Ultrasound Assisted Spinal Anesthesia: A Safety Procedure or a Lost of Time?

The role of ultrasound (US) in central neuraxial blocks is often underestimated for the difficulty in ultrasound visualization. The utility of ultrasound technique is more evident in expected difficult cases for the relative efficacy of blind techniques. Studies with larger sample size are necessary to improve spinal ultrasound use in neurassial anesthesia. partly to on to the the the by in


Introduction
The role of ultrasound (US) in central neuraxial blocks has often been underestimated, partly due to the relative efficacy of blind techniques (based on the exploration of the classic anatomical landmarks), partly due to the difficulty in the ultrasound visualization due the acoustic window produced by spine bony structures. Nevertheless, in the last decade, ultrasound usage in neurassial anesthesia has aroused greater interest, due to its great usefulness [1]. The first description of an echo-assisted lumbar puncture dates to 1971. More recently, neurassial ultrasound was used both as a pre-procedural aid and for real-time needle placement (eco-guided technique). Imaging in the adults is possible only through the interlaminar space between adjacent elements.
Combining various ultrasound scans allows to study the patient's vertebral column, exploring the anatomical structures to be crossed and recording relevant parameters for the anesthesiologist [2][3][4] such as intervertebral levels, depth of the epidural and subarachnoid space and to trace the optimal spinal needle entry point. The utility of ultrasound technique is more evident in expected difficult cases, when only superficial anatomical landmarks are poor, such as obesity or excessive thinness, and previous spinal surgery, or in case of deformity of the vertebral column [5].
Conventional palpation of the surface anatomy may be insufficient for an exact neurassial anesthesia [6]. For example, in elderly, midline canal access may be more difficult due to narrowing or calcification of the interspinous space, heterotopic ossification of the interspinous ligaments, or joint hypertrophy. Most of the evidence related to neurassial ultrasonography are derived from obstetric anesthesia performed in limited specialized centers [7]. Those evidences show that ultrasound may reduce attempts number, can accurately predict epidural space depth and can therefore significantly improve the success rate of anesthesia, even in young specialists. Similarly, to other loco-regional anesthesia techniques, the real-time ultrasound-guided spinal needle penetration in the subarachnoid space requires technical ability and has a learning-curve, in order to maneuver coherently the ultrasound probe and the needle. The ultrasound-guided technique consists of an accurate pre-procedural patient's back scan and has usually a shorter learning-curve. Ultrasound-guided single shot subarachnoid anesthesia is less studied. An observational study in orthopedic patients showed that ultrasounds may accurately predict dura mater depth [8].
Moreover, pre-procedural US has been decisive in clinically difficult situations, such as obesity, kyphoscoliosis and cases of previous spinal surgery [9,10]. The echo-guided subarachnoid anesthesia has been described in technically difficult and prone position patients. via the Taylor approach [11]. Perlas et al. [12,13],  [14]. The failure rate was significantly lower in the USguided group. The second aspect was the forecast of difficulties.
The difficulty of a neurassial block is quantifiable through two main parameters: the number of attempts to introduce/realign the needle and the time to block. Of the two, we consider the number of attempts/realignments the most important factor, since a complicated insertion of the spinal needle is an independent risk factor for possible complications as vascular puncture, paresthesia, hematomas, long term neurological damage [15,16].
In a randomized trials US usage reduced the number of spinal needle movements by 50% and significantly increased the first attempt success rate [17]. As a further support to the safety with US-technique, Shaikh et al. reported a reduced rate of realignment and spinal punctures for neurassial procedures, including epidural catheterizations, in a 2013 meta-analysis [18]. Several case reports have also described neurassial blocks successfully carried out in markedly obese patients, elderly, patients with spinal deformities and with a history of vertebral surgery (once considered a contraindication to the subarachnoid procedure), by means of US support [19][20][21]. A randomized controlled trial conducted on 120 variants. The correlation between US depth and that detected with needle insertion was evaluated in several studies [24][25][26]. Based on measurements conducted in different ultrasound views, the correlation was excellent in all studies. Albeit rarely, an incorrect intervertebral spaces identification can lead to serious complications, such as spinal cord injury [27]. In adults (unlike in pediatric patient) the medullary cone is hardly appreciated, due to its depth and to a more restricted acoustic window at the vertebral canal level. In this case, US can identify intervertebral levels more accurately, starting from the sacral junction (clearly distinguishable from other spaces) and identifying the spinous processes and/or laminae in a cranial direction.
This scanning mode is more accurate than the one with the use of intercaecal line as external anatomical reference point.
Schlotterbeck et al. [28,29], have found that, in case of discordance between real and US anatomical landmarks, the intervertebral space was generally lower than the one identified by US. In contrast Locks et al. [30] observed that, the real level was cranial than the US reference. US is not unerring. When compared with other imaging modalities, such as MRI or CT, US can correctly identify structures in only 68-76% cases. Nevertheless, the US inaccuracy degree is considerably lower than the manual palpation of anatomical landmarks in the order of one intervertebral space versus 3 intervertebral spaces, respectively [31]. Moreover, it should be considered that US errors are operator-dependent and are therefore more likely at the beginning. Precision level greater than 90% can be achieved with an appropriate training and experience; mannequins and phantoms have proved to be useful for the training of young anesthesiologists or specialists with little experience in spinal US [32,33].
The most common errors usually derive from the misidentification of the L5-S1 junction or lumbosacral junction anomalies, which occur in approximately 12% of the general population. The most common anatomical variant is the sacralization of L5 in which L5 is fused to the sacrum and one or both transverse processes are involved. Less frequently, S1 can resemble a lumbar vertebra (lombarization). In this sense, counting vertebrae from D12, identifying the twelfth rib through US, to the lumbo-sacral junction, represents a good strategy. Kim et al. [34] have shown that the distance between the medullary cone and the Tuftier line is similar both in patients with lumbosacral junction abnormalities and in those without. In conclusion, the vertebral counting starting from the lumbosacral junction seems an adequate and sufficiently safe US technique. The most common pitfall of the US approach is the poor image quality with difficulties in identifying deeper structures such as the epidural space, dura mater, intrathecal space and anterior complex in markedly obese patients and in the elderly.
Furthermore, in obese patients there may be an image attenuation due to the abundance of soft tissues Moreover, it can be an image aberration due to the different sound propagation speed through irregular and deformed adipose tissue [35]. However, the identification of spinous processes and the midline is almost always possible, even in these cases. In the elderly, ligaments ossification and facet joints hypertrophy cause a narrowing of the interspinous and interlaminar spaces. In skinny patients, prominent and sharp spinous processes could reduce correct probe-skin contact and determine a poor image quality. In such patients, the oblique view structures and image quality [36]. An initial depth of 7-8 cm is appropriate for most patients, bearing in mind that depth, focus and gain must be adjusted by the operator during the scanning process to produce a high-quality image (Figure 1). Spine can be studied with five basic US views [37]:

Visualization of Transverse Processes in Sagittal Paramedian
The probe is positioned 3-4 cm lateral to the midline, just above the sacrum. In this image, the transverse processes of the subsequent lumbar vertebrae are visible as small hyperechoic curvilinear structures, the "trident sign". The psoas muscle is visible between the acoustic shadows, deeply to the transverse processes.

Visualization of Articular Processes in Sagittal Paramedian
The probe is slides medially, until it meets a hyperechoic line of "humps" corresponding to the facet joints (at a lower depth than the transverse processes) of two adjacent vertebrae.

Visualization in Oblique Sagittal Paramedian
Once the transverse processes have been identified, the probe is tilted to put the beam in a lateral-medial direction, towards the midline. The hyperechoic sloping lumbar vertebrae laminae form a "saw-tooth" pattern among which the paramedian interlaminar

Visualization of the Spinous Processes in Transversal
Once completed the paramedian sagittal plane examination, the probe is rotated 90° in a transverse orientation and then moved to the midline. If the probe is above a spinous process, this will appear as a superficial hyperechoic line above a hypoechoic shadow. The lamina is visible on both sides of the spinous process as a hyperechoic wing, while all the other structures of interest are covered by shadow cones generated by the bony tissue.

Visualization of the Interlaminar Space in Transversal
Slide the probe in the cranial or caudal direction starting from the spinous process, align the US beam with the interspinous and interlaminar spaces, to visualize the vertebral canal in transverse orientation. Typically, the acoustic shadow generated by the spinous process gives way to an iperechoic vertical line (the interspinous ligament surrounded by the erector muscles) and, more deeply, two parallel hyperechoic lines separated by a hypoechoic space When the beam passes through the vertebral canal, the vertebral body complex is visible and the interlaminar space has been correctly identified [38]. The transverse processes and the articular processes are additional reference points, since they lie approximately on the same level as the interlaminar space, resulting useful for the needle orientation in difficult anatomical cases. The midline and the interlaminar space can therefore be marked on the patient skin at the midpoint of each side of the convex probe (long and short, respectively). The intersection of the lines passing for these two landmarks indicates therefore the optimal puncture site for a median approach, both for subarachnoid anesthesia and for other neurassial blocks. Furthermore, the spinal needle angle can be estimated from the probe tilting degree necessary to obtain a good interlaminar view. Our experience concerns orthopedic surgery. Locoregional anesthesia is the gold standard, due to the many advantages in terms of intra and postoperative pain control, and to the mortality and morbidity reduction.
The US-guided technique requires a first US look on a sterile field, before proceeding to the puncture and subarachnoid anesthesia. Progress in the locoregional field has improved efficacy and safety of peripheral and central nerve blocks. In expert hands, loco-regional techniques can result in greater patient comfort, In cases of reduced intervertebral spaces, another factor that affect the success of a subarachnoid anesthetic, US might be superior to blind technique for two reasons. Firstly, manual palpation alone provides only a rough estimate of the passage through which the spinal needle will passed. Very often, the real perception of the width of the space comes at the very moment in which the spinal needle is aligned or ongoing while the introducer is inserted. The operator can therefore under or overestimate the space amplitude, but recognize a space narrowing only when the procedure started. In summary, there could be a mismatch between the parameter recorded before the procedure and the real width of the intervertebral space. Secondly, the US anatomical study helps the operator to view and calculate intervertebral space width, ratios and inclinations of the spinous processes and the articular processes, to identify the needle insertion point, estimating its real dimensions before the procedure. The most obvious limitation of the US-technique for a neurassial block is represented by the increased overall procedural time, defined as the time between the preparation of the sterile field and the end of the procedure.
Moreover, the US scan performed with a sterile probe cover needs several minutes more than the blind technique, while keeping the patient in an adequate position for the entire procedure. The prolongation of US scan, especially in difficult anatomical cases or for reasons of inexperience, could lead to patient discomfort or compromise his correct positioning at the time of puncture. These factors that must be carefully considered and monitored. However, comparing timing differences between the two techniques, analyzing the needle insertion time and the liquor obtaining, US is faster as compared to the blind technique and compensate in a certain way the longer time required for the US study of the column. Moreover, it should be mentioned that the increase in non-operating time can be overcome by a totally ecoguided approach with introducer-free spinal needles, such as the Quincke, performing US scan and puncture together, thus ensuring US viewing together with a reduced overall procedural time. In our clinical practice we use primarily an atraumatic needle such as the 25G Whitacre, suitable for US-guided method. This choice, reducing the risks of dura mater trauma, ensure a greater patient safety together with pre-procedural US guidance.

Conclusion
In conclusion, studies with larger sample size are necessary to analyze further implications and aspects on the use of spinal ultrasonography and the efficacy of neurassial blocks. In support of these studies, especially in a future perspective of a more systematic approach, the US of the neurassial district showed to be an accurate and valid aid for subarachnoid anesthesia in terms of success and safety.