a dorsolateral
VetBooks.ir the palmar distal aspect of the accessoriometacarpal liga­
direction distal to the accessory carpal bone and runs along
ment (Figure 2.153d). At the proximal end of the fourth
metacarpal bone, the lateral palmar nerve gives off its deep
branch that detaches branches to the origin of the sus­
pensory ligament and divides into the lateral and medial
palmar metacarpal nerves (Figure 2.153b and b ).
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1
2
The lateral palmar nerve can be anesthetized just below
the accessory carpal bone (lateral approach) or axial to the
accessory carpal bone in a more proximal location (medial
approach). This block desensitizes the origin of the suspen­
sory ligament and other deep structures of the palmar
metacarpus. Performing this block avoids the necessity of
direct infiltration of the suspensory ligament and anesthe­
sia of the palmar and palmar metacarpal nerves
independently. 24,34
The lateral palmar nerve is anesthetized with 4–6 mL
of anesthetic administered through a 1‐inch (2.5‐cm),
20‐gauge needle midway between the distal border of
D
the accessory carpal bone and the proximal end of the
fourth metacarpal bone on the palmar border of the
accessoriometacarpal ligament (Figures 2.152c and
C2 2.153d). 3,75 The needle is directed in a palmarolateral‐
C1
to‐dorsomedial direction and must penetrate the 2‐ to
3‐mm thickness of the flexor retinaculum of the carpus
A2 (Video 2.12). 3,34 This block may be performed with the
A1 horse standing or with the carpus slightly flexed. 3,34 Skin
B1 B2 sensation is not useful to evaluate the effect of the block.
Instead, lack of any response to deep palpation of the
proximal suspensory ligament often suggests an effec­
tive block.
Figure 2.153. Palmar view of the carpometacarpal region of the
left forelimb showing the synovial outpouchings of the carpometa- Lateral Palmar Block (Medial Approach)
carpal joint (arrows). (A1 and A2) Sites for injection of the palmar The lateral palmar nerve may also be blocked medial
nerves. (B1 and B2) Sites for injection of the palmar metacarpal to the accessory carpal bone. This medial technique is
10
nerves. (C1 and C2) Sites for direct infiltration of the origin of the thought to reduce the risk of inadvertent injection into
suspensory ligament. (D). Site for injection of the lateral palmar the carpal canal, which may occur with the lateral
nerve using the lateral approach.
approach to the lateral palmar nerve. 24,48,49 The site of
injection is a longitudinal groove in the fascia palpable
and lateral palmar metacarpal nerves innervate the over the medial aspect of the accessory carpal bone, pal­
interosseous ligaments of the second and fourth meta­ mar to the insertion of the flexor retinaculum that forms
carpal bones, the interosseous lateralis and medialis the palmaromedial aspect of the carpal canal. 10,48 With
muscles, and the suspensory ligament (interosseous the limb weight‐bearing, a 25‐gauge, 5/8‐inch needle is
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muscle). The palmar nerves innervate the flexor ten­ inserted into the distal third of the groove in a mediolat­
dons and the inferior check ligament. Horses that eral direction perpendicular to the limb. The needle
48
become sound after this block warrant diagnostic imag­ should contact the bone and 3–4 mL of anesthetic is
ing of the metacarpal region. Lack of improvement with injected (Figure 2.154). In some cases the injection may
a high palmar block does not necessarily rule out a be difficult until the needle is withdrawn slightly or redi­
problem at the origin of the suspensory. Because of the rected. The author uses a 22‐gauge, 1‐inch (2.5 cm) nee­
difficulty of performing this block and its lack of speci­ dle and 3–4 mL of anesthetic for this technique (Video
ficity, use of the high palmar block has fallen out of 2.13). A recent contrast study indicated that anesthetic
favor with many clinicians. The lateral or medial may be distributed proximally to the distal third of the
approach to the lateral palmar nerve is usually per­ antebrachium following this block. Skin sensation is
49
formed instead especially if a proximal suspensory prob­ not useful to evaluate the effect of the block, and lack of
lem is suspected. Alternatively, a high 2‐point block can pain on palpation of the suspensory ligament is often
be performed to desensitize the metacarpal region. the best indicator of success.

Lateral Palmar Block (Lateral Approach) High 2‐Point Block
The lateral palmar nerve originates at a variable dis­ The high 2‐point block is a combination of the lateral
tance proximal to the carpus and represents a continuation palmar block and the high medial palmar block (1 nerve