ltrasound as a Screening Tool for Proceeding With Caudalpidural Injections
arl P. Chen, MD, PhD, Alice M. Wong, MD, Chih-Chin Hsu, MD, PhD, Wen-Chung Tsai, MD, PhD,hen-Nen Chang, MD, Shih-Cherng Lin, MS, Yin-Cheng Huang, MD, PhD, Chih-Hsiang Chang, MD,
imon F. Tang, MD
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ABSTRACT. Chen CP, Wong AM, Hsu C-C, Tsai W-C,hang C-N, Lin S-C, Huang Y-C, Chang C-H, Tang SF.ltrasound as a screening tool for proceeding with caudal
pidural injections. Arch Phys Med Rehabil 2010;91:358-63.
Objective: To study the anatomical structure of the sacraliatus using ultrasound. Based on the sonographic images of theacral hiatus, the feasibility of caudal epidural injection can thene assessed.
Design: Case-controlled study.Setting: Rehabilitation outpatient clinic in a tertiary medical
enter.Participants: Patients (N�47; 20 women, 27 men) with low
ack pain and sciatica who were to receive caudal epiduralnjection treatments were recruited into this study.
Interventions: Sonographic images of the sacral hiatus werebtained from all the patients. An ultrasound machine capable ofxamining musculoskeletal tissues with real-time linear-array ul-rasound transducer was used to measure the distance between thenterior wall and posterior wall of the sacral hiatus (diameter ofhe sacral hiatus) and the distance between bilateral cornua.
Main Outcome Measures: Diameter of the sacral canal andistance between bilateral cornua measured in millimeters.Results: The mean diameter of the sacral canal was measured
o be 5.3�2.0mm in our recruited patients. The mean distanceetween bilateral cornua was measured to be 9.7�.9mm. Caudal epidural injections failed in 7 patients. In these 7atients, 4 had very small diameter of the sacral canal (1.6, 1.2,.4, and 1.5mm). In 1 man, sonographic images revealed a closedacral hiatus (no sacral canal diameter can be measured). Twoatients revealed flow of fresh blood into the syringe while check-ng for the escape of cerebrospinal fluid after the needles werenserted into the sacral canal. For safety reasons, steroid injectionsere not performed in these 2 patients.Conclusions: Ultrasound may be used as an effective
creening tool for caudal epidural injections. Anatomic varia-ions of the sacral hiatus can be clearly observed using ultra-ound. Sonographic images indicating a closed sacral canal andacral diameters ranging from 1.2 to 1.6mm may suggest aigher failure rate in caudal epidural injection.
From the Departments of Physical Medicine and Rehabilitation (Chen, Wong, Tsai,in, C-H Chang, Tang) and Neurosurgery (C-N Chang, Huang), Chang Gung Me-orial Hospital at Linkou, Taiwan, and College of Medicine, Chang Gung University,wei-Shan, Tao-Yuan County, Taiwan; Department of Physical Medicine and Re-abilitation (Hsu), Chang Gung Memorial Hospital at Keelung, Taiwan, and Collegef Medicine, Chang Gung University, Kwei-Shan, Tao-Yuan County, Taiwan.No commercial party having a direct financial interest in the results of the research
upporting this article has or will confer a benefit on the authors or on any organi-ation with which the authors are associated.
Reprint requests to Simon F. Tang, MD, Professor and Program Director, Deptf Physical Medicine and Rehabilitation, Chang Gung Memorial Hospital atinkou, 5, Fu-Hsin ST, Kwei-Shan, Tao-Yuan County, 333, Taiwan, e-mail:[email protected].
HE CAUDAL APPROACH to the epidural space via thesacral hiatus is often the preferred injection method in the
reatment of low back pain caused by lumbosacral root com-ression.1 Many nonanesthetists prefer this injection methodecause it carries a lower risk of inadvertent thecal sac punc-ure and intrathecal injection.2 Successful caudal epidural in-ection relies on the proper placement of the needle in thepidural space.3 The most common method used to identify theaudal epidural space is by detecting the characteristic “give”r “pop” when the sacrococcygeal ligament is penetrated.4 Inhe event of unaided or blind needle insertion, incorrect needlelacement has been reported to occur in 25% to 38% of cases,ven in the hands of experienced physicians.5 Furthermore,ven when physicians are confident with their injection tech-ique, incorrect needle placement has been observed in about 1f 7 caudal injection procedures.2 An incorrect needle positionould most likely result in deep subcutaneous injections.5
In clinical practice, the “whoosh” test,6 nerve stimulation,4
nd fluoroscopy are the 3 methods that can be used to identifyhe caudal space before the injection of medications.3 In ourrevious study, ultrasound has been proven to be easy to usend can provide real-time images in guiding the needle into theaudal space.3 Ultrasound has also been shown to be an effec-ive tool in performing ultrasound-guided sacroiliac joint in-ections7 and in determining the optimal angle for needlensertion in performing caudal block in children.8 The sacraliatus is a small aperture at the base of the sacrum and isordered by 2 bony prominences—the sacral cornua.2 Theacral hiatus can be identified under the sonographic transverseiew (fig 1), while the real-time needle advancement imagesan be observed under sonographic longitudinal view (fig 2).3
If ultrasound can provide clear images of the sacral hiatusnd can effectively assist the physician in guiding the needlento the sacral canal, then it should be able to provide crucialonographic images of the anatomic variations of the sacraliatus that may lead to unsuccessful caudal epidural injections.pproximately 3% of the studied population have closed sacral
anals, thus making caudal epidural injections impossible forhese subjects.9 It is hypothesized that ultrasound can provides with sonographic images that reveal the anatomic variationsf the sacral hiatus such as closed sacral canals. As a result, theurpose of this study was to examine whether ultrasound cane used as an effective screening tool for successful caudalpidural injections.
359ULTRASOUND AS A SCREENING TOOL IN CAUDAL INJECTIONS, Chen
METHODSSonographic images of the sacral hiatus were obtained from
7 patients (20 women and 27 men) with low back pain andciatica who were to receive caudal epidural injection treat-ents. These study participants all signed the informed consent
or caudal epidural injections. This study was approved by theocal medical ethics and the human clinical trial committeeChang Gung Memorial Hospital, Taiwan).
The LOGIQ 9 ultrasound machinea was used in this study.he M12L real-time linear-array ultrasound transducera with aandwidth of 5.0 to 13.0MHz was selected. Sonographic im-ges of the sacral hiatus area were obtained from a physiatristho had years of experience in handling and interpretingusculoskeletal ultrasound images. This physiatrist was a cer-
ified musculoskeletal ultrasound user and a member of the
ig 1. (A) Transverse plane placement of the ultrasound transducer.B) Transverse plane ultrasound image of the sacral hiatus. Betweenhe 2 cornua, there are 2 hyperechoic band-like structures. Theand-like structure on top is the apex of the sacral hiatus. Theand-like structure at the bottom is the dorsal bony surface ofhe sacrum or the posterior wall of the sacral hiatus. The sacraliatus is the hypoechoic region observed between these 2 hypere-hoic band-like structures. The distance between bilateral cornuaas measured from the apex of one sacral cornu to the other.
aiwan Society of Ultrasound in Medicine.ha
Patients were placed in prone position for ultrasonographicxamination of the sacral hiatus. In some patients, the sacraliatus is located at the level between the 2 gluteal masses,aking sonographic examination of the sacral hiatus diffi-
ult because the transducer cannot achieve a flat skin con-act. Help from an assistant is needed to push the glutealasses apart for ultrasonographic examination and subse-
uent caudal epidural injection.3 The assistant’s arms arelaced horizontally out of the physician’s way to hold theluteal masses apart to achieve a flatter skin surface at the sacraliatus area for the placement of ultrasound transducer. Putting aillow at the inguinal areas can increase backward tilt of theelvis, enabling the sacral bone to move in a cephalad directionway from the gluteal masses. As a result, better sonographiciewing of the sacral hiatus can then be achieved because theacral hiatus is now located at the level where the overlying skins flatter for the placement of ultrasound transducer.
ig 2. (A) Longitudinal plane ultrasound image of the sacral hiatus.B) Schematic illustration of figure 2A. The apex of sacral hiatus,acral canal, sacrococcygeal ligament, posterior wall of the sacral
iatus, and how the diameter of the sacral canal is measured at thepex of the sacral hiatus are clearly shown.
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360 ULTRASOUND AS A SCREENING TOOL IN CAUDAL INJECTIONS, Chen
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easurementsBody mass index. BMI was calculated for every patient
sing the following formula:
Weight (in kg)
Height2 (in m2)
ccording to the World Health Organization BMI categoriza-ion in Asian populations, patients with a BMI of 23 to 24.9
Table 1: Patient Character
PatientNumber Sex Age (y) BMI
Dis
1 W 42 22.4
2 M 58 22.13 M 52 24.24 W 48 22.55 M 61 21.66 W 64 24.77 W 61 26.68 M 60 23.69 W 57 21.5
10 M 59 23.711 M 47 21.112 W 36 19.613 M 57 21.414 M 62 24.115 M 64 25.7
16 M 68 26.917 W 49 21.518 M 55 21.619 W 65 22.120 M 60 23.8
21 M 63 20.222 M 64 23.423 W 70 24.624 W 40 22.325 M 37 20.726 M 41 2127 W 28 20.128 M 68 24.529 W 77 26.1
30 W 65 23.831 M 63 23.932 W 66 24.3
33 M 51 21.234 M 44 20.435 M 58 23.836 W 70 25.937 M 69 24.7
38 W 70 25.139 W 28 20.340 M 36 19.741 W 39 22.742 M 44 21.643 W 60 24.644 W 61 22.445 M 57 24.946 M 50 21.747 M 66 22.9
Mean � SD 22.9�1.9
OTE: Boldface values indicate the patients in which caudal epidural injbbreviations: M: man; NO: Failed caudal epidural injection; W, woman;
rch Phys Med Rehabil Vol 91, March 2010
ere considered mildly overweight, and patients with a BMI of5 to 27.4 were considered pre-obese.10
Distance between bilateral cornua and diameter of theacral canal. The distance between bilateral cornua waseasured (see fig 1). This distance was measured from the
pex of one sacral cornu to the other and under sonographicransverse view. The distance between the anterior wall andosterior wall of the sacral hiatus was measured in the sacraliatus apex area (see fig 2)11 using the sonographic longitudinal
361ULTRASOUND AS A SCREENING TOOL IN CAUDAL INJECTIONS, Chen
iew. Both distances were measured in millimeters (mm).ean distances were expressed as � SDs.
RESULTSPatients’ age ranged from 28 to 77 years. Six patients had
MI between 25.7 and 26.9 (preobese) and 16 had BMI be-ween 23.4 and 24.9 (mildly overweight). The mean BMImong all the patients was calculated to be 22.9�1.9 (table 1).espite having 16 patients with BMI greater than 23.4, none of
hem had thick subcutaneous fat layer covering the sacraliatus that would interfere with the clarity of the sonographicnatomic images of the sacral hiatus. As a result, curvilinearransducer was not used in this study.
In these 47 patients, the diameter of the sacral canal at thepex of the sacral hiatus was measured to have a mean of.3�2.1mm. The distance between bilateral cornua was mea-ured to have a mean of 9.7�1.9mm. Bilateral cornua werelearly visible under ultrasound examination in all of theseatients. Caudal epidural injections failed in 7 of these patients15% failure rate). Two of them revealed flow of fresh bloodnto the syringe when checking for the escape of cerebrospinaluid for possible dura tear after the needles were inserted into
he sacral canal. Therefore, steroid injections were not pursuedn these 2 patients for safety reasons. In the remaining 5atients, 4 (2 men, 2 women) had very small diameter of theacral canal at the apex of the hiatus (fig 3). Their sacral canaliameters were 1.6mm (patient 1), 1.2mm (patient 15), 1.4mmpatient 20), and 1.5mm (patient 37). Needles with smalleriameters (eg, 24-gauge) were also tried in these 4 patients, but
ig 3. Sonographic images on thenatomical variations of sacral hia-us that caused failure in caudal epi-ural injections. (A) Small sacral hia-us was observed under transverseiew. The diameter of the sacral hi-tus was measured to be about.2mm. (B) Small sacral canal at thepex of the sacral hiatus under sono-raphic longitudinal view. The entire
acrococcygeal ligament lies directlyn top of the base of the sacrum.
ifficulties in steroid injections were still observed. In 1 man,onographic images revealed a closed sacral hiatus (0mm sa-ral canal diameter), making caudal epidural injection impos-ible. Lateral sacral bone roentgenogram examination of thisatient confirmed the sonographic finding of a closed sacraliatus (fig 4).
DISCUSSIONAnatomic variation of the sacral hiatus can affect the success
ate in performing caudal epidural injections.9 The sacral hiatuss the most important bony landmark for performing caudalpidural injections because the apex of the sacral hiatus showshe existence of a sacral canal.12 Caudal epidural injections willot be possible in the anatomic variations of absent hiatus andomplete agenesis of the sacral hiatus.9
In the current study, the obtained sonographic images sug-est that ultrasound may be used as an effective tool in eval-ating the anatomic variations of the sacral hiatus and henceudging whether caudal epidural injection can be performeduccessfully on a patient. Ultrasound is an important tool forodern physiatrists in diagnosing soft-tissue lesions13 and in
erforming ultrasound guided injections.3 Under ultrasoundxamination, anatomical structures of sacral cornua, apex ofhe sacral hiatus, base of sacrum (posterior wall of the sacraliatus), and sacrococcygeal ligament can be clearly detected.s a result, the diameter of the sacral canal at the apex of the
acral hiatus and the distance between bilateral cornua can berecisely measured.
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362 ULTRASOUND AS A SCREENING TOOL IN CAUDAL INJECTIONS, Chen
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In this study, the diameter of the sacral canal at the apex ofiatus was measured to have a mean of 5.3�2.1mm. This wasimilar to the findings discovered by Sekiguchi et al9
6.0�1.9mm). In our 4 patients with failed caudal epiduralnjections, the canal diameters were measured to be 1.6mm,.2mm, 1.4mm, and 1.5mm. In these patients, it was difficult tonsert the needles into the sacral canal. They complained ofevere soreness and pain at the injection site when steroidnjection was tried after the injection needle just penetratedhrough the sacrococcygeal ligament. Strong resistance was felturing the injection procedure, and it was nearly impossible tonject the medication into the sacral canal. Injection failure inhese patients can be expected because the diameter of their sacralanal is nearly the same as the diameter of the injection needle
tself (the diameter of the 21-gauge injection needle is approxi- s
rch Phys Med Rehabil Vol 91, March 2010
ately 1mm and the diameter of the 24-gauge is approximately.8mm). It has been reported that a diameter of less than 2mm canesult in the increased failure rate of caudal epidural injections.9
ased on the data obtained in this study, the measured sacraliatus diameter of 2.9mm (patient 23) is adequate for successfulaudal epidural injection to be performed (see table 1).
One study has mentioned that bilateral cornu was not pal-able in approximately 14.3% of the cases they have studied.12
n this study, all our recruited patients revealed the existence ofilateral sacral cornua. The distance between bilateral cornuaas been reported to be approximately 10.2mm.14 This studyevealed similar mean distance finding of 9.7�1.9mm with.9mm as the smallest distance measured. Therefore, based onhe results we have obtained, the distance between bilateral cornua
Fig 4. Roentgenogram and sono-graphic images showing closedsacral hiatus. (A) Lateral roentgeno-gram of the sacrococcygeal bone.Enlarged image of the sacral boneconfirmed a closed sacral hiatus. (B,C) Both sonographic transverse andlongitudinal views showing nonex-istent sacral hiatus.
hould not be the factor in causing failed caudal epidural injections.
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363ULTRASOUND AS A SCREENING TOOL IN CAUDAL INJECTIONS, Chen
he distance should be sufficiently wide in most of the generalopulation for the injection needle to pass through easily.In the study by Sekiguchi et al,9 they reported closed sacral
anals in 3% of their studied population. In this study, closedacral canal was observed in 1 of the recruited patients (about%). Closed sacral canal was immediately impressed based onhe sonographic images obtained for this patient. Meticulousltrasound examination of the sacral bone of this patient re-ealed no evidence of a sacral hiatus. Lateral roentgenogram ofacrococcygeal bone was then performed and confirmed theresence of a closed sacral canal. As a result, it was notossible to perform caudal epidural injection on this patient.ometimes a curvilinear transducer is required to examine theusculoskeletal structures that are covered with excessive fat
issue.15 Although 16 of our patients met the World Healthrganization BMI categorization for Asian populations10 asildly overweight or preobese, we did not observe excessive
at tissue overlying the sacral hiatus in these patients. Theinear-array ultrasound transducer used in this study was able toonographically view the anatomical structures of the sacraliatus clearly in all the patients.
CONCLUSIONSUltrasound can be used not only as an effective needle
uiding tool in performing spine injections, but also to deter-ine the likelihood of success of caudal epidural injections.natomic variations capable of causing failed caudal epidural
njections can be clearly observed using ultrasound. Sono-raphic images indicating a closed sacral canal and a sacraliatus diameter in the range of around 1.5mm may suggest aigher incidence of failed caudal epidural injection.
Acknowledgments: We thank Henry L. Lew, MD, PhD ofarvard Medical School for his expert advice in experimental design
nd in caudal epidural injection techniques. We also thank Max. Chen, MD of Chang Gung Memorial Hospital for his expertise inenerating computer graphics.
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