Page 307 Guide to Pain Management in Low-Resource Settings

Diagnostic and Prognostic Nerve Blocks 295

Finally, it must be remembered that the pain Neuroaxial diagnostic nerve blocks
and anxiety caused by the diagnostic nerve block it-
Diff erential spinal and epidural blocks have gained
self may confuse the results of an otherwise technically a modicum of popularity as an aid in the diagnosis of
perfect block. Th e clinician should be alert to the fact
pain. Popularized by Winnie [9], diff erential spinal and
that many pain patients may premedicate themselves epidural blocks have as their basis the varying sensitivity
with alcohol or opioids because of the fear of procedur-
of sympathetic and somatic sensory and motor fi bers to
al pain. Th is situation also has the potential to confuse blockade by local anesthetics. While sound in principle,
the observed results. Obviously, the use of sedation or
these techniques are subject to some serious technical
anxiolysis prior to the performance of diagnostic nerve diffi culties that limit the reliability of the information
block will further cloud the very issues the nerve block
obtained. Th ey include:
is in fact supposed to clarify. 1) Th e inability to precisely measure the extent that
each type of nerve fi ber is blocked;
What are important and useful 2) Th e possibility that more than one nerve fi ber
specifi c diagnostic nerve blocks? type is simultaneously, blocked leading the clinician to
attribute the patient’s pain to the wrong neuroanatomi-
Early proponents of regional anesthesia such as La-
cal structure;
bat and Pitkin [3] believed it was possible to block just
3) Th e impossibility of “blinding” the patient to the
about any nerve in the body. Despite the many technical
sensation of warmth associated with sympathetic block-
limitations these pioneers were faced with, these clini-
ade as well as the numbness and weakness that accom-
cians persevered. Th ey did so, not only because they be-
pany blockade of the somatic sensory and motor fi bers;
lieved in the clinical utility and safety of regional nerve
4) Th e fact that in clinical practice, the construct
block, but because the available alternatives to render
of temporal linearity, which holds that the more “sensi-
a patient insensible to surgical pain at their time were
tive” sympathetic fi bers will become blocked fi rst, fol-
much less attractive. Th e introduction of the muscle
lowed by the less sensitive somatic sensory fi bers and
relaxant curare in 1942 by Dr. Harold Griffi th changed
lastly by the more resistant motor fi bers, breaks down.
this construct [2], and in a relatively short time, region-
As a practical matter, it is not uncommon for the patient
al anesthesia was relegated to the history of medicine,
to experience some sensory block prior to noticing the
with its remaining proponents viewed as eccentric at
warmth associated with block of the sympathetic fi bers,
best. Just as the Egyptian embalming techniques were
rendering the test results suspect;
lost to modern man, many regional anesthesia tech-
5) Th e fact that even in the presence of a neuroaxial
niques that were in common use were lost to today’s
block dense enough to allow a major surgical procedure,
pain management specialists.
aff erent nociceptive input can still be demonstrated in
What we have left are those procedures which
the brain;
have stood the test of time for surgical anesthesia. For
6) Th e fact that the neurophysiological changes as-
the most part, these were the nerve blocks that were not
sociated with pain may increase or decrease the nerves’
overly demanding from a technical viewpoint and were
fi ring threshold, suggesting that even in the present of
reasonably safe to perform. Many of these techniques
sub-blocking concentrations, there is the possibility that
also have clinical utility as diagnostic nerve blocks. Th ese
the sensitized aff erent nerves will stop fi ring;
techniques are summarized in Table 2. Th e more com-
7) Th e fact that modulation of pain transmission at
monly used diagnostic nerve blocks are discussed below.
the spinal cord, brainstem, and higher levels is known to
exist and may alter the results of even the most carefully
Table 2
Common diagnostic nerve blocks performed diff erential neural blockade; and
Neuroaxial blocks: epidural, subarachnoid 8) Th e fact that there are signifi cant behavioral
Peripheral nerve blocks: greater and lesser occipital, trigeminal, components to a patient’s pain, which may infl uence the
brachial plexus, median, radial and ulnar, intercostal, selective subjective response the patient reports to the clinician
nerve root, sciatic
performing diff erential neuroaxial blockade.
Intra-articular nerve blocks: facet
In spite of these shortcomings, neuroaxial dif-
Sympathetic nerve blocks: stellate ganglion, celiac plexus, lumbar,
ferential block remains a clinically useful tool to aid in
hypogastric plexus and ganglion impar