Spinal anesthesia can be affected by gaps in the ligamentum flavum that may confuse epidural needle placement; cause Quincke needle deformation if the vertebral bone is hit, expanding the dural sac hole and leading to cerebrospinal fluid (CSF) leakage and post-dural puncture headache; and variability in CSF volume, altering intrathecal drug dilution and anesthetic efficacy. Paramedian approaches with previously calculated optimal angles can be adopted, and CSF volumes may be estimated using threshold selection within the grayscale range following anatomical criteria. Angles can be determined by previously quantifying the skin-dural sac distance using ultrasound. Here, the relevant anatomical structures are illustrated (ligaments, the ligamentum flavum gap, vertebrae, meninges, cauda equina roots) using a freely available 3D PDF tool (http://hdl.handle.net/2445/55225), the additional inconvenience of variability in vertebra rotation is quantified, a free guide for optimizing spinal puncture is presented (http://hdl.handle.net/2445/179594), and the possibility of individualizing intrathecal drug dosages is discussed.
TopBackground
The deformation of the tip of the needles when touching bone during a spinal puncture is of special relevance when Quincke needles are used (Reina et al., 2002) which will cause a larger dural disruption after redirection of the needle and a new attempt of dural puncture. The leakage of cerebrospinal fluid (CSF) through the dural disruption can produce a post-dural puncture headache (PDPH) (Buddeberg et al., 2019). The hole size is known to be related to the needle type (Reina et al., 2017). Furthermore, accidental dural punction when performing epidural analgesia during labour occurs in as many as 1.5% of cases (Maranhao et al., 2020) and PDPH may last for months. Given that intrathecal or epidural procedures are performed thousands of times a day worldwide, PDPH affects thousands of patients every year. In the other hand, the variability in dilution volume due to the variability in CSF volume per segment (Prats-Galino et al., 2012; Puigdellívol-Sánchez et al., 2015a, 2015b), might also produce a variability in the anesthetic effect.
Medial and paramedian epidural / spinal approaches will be analyzed. In the medial approach, the needle must traverse either the supraspinous ligament, the interspinous ligament and the ligamentum flavum before arriving to the epidural space and dural sac. However, sometimes a gap is present at the midline of the ligamentum flavum (Lirk et al., 2004; 2005; Hermanides et al., 2012), removing the usual midline resistance that is felt, and introducing the potential to misinterpret the dural sac as being at the ligamentum flavum level (Reina et al., 2016). A paramedian approach avoids this, ensuring that the ligamentum flavum is identified first and that the needle is located in the epidural space, after traversing the ligamentum flavum, either in epidural procedures or in spinal procedures, before penetrating the dural sac. This approach is limited by the choice of the correct angle to avoid hitting the articular process. Freely available 3D PDF tools can help to understand the needle trajectory and the ligamentum flavum gap inconvenience. In this chapter, illustrative images have been prepared to show those anatomical aspects.
Skin–dural sac distance by means of ultrasound evaluation can be used to calculate the optimal angles of the needle for paramedian approaches by means of an easy formula to avoid a touching bone structures (Puigdellívol-Sánchez et al., 2016). An additional issue is the frequent vertebral body rotation, which may also affect the final angle of incidence to vertebral canal. The rotation of vertebral bodies in a sample of patients will be calculated in this chapter. Axial rotation of vertebral bodies may affect the optimal angle of incidence, even if the skin-sac distance is calculated. Rotations have extensively been assessed for scoliosis studies, but axial rotation is frequently mistaken (Illés et al., 2019) and no detailed studies relating rotations and anesthesia exist. Furthermore, given that additional rotation may appear when punctures are performed in a lateral decubitus position, quantification of that rotation will be presented in an illustrative case.