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7/23/2019 Subdural Injection
1/5
ANESTH ANALG 175
YXS.67.17i9
Inadvertent Subdural Injection:
A
Complication
of
an Epidural Block
Timothy Lubenow,
MD,
Elisa Keh-Wong, MD, Kathy Kristof,
BSN
Olga Ivankovich, MD,
and Anthony
D.
Ivankovich,
MD
LUBENOW
T,
KEH-WONG E, KRlSTOF K
IVANKOVICH 0 , IVANKOVICH AD. Inadvertent
subdural injection: a complication
of
an epidural block
Anesth Analg 1988;67:175-9.
Tzilenty-one h i d r e d eighty two consecutizle lzirribar epi
dural injections
zcwe
studied to determine
the
incidence
of
iriaiiziertent
subdural
hlock retrcispectiuel /. sitbtliiral
block is defined s an estensiz1e
neurd
block in
the
absence
of
S I ~ J U Y ~ C ~ ~ J O ~ ~
zriicturc,
that
is
out
of
proportion
to the
amount of local anesthetic injected. Siibdural injection s
conr;ilicntioii of epidural block that probabl y occiirs t l ow
frequently than previously recognized.
n
earlier report ins
esf i innted the incidence of siibdlird block
to
be
0 . 7
5 . This
study, liozuezlcr, reports n incidence of 0.82% froni a
s m p l e size of 2182 patients.
Cndarwic
dissection
700s
also
~~erfor ir ied,iirtlier
clarifying tlic
presence a n d riiztcnnic
position
of
the siibdurnl
spare.
Key
Words:
ANESTHETIC
T E C H N I Q U E S -
epidural.
It is generally accepted that the subdural space exists
in the cerebral meninges. The potential extension of
this subdural space, however, down into the spinal
segment of the meninges has not been well appreci-
ated. This subdural space can have clinical signifi-
cance when local anesthetics are inadvertently depos-
ited there, causing unexpected sensory, sympathetic,
and motor blocks.
Clinically the extraarachnoid space has been dem-
onstrated during myelograms with an incidence re-
ported between 1and 13%(1-3). This extraarachnoid
subdural space lies between the dura and arachnoid
membranes. It contains a small amount of serous
fluid to moisten the surfaces of the opposing mem-
branes. While not communicating with the subarach-
noid space, the extra-arachnoid subdural space does
continue for a short distance along the cranial and
spinal nerves (4). It is larger in the cervical than in
lumbar region, and is widest in its lateral and dorsal
aspects (5). Here, there is free communication with
the lymphatic vessels of the spinal nerves. Moreover,
there are isolated connective-tissue trabeculae, espe-
cially on the posterior aspect, which contact the
Presented a t the 61st Congress of the Interna tional Anesthes ia
Research Society, March 1418, 1987, Orlando, Florida.
Received from the Department
of
Anesthesiology, Rush Prys-
byterian St.
Lukes
Medical Center, Chicago, Illinois. Accepted for
publication
on
Oct.
1
1987.
inside surface of the dura and the outside surface of
the arachnoid.
Accidental subdural injections were first described
by de Saram
(6)
and Dawkins
7) ,
but no large series
have examined its occurrence. There has been several
case reports of accidental subdural catheterizations
that have been radiographically confirmed (&lo) .
Dawkins description of a massive epidural fits the
clinical presentation of an inadvertent subdural injec-
tion. He describes an unexpected widespread nerve
block occurring after a negative aspiration test asso-
ciated with symptoms such as pupillary dilation,
consistent with a high sympathetic block. In addition,
the patients experienced a 20-minute delay in the
onset of symptoms. This is in contrast to an acciden-
tal subarachnoid injection in which symptoms char-
acteristically develop in
1-2
minutes. The purpose of
this study is to retrospectively evaluate
a
large series
of epidural injections to determine the incidence of
inadvertent subdural block.
Methods
During the 30-month study period (March
1984-
September
1986),
2182 lumbar epidural steroid injec-
tions were performed at the Pain Center for various
forms of low back pathology. During this period any
patient whu exhibited any untoward or unpleasant
7/23/2019 Subdural Injection
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176
A N E S l H
ANALG
1988:67
759
L UB E NOW
ET
AL.
side effects from the injection (e .g ., headache, hypo-
tension, nausea, motor or extensive sensory block)
was identified for follow-up. The patients ranged in
age from 17 to 86 years and each received a single
epidural injection via a lumbar interspace, between
L1
and L5. The blocks were performed by an attend-
ing anesthesiologist or
a
supervised resident using
bupivacaine, 4-6 cc of 0.25% or 6-8 cc of 0.125%, in
combination with methylprednisolone acetate 80-120
mg (Depo Medrol, Upjohn Company, Kalamazoo,
Michigan). The epidural space was identified by the
loss-of-resistance technique. After a careful negative
aspiration test, injections were performed with dis-
posable 17- or 18-gauge Touhy point needles. Aspi-
ration was routinely done before, during, and after
each injection. After the injections, the patients were
observed for approximately 1 hour before discharge
from the center.
Records were evaluated in the following manner
for the presence or absence of clinical findings con-
sistent with subdural injection. I n any patient exhib-
iting a complication as mentioned above, a detailed
description of the complication and clinical findings
was obtained and recorded in the patients chart at
the time
of
occurrence. Clinical findings were classi-
fied into two levels of criteria, major and minor.
Findings considered major criteria were:
1)
a negative
aspiration test, or 2) an unexpected widespread sen-
sory block after epidural injection. The three minor
criteria were:
1)
a delayed onset of
10
minutes or more
of
a
sensory or motor nerve block, 2)
a
variable motor
blockade occurring, despite use of low doses of
bupivacaine, or
3 )
sympatholysis out of proportion to
the administered dose of local anesthetic. A positive
subdural injection was judged to have occurred in
both of the major criteria and at least one minor
criteria were present. With the criterion of negative
aspiration test we excluded any patient who had a
wet tap before the apparent successful epidural injec-
tion. All of these records of morbid events were then
retrospectively evaluated by one reviewer (TL)
to
determine if criteria for a subdural block were
present. From 38 potential subdural injections, 18
were judged by an additional investigator (ADI) as
having met the criteria for
a
subdural injection.
Results
Eighteen patients met the criteria for
a
subdural
block, establishing an incidence of 0 .82%. One pa-
tient exhibited all three minor criteria, while an
additional seven patients displayed two of the minor
criteria (Table
1 .
All 18 patients developed sensory levels much
higher than would be expected from the amount of
local anesthetic administered. One patient had a
sensory level of C4 after injection of 6 cc of 0.25%
bupivacaine. In none of the 18 patients was CSF
aspirated. Ten of the 18 patients developed motor
block. Delayed onset times of greater than 10 minutes
were noted in
11
patients (61%)with the longest time
to onset of symptoms being
30
minutes. Hypoten-
sion, defined as a drop in systolic pressure of at least
30 from baseline, occurred in 11 patients. Eight of
the 11 patients had moderate to severe hypotension
with a drop in pressure greater than 40 of the
baseline. Six of these patients had severe decreases in
blood pressures. In all cases, hypotension responded
to fluids or ephedrine
( 5 1 5
mg).
Five of the 18 patients (2870) had had previous
back surgery. These five patients represent
a
higher
percentage of patients than what is seen in our
overall patient population (12%)).Six of the 18 re-
ceived 0.25% bupivacaine, while 12 received 0.125%
bupivacaine.
Further studies were also performed on cadavers
to provide additional information on the subdural
space. The existence
of
the subdural space was con-
firmed by cadaveric dissection. A lumbar laminec-
tomy was performed and the spinal cord and me-
ninges were exposed from the S1 to the L 1 levels.
Dissected dura mater
was
found to have two layers:
an outer, thicker, opaque layer and an inner, more
translucent layer. Deep to these layers there existed a
potential space easily identified after reflecting the
dura mater. The arachnoid mater was noted to be
a
translucent membrane separating the subdural space
from the subarachnoid space. Deep to the arachnoid
mater the spinal nerves and subarachnoid space were
identified. Our depiction of the anatomy is similar to
the description made 23 years ago
by
Sechzer
(11).
Discussion
Epidural nerve blocks occasionally exhibit an atypical
pattern of spread. This may be caused by relative
overdose or accidental injection into the subdural or
subarachnoid spaces . Several investigators have
demonstrated radiological confirmation of catheters
present in the subdural space, especially in cases of
massive epidurals (8,12). A recent report describes
the ease of intentional subdural puncture and further
suggests that accidental subdural punc ture may occur
in attempted epidural block even in experienced
hands
(13).
Consequently, it appears that accidental
subdural injection probably occurs more frequently
than previouslv recognized.
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INADVERTENT SUBDURAL INlECTlON OF EPIDURAL BLOCK
ANE S T H ANAL G 177
1988,67 175-9
Table
1.
Summary
of Patient
Data
Bupivacaine Onset Previous Major Minor
Patient concentrat ion Aspiration Level Sensory Motor Degree of time Recovery back criteria criteria
no.
( )
Vol. test injected level block hypotens ion (min) time (hr) surgery met met
1
2
3
5
6
7
8
9
10
11
12
13
4
15
16
17
18
0 .25
0.25
0.25
0.125
0.125
0.125
0.125
0.125
0.125
0.125
0.125
0 .25
0.125
0.125
0.125
0.125
0.25
0.25
6
4
6
8
8
8
8
8
8
8
8
6
8
8
8
8
6
6
T12-Ll T4
L4-5 L2
L 3 4 T4
L 3 4
T2
L 3 4 T10
L 3 4 T I2
L 3 4 T6
L 3 4 T I2
L 4 5 T8
L2-3 T10
L M L10
L 3 4
T10
L1-2 T6
L4-5 T10
L2-3 T4
L 3 4 T9
L 4 5 c 4
L3-4 T2
Dense,
LE
bilateral
Dense, LE
bilateral
Moderate,
LE
bilateral
Moderate,
LE bilateral
None
Mild LE
bilateral
Dense,
LE
bilateral
None
None
Mild,
LE
bilateral
None
None
None
Dense, Le
bilateral
Moderate,
LE
bilateral
Dense, LE
bilateral
None
None
40 )
None
50
50
None
None
None
30
50
None
None
30%
4 0 4
None
50
None
50
50
1 0 3 . 5
10 4.0
10 3 .0
20 6.0
5 3.0
5 2.0
5 3.0
30 2.0
30 2.0
10 4.0
10 2 .0
5 1 .5
20 3.0
5
3 .0
5 3.0
5 2 .0
10 3 .0
15 3 .5
Yes, fusion
L 4 5 , 5 -s 1
No
NO
No
No
No
No
No
Yes, LAM
x 2
Yes, LAM
Yes, LAM
N o
Yes, LAM
No
No
N
No
No
2 2
- 1
2 2
-
3
1
2 1
2
1
2
2 2
2 1
-
1
2 1
2 2
2
I
2 2
2 1
- 1
2 2
7
7
7
3
L AM , laminectorny, LE,
lower
extrcmity
Intentional neurolytic subdural puncture has been
previously described (14). This technique involves
identification of the epidural space using the loss-of-
resistance technique. The needle is then rotated
through an arc of 180 with applied gentle pressure.
In order to avoid accidental subdural puncture, the
authors believe that a properly placed epidural nee-
dle should never be rotated to point the bevel in a
superior or inferior position. If one rotates the needle
to produce an intentional subdural puncture, this
same practice, if repeated for an epidural block, may
produce an accidental subdural puncture.
The three most common features noted in this
study were: 1) an unexpectedly high sensory block, 2)
exaggerated hypotension, and 3) unexpected motor
block. An interesting characteristic
of
subdural blocks
in the study is the variability in onset time. The
fastest onset time was between 5 and 10 minutes,
while other patients did not notice symptoms or
exhibit signs until 30 minutes after injection. These
findings do not differ significantly from other studies.
Case reports have documented the onset of symp-
toms to be as long as 30 minutes. Other descriptions
of accidental subdural injections have reported onsets
to be
as
short as
7
minutes (15). We believe that
subdural blocks do exhibit
a
variability in onset time.
This is dependent, perhaps, upon the relative
amount of local anesthetic deposited in the subdural
space, and may
also
be responsible for the wide-
spread sensory block and exaggerated hypotension.
Another explanation for the unexpected high sensory
and sympathetic blocks may be that previous back
surgery produced scarring and cicatrization, thereby
partly obliterating the epidural space in the lower
lumbar area. This partial obliteration of the epidural
space may cause marked cephalad spread. There are,
however, many exceptions to this hypothesis. Only
5
of the 19 patients had had previous back surgery. The
patients who had the most dramatic symptoms (pa-
tients 3, 4, and 17)had had no back surgery. Three of
the six patients who had previous back surgery
(Patients 10, 11, and 13) were among those who
exhibited the mildest symptoms. The presence of
previous back surgery with deformity of the epidural
space does not explain all of the observed events.
However, it appears that patients who have had back
7/23/2019 Subdural Injection
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178 ANESTH ANALG
1988.67:17'59
LUBENOW ET A1
surgery are more prone to accidental subdural injec-
tion. This is likely because the anatomy may be
altered secondary to scarring and retraction, produc-
ing
a
thin epidural and wide subdural space.
Epidural blocks seem more likely to produce acci-
dental subdural injection than do spinal blocks. This
may be due to differences in technique and the type
of needle used. Epidural injections use a large, blunt-
tipped, long-bevel needle that is introduced very
slowly, sometimes
a
millimeter at
a
time. In contrast,
for a subarachnoid puncture,
a
thinner, sharper nee-
dle is introduced, usually at
a
much faster rate.
I t
is
more likely that the blunt needle tip will pierce the
dura without piercing the arachnoid. The large open-
ing of the epidural needle may straddle the subdural
and epidural, allowing part of the local anesthetic to be
injected into the subdural space while some of
it
could
be deposited in the epidural space (Fig.
1).
This
partitioning of anesthetic may explain the difference in
degree of symptoms. Patients experiencing profound
sensory and motor block obviously would have had
more anesthetic deposited in the subdural space.
Another explanation regarding the difference in
symptomatology may relate in part to the anatomic
distribution of sensory, sympathetic, and motor
nerve fibers. The anterior nerve roots carry motor and
sympatlietic nerve fibers, while sensory fibers are
within posterior nerve roots. Because the subdural
space has more potential capacity posteriorly and
laterally, one should expect a sensory block. Mean-
while,
a
motor or sympathetic block would be present
only if local anesthetic traveled anteriorly within this
Figure 1. This i l lustration depicts rel-
ative position
of
intrathecal, epidural,
and subdural needle placement . Note
that if the needle is in the subdural
space, w i th the dura s t raddl ing the
bevel, some
of
the local anesthetic
may be deposited in the subdural
space while so me will be placed in the
epidural space.
subdural space (Fig. 2). Therefore, positioning of the
patient after the block would influence the type of
block to
a
large extent. Moreover, a motor and
sympathetic block would occur more readily if a
patient were in the lateral position, whereas sensory
block would predominate
if
the patient were supine
after the injection.
The absence of significant hypotension, in con-
junction with
a
profound motor block as demon-
strated by some of our patients, may reflect hydration
status more than anything else. The hypotension
seen in our patients was dramatic in certain cases, but
was easily treated in
all
cases with relatively
small
amounts of fluid (250-500 cc) and small doses of
ephedrine. Only one patient required
15
mg ephed-
rine. All others with hypotension responded to 5 or
10 mg ephedrine. Hypotension, which is easily
treated, has been a feature of all previously confirmed
subdural injections (16). This contrasts accidental
subarachnoid injection where hypotension is charac-
teristically more profound and difficult to correct.
The cadaveric dissection was perforined to further
exemplify the presence and anatomic proportion of
the subdural space. It
is
well accepted that the sub-
dural space exists in the cerebral meninges, and that
certain clinical entities are seen when pathology is
present in the subdural space (e.g., subdural hema-
toma). However, the extension of the subdural space
down into the spinal segment of th e meninges has
been previously regarded b y some authors as having
questionable clinical significance
(4).
Our dissection
support s the presence of the subdural space within
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INADVERTENT SUBDURAL INJECTION
OF
EPIDURAL BLOCK
ANESTH ANALG
19SS;h7
175-9
179
Figure
2.
This illustration shows the
anatomic relationship
of
dura and
arachnoid. The subdural space exists
as
a
potential space encircling the
arachnoid membrane and contained
within the dura.
the spinal cord segment of the meninges. A previous
study on autopsy subjects has also portrayed the
subdural space as a readily identifiable potential
space. In our dissection, the potential subdural space
and its relationship to the dura a nd arachnoid mem-
branes was found to be similar to its portrayal by
other authors (4,11,13). A s depicted in Figure
1,
a
needle may pierce the dura but not the arachnoid and
be contained within the subdural space. Local anes-
thetics, if deposited here, can travel cephalad and
caudad in this narrow potential space, producing the
unexpected extensive sensory, sympathetic, and mo-
tor blocks encountered in this series of therapeutic
epidural drug depositions.
In conclusion, after subdural deposition of a local
anesthetic, the development of an extensive sensory
and motor block, with or without hypotension, may
occur up to 30 minutes after the injection. The differ-
ential diagnosis
of
a possible subdural injection
should be entertained as readily as one would sus-
pect a subarachnoid injection. subdural block
should be considered when there has been extensive
sensory or motor blockade after a negative CSF
aspiration test when small volumes and dilute con-
centrations of local anesthetics are utilized. We rec-
ommend that outpatients receiving epidural injec-
tions of any amount of local anesthetics be observed
for at least 1 hour before discharge because
of
poten-
tial for a subdural injection.
References
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