Regional Anesthesia
Section Editor: Terese T. Horlocker
Peripheral Nerve Injury After Local Anesthetic
Injection
Scott J. Farber, MD, Maryam Saheb-Al-Zamani, MD, MS, Lawrence Zieske, Osvaldo Laurido-Soto,
Amit Bery, Daniel Hunter, RA, Philip Johnson, PhD, and Susan E. Mackinnon, MD
BACKGROUND: A well-known complication of peripheral nerve block is peripheral nerve injury,
whether from the needle or toxicity of the medication used. In this study, we sought to determine
the extent of damage that results from intrafascicular injection of various commonly used local
anesthetics (LAs).
METHODS: Sixteen Lewis rats received an intrafascicular injection of saline (control) or 1 of 3
LAs (bupivacaine, lidocaine, or ropivacaine) into the sciatic nerve (n = 4). At a 2-week end point,
the sciatic nerves were harvested for histomorphometric and electron microscopic analysis.
RESULTS: Animals that received intrafascicular LA injections showed increased severity of injury
as compared with control. In particular, there was a significant loss of large-diameter fibers as
indicated by decreased counts (P < 0.01 for all LAs) and area (P < 0.01 for all LAs) of remaining fibers in severely injured versus noninjured areas of the nerve. There was a layering of
severity of injury with most severely injured areas closest to and noninjured areas furthest
from the injection site. Bupivacaine caused more damage to large fibers than the other 2 LAs.
In all groups, fascicular transection injury from the needle was observed. Electron microscopy
confirmed nerve injury.
CONCLUSIONS: Frequently used LAs at traditional concentrations are toxic to and can injure
the peripheral nerve. Any combination of motor and/or sensory sequelae may result due to the
varying fascicular topography of a nerve. (Anesth Analg 2013;117:00–00)
N
eurotoxicity after the use of local anesthetics (LAs)
for peripheral nerve block has been recognized for
decades.1–4 Many experimental studies have documented that the intraneural injection of drugs, including
LAs, is neurotoxic.2,3,5–15 Similarly, loss of sensation, motor
function, pain, and causalgia have been reported as a result
of intraneural injection of LAs.1,16–22 More recently, some
studies have claimed that the inadvertent intraneural injection of LAs does not always result in lasting neurological
injury or functional impairment.23–27 In addition, some anesthesia textbooks and technical manuals, in addition to minimizing the toxicity of certain LAs, suggest that nerve injury
due to inadvertent injection of LAs is extremely rare.4,28,29
Given these contradictory opinions, this study reevaluated the effect of currently used LA drugs on the rat sciatic
nerve using histological assessment techniques.
METHODS
Animals
Sixteen inbred adult male Lewis rats, weighing 200 to 250 g
(Charles River Lab, Wilmington, MA), were used. Rodents
From the Division of Plastic and Reconstructive Surgery, Washington University in St. Louis, St. Louis, Missouri.
Accepted for publication May 29, 2013.
Funding: No funding.
The authors declare no conflicts of interest.
Reprints will not be available from the authors.
Address correspondence to Susan E. Mackinnon, MD, Division of Plastic
and Reconstructive Surgery, Washington University in St. Louis, 660 S Euclid
Ave., 1150 NW Tower Campus Box 8238, St. Louis, MO 63110. Address e-mail
to [email protected].
Copyright © 2013 International Anesthesia Research Society
DOI: 10.1213/ANE.0b013e3182a00767
September 2013 • Volume 117 • Number 3
were housed in a central animal care facility with a 12-hour
light–dark cycle. Food and water were provided ad libitum.
Animals were monitored for appropriate postsurgical recovery and weight gain. All study procedures were approved
by the Washington University Animal Studies Committee.
Experimental Design
Animals were randomized to 4 groups (n = 4), corresponding to injected drug. Four different drugs were evaluated
in this study, including 3 LAs: lidocaine HCl (1%, Hospira
Inc., Lake Forest, IL), bupivacaine (0.5%, Hospira Inc.),
and ropivacaine (0.5%, Naropin®; APP Pharmaceuticals,
Schaumburg, IL). Saline (0.9%) was used as negative control. All drugs were administered intrafascicularly in a 50
µL volume to the sciatic nerve 5 mm proximal to its trifurcation (Fig. 1). Each animal received a single treatment involving 1 drug in the right sciatic nerve. A 2-week end point
was chosen to evaluate the complete histological effects of
Wallerian degeneration and early regeneration through the
site of injury. At the 2-week end point, the sciatic nerve was
harvested to evaluate the extent of nerve injury using light
microscopy, electron microscopy (EM), and quantitative
histomorphometry.
All anesthetics used were confirmed to be current, eliminating any confounding factors that might be present in
expired drugs. Treatment volume and the concentration of
the injectables were based on the concentrations described
in the literature for major peripheral nerve block30–32 and
prior published work on LA injection injury.3,17
Surgical Procedure
All surgical procedures were performed aseptically under
an operating microscope (Wild Heerbrugg, Heerbrugg,
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Peripheral Nerve Injury After Local Anesthetic Injection
Figure 1. Demonstration of intrafascicular injection technique.
Switzerland). Animals were anesthetized by subcutaneous administration of a mixture of 75 mg/kg ketamine
hydrochloride (Fort Dodge Animal Health, Fort Dodge, IA)
and 0.5 mg/kg dexmedetomidine hydrochloride (Orion
Corporation, Espoo, Finland).
The right hindquarters of the animals were shaved and
sterilized with povidone iodine solution. The sciatic nerve
was exposed via a gluteal muscle-splitting incision. An area
5 mm proximal to the takeoff of the tibial nerve from the sciatic nerve trifurcation was used as a reproducible injection
site. The intrafascicular injections were performed with an
insulin syringe fitted with a 29-gauge × 1/2″ needle (Terumo
Medical Co., Elkton, MD) and aimed deep into the fascicles
and distally using constant finger pressure. Intrafascicular
injections are administered directly into the nerve fascicles,
and placement was confirmed under the microscope by an
obscured needle bevel. Caution was taken not to impart a
“through and through” injury of the nerve with the needle.
A symmetric fusiform ballooning of the nerve after injection
confirmed that our intrafascicular injections were properly
placed. To minimize the issue of injury imparted by the needle, the same method of injection was used for every animal
and was performed by the same operator. The injury site
was marked with a single 10-0 nylon microsuture. Muscle
and skin were reapproximated with 6-0 Vicryl (Ethicon
Inc., Somerville, NJ) and 4-0 nylon, respectively. Anesthesia
was then reversed with 0.2 mg/kg subcutaneous injection
of atipamezole HCl (Pfizer Animal Health, Exton, PA). At
the 2-week end point, animals were reanesthetized and
the incision site was reopened. Neurolysis was performed
to expose the sciatic and tibial nerves. The nerves were
sharply excised, ensuring that a 10 mm section both proximal and distal to the injection site was removed en bloc. The
nerve was promptly placed in 3% glutaraldehyde solution
at 4°C to prepare it for histomorphometric analysis. Once
the nerve was harvested, the animal was euthanized with
intracardiac injection of 200 mg/kg sodium pentobarbital
(Euthasol; Delmarva Laboratories, Des Moines, IA) while
anesthetized.
Light Microscopy and Histomorphometry
Harvested sciatic nerves from the Lewis rats were processed
in a manner previously described.3,33 First, they were all
immersion-fixed by placing the samples overnight in 3%
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glutaraldehyde at 4°C. They were then postfixed with 1%
osmium tetroxide, dehydrated in graded concentrations
of ethanol, and embedded in Araldite 502 (Polysciences,
Inc., Warrington, PA). Cross-sections, 1-µm thick, from the
injured region of the nerve were stained with toluidine
blue. Nerves underwent serial sectioning to identify the
injection site marked by a suture. Images taken at 1000×
magnification were measured to determine fiber area along
with density of myelinated fibers. These images were analyzed using a digital image analysis system linked to morphometric software (Leco Instruments, St. Joseph, MI) and
algorithms designed by Hunter et al.34 A single histologist,
blinded to treatment group, performed image analysis. In
some samples, needle track site was directly confirmed by
light microscopy. The size of the affected region varied from
specimen to specimen, and to account for this variable,
nerve fiber counts were expressed as a density rather than
an absolute number.
Electron Microscopy
Samples embedded in Araldite 502 (Polysciences Inc.) were
cut into sections with an ultramicrotome, stained with
uranyl acetate and lead citrate, and evaluated with a Jeol
1200EX electron microscope (Jeol USA Inc., Peabody, MA)
for qualitative assessment of myelination, perineurial competence, extent of fibrosis, and cellular damage. EM was
performed by a nonblinded operator.
Statistical Analysis
Statistical analysis was performed with Statistica version 7
(StatSoft, Tulsa, OK) Sample size was determined by power
calculation. Four animals were used per group to achieve
a power of 0.80 and to detect a decrease in fiber density of
more than 30% or 6000 fibers per mm2 with a standard deviation of about 2000 fibers per mm2.3 Four animals per group
were all that was necessary to show the significant histological damage as determined by the experienced blinded
observer (DH).
All groups and injury parameters were independently
compared using analysis of variance . Any statistically significant differences in sample means were further evaluated
by Dunnett test, and the corrected P value was reported.
Given the small sample size and consequently the inability to test for normality and equal variance, a stringent P
value <0.01 was considered to be statistically significant in
all comparisons. After treating the animal as a fixed effect,
there was no effect of the animal on any dependent variables (minimum P > 0.378).
Determination of Injury and Toxicity
A characteristic LA injection injury caused neuronal damage from needle trauma, increased intraneural pressure
with injection, and toxicity intrinsic to a given drug. The
combination of these factors can result in variable degrees of
neuronal injury and was classified as intermediate or severe
(Table 1). Intermediate nerve injury was defined by a focal
area of partial loss of large myelinated fibers with endoneurial expansion secondary to edema. Severe nerve injury was
defined as a focal area of total loss of large myelinated fibers
along with extensive fibrosis (Fig. 2).
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Table 1. Description of Nerve Injection Injuries
Degree of injury
Intermediate
Severe
Histologic features
Focal areas of partial loss of large myelinated
fibers with endoneurial expansion secondary
to edema
Focal areas of total loss of large myelinated fibers
along with extensive fibrosis and collagen
disarray
Increasing toxicity of a drug is reflected in increased
severity of injury as well as a more expansive region of
injury. Severity of injury was determined by degree of loss
of myelinated fibers and fibrosis within the injected nerve
as described earlier.
RESULTS
Histomorphometry and Light Microscopy
Injection Injury
All histomorphometric results are summarized in Table 2.
The noninjured zone of saline-injected nerves was used as a
negative control. The insult from administration of an anesthetic by injection resulted in a significant decrease in the
number of nerve fibers in the zone of intermediate injury
(bupivacaine P < 0.0001, lidocaine P < 0.0001, ropivacaine
P < 0.0001, and saline P < 0.0001) and smaller fiber percent
per nerve (bupivacaine P < 0.0001, lidocaine P < 0.0001,
ropivacaine P < 0.0001, and saline P < 0.0001) in comparison
with the control noninjured zone in saline-injected nerves.
In particular, there was a loss of large--diameter fibers as
exhibited by a decrease in fiber area of the remaining nerves
(bupivacaine P < 0.0001, lidocaine P < 0.0001, ropivacaine
P < 0.0001, and saline P < 0.0001) compared with noninjured fibers in the control zone. The area of intermediate
injury was smaller than the noninjured area (bupivacaine
P < 0.0001, lidocaine P < 0.0001, ropivicaine P < 0.0001, and
saline P < 0.0001), which explains the lack of difference in
fiber density within the zone of intermediate injury relative to fiber density in uninjured control areas (bupivacaine
P = 0.0700, lidocaine P = 0.1493, ropivacaine P = 0.0019, and
saline P = 0.6106).
Cross-sections of whole nerves were analyzed, and a
comparison of intermediately injured zones among groups
injected with bupivacaine, lidocaine, ropivacaine, and
saline revealed that a comparable number of axons were
Figure 2. Light microscopy at 2 weeks postinjection (100×): Demonstration of the 4 injectate groups. S = severe injury area; I = intermediate
injury area; N = normal area. Black arrow indicates direction of needle track. The intrafascicular injection of saline produced little nerve fiber
damage. By contrast, the intrafascicular injection of the local anesthetic agents caused variable damage relating to the injection site with the
most significant axonal injury closest to the injection site.
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Peripheral Nerve Injury After Local Anesthetic Injection
Table 2. Summary of Histomorphometric Analysis of Nerves Following Injection Injuries
No. of
nerves with
injury
Zone of injury
Injected drug
Severe
Bupivacaine (n = 4)
4
Intermediate
4
None
4
Severe
Lidocaine (n = 4)
3
Intermediate
4
None
4
Severe
Ropivacaine (n = 4)
4
Intermediate
4
None
4
Severe
Saline (n = 4)
0
Intermediate
2
None
4
Mean nerve
Mean percent
Mean total nerve Mean fiber area
fiber density
area of fibers per
fibers per nerve
(μm2)
(fibers/mm2)
nerves (%)
11.23 ± 4.03a 12,104 ± 10,246 16.06 ± 17.10a
254 ± 229a
803 ± 485a
14.59 ± 2.51a 12,477 ± 3658
18.70 ± 7.77a
6783 ± 1830
28.98 ± 0.86
20,750 ± 2701
60.01 ± 6.79
6.33 ± 0.93a
2306 ± 2261a
1.59 ± 1.75a
48 ± 20a
1049 ± 668a
13.66 ± 2.55a 13,465 ± 2188
18.69 ± 5.50a
7333 ± 927
33.52 ± 1.70
16,842 ± 1416
56.35 ± 3.63
11.62 ± 3.24a
3322 ± 3341a
4.14 ± 4.09a
127 ± 139a
482 ± 368a
19.66 ± 3.20a
8039 ± 4334
16.01 ± 8.70a
8535 ± 568
45.44 ± 4.35
14,010 ± 1135
63.38 ± 3.81
—
—
—
—
13.10 ± 4.26a 15,107 ± 3217
19.29 ± 3.40a
214 ± 120.33a
8400 ± 1144
31.02 ± 1.35
18,390 ± 2434
56.96 ± 7.06
Mean total area of
injury (μm2)
27,261 ± 27,815a
74,226 ± 64,321a
329,127 ± 86,330
29,733 ± 16,767a
76,712 ± 42,247a
434,947 ± 30,769
55,127 ± 51,551a
57,390 ± 16,233a
614,641 ± 90,389a
—
13,748 ± 5857a
456,812 ± 11,736
Values represent mean ± SD.
Intermediate nerve injury is defined by a focal area of partial loss of large myelinated fibers with endoneurial expansion secondary to edema. Severe nerve injury
is defined as a focal area of total loss of large myelinated fibers along with extensive fibrosis and collagen disarray.
a
Significantly different (P < 0.01) from zone of no injury in saline-injected nerves (negative control).
affected as measured by mean total fiber count, size of
remaining fibers, fiber density, area of injury, and percent
nerve (P > 0.01 for all group comparisons across all measures, Tables 3 and 4). Needle track injury was observed
in all groups, resulting in a sixth-degree injury, or one that
includes Sunderland first- through fifth-degree injuries, further detailed in the Discussion section.
The degree of injury decreased with increasing distance
from site of injection, with the zone of severe injury (if present) surrounded by zones of intermediate and no injury, in
succession (Fig. 3). As the concentration of the LA decreased
as it dispersed, a progressive decline in damage away from
the injection site was seen. Administration of saline resulted
in intermediate damage to nerves, indicating a baseline level
of injury associated with injecting any agent into a nerve.
Drug Toxicity
In the negative control group, administration of 50 µL saline
resulted in no severe injury to nerve fibers. This is in contrast to bupivacaine, lidocaine, and ropivacaine, where
nearly all of the injected nerves showed areas of severe
injury, defined by total loss of large myelinated fibers and
extensive fibrosis. Regardless of the drug used, there was
a significant decrease in number of fibers (bupivacaine P <
0.0001, lidocaine P < 0.0001, and ropivacaine P < 0.0001) and
percent nerve in severely injured nerve zones as compared
with noninjured control zones (bupivacaine P = 0.0003, lidocaine P < 0.0001, and ropivacaine P < 0.0001) and decrease
in fiber area (bupivacaine P = 0.0002, lidocaine P = 0.0002,
and ropivacaine P = 0.0002) denoting a drop-off of largediameter fibers. There was also a trend toward smaller fiber
density in lidocaine-injected (P = 0.0110) and ropivacaineinjected (P = 0.0102) groups relative to bupivacaine-injected
nerves (P = 0.3433). No differences were observed between
the drug-injected nerves across any measurements of
severely injured areas (Fig. 4). No zone of severe injury was
observed in saline-injected nerves as defined by the variables outlined earlier.
Axon–myelin ratios, previously described,35,36 were
measured in all groups. The normal ratio is approximately
0.6, with a higher ratio indicating a decreased amount of
myelin surrounding the axon. In the bupivacaine, lidocaine,
and ropivacaine groups, the intermediate injury zones had
an axon–myelin ratio of 0.68, 0.66, and 0.77, respectively.
The severely injured zones had axon–myelin ratios of 0.76,
0.81, and 0.83, respectively. As indicated by the increasing
ratios, there was a decrease in myelin thickness as injury
progressed from the intermediate zone to the severe zone in
all 3 experimental groups.
Electron Microscopy
Cross-sections of the nerve were analyzed via EM and
revealed that the extent and location of fascicular injury varied
widely among the LA groups. In the saline group, no injury
was observed apart from the occasional injury caused by the
needle. Figure 5 represents an EM of an uninjured section of
nerve in the saline group with normal-appearing myelinated
Table 3. Statistical Comparisons of Nerve Parameters Between Injected Drugs in Severe Zones of Injuries
No. of
Mean total
Comparison with
Mean nerve
Mean percent
nerves
Mean total nerve Mean fiber
area of injury
severe zone of
fiber density
area of fibers per
Severe zone of injury
area (μm2)
(μm2)
injury of injectate with injury fibers per nerve
(fibers/mm2)
nerves (%)
from injectate
Bupivacaine (n = 4)
4
254 ± 229
11.23 ± 4.03 12,104 ± 10,246 16.06 ± 17.10a 27,261 ± 27,815
Lidocaine
3
P = 0.9753
P = 0.2034
P = 0.2281
P = 0.3298
P = 0.9185
Ropivacaine
4
P = 0.9907
P = 0.9972
P = 0.2053
P = 0.3693
P = 0.4869
Lidocaine (n = 4)
3
48 ± 20
6.33 ± 0.93
2306 ± 2261
1.59 ± 1.75
29,733 ± 16,767
Bupivacaine
4
P = 0.9753
P = 0.2034
P = 0.2281
P = 0.3298
P = 0.9185
Ropivacaine
4
P = 0.9986
P = 0.1573
P = 0.9964
P = 0.9887
P = 0.3042
Ropivacaine (n = 4)
4
127 ± 139
11.62 ± 3.24
3322 ± 3341
4.14 ± 4.09
55,127 ± 51,551
Bupivacaine
4
P = 0.9907
P = 0.9972
P = 0.2053
P = 0.3693
P = 0.4869
Lidocaine
3
P = 0.9986
P = 0.1573
P = 0.9964
P = 0.9887
P = 0.3042
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Table 4. Statistical Comparisons of Nerve Parameters Between Injected Drugs in Intermediate Zones of Injuries
Comparison with
No. of
intermediate
nerves
Mean total nerve Mean fiber area
zone of injury of
Intermediate zone
with injury fibers per nerve
(μm2)
injectate
of injury from injectate
Bupivacaine (n = 4)
4
803 ± 485
14.59 ± 2.51
Lidocaine
4
P = 0.9864
P = 0.9868
Ropivacaine
4
P = 0.9641
P = 0.1115
Saline
2
P = 0.9134
P = 0.9788
Lidocaine (n = 4)
4
1049 ± 668
13.66 ± 2.55
Bupivacaine
4
P = 0.9864
P = 0.9868
Ropivacaine
4
P = 0.7816
P = 0.0479
Saline
2
P = 0.7570
P = 0.9995
Ropivacaine (n = 4)
4
482 ± 368
19.66 ± 3.20
Bupivacaine
4
P = 0.9641
P = 0.1115
Lidocaine
4
P = 0.7812
P = 0.0479
Saline
2
P = 0.9950
P = 0.9995
Saline (n = 4)
2
214 ± 120.33
13.10 ± 4.26
Bupivacaine
4
P = 0.9134
P = 0.9788
Lidocaine
4
P = 0.7570
P = 0.9995
Ropivacaine
4
P = 0.9950
P = 0.1613
and unmyelinated fibers. In the intermediate zone of injury,
normal Wallerian degeneration was seen alongside smaller,
uninjured fibers (Fig. 6). EM demonstrated decreased axon–
myelin ratios along with some axons that maintained normal axon–myelin ratios. In the severe zone, EM revealed
near complete loss of myelin along with increased fibrosis
indicated by an increase in fibroblasts (Fig. 7). There was
also increased collagen disorganization evident in this zone.
Mean nerve
fiber density
(fibers/mm2)
12,477 ± 3658
P = 0.9917
P = 0.3386
P = 0.9201
13,465 ± 2188
P = 0.9917
P = 0.1790
P = 0.9848
8039 ± 4334
P = 0.3386
P = 0.1790
P = 0.2387
15,107 ± 3217
P = 0.9201
P = 0.9848
P = 0.2387
Mean percent
area of
fibers per
nerves (%)
18.70 ± 7.77
P = 1.0000
P = 0.9820
P = 1.0000
18.69 ± 5.50
P = 1.0000
P = 0.9823
P = 1.0000
16.01 ± 8.70
P = 0.9820
P = 0.9823
P = 0.9897
19.29 ± 3.40
P = 1.0000
P = 1.0000
P = 0.9897
Mean total area
of injury (μm2)
74,226 ± 64,321
P = 1.0000
P = 0.9688
P = 0.5328
76,712 ± 42,247
P = 0.9998
P = 0.9494
P = 0.496125
57,390 ± 16,233
P = 0.9688
P = 0.9495
P = 0.7826
13,748 ± 5857
P = 0.5328
P = 0.4961
P = 0.7826
Injury was also evident in the perineurium surrounding this
zone, evident by perineurial thickening. In addition, edema
and fibrosis were evident on EM throughout the endoneurium in all of our experimental groups.
DISCUSSION
Peripheral nerve block with LA is a common practice in providing pain control for a wide range of surgical procedures
Figure 3. Light microscopy at 2 weeks postinjection (400×): Higher magnification areas of severe axonal injury are noted in the nerves injected
with the local anesthetic drugs.
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Peripheral Nerve Injury After Local Anesthetic Injection
Figure 4. Nerve injury area: The relative injury areas (µm2) were measured using a binary imaging technique. Layering of injury with severe
injury occurring closest to injection site (indicated by black arrow) is seen in the adjacent diagram.
and pain syndromes. Inadvertent intrafascicular injection
of an LA can generate a variety of nerve injuries, some of
which may result in long-term disability.17 All anesthetics used in this study produced some degree of damage
to the nerve when injected intrafascicularly, as evidenced
by demyelination and Wallerian degeneration. The most
severe injury occurred closest to the site of injection, and
the extent of injury tapered off as distance from the injection
site increased. This led to mixing of the injuries, evident in
all LA groups on histological analysis as seen in Figure 4.
Although there were minor differences in the extent of
injury caused by the 3 LAs, there was an overall reduction
of fiber quantity and fiber area in both the intermediate and
severe injury zones. EM confirmed the severe injury zones
by demonstrating absence of nerve fibers associated with
severe fibrosis and collagen disarray. Intermediate injury
was seen in all LA groups as areas of Wallerian degeneration alongside normal myelinated fibers. All 3 LAs used
Figure 5. Electron microscopy (3690×) 2 weeks after intrafascicular saline injection: Demonstrating normal area of myelinated and
unmyelinated fibers, with no injury.
6 www.anesthesia-analgesia.org
in this study—ropivacaine, lidocaine, and bupivacaine—
caused considerable histological abnormalities when
injected intrafascicularly.
Nerve injury from peripheral nerve blockade is well
described; however, case reports of incidence, duration, and
extent of sequelae are varied. Nerve injection injury is considered to be multifactorial in nature. Factors such as needle
type and size, site of insertion, angle of insertion, pressure
achieved during injection, type, and dose of medication
injected (toxicity) affect degree of nerve injury according to
various studies.3,6,9,10,12,18,37–39 All of these variables account
for the disparity in the incidence of nerve injury with anesthetic injection reported in the literature.21,22,28,40
The effect of intrafascicular and extrafascicular injection
of numerous drugs in common use has been extensively
Figure 6. Electron microscopy (4610×) at 2 weeks after intrafascicular ropivacaine injection: Demonstrating a normal myelinated axon
(N) adjacent to axons in the process of Wallerian degeneration (W)
and a regenerating unit (R) in the intermediate zone of injury.
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Figure 7. Electron microscopy (4610×) 2 weeks after ropivacaine
intrafascicular injection: Demonstrating collagen fiber disarray (C) and
fibroblast (F) invasion in the severe injury zone. No nerve fibers noted.
studied by our laboratory.2,3,5–7,9,10,12,38 In general, most drugs
caused nerve injury when injected intrafascicularly, and, in
contrast, extrafascicular injections produced little to no damage. A few exceptions, dexamethasone, botulinum toxin,
and bovine collagen, demonstrated little to no axonal damage after intrafascicular injection.7,9,38 LAs have been shown
to have a direct toxic effect on the nerve. Several studies
have demonstrated that LAs can lead to fragmentation of
DNA and disrupt the membrane potential in mitochondria,
resulting in the uncoupling of oxidative phosphorylation.
This in turn releases cytochrome c and activates caspase,
which may result in apoptosis.41,42 The activation of proapoptotic enzymes in neuronal cells such as p38 mitogenactivated protein kinase and Jun N-terminal kinase have
been shown to be initiated by lidocaine, bupivacaine, and
ropivacaine.41,43 Generation of reactive oxygen species in
Schwann cells by bupivacaine results in apoptosis.44 LAs
have been shown to cause rapid necrosis of human neuronal cells. In addition, there is a direct correlation between
concentration of the LA and time of exposure to the nerve
with Schwann cell death.42,45 Rat, porcine, and canine sciatic
nerves with exposure to LAs demonstrate infiltration of the
nerve with macrophages and associated myelin damage.45
In addition to inducing apoptotic pathways and signals,
LAs have been shown to constrict vasculature and decrease
the blood flow to the nerves.46–48 There are a number of previous studies that have demonstrated differing levels of toxicity among various LAs.42,45,49 The observed toxicity from
these previous studies is consistent with our study demonstrating that all 3 LAs produced ­neural damage.
In addition to toxicity of the medication used, the intraneural pressures achieved during accidental injection of
drug into the nerve have been shown to be damaging.
Hadzic et al.18 found that when using the same injectate volumes, intrafascicular injections resulted in higher pressures
September 2013 • Volume 117 • Number 3
and were associated with lasting motor deficits and histological damage. More recently, Orebaugh et al.50 reported
that all intrafascicular injections at the level of the brachial
plexus root in fresh human cadavers result in high injection pressure. The saline control in this study demonstrated
some areas of intermediate damage, comparable with that
of the 3 anesthetic groups. Although we did not measure
intraneural pressure, this may reinforce the idea that intermediate level injury can be largely attributed to the trauma
of increased intraneural pressure on injection. However, the
role of drug toxicity in producing intermediate level injury
cannot be completely discounted as only 2 of the 4 nerves
in the saline group showed intermediate injury whereas
all 4 nerves in the bupivacaine, lidocaine, and ropivacaine
groups showed intermediate injury.
In this study, intrafascicular injection resulted in varying
grades of injury to the nerve as indicated by histomorphometry. Traditionally, peripheral nerve injury has been classified into 5 different degrees, as described by Sunderland.51
A first-degree injury, or neurapraxia, involves segmental demyelination. Second-degree injury, or axonotmesis,
involves injury to both the axon and the myelin; however,
the endoneurial tissue is not damaged. Third-degree injury
involves an injury to the axon, myelin, and endoneurium
and leaves the perineurium intact. A fourth-degree injury
involves injury to the axon, myelin, endoneurium, and perineurium; the fascicle is scarred with only the epineurium
being intact. Fifth-degree injuries occur when the fascicle
or entire nerve has been transected. Sixth-degree injury
was first described by Mackinnon et al.52,53 and involves a
combination of first- through fifth-degree injuries. Fourththrough sixth-degree injuries require operative resection
and repair. These types of injuries have been demonstrated
by Sala-Blanch et al.38 where complete transection of fascicles (fifth- or sixth-degree injury) was observed. Our results
demonstrate that all 3 LA groups demonstrated sixth-degree
injury, as the nerves exhibited signs of first- to fifth-degree
injuries. Saline-injected nerves (control) demonstrated areas
of third-degree injury, with occasional axonal transection, or
fifth-degree injury.
In summary, peripheral nerve injury from LA injection
into the nerve occurs and remains a real clinical danger.
The sequelae from such an injury may be long lasting and
require surgical intervention. Intentional intraneural injection, as recommended by some clinicians to hasten the onset
of the block, is not recommended. E
DISCLOSURES
Name: Scott J. Farber, MD.
Contribution: This author helped design and conduct the
study, analyze the data, and write the manuscript.
Attestation: Scott J. Farber has seen the original study data,
reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study
files.
Name: Maryam Saheb-Al-Zamani, MD, MS.
Contribution: This author helped analyze the data and write
the manuscript.
Attestation: Maryam Saheb-Al-Zamani has seen the original
study data, reviewed the analysis of the data, and approved the
final manuscript.
www.anesthesia-analgesia.org 7
Copyright © 2013 International Anesthesia Research Society. Unauthorized reproduction of this article is prohibited.
Peripheral Nerve Injury After Local Anesthetic Injection
Name: Lawrence Zieske.
Contribution: This author helped design the study and analyze
the data.
Attestation: Lawrence Zieske has seen the original study data,
reviewed the analysis of the data, and approved the final
manuscript.
Name: Osvaldo Laurido-Soto.
Contribution: This author helped design the study and analyze
the data.
Attestation: Osvaldo Laurido-Soto has seen the original study
data, reviewed the analysis of the data, and approved the final
manuscript.
Name: Amit Bery.
Contribution: This author helped design the study and analyze
the data.
Attestation: Amit Bery has seen the original study data, reviewed
the analysis of the data, and approved the final manuscript.
Name: Daniel Hunter, RA.
Contribution: This author helped design the study and analyze
the data.
Attestation: Daniel Hunter has seen the original study data,
reviewed the analysis of the data, and approved the final
manuscript.
Name: Philip Johnson, PhD.
Contribution: This author helped design and conduct the
study and write the manuscript.
Attestation: Philip Johnson has seen the original study data,
reviewed the analysis of the data, and approved the final
manuscript.
Name: Susan Mackinnon, MD.
Contribution: This author helped design the study and write
the manuscript.
Attestation: Susan Mackinnon has seen the original study data
and approved the final manuscript.
This manuscript was handled by: Terese T. Horlocker, MD.
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