Pediatric Anesthesiology
Section Editor: Peter J. Davis
Defining the Reliability of Sonoanatomy Identification by
Novices in Ultrasound-Guided Pediatric Ilioinguinal and
Iliohypogastric Nerve Blockade
Simon Ford, MB, ChB, FRCA*
Maryam Dosani, BSc†
Ashley J. Robinson, MB, ChB‡
G. Claire Campbell, MB, ChB,
FRCA†
J. Mark Ansermino, MBBCh,
MSc(Inf), FFA†
Joanne Lim, MASc†
Gillian R. Lauder, MBBCh,
FRCA†
BACKGROUND: The ilioinguinal (II)/iliohypogastric (IH) nerve block is a safe, frequently
used block that has been improved in efficacy and safety by the use of ultrasound
guidance. We assessed the frequency with which pediatric anesthesiologists with
limited experience with ultrasound-guided regional anesthesia could correctly identify
anatomical structures within the inguinal region. Our primary outcome was to
compare the frequency of correct identification of the transversus abdominis (TA)
muscle with the frequency of correct identification of the II/IH nerves. We used 2
ultrasound machines with different capabilities to assess a potential equipment effect
on success of structure identification and time taken for structure identification.
METHODS: Seven pediatric anesthesiologists with
6 mo experience with ultrasound-
guided regional anesthesia performed a total of 127 scans of the II region in
anesthetized children. The muscle planes and the II and IH nerves were identified and
labeled. The ultrasound images were reviewed by a blinded expert to mark accuracy
of structure identification and time taken for identification. Two ultrasound machines
(Sonosite C180plus and Micromaxx, both from Sonosite, Bothell, WA) were used.
RESULTS: There was no difference in the frequency of correct identification of the TA
muscle comparedwith the II/IH nerves (
2 test, TA versus II, P 0.45; TA versus IH,
P  0.50). Ultrasound machine selection did show a nonsignificant trend in
improving correct II/IH nerve identification (II nerve
2 test, P  0.02; IH nerve
2
test, P  0.04; Bonferroni corrected significance 0.17) but not for the muscle planes
(
2 test, P  0.83) or time taken (1-way analysis of variance, P  0.07). A curve of
improving accuracy with number of scans was plotted, with reliability of TA
recognition occurring after 14–15 scans and II/IH identification after 18 scans.
CONCLUSIONS: We have demonstrated that although there is no difference in the
overall accuracy of muscle plane versus II/IH nerve identification, the muscle
planes are reliably identified after fewer scans of the inguinal region. We suggest
that a reliable end point for the inexperienced practitioner of ultrasound-guided
II/IH nerve block may be the TA/internal oblique plane where the nerves are
reported to be found in 100% of cases.
(Anesth Analg 2009;109:1793–8)
The ilioinguinal (II)/iliohypogastric (IH) nerve block
is a procedure frequently performed in children. It is
considered a safe, low-risk, and effective local anes-
thetic block that provides postoperative pain relief for
children undergoing inguinal surgery.1 The anatomi-
cal landmark technique for II/IH nerve blockade, with-
out ultrasound guidance, has a reported failure rate of
20%–30%.2 It is associated with incorrect local anesthetic
placement in 14% of cases,3 inadvertent femoral nerve
block (reported incidence of 8.8%),4 and the rare but
serious complication of small bowel puncture.5,6
Ultrasound imaging to visualize the sonoanatomy
of the inguinal region improves the success rate and
speed of onset of the II/IH nerve block.7 The volume
of local anesthetic required to produce an effective
II/IH nerve block8 is also reduced reflecting closer
placement of local anesthetic to the nerves. Local anes-
thetic blockade of the II/IH nerves is considered to be
less technically demanding than other ultrasound-
guided nerve blocks9 and thus may be attempted by
inexperienced practitioners of ultrasound-guided re-
gional anesthesia (UGRA).
Accurate identification of anatomical structures
represented by ultrasound imaging is fundamental to
the safe and effective practice of UGRA. In this study,
From the *Department of Anaesthesia, Morriston Hospital,
Swansea, Wales, United Kingdom; and Departments of †Anesthe-
sia, and ‡Radiology, British Columbia Children’s Hospital, Vancou-
ver, British Columbia, Canada.
Accepted for publication July 21, 2009.
Supported by Canadian Institutes of Health Research (salary
support for SF and JL), Michael Smith Foundation for Health
Research (salary support for JMA).
The preliminary results of this study were presented at the
Association of Paediatric Anaesthetists of Great Britain and Ireland
(APA) London 2008 meeting.
Both ultrasound machines were lent for the duration of the study
by Sonosite (Bothell, WA).
Reprints will not be available from the author.
Address correspondence to Gillian R. Lauder, MBBCh, FRCA,
Department of Anesthesia, British Columbia Children’s Hospital, Van-
couver, British Columbia, Canada. Address e-mail to glauder@cw.bc.ca.
Copyright © 2009 International Anesthesia Research Society
DOI: 10.1213/ANE.0b013e3181bce5a5
Vol. 109, No. 6, December 2009 1793

we assessed the frequency with which pediatric anes-
thesiologists with limited experience with UGRA
could correctly find and identify anatomical structures
within the inguinal region. Our primary outcome was
to compare the frequency of correct identification of
the transversus abdominis (TA) muscle of the abdomi-
nal wall with the frequency of correct identification of
the II/IH nerves. Our secondary end point was the
frequency and time taken for correct identification
using 2 different ultrasound machines with different
imaging capabilities. Differences in machines may
play a role in a practitioner’s ability to accurately
identify anatomical structures.
METHODS
After receiving IRB approval, written and informed
consent was obtained from 7 staff pediatric anesthesi-
ologists. All subjects regularly performed anatomical
landmark technique II/IH nerve blocks as part of their
clinical practice. All subjects had
6 mo experience
with UGRA.
The study had 2 phases. The training phase con-
sisted of peer-reviewed e-learning tutorials on the
physics of ultrasound, anatomy of the inguinal region,
and a structured approach to scanning of the region.
Subjects then practiced scanning the inguinal region
on healthy adult volunteers to gain experience in
scanning dexterity, structure recognition, and ma-
chine familiarization. All subjects had at least 1 h of
training using adult volunteers.
In the second phase, subjects were required to scan
the abdomen of anesthetized children before the start of
surgery. After receiving written informed consent from
parents/guardians (and assent from children if appro-
priate), healthy children between the ages of 1 mo and 8
yr attending British Columbia Children’s Hospital for
day case surgery were recruited. Children were ex-
cluded if they had had previous abdominal surgery or
were known to have abnormal abdominal anatomy. No
II/IH blocks were performed as part of this study.
Anesthesiologist subjects were required to identify,
in order, the anterior superior iliac spine (ASIS), the
external oblique (EO) muscle, the internal oblique (IO)
muscle, the TA muscle, the II nerve, and the IH nerve.
Identification was achieved by “freezing” the ultra-
sound image and highlighting the required structure
using the on-screen pointer and then labeling with
preloaded labels. Subjects were then able to unfreeze
the image and continue scanning for the next struc-
ture. The time taken, in seconds, from the start of
scanning to the point at which the on-screen image
was frozen was recorded. If the image was unfrozen
and further scanning performed, the duration of this
extra scanning period was recorded. The on-screen im-
age was digitally encoded using an analog-to-digital
converter (DVD Xpress DX2, ADS Technologies, Cerri-
tos, CA) attached to a laptop (Inspiron 6000, Dell Com-
puter Corporation, Round Rock, TX). Scanning was
performed with a high-frequency 5- to 10-MHz linear
hockey stick probe (Sonosite, Bothell, WA) attached to
either machine A, with a 5-in. display with basic
focusing capabilities (Sonosite 180plus, Sonosite), or
machine B, with a 10.4-in. display and advanced
image processing capabilities (Micromaxx, Sonosite).
The machines and side (left or right) of the subject to
be scanned were assigned using a block randomization
protocol (StatsDirect statistical software, StatsDirect,
Ltd., Manchester, UK).
The recorded video images were edited to remove
any on-screen indicators of which machine had been
used. The edited images were analyzed by a radiolo-
gist (AR) with extensive experience with ultrasound
scanning. AR was blinded to who performed the scan
and the machine used. Labeling was marked correct or
incorrect for each structure identified.
Comparison of frequency of correct structure iden-
tification was performed by a
2 test using a 2  2
table construct of structure against identification. This
was performed for both TA and II, and TA and IH.
Additional 2  2 tables were constructed for each
structure with correct or incorrect identification
against machine A or B,
2 test was performed, and a
Bonferroni correction for multiple comparisons was
applied. The time taken to identify the TA, II, and IH
nerves using the 2 machines was compared using a
1-way analysis of variance test.
A power calculation for this study was performed
by a statistician using the first 6 scans of each subject
to estimate the sample size because there were no
comparative studies available. The study was pow-
ered to find a 25% difference in the frequency of
correct identification of the TA, II, and IH nerves
between the 2 machines and a 25% reduction in time
taken to identify the TA, II, and IH nerves between the
2 machines (Type 1 error of 0.05 and a Type 2 error of
0.2). A sample size of 126 scans was calculated.
RESULTS
The pediatric patient demographics for sex, age,
and side of scanning are shown in Table 1. The side of
scanning was not recorded for 1 female patient
scanned with machine A. The 7 anesthesiologist sub-
jects performed 132 scans. Five scans could not be
analyzed because of failure of video recording. The
remaining 127 scans were analyzed providing 18 scans
from 6 subjects and 19 scans from 1 subject. In some
cases, identification of a structure was not attempted,
and the next structure in order was identified. This
Table 1. Sex, Age, and Side of Pediatric Subject Scanned
Sex Side Age median
(interquartile
range) (yr)Male Female Right Left
Machine A 39 23 28 33 3.7 (2.0–5.2)
Machine B 32 33 34 31 3.6 (1.8–4.7)
1794 Pediatric Inguinal Sonoanatomy ANESTHESIA & ANALGESIA

was recorded as an incorrect identification for the
omitted structure.
Subjects correctly identified the ASIS in 123 scans
(97%), EO in 97 scans (76%), IO in 90 scans (71%), TA
in 88 scans (69%), II in 81 scans (64%), and the IH in 82
scans (65%). There was no difference in the total
frequency of correct identification of the TA compared
with the II or IH nerves (
2 test [Yates corrected], P 
0.45 and P  0.50, respectively). Sequential scan
number was plotted against the number of subjects
correctly identifying the structure (Figs. 1–6). The
figures show an increase in successful identification with
increasing scan number. The bony landmark of the ASIS
was reliably identified after 7 scans. Approximately 15
scans were required before subjects were able to cor-
rectly identify the TA muscle with 71% or more chance.
However, consistent correct identification of the II and
IH nerves did not occur even after 18 scans, with the
percentage correctly identified varying between 29%
and 86% during scans 17 and 18, respectively.
The frequency of correct TA identification showed
no difference between machine A and B (44 and 45,
respectively;
2 test, P  0.83). However, machine
selection did result in a difference when looking for
the nerves, with machine B allowing subjects to cor-
rectly identify the II and IH nerves with greater fre-
quency (II nerve: machine A 34 correct, machine B 48
correct,
2 test, P 0.02; IH nerve: machine A 35 correct,
machine B 48 correct,
2 test, P 0.04), although this did
not reach significance when corrected for multiple com-
parison (Bonferroni correction, P  0.17).
The average time taken to identify each structure
from the start of scanning was 46 s for the ASIS, 58 s
for the EO, 56 s for the IO, 58 s for the TA, and 75 s
for both the II and IH nerves (Table 2). For those
cases in which TA, II, and IH were correctly identi-
fied, there was no difference in the time taken using
machine A or machine B (1-way analysis of vari-
ance, P  0.07).
Figure 1. Number of clinician subjects correctly identifying the anterior superior iliac spine against sequential scan number.
Figure 2. Number of clinician subjects correctly identifying the external oblique muscle against sequential scan number.
Vol. 109, No. 6, December 2009 © 2009 International Anesthesia Research Society 1795

Figure 3. Number of clinician subjects correctly identifying the internal oblique muscle against sequential scan number.
Figure 4. Number of clinician subjects correctly identifying the transversus abdominis muscle against sequential scan number.
Figure 5. Number of clinician subjects correctly identifying the ilioinguinal nerve against sequential scan number.
1796 Pediatric Inguinal Sonoanatomy ANESTHESIA & ANALGESIA

DISCUSSION
In this study, we investigated the ability of pediat-
ric anesthesiologists who were novice UGRA practi-
tioners to correctly identify inguinal structures using
ultrasound imaging. We anticipated that larger struc-
tures would be more reliably identified and would
potentially be more appropriate targets for II/IH
nerve blockade. We have demonstrated that there is
no difference in the number of correct identifications
of the TA muscle as opposed to the II or IH nerves.
The rate of acquisition of correct structure recognition
does show a trend toward the TA muscle being
reliably identified after fewer scans than the II/IH
nerves. It is important to note that reliable II/IH nerve
identification did not occur even after 18 scans of the
region. The machine used to perform the scan did
seem to make a difference when looking for the
smaller II/IH nerves, with machine B facilitating an
increase in the number of correctly identified nerves
compared with machine A, but not reducing the time
taken to identify the nerves.
UGRA relies on correct anatomical target structure
identification on ultrasound imaging and accurate
hand-eye coordination. Work has been performed
evaluating hand-eye coordination in both phantom
models and the clinical setting.10,11 To our knowledge,
there is no work describing how quickly novice UGRA
practitioners attain correct structure recognition skills.
We anticipated that the muscle belly of TA would be
correctly identified more frequently than the smaller
nerves. We chose the TA muscle because it lies next to
the ASIS and immediately above the peritoneum with
a reliable relationship. Failure to see an improved
correct identification rate may have resulted because
the ultrasound probe was allowed to slide more
medially toward the umbilicus, making it difficult or
impossible to identify the TA. This error is easily eradi-
cated by placing the little finger of the hand holding the
ultrasound probe on the skin of the patient, allowing
detection of anymovement of the probe in relation to the
patient.
We did not use EO and IO muscle identification in
the analysis because the EO muscle can be aponeurotic
in up to 50% of patients in this age group,7 potentially
causing confusion when labeling the EO and IO
muscles. The presence of EO was higher in our study
than previously reported, despite a similar age range,
and may represent a difference in the point of scan-
ning. The more EO muscle belly is seen, the more
cephalad and lateral the ultrasound probe is directed.
The figures of correct structure identification
against number of attempts demonstrate an interest-
ing learning progression. As anticipated, bony land-
marks were reliably identified early, whereas muscle
planes were reliably identified sooner than nerves.
This is at odds with our overall finding that there was
no difference in the frequency of correct structure
identification between the TA and II/IH. Because of
the small sample size, 1 subject making an incorrect
identification made a significant change to the curve
and the percentage success rate. Our identification
success curves show a tendency to earlier reliable
recognition of the TA, but there is no increase in
Figure 6. Number of clinician subjects correctly identifying the iliohypogastric nerve against sequential scan number.
Table 2. Frequency and Time Taken to Correctly Identify
Structures, n  126
Correct
structure
identification
Number of times
structure was
correctly
identified from
all scans
Time taken
to correctly
identify
structure
(mean sd) (s)
Anterior superior
iliac spine
123 (97%) 46 (36)
External oblique 97 (76%) 58 (43)
Internal oblique 90 (71%) 56 (39)
Transversus
abdominis
88 (69%) 58 (45)
Ilioinguinal nerve 81 (64%) 75 (51)
Iliohypogastric
nerve
82 (65%) 75 (46)
Vol. 109, No. 6, December 2009 © 2009 International Anesthesia Research Society 1797

overall frequency of correct identification compared
with the nerves. This may reflect that the lag between
the correct identification curves for TA and II/IH is
only small and therefore does not affect overall struc-
ture identification frequency, or it may represent an
undersized study group failing to detect a genuine
difference in the frequency of correct TA identification
compared with nerve identification.
If our study had been conducted over a larger number
of scans, we may have seen an increase in consistency of
II/IH identification. It is this reliability in ultrasound-
guided structure identification that is required before
UGRA can be performed. The identification curves sug-
gest that a reliably identifiable target for novice UGRA
practitioners may be the muscle planes when initially
performing an II/IH nerve block.
Our study took place over a period of 8 mo when
subjects intermittently scanned children who met the
eligibility criteria when they appeared on their oper-
ating list. The study was not part of a concentrated
teaching program with continual feedback, but re-
flects the reality of some clinicians learning UGRA.
Many clinicians will attend a course, study day, or
workshop and acquire basic knowledge covering sev-
eral nerve blocks over a short period, and then gradu-
ally introduce these skills into their clinical practice.
Our study emphasizes the importance of performing
at least 18 scans of the inguinal region before consid-
ering any invasive procedure. If learning is part of a
continuing intensive educational program as part of a
residency or fellowship, then this number may change.
Although there was a trend of increased frequency
of correct nerve identification using machine B, there
was no difference in the time taken to identify struc-
tures. This may be an effect of the study design,
because time taken to mark the structures once the
picture was frozen was not measured. This was delib-
erately not measured, so that machine usability and
the subject’s familiarity with the keyboard and screen
navigation keys would not interfere with timing of
structure identification. Subjects may have frozen the
image only when they had a good picture of all 3 muscle
planes to label, despite recognizing some structures
sooner. A similar identification time was then recorded
for all 3 structures. Subjects were also required to find
structures in a predefined order, artificially prolonging
the time taken to identify the nerves.
Willschke et al.7,8 used a small-screen basic ultra-
sound machine for their studies, and reported that the
II and IH nerves were identifiable on all scans. Our
subjects were able to identify the II/IH nerves in 64%
and 65% of cases, respectively. In agreement with
Willschke et al.,8 our expert marker was able to
identify the nerves on all scans presented, thus em-
phasizing the impact of experience on structure iden-
tification. Our results suggest that machine B with a
larger screen and improved imaging processing helps
in correct identification of the smaller structures. Nov-
ices should perhaps be aware of the capabilities and
specifications of the machine that they will use and
adapt their intended end point to ensure a realistic
chance of accurate structure identification.
In conclusion, we have shown that there is a
significant learning curve to correctly identifying ana-
tomical structures necessary to perform II/IH nerve
block by ultrasound. Bony landmarks are quickly and
reliably identified, followed by muscle planes, and
finally the smaller II/IH nerves. This is an important
aspect of safe and successful UGRA and is often
overlooked. This study suggests that UGRA novices
starting to perform II/IH blocks should scan the
region at least 14–15 times before performing a nerve
block using the muscle planes as an end point. As the
practitioner becomes more reliable in nerve identifica-
tion, the nerves can be used as the target structures. It
is imperative that competence in both accurate struc-
ture identification and hand-eye coordination is at-
tained before performing any UGRA technique, if an
increase in reported complications is to be avoided.
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1798 Pediatric Inguinal Sonoanatomy ANESTHESIA & ANALGESIA