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. 2021 Aug 19;21(10):369–375. doi: 10.1016/j.bjae.2021.06.003

Ultrasound-facilitated neuraxial anaesthesia in obstetrics

A Sadeghi 1, R Patel 2,∗, JCA Carvalho 3,4
PMCID: PMC8446248  PMID: 34567791

Learning objectives.

By reading this article, you should be able to:

  • •

    Explain the principles of how to perform preprocedural spinal ultrasound.

  • •

    Discuss the inaccuracy of the traditional landmark palpation approach in identifying the intervertebral level and the associated clinical and medicolegal consequences.

  • •

    Detail the evidence in favour of routine preprocedural ultrasound in pregnant women undergoing neuraxial anaesthesia.

Key points.

  • •

    Landmark palpation for neuraxial anaesthesia is inaccurate and can be challenging in pregnant women.

  • •

    Preprocedural ultrasound allows accurate identification of the intervertebral level and insertion point.

  • •

    Preprocedural ultrasound enables visualisation of the ligamentum flavum and dura mater, and estimation of the depth to the epidural space.

  • •

    Time required to perform preprocedural ultrasound can abbreviate procedural time, with fewer needle punctures and redirections.

  • •

    Ultrasound is a valuable teaching tool and improves the learning curve of neuraxial procedures.

There are more than 700,000 deliveries in NHS hospitals in the UK every year, and more than 270,000 of these involve neuraxial analgesia or anaesthesia for labour and delivery.1 There is a high incidence of weight gain and obesity in obstetric patients, which makes neuraxial anaesthesia more challenging. Medical disorders of pregnancy such as preeclampsia may increase the risk of vertebral canal haematoma, and neuraxial anaesthesia may be required in a time-critical manner, for example at emergency Caesarean delivery. Furthermore, obstetric patients are a high-risk group for general anaesthesia and so neuraxial anaesthesia may be the best option available.

The National Institute for Health and Care Excellence (NICE) has recommended using neuraxial ultrasound for epidural catheterisation since 2008.2 Over the past decade, evidence to support the benefit of ultrasound-assisted neuraxial anaesthesia has increased. A previous article in this journal gave an overview of using ultrasound for lumbar neuraxial anaesthesia in non-obstetric patients.3 In this article, we focus on the practical steps of performing preprocedural ultrasound and the evidence for its role as standard practice in obstetric patients.

Preprocedural ultrasound scanning technique

Spinal ultrasound involves imaging structures that are sheathed by a complex, articulated encasement of bones. The ultrasound beam needs to penetrate very narrow acoustic windows within this bony complex to obtain the best view. The structures are located deeper to the surface of the skin compared with those we image with ultrasound for peripheral nerve blocks and placement of vascular access. The ultrasound probe used for spine assessment should be a low-frequency (2–5 MHz) curved probe, which has lower image resolution but penetrates deeper, compared with the high-frequency (10–15 MHz) linear probe, which in turn has better resolution but penetrates to a lesser degree.

With spinal ultrasound, two planes of approach enable penetration of the ultrasound beam through two narrow acoustic windows. These are the longitudinal paramedian oblique approach and the transverse approach; the images seen on these planes have been described as the ‘saw sign’ and ‘flying bat’, respectively.4

The longitudinal paramedian oblique approach is performed by positioning the ultrasound probe vertically in line with the long axis of the spine. The probe is placed over the sacral area, 1–3 cm to the left of the midline and then angled medially to target the centre of the spine until a continuous hyperechoic (bright) white line (sacrum) is seen. The probe is then moved cephalad until a hyperechoic sawlike image is shown (Fig. 1). The ‘saw’ represents the vertebral laminae (teeth of the saw) and the interspaces (spaces between the teeth), where the ligamentum flavum and posterior dura (posterior complex), and the anterior dura, posterior longitudinal ligament and vertebral body (anterior complex) can be visualised. The precise level of the interspaces, starting with L5–S1, can then be counted above the sacrum and marked.

Fig 1.

Fig 1

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Longitudinal paramedian approach with the typical saw sign. (a) Ultrasound probe over the sacrum and lower lumbar spine. (b) Hyperechoic image of the sacrum and the saw sign, which represents the laminae of the lumbar vertebrae and the interspaces. (c) Hyperechoic image of the laminae, the ligamentum flavum and posterior dura mater (posterior unit), the anterior dura mater, posterior longitudinal ligament and vertebral body (anterior unit). Reprinted from Talati et al., Anesthesiol Clin 2017; 35: 35–58, with permission.5

The transverse approach is performed by placing the ultrasound probe horizontally, perpendicular to the long axis of the spine, at the marked levels obtained with the longitudinal paramedian oblique view. Assuming that the spine is aligned with no scoliosis, the midline of the spine can be identified by a small hyperechoic signal immediately underneath the skin (spinous process), which continues as a long triangular hypoechoic (dark) acoustic shadow. The probe is moved cephalad or caudad until an acoustic window reveals an interspace. Within the interspace, two hyperechoic lines (‘equal sign’) corresponding to the anterior and posterior complex will be seen, along with the articular processes (bat's ears) and transverse processes (bat's wings) to give an image of a ‘flying bat’ (Fig. 2). Importantly, the angle at which the probe is held against the skin to give the optimal view of the ‘flying bat’ is also the angle at which the needle should subsequently be inserted.

Fig 2.

Fig 2

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Transverse approach at a lumbar interspace with the ‘flying bat’ sign. (a) Ultrasound probe placement. (b) Arrows indicate ligamentum flavum and vertebral body, which are seen as hyperechoic structures in the midline. Articular and transverse processes are seen as paramedian hyperechoic structures. (c) Artistic impression of the ‘flying bat’. Reprinted from Carvalho, Anesthesiol Clin 2008; 26: 145–58, with permission.4

The middle of the probe along the horizontal and vertical edges can now be marked on the skin, corresponding to the midline and interspace level, respectively. The ultrasound image can be frozen and the distance from the skin to the epidural space measured, with the built-in caliper. The needle puncture site is determined by the intersection of these two marks on the skin (Fig. 3).4 This can be marked with an indentation on the skin, for example using the circular hub of a blunt drawing-up needle or the tip of a 1 ml syringe. The pen marks on the skin can then be wiped clean and the back surgically cleaned with antiseptic solution, as per unit policy, leaving the skin indentation to indicate the needle puncture point.

Fig 3.

Fig 3

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The insertion point is determined by the intersection of the two marks on the skin in the vertical (midline) and horizontal (interspace) planes. Reprinted from Carvalho, Anesthesiol Clin 2008; 26: 145–58, with permission.4

In summary, prepuncture ultrasound scanning provides accurate and reliable information on several critical landmarks needed for successful epidural catheter placement. These are the exact interspace (level) at which the puncture will be performed; the interspace with the clearest sonoanatomy; the midline; the optimal insertion point; the optimal angle for needle insertion; the depth to the epidural space; and any abnormalities of the spine (e.g. scoliosis).4,6

Identification of intervertebral level

The level at which the spinal cord terminates is highly variable, ranging from T12 to L3, and is at a lower interspace in women.7, 8, 9 Using MRI scanning, the conus was found to terminate below the body of the L1 vertebra in a significant number of patients, and in 28–58% by anatomists in a series of dissection studies.10 This becomes acutely relevant when intrathecal ‘needling’ is performed, which can cause direct nerve injury to the spinal cord and may result in litigation against the anaesthetist.11

The intercristal line (Tuffier's line) is a horizontal line drawn across the highest points of both iliac crests in an anteroposterior (AP) lumbar radiograph. It usually intersects the vertebral body of L4 in men and L5 in women. In the approach traditionally taught, palpation of the bony iliac crests is used to identify an imaginary line, as a substitute for the true radiological Tuffier's line. The accuracy of this conventional teaching has been investigated and found to be highly unreliable, both in pregnant and non-pregnant patients.10,12

Broadbent and colleagues studied the ability of anaesthetists to identify a marked lumbar interspace accurately by palpation.10 They found that experienced anaesthetists incorrectly identified interspaces in 71% of cases, when confirmed by MRI. The interspace identified was often higher than the actual interspace, usually by one or two levels, but up to four interspaces higher was reported and accuracy was lower in patients with obesity.10 This was confirmed with a study involving pregnant women at term, where the intercristal line determined by palpation was identified above the L4–L5 interspace in all cases, when assessed with ultrasound. The level of intersection ranged from immediately above L1–L2 to L4–L5.12 Unlike palpation, the use of ultrasound identification of the L3–L4 interspace has been found to correlate more accurately (~71–76%) with spinal imaging and further supports ultrasound as a useful adjunct to safe intrathecal ‘needling’.13,14

A case report by Reynolds highlighted injury to the spinal cord after spinal or combined spinal–epidural, where the anaesthetists believed the needle was inserted at the L2–L3 interspace.15 The author concluded that given the inaccuracy of methods of identifying lumbar interspaces and the variability of the position of the conus, an intrathecal needle should not be inserted above the level of L3. In a subsequent editorial, Bogod supported this recommendation and described the area below L2–L3 as ‘Reynold's zone’, a safe space for intrathecal needle placement.16 He reiterated that relying on Tuffier's line as being at the level of L4 or below may allow the error of choosing a higher interspace to perform the puncture, thus risking damage to the spinal cord. Of note, all these recommendations become pointless if neuraxial anaesthesia is practiced without ultrasound assistance, given the inaccuracy of the palpatory technique.

Insertion point, midline and depth to the epidural space

In addition to the lumbar spinal level, ultrasound allows identification of the midline, the optimal insertion point, and estimation of depth to the epidural space. Successful identification of the epidural space and avoiding unintentional dural puncture relies on the loss of resistance technique either to saline or air. This is a ‘blind’ technique based on tactile feel. An unintentional dural puncture with the Tuohy needle is a complication of epidural insertion, with a reported incidence of 1%, that may be associated with postdural puncture headache and other serious neurological complications, including subdural haematoma.17

The ability to visualise the neuraxial structures by preprocedural ultrasound assessment can help to access the epidural space safely with the Tuohy needle. Several studies have shown that the depth to the epidural space estimated using ultrasound and the actual needle depth are closely related.18, 19, 20 The ability to identify the correct intervertebral level and optimal insertion point accurately, and to estimate depth to the ligamentum flavum is particularly useful in cases where anatomical landmarks may be difficult, for example in the presence of obesity, scoliosis or previous spinal surgery.

Obesity

The identification of bony landmarks in women with obesity can be particularly challenging, because of increased subcutaneous tissue and deeply located structures, making palpation of the lumbar spinous processes and identification of intervertebral level and midline almost impossible. Spinous processes may be easier to palpate at higher intervertebral levels in women with obesity, and without the use of ultrasound, may encourage the practice of inserting spinal needles above L3–L4. Using ultrasound to estimate depth to the ligamentum flavum in women with obesity can also guide the decision on how far to insert the needle, and when to use a longer needle.

Several studies have investigated whether using ultrasound translated into improved technical performance in women with obesity. The actual depth to the epidural space can be predicted within a range of 0.7 cm in women of normal weight and 1 cm in women with obesity.6 An RCT studied women scheduled for Caesarean delivery under spinal anaesthesia and reported a reduced number of puncture attempts and fewer puncture levels, with a first attempt success rate in women with obesity (BMI >30) of 92% in the ultrasound group compared with 44% in the landmark group.20

While performing spinal ultrasound in women with obesity, it is important to appreciate the degree of subcutaneous tissue compression, which can significantly contribute to the underestimation of the depth from the skin to the epidural space. A significant improvement in the technique can be accomplished by relieving the pressure on the skin, freezing the image and measuring this distance. Although in most patients the ligamentum flavum is shallower than 8 cm, further distances can be found. This is less likely if the optimal insertion point and angle are used; thus, the use of ultrasound may avoid the need for use of an extra-long needle.

Scoliosis and spinal surgery

Scoliosis is an anatomical variation in the spine that leads to lateral curvature and rotation around the longitudinal axis, with a higher incidence and severity in females.21 The interspaces themselves can be abnormal, showing asymmetry of bony structures, including the facet joints, transverse and articular processes. In these cases, rotating the ultrasound transducer angulation until these structures appear symmetrical and the anterior and posterior complexes appear horizontal and parallel (equal sign) can further help to estimate the degree of spinal rotation and aid the correct needle trajectory. By scanning several interspaces, ultrasound can also be used to avoid severely affected interspaces and aid in selection of a preferred level.4,6

Further causes of abnormal anatomy include previous spinal surgery where spinal fixation with rods and screws can grossly obscure anatomy resulting in high rates of complications and failure of neuraxial techniques. Ultrasound can also be used to try and identify a space with preserved anatomy to facilitate regional anaesthesia, in particular spinal anaesthesia, thus avoiding general anaesthesia.4,6

Improved technical performance, clinical efficacy and patient satisfaction

A number of RCTs and meta-analyses have shown that preprocedural ultrasound improves technical performance and this translates into improvements in efficacy and patients’ satisfaction.19,22, 23, 24 Grau and colleagues investigated the use of preprocedural ultrasound and maternal discomfort or satisfaction with the procedure.19,23 They observed that preprocedural ultrasound significantly reduced the number of puncture levels, pain on insertion and increased complete analgesia and patients acceptance of the procedure.19,23 Arzola and colleagues found that the success of insertion was almost 92% using the transverse approach with ultrasound, with no need to redirect the needle in almost 74% of patients.18

A recent systematic review and meta-analysis examined the efficacy, the time taken and the safety of neuraxial anaesthesia using preprocedural ultrasound in obstetric patients, compared with the landmark palpation method. Young and colleagues reported an increase in the first-pass success rate with preprocedure ultrasound, with no difference in the total procedural time.24 Subgroup analysis demonstrated increased benefit of preprocedural ultrasound in cases where the neuraxial procedure was predicted to be difficult. Furthermore, preprocedural ultrasound decreased the incidence of complications, including: technical inability to perform the neuraxial technique; failure of analgesia or anaesthesia; ‘bloody tap’ or vascular cannulation; postpartum back pain, and headache.24

Use of spinal US for teaching and training

Spinal ultrasound can be used to improve the learning curve for epidural anaesthesia at all stages of learning. In an RCT of 600 epidural anaesthetics, Grau and colleagues found that trainees who undertook their first 60 epidurals with ultrasound guidance (compared with palpation of landmarks) had a significantly greater success rate at all time points in the study.25 A systematic review of the literature on teaching neuraxial anaesthesia to trainees highlighted the need for further research in the area and concluded that ultrasound should be considered as a first-line teaching tool for novice trainees.26

In general, most authors agree that more than 20 supervised trials are required to learn the technique of lumbar spine ultrasound assessment, and that 30–40 supervised trials are required to attain competency.27,28 In a learning curve study, using CT as the gold standard for intervertebral identification, Halpern and colleagues observed that two anaesthetists with no prior experience using neuraxial ultrasound achieved 90% or greater accuracy with ultrasound use after 22 and 36 procedures, respectively.28 Practical learning should be supplemented with lectures and hands-on workshops to ensure a good understanding of the underlying sonoanatomy and technical skills.12

Limitations and challenges for the future

Although there is now significant evidence to encourage the use of ultrasound in obstetric neuraxial anaesthesia, its uptake in clinical practice remains poor. This may partially result from the low baseline rate of failed procedures amongst experienced anaesthetists, who question the need for this additional technology. Several studies have investigated whether technical performance is improved with ultrasound in patients with easily palpable anatomical landmarks. These studies are conflicting, with some concluding that in patients with easily palpable landmarks, experienced anaesthetists may not improve their success rate with ultrasound use.29, 30, 31 A recent large RCT trial found evidence that the use of ultrasound did improve first-pass success in patients with easily palpable spinous processes; however, these results were from sub-group analysis and the study was not powered for this outcome.32 In the authors' opinion, the use of preprocedure ultrasound regularly on all patients, including cases where difficulty is not anticipated, is required to gain and maintain competence, so that the technique can be used effectively in difficult cases. Furthermore, spinous processes that are easy to palpate do not always translate into ease of neuraxial insertion. Patients with mild undiagnosed scoliosis may have palpable spinous processes, but the degree of lateral curvature will not be visualised if ultrasound is not used routinely. In addition, we know that even experienced anaesthetists cannot always accurately identify a particular lumbar interspace by palpation.10 This risks spinal damage from inserting spinal needles at a high lumbar interspace. For these reasons, all obstetric patients and not just those predicted to be difficult, can benefit from preprocedural ultrasound.

The additional time required to complete preprocedural ultrasound in busy obstetric units is commonly given as a reason for not using ultrasound, particularly for cases with clinical urgency, such as category 1 Caesarean section. There is evidence that the time saved with improved first pass success and reduced number of times the needle needs to be redirected balances the time taken for preprocedural ultrasound (2–3 min), and can even shorten the overall procedure time.20,24 The availability of an ultrasound machine for dedicated anaesthetic use on the labour ward will further reduce delays in accessing this valuable tool. Where resources are limited, it is worth highlighting that the curved probe used by obstetricians for obstetric scanning is the same as the probe used for lumbar ultrasonography and may be shared.

More recent studies have compared handheld ultrasound devices with traditional high-end machines. Handheld devices are less expensive and are increasingly portable, and show good correlation compared with high-end machines across several medical specialties, including estimated epidural depth in labouring women.33,34 Both ultrasound devices demonstrated accuracy in estimating the actual depth to the epidural space; however, they consistently underestimated the measurement. This may translate into added safety in clinical practice, informing the operator to use additional caution with advancement of the Tuohy needle as they approach the estimated depth to the epidural space. However, there is currently no evidence that the information of the estimated epidural depth by preprocedural ultrasound results in fewer accidental dural punctures.34

Summary

Neuraxial ultrasound is a highly effective and evidence-based tool for accurately visualising spinal anatomy. This quick and noninvasive procedure has been shown to improve technical performance, clinical efficacy and patient experience of neuraxial anaesthesia in obstetrics. It is of particular value in reducing complications in patients with difficult anatomy, or at increased risk of bleeding. We recommend that structured training on the use of preprocedural lumbar ultrasound is integrated into regional and local obstetric anaesthetic training across every maternity unit, and that it is used routinely to maintain technical skills.

Declaration of interests

The authors declare that they have no conflict of interest.

Biographies

Abtin Sadeghi BA (Hons) MA (Cantab) FRCA is a specialty registrar in anaesthesia at Imperial School of Anaesthesia.

Ruchira Patel BSc FRCAPG Cert Med Ed is a consultant anaesthetist and lead for obstetrics at West Middlesex University Hospital, Chelsea & Westminster NHS Trust. She has completed a fellowship in obstetric anaesthesia at Mount Sinai Hospital, Toronto.

Jose Carvalho MD PhD FANZCA FRCPC is director of Obstetric Anaesthesia at Mount Sinai Hospital and professor of anaesthesia and obstetrics and gynaecology at the University of Toronto. He is a pioneer for ultrasound in obstetric anaesthesia and has published widely in this field.

Matrix codes: 1A03, 2B04, 3B00

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