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LOCAL ANESTHETICS
Neuropathy – overview 295
By stabilizing sodium channels, local anesthetics block spontaneous ectopic
impulses in the axons of peripheral first-order neurons. They may also have
opioid-like effects, and may alter N-methyl-D-aspartate (NMDA) receptor
activity. This class of drugs may be most effective in a partially injured
nerve with excess sodium channnels. Clinically, patients with allodynia are
potential candidates for local anesthetic use, since allodynia implies presence
of functional, yet damaged, peripheral nocioceptors. If there are no
remaining peripheral nocioceptors (i.e. complete deafferentation), local
anesthetics will not work. The concentrations at which local anesthetics
suppress nocioceptive impulses are lower than those in which they suppress
normal sensorimotor impulses. Cochrane review found that oral or parenteral
local anesthetics exhibit both safety and efficacy in non-ED treatment
of the the general NP population. 28
Of the local anesthetics, lidocaine is the most studied. Other agents
(e.g. tocainide) have occasionally been discussed for NP therapy, but the
lack of supporting evidence and potential risk of side effects relegate these
agents to use outside the acute care setting. The most-cited use of topical
lidocaine in NP is for post-herpetic neuralgia. 29 However, the topical lidocaine
approach has also been the found effective in other NP conditions.
A trial in mixed-etiology focal painful polyneuropathy found that lidocaine’s
number needed to treat to achieve 50% pain relief was 4.4 (95% confidence
interval, 2.5–17.5). 30 Advantages of the lidocaine patch are its size and shape
flexibility and the physical protection that allodynic skin receives from the
covering patch.
Although mild local skin reactions may occur after topical application of
local anesthetics, systemic absorption of lidocaine is minimal when the drug
is used in topical form for NP. Intravenous lidocaine (5 mg/kg) is sometimes
used for NP, but the data are insufficient to warrant a recommendation for
ED use of this approach. 31
Local anesthetics may also be useful in NP when administered in the form
of regional nerve blocks. The regional block approach has some appeal for
acute care, especially for those nerve blocks commonly provided in the ED.