MISINTERPRETING DIAGNOSTIC ANALGESIAWHAT ARE MY BLOCKS TELLING ME?
Michael Schramme, John Schumacher, Jim Schumacher and Ray Wilhite
Département Hippique, Ecole Nationale Vétérinaire de Lyon, France
College of Veterinary Medicine, Auburn University, Alabama, USA
College of Veterinary Medicine, University of Tennessee, Knoxville, Tennessee
Summary
A palmar digital nerve block anaesthetises the entire foot, rather than just its palmar
half. To avoid partially anaesthetising the proximal interphalangeal joint, the palmar
digital nerves should be anaesthetised near or distal to the proximal margin of the
collateral cartilages. A basilar sesamoid nerve block may ameliorate pain within the
metacarpo(metatarso)phalangeal joint. Mepivacaine administered into the distal
interphalangeal (DIP) joint desensitises the DIP joint and the subsynovial palmar
digital nerves thereby also anesthetising the navicular apparatus, the toe region of
the sole and the digital portion of the deep digital flexor tendon (DDFT). Even though
analgesia of the DIP joint results in analgesia of the navicular bursa, analgesia of the
navicular bursa does not result in analgesia of the DIP joint. Analgesia of the DDFT
sheath likely desensitises only structures within the sheath and the DDFT distal to
the sheath.
Anaesthesia of the palmar digital nerves (Palmar digital nerve block-PDNB)
The PDNB is sometimes referred to as the “heel block” because clinicians have long
believed that a positive response to anaesthesia of the palmar digital nerves
localizes pain to the palmar third or half of the foot, including the palmar portion of
the DIP joint. Easter et al.1 found, however, that anaesthesia of the palmar digital
nerves just proximal to the bulbs of the heel alleviated lameness caused pain in the
DIP joint, indicating that the palmar digital nerves innervate the entire DIP joint.
The PD nerves should be anaesthetised near or distal to the proximal margin of the
collateral cartilage.2,3 One theoretical advantage of anaesthetizing the palmar digital
nerves as far distally as possible is that anaesthesia of the dorsal branches of the
palmar digital nerve is more likely to be avoided (Beeman 1988). According to
results of the study by Sack4 and that of Easter et al.,1 however, the dorsal branches
are unlikely to contribute much more than sensory innervation to the dorsal aspect of
the coronary band and dorsal laminae of the foot. Recent research suggests the
even the dorsal laminae of the foot are desensitised by a PDNB.5 Research
indicates, therefore, that a PDNB anaesthetises the entire foot.
A more important reason for depositing local anaesthetic solution as far distally in the
pastern as possible when performing a PDNB is that more proximal deposition of
local anaesthetic solution increases the likelihood of causing analgesia of the pastern
joint.6
Proceedings
of
the
South
African
Equine
Veterinary
Association
Congress
2016
43