BACKGROUND : The ilio-inguinal/iliohypogastric nerve block (INB) is one of
the most common peripheral nerve block techniques in pediatric anesthesia,
which is largely due to the introduction of ultrasound (US) guidance. Despite
the benefits of US guidance, the absence of an US machine should not deter
the provider from performing INB, considering that many institutions, especially
in developing countries, cannot afford to provide ultrasound machines
in their anesthesiology departments. The aim of this study was to revisit the
anatomical position of the ilio-inguinal and iliohypogastric nerves in relation
to the anterior superior iliac spine (ASIS), in a large sample of neonatal
cadavers, and compare the results with a similar group in a previously published
US-guided study.
METHODS :  With Ethics Committee approval, the ilio-inguinal and iliohypogastric
nerves were carefully dissected in 54 neonatal cadavers.
RESULTS : In the total sample, the ilio-inguinal nerve was found to be
2.2   1.2 mm from the ASIS, on a line connecting the ASIS to the umbilicus.
The iliohypogastric nerve was on average 3.8   1.3 mm from the ASIS. For
the entire sample, the optimal needle insertion site was 3.00 mm from the
ASIS. Although there is a strong correlation between the needle insertion point
and the weight of the neonate, this will only ‘fit’ for 60%of the population.
CONCLUSION :  The linear regression formula; needle insertion distance
(mm) = 0.6 9 weight + 1.8 can be used as a guideline for the position of the
ilio-inguinal and iliohypogastric nerves.http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1460-9592hb201

Bosenberg, A. T.

Bosman, Marius C.

Van Schoor, Albert-Neels

UPSpace at the University of Pretoria

1Original articleRevisiting the anatomy of the ilio-inguinal / iliohypogastric nerve blockAlbert-Neels van Schoor 1, Marius C Bosman 1, Adrian T Bosenberg 2,1 Department of Anatomy, School of Medicine, Faculty of Health Sciences, University of Pretoria, Pretoria, SouthAfrica.2Department of Anesthesiology and Pain Management, University Washington and Seattle Children’s Hospital,Seattle, USA.*Correspondence to:Dr. A. van Schoor,Department of Anatomy, Section of Clinical Anatomy, School of Medicine, Faculty of Health Sciences, Universityof Pretoria, Pretoria 0001, South AfricaTel: +27 12 3192315Telefax: +27 12 3192240E-mail: albert.vanschoor@up.ac.zaAbstractBackground: The ilio-inguinal / iliohypogastric nerve block (INB) is one of the most common peripheralnerve block techniques in pediatric anesthesia, which is largely due to the introduction of ultrasound(US) guidance. Despite the benefits of US guidance, the absence of an US machine should not deterthe provider from performing INB, considering that many institutions, especially in developingcountries, cannot afford to provide ultrasound machines in their anesthesiology departments. The aimof this study was to revisit the anatomical position of the ilio-inguinal and iliohypogastric nerves inrelation to the ASIS, in a large sample of neonatal cadavers, and compare the results with a similargroup in a previously published US guided study. Methods: With Ethics Committee approval the ilio-inguinal and iliohypogastric nerves were carefully dissected in 54 neonatal cadavers. Results: In thetotal sample the ilio-inguinal nerve was found to be 2.2mm±1.2mm from the ASIS, on a line connecting2the ASIS to the umbilicus. The iliohypogastric nerve was on average 3.8mm±1.3mm from the ASIS.For the entire sample, the optimal needle insertion site was 3.00mm from the ASIS. Although there isa strong correlation between the needle insertion point and the weight of the neonate, this will only “fit”for 60% of the population. Conclusion: The linear regression formula; needle insertion distance (mm) =0.6 x weight + 1.8 can be used as a guideline for the position of the ilio-inguinal and iliohypogastricnerves.Key words: Pediatric, ilio-inguinal blocks, trunk, linear regression, inguinal surgery, ultrasoundIntroductionThe ilio-inguinal / iliohypogastric nerve block (INB) provides effective analgesia for inguinalsurgery (L1 dermatome). It has been well described for common surgical procedures such as inguinalhernia repair, orchidopexy, hydrocele or varicocele repair (1-3). Performing these proceduressuccessfully and without complications is an integral part of modern pediatric surgical practice.General anesthesia is still widely utilized in children undergoing these procedures.  An exceptionwould be premature infants who may have the procedure performed under spinal anesthesia, caudalepidural anesthesia with or without mild sedation.The INB has become one of the most common peripheral nerve block techniques in pediatricanesthesia largely due to the introduction of ultrasound (US) guidance. US allows the anesthesiologistto visualize the nerves within the correct tissue plane (i.e. between the internal oblique and transverseabdominis muscles) and to introduce an appropriate sized needle within that tissue plane by usingreal-time tracking and hydro-dissection (1-4).Despite the benefits of US guidance, the absence of an US machine should not be acontraindication to performing INB, considering that many institutions, especially in developingcountries, cannot afford to provide ultrasound machines for their anesthesiology departments. Withthis in mind, a thorough knowledge of the anatomy of the ilio-inguinal and iliohypogastric nervestechnique is essential, as well as knowledge of the technique.3In 2005, the authors conducted a study on 25 embalmed neonatal cadavers in order todetermine the precise anatomical positions of both the ilio-inguinal and the iliohypogastric nerves. Theresults showed that the ilio-inguinal and iliohypogastric nerves lie much closer to the ASIS than waspreviously thought (5).The aim of this study was to revisit the anatomical position of the ilio-inguinal andiliohypogastric nerves in relation to the ASIS in a larger sample, as well as to compare the results ofthe cadaveric study with a similar US guided study (1).Materials & methodsThe ilio-inguinal and iliohypogastric nerves were carefully exposed in a further 29 embalmedcadavers giving a total of 54 neonatal and infant cadavers (51 left and 52 right sides; mean length:43cm; mean weight: 1.9kg) that was studied. A specimen was excluded when both nerves were notfound for any given side. The nerves were exposed by first making a midline incision from theumbilicus to the pubic symphysis. From there, two horizontal incisions were made on both sides ofeach cadaver that extended laterally from the umbilicus and pubic symphysis. The skin was thenreflected laterally, leaving the superficial fascia of the anterior abdominal wall intact. The superficial fatlayer was carefully removed to expose the superficial muscles and the rectus sheath of the anteriorabdominal wall (see Fig 1).A midline incision through the aponeurosis of the external oblique muscle was made andcarefully reflected laterally to expose the underlying internal oblique and rectus abdominis muscles.The ilio-inguinal nerve was then identified as it courses through the deeper layers and travels withinthe inguinal canal (see Figure 2). The nerve was then followed superolaterally to the point where itpenetrates the internal oblique muscle. The latter was then carefully removed in sections to exposethe course of the ilio-inguinal nerve back to a point superolateral to the ASIS. The iliohypogastricnerve, which courses superior to the ilio-inguinal nerve, was exposed in a similar manner (see Figure3).4Figure 1: Superficial and deeper dissections of the anterior abdominal wall of a neonatalcadaver with (a) the external oblique muscle, (b) the ASIS, (c) the rectus sheath, (d) testis, (e)the umbilicus, (f) internal oblique muscle, (g) rectus abdominis muscle, and (h) the conjointtendon (also indicated with the smaller, curved dotted line). The inguinal ligament is indicatedwith a dashed line.Figure 2: Dissection of (i) the ilio-inguinal nerve. The nerve can be seen piercing (f) the internaloblique muscle running with the spermatic cord towards (d) the testis.5Figure 3: Dissection of the (i) ilio-inguinal and (j) iliohypogastric nerves. As the nerves passinferomedial past the (b) ASIS, they are still running in the plane between the (k) transverseabdominis muscle and (f) internal oblique muscle.The ASIS was identified on both sides and the distance of both nerves from the ASIS - on aline connecting the ASIS to the umbilicus - was measured with a mechanical dial sliding caliper(accuracy of 0.01mm).Statistical analysis included a comparison between the left and right sides by using a paired t-test. A Pearson’s correlation coefficient test (correlation coefficient or R) was conducted to determinethe correlation coefficient or the strength of the correlation between the dependent variables, i.e., thedistance of the ilio-inguinal and iliohypogastric nerves from the ASIS and the distance of the point ofneedle insertion from the ASIS, and the independent variables, i.e., the length and weight of thesample population.In the case where a strong or high, moderate correlation (R > 0.6) between the dependent andindependent variables was found, the coefficient of determination (or R2) was determined. This is astatistical measure of how well the regression line, that was also determined, approximates the realdata points.6ResultsThe distances from the ASIS to the ilio-inguinal and iliohypogastric nerves, on both the left andright sides of the cadavers, are shown in Table 1. The upper and lower ranges stated in Table 1 weredetermined with a 95% confidence level.Table 1: Distances (in mm) of the right and left ilio-inguinal and iliohypogastric nerves (II-IH)from the ASIS (SD = Standard Deviation, CI95% = 95% confidence interval)Right LeftIlio-inguinalnerveIliohypogastricnerveII-IHIlio-inguinalnerveIliohypogastricnerveII-IHn 52 51Mean 2.3 3.9 1.6 2.2 3.8 1.6SD 1.2 1.3 0.5 1.2 1.4 0.6CI 95% 0.3 0.3 0.1 0.3 0.4 0.2Lower range* 2.0 3.6 1.5 1.8 3.4 1.4Upper range* 2.6 4.2 1.7 2.5 4.1 1.8Insertion site 3.1 3.0The correct distance for needle insertion was then defined as a point midway between the ilio-inguinal and iliohypogastric nerves (needle insertion from ASIS = distance to ilio-inguinal nerve + ½(distance between the iliohypogastric and ilio-inguinal nerves). The needle insertion site on the rightand left sides were found to be 3.1mm and 3.0mm from the ASIS, on a line connecting the ASIS withthe umbilicus, respectively.A paired t-test revealed that there was no significant difference between the distance from theASIS to the left and right ilio-inguinal or iliohypogastric nerves or needle insertion site. There was alsono significant difference between the distance between the ilio-inguinal and iliohypogastric nerves onboth sides (p > 0.05 for all comparisons). All the data was therefore compiled for a total sample of 103(see Table 2).7Table 2: Distances (in mm) of the ilio-inguinal and iliohypogastric nerves from the ASIS for thetotal neonatal sample.TotalIlio-inguinalnerveIliohypogastricnerveII-IHn 103Mean 2.2 3.8 1.6SD 1.2 1.3 0.6CI 95% 0.2 0.3 0.1Lower range* 2.0 3.6 1.5Upper range* 2.4 4.1 1.7Insertion site 3.00Looking at the total of 103 inguinal regions, the ilio-inguinal was found to be 2.2mm±1.2mmfrom the ASIS, on a line connecting the ASIS to the umbilicus. The iliohypogastric nerve was3.8mm±1.3mm from the ASIS. For the entire sample, the optimal needle insertion site was 3.00mmfrom the ASIS.The Pearson’s correlation coefficient test revealed that a moderate correlation exists betweeneither the distance of the ilio-inguinal nerve (R=0.4032), iliohypogastric nerve (R=0.5161) and point ofneedle insertion (R=0.4776), to the ASIS (dependent variables) or the length of the cadaver specimen(independent variables). There was however a strong correlation between the distance of the ilio-inguinal nerve (R=0.7340), iliohypogastric nerve (R=0.7647) and point of needle insertion (R=0.7707),to the ASIS and the weight of the cadaver specimen. Because of this strong correlation, a linearregression formula was then calculated for determining the correct distance, from the ipsilateral ASIS,to the ideal point of needle insertion when the weight of the cadaver is known (see Figure 4).8Figure 4: Linear regression formula for the distance of the point of needle insertion from theASIS as the dependent variable, and the weight of the sample as the independent.The coefficient of determination for these linear regression formulae revealed that there is amoderate “fit” (R2=0.59) when determining the distance to the ideal point of needle insertion.DiscussionSound knowledge and understanding of the anatomy is vitally important for successful nerveblocks, even when using US guidance. Extrapolation of anatomical findings from adult studies andsimply “downscaling” these findings in order to apply them to infants and children is inappropriate. Theideal situation would be to study the anatomy of neonatal or infant cadavers that are similar in age ofthe living patient. This unfortunately wasn’t (and very often isn’t) possible. A clear limitation of thisstudy was therefore the fact that only nine cadavers were above 2.5kg (normal birth weight) and therewas also lack of specific information regarding the neonatal sample (i.e. gestational age). It wasimpossible to determine whether the sample of neonatal cadavers, which ranged from very low birthweight (<1.5kg), low birth weight (1.5kg-2.5kg) and normal birth weight (>2.5kg), were growthrestricted term or preterm infants. However, the intent of this study was to provide accurate,anatomical measurements for a neonatal sample that could be extrapolated and used to identify theilio-inguinal and iliohypogastric nerve on older infants. This study further emphasized that the ilio-inguinal and iliohypogastric nerves in neonates and infants lie very close to the ASIS. A recent review9still suggests that the needle insertion point should be at the junction of the lateral one third andmedial two thirds on a line connecting the ASIS with the umbilicus (2). In neonates, this point is farmore cranial than the actual position of the nerves and requires that the local anesthetic spreadinferiorly in order to reach both nerves and provide effective analgesia.In this study, the ilio-inguinal nerve was found to be between 2.0mm and 2.4 mm (95%confidence level), while the iliohypogastric nerve was between 3.6mm and 4.1mm from the ASIS. Bothnerves ran in the plane between the transverse abdominis and internal oblique muscles to a pointinferomedial to the ASIS, where the ilio-inguinal nerve then pierced the internal oblique muscle andentered the inguinal canal. The iliohypogastric nerve pierced the internal oblique muscleapproximately at the semi-lunar line or the lateral border of the rectus sheath.The ideal point of needle insertion, according to our data, should be halfway between the ilio-inguinal and iliohypogastric nerves, which was between 2.8mm and 3.2mm (95% confidence level)from the ASIS. The average distance is 3.0mm. Willschke and co-workers performed US guided INBon one hundred pediatric patients. They determined that the mean distance of the ilio-inguinal nervefrom the ASIS was 6.7mm ± 2.9mm. The mean weight of the population was 13kg (1). Thiscorresponds well with the data obtained from this study, where the mean distance of the ilio-inguinalnerve was found to be 2.2mm in a much smaller population with a mean weight of only 1.9kg. Thedistance of the ilio-inguinal nerve from the ASIS of two larger infants in the sample, weighing 8.1kgand 9.1kg, were 5.1mm and 8.0mm, respectively, which falls within the range described by Willschkeand co-workers. They also looked at the skin to ilio-inguinal nerve and ilio-inguinal nerve toperitoneum distance and found them to be 8.0mm ± 2.2mm and 3.3mm ± 1.3mm (shortest distance, 1mm), respectively. This emphasizes the risk of undetected peritoneal puncture when using the “fascialclick” method (1). This may contribute to failed blocks and is a strong argument for the use ofultrasound guidance in young children.There are several reasons why INB should preferably be done under US guidance. Theliterature describes several complications occurring when using the “blind” technique. The first is ahigh incidence of complete or partial failure to block the ilio-inguinal and iliohypogastric nerves. It is10estimated that, even in experienced hands, complete failure of this block could occur in about 10% ofcases. Partial failure to block these nerves occurs even more frequently - between 10 and 15% (6-9).More recently, Eichenberger and co-workers reported a failure rate as high as 20%–30% in childrenwhen using the classic technique for blocking the ilio-inguinal and iliohypogastric nerves (9). Aninadvertent femoral nerve block is another recognized complication, (8, 10, 11) which was found tooccur in 11% of cases in a prospective study in children between 2 and 12 years (11). An adultcadaver study has shown that the fascial plane between the transverse abdominis muscle and thetransversalis fascia was in continuity with the space around the femoral nerve posteriorly. Injection ofmethylene blue into this plane resulted in pooling of dye around the femoral nerve (10). This maypartly explain the relatively high incidence of femoral nerve palsy, particularly if a relatively largevolume of local anesthetic is used. Incorrect placement of the needle below or close to the inguinalligament using incorrect landmarks is more likely to involve the femoral nerve, even if small volumes oflocal anesthetic are used. If the needle is inserted too deeply, unintentional intraperitoneal injectionmay occur. Jöhr and Sossai reported a case of accidental colonic puncture after INB was performedon a child (12).US guidance is therefore strongly recommended when performing the INB wheneveravailable. Precise administration of small volumes of local anesthetic is possible under US guidance toprovide effective blockade with a low failure rate and little or no complications (1).ConclusionThe findings of this study suggest that the needle should be placed much closer to the ASISthan previously described. In neonates, the local anesthetic needle should be inserted approximately3mm from the ASIS on a line drawn between the ASIS and the umbilicus. Although there is a strongcorrelation between the needle insertion point and the weight of the sample, this will only “fit” for 60%of the population. The linear regression formula; needle insertion distance (mm) = 0.6 x weight +1.8 can therefore be used as a guideline for the position of the ilio-inguinal and iliohypogastric nerves,with or without US guidance.11Ethical approvalEthical approval to conduct dissections on a sample of neonatal cadavers within the Department ofAnatomy, University of Pretoria, was obtained from the Faculty of Health Sciences, University ofPretoria Main Ethics Committee. All dissections were also conducted in the Department of Anatomyunder the rules and regulations stipulated in the South African National Health Act, number 61 of2003.Conflict of interestsNone of the authors has any conflicts of interest to declare.12References1. Willschke H, Marhofer P, Bosenberg A, Johnston S, Wanzel O, Cox SG, Sitzwohl C, Kapral S.Ultrasonography for ilioinguinal / iliohypogastric nerve blocks in children. Br J Anaesth 2005;95:226-230.2. Aziz A, Farid N, Abdelaal A. Ultrasound-guided regional anaesthesia and paediatric surgery.Curr Anaesth Crit Care 2009; 20:74-79.3. Weintraud M, Lundblad M, Kettner SC, Willschke H, Kapral S, Lönnqvist P, et al. UltrasoundVersus Landmark-Based Technique for Ilioinguinal-Iliohypogastric Nerve Blockade inChildren: The Implications on Plasma Levels of Ropivacaine. Anesth Analg 2009; 108:1488-1492.4. McCormack JG, Malherbe S. Applications of ultrasound in paediatric anaesthesia. CurrAnaesth Crit Care 2008; 19:302-308.5. Van Schoor A, Boon JM, Bosenberg A, Meiring JH. Anatomical considerations of thepaediatric ilioinguinal/iliohypogastric nerve block. Paediatr Anaesth 2005; 15: 371-377.6. Dalens BJ. Nerve blocks of the trunk. In: Dalens BJ (ed.). Regional Anesthesia in Infants,Children, and Adolescents. London: Williams & Wilkens, 1995:476-479.7. Markakis DA. Regional anaesthesia in paediatrics. Anesth Clin North Am 2000; 18:355-381.8. Lim SL, Ng Sb A, Tan GM. Ilioinguinal and iliohypogastric nerve block revisited: single shotversus double shot technique for hernia repair in children. Paediatr Anaesth 2002; 12:255-260.9. Eichenberger U, Greher M, Kirchmair L, Curatolo M, Moriggl B. Ultrasound-guided blocks ofthe ilioinguinal and iliohypogastric nerve: accuracy of a selective new technique confirmed byanatomical dissection. Br J Anaesth 2006; 97:238-243.10. Rosario DJ, Jacob S, Luntley J, Skinner PP, Raftery AT. Mechanism of femoral nerve palsycomplicating percutaneous ilioinguinal field block. Br J Anaesth 1997; 78:314-316.11. Lipp A, Woodcock J, Hensman B, Wilkinson K. What is the incidence of leg weakness afterilio-inguinal block in children? Paediatr Anaesth 2000; 10: 687-688.12. Jöhr M, Sossai R. Colonic puncture during ilioinguinal nerve block in a child. Anesth Anal1999; 88:1051-1052.

Revisiting the anatomy of the ilio-inguinal/iliohypogastric nerve block

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Revisiting the anatomy of the ilio-inguinal/iliohypogastric nerve block

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