Department of Anaesthesiology, University of Heidelberg, Im Neuenheimer Feld 110, D-69120 Heidelberg, Germany*Corresponding author
Accepted for publication: February 14, 2001
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Abstract |
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Br J Anaesth 2001; 86: 798804
Keywords: pregnancy; measurement techniques, Doppler ultrasound
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Introduction |
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Ultrasound imaging is widely used in medicine, but identification of the epidural space by ultrasonography is hampered by the surrounding bony structures.4 The great improvement in ultrasound technology in recent years has increased the possibility of it becoming a valuable tool before the performance of neuraxial anaesthesia. The first successful studies in the field of ultrasonic measurement of the epidural space date back 20 yr, when Cork and colleagues5 and Currie6 observed a strong relationship between the epidural space depth seen on ultrasonic images and the distance to the point of the epidural needle. Wallace7 confirmed that needle depth is predictable from ultrasound depth measurement. In a more recent study, Bonazzi and de Gracia8 were the first to identify the lumbar ligamentum flavum. They established the usefulness of ultrasound examination before epidural space puncture.
However, detailed knowledge of the pregnancy-related variability of the anatomy of the lumbar spinal column and its flanking ligaments is not available. In our clinic, parturients often present with abnormal anatomical conditions such as scoliosis, hyperlordosis or kyphosis of the lumbar spine, severe obesity or local oedema, which can obscure anatomical landmarks. Additionally, hormonal changes during pregnancy result in an alteration in tissue consistency. Collagen concentration and organization, relative molecular proportions and the water content of the tissue are altered.9 10 Subsequently, the structure of the interspinous ligament becomes softer and inhomogeneous. This often causes false loss of resistance, which can lead to malplacement of the epidural catheter.
Our aim was to investigate the influence of the tissue alterations of pregnancy on epidural technique. We scanned the lumbar epidural space of women during and after pregnancy. Variability in size of the L3/4 intervertebral space and changes in the localization of the ligamentum flavum and dura mater were recorded. We also graded how well the main anatomical landmarks were visualized by ultrasonography.
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Materials and methods |
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Parturients who did not require epidural anaesthesia or in whom it was impossible for medical reasons were excluded from the study, as were obstetric emergencies. None of the patients had a history of spinal surgery or trauma, or evidence of congenital spinal abnormalities. In a short clinical examination, we looked for signs of scoliosis, kyphosis or hyperlordosis, and local oedema (Table 1). The time taken for ultrasonic measurement was noted.
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The needle length for epidural space puncture is shortest when the needle is advanced perpendicularly to the skin surface. This distance was measured in the transverse scanning plane. The distance is longest in a maximal oblique trajectory, which is the path along the spinal process. This distance was assessed in the longitudinal scanning plane. We also measured the angle between these trajectories.
The quality of ultrasonic depiction of the epidural space and the surrounding landmarks was analysed by a numeric scoring system (Table 2). The extent of the acoustic shadows was evaluated similarly (Table 2). In both scanning planes, the diameter of the intrathecal space (i.e. the distance between ligamentum flavum and vertebral body) was measured. We assessed the visible part of the ligamentum flavum between the vertebral arches (longitudinal extent) as the size of the target area.
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Statistics
We recorded all data on standardized forms using tick boxes. Data were analysed using the chi-squared test with Yates correction where appropriate, or using Students t-test. Excel 97TM MicrosoftTM, SPSS 7.5TM and Primer Biostatistik 4.04TM (S. A. Glantz) software were used for statistical analysis. Data are given as mean (SD) unless stated otherwise.
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Results |
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At the first clinical examination, 11 patients (20.8%) presented with axis deviation of the spine and 14 (26.4%) showed lumbar oedema. One parturient suffered from pre-eclampsia and presented with severe oedema. The number of pathological findings decreased at the follow-up examination, but no significant difference could be established (Table 3).
In the first examination, the visibility of the ligamentum flavum, the major target of epidural space ultrasonography, was scored as very well detectable (2.7 (0.5); for all numerical scoring see Table 2). The visibility of the epidural space was scored as relatively poor (1.4 (0.5)) (Fig. 1), and the visibility of the dura mater (1.0 (0.4)) and intrathecal space (0.9 (0.4)) (Fig. 2).
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Figures 1 and 2 indicate that the ligamentum flavum was detectable by ultrasound in both the pregnant and non-pregnant states. But the epidural space was hardly detectable in 57% of pregnant patients yet was detectable in 75% of those who were not pregnant (Fig. 1). The dura was hardly detectable in 88% of the pregnant and 56% of the non-pregnant group (Fig. 2).
The visibility of the intrathecal space post-partum was rated as 1.0 (0.3; n.s.) (Table 4). The visibility of the vertebral body was significantly better (2.8 (0.4); P<0.008), and the visibility of the spinous process was 1.7 (0.5; n.s.). As in pregnancy, the vertebral discs could not be detected satisfactorily (0.3 (0.5)). The intensity of the acoustic shadows showed no significant alteration post-partum (Table 4). Figure 4 shows typical findings.
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Discussion |
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In this study, pregnancy-associated weight gain and tissue changes resulted in significant changes in spinal anatomy. These alterations were readily observed by the use of ultrasound imaging. During pregnancy, the epidural space was located deeper under the skin surface in all patients. The upward gradient of the space between the spinal processes was steeper and, hence, so was the angle of epidural space puncture. The size of the ligamentum flavum target area did not seem to alter yet the diameter of the L3/4 intervertebral space was reduced. The explanations for these findings are probably oedema and loosening of connective tissue, causing an increase in the bone volume of the spinal processes. Thus, the optimum puncture area on the skin for epidural space cannulation was smaller and the soft-tissue channel between the spinal processes was narrower by the end of pregnancy. The distance from the skin to the epidural space was longer and the epidural space was narrower. Therefore, the safety zone between transfixation of the ligamentum flavum and inadvertent dural puncture was smaller.
The quality the sonographic depiction of the key structures was reduced by pregnancy. Nevertheless, ultrasonography can be a valuable guide to epidural space puncture in parturients. The average quality of depiction is diminished in pregnancy 5070% from the non-pregnant state, but was sufficient to identify the flaval ligament and the epidural space in all cases.
Segal and colleagues were one of the more recent groups attempting to relate the depth of the epidural space to the physical characteristics of the patients.15 Like the preceding studies in this field,16 17 their findings did not prove adequate for clinical use. Palpation is the only diagnostic method established for use during epidural puncture. Distance measurements other than by ultrasonography can only be obtained from a lateral radiograph of the lumbar spine or by computerized axial tomography, but radiography is an inappropriate diagnostic tool in pregnancy. Magnetic resonance imaging has expanded the diagnostic capabilities,18 19 but is impractical in labour.
Ultrasonography is a promising technique for the assessment of spinal anatomy.4 20 It is widely used during pregnancy, so parturients are familiar with it. The technique is free from adverse effects to mother and fetus. It is a mobile tool, which is readily available, even in the delivery room. There have been many advances in ultrasound imaging technology since earlier studies in this field were reported.57 By the use of an advanced sonographic system we were able to identify the flaval ligament in nearly all cases, and the dura mater and, subsequently, the epidural space in many cases. Often, a three-dimensional impression could be acquired using two perpendicular scanning planes; thereby, enabling us to preview the anatomy of the lumbar spine. Thus, modern ultrasonography can be a valuable support before performing regional anaesthesia.
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References |
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