The Spinal Cord, Spinal Canal, and Meninges

The cephalad aspect of the spinal cord is continuous with the brainstem through the foramen magnum. The spinal cord terminates as the conus medullaris, most often at the level of the lower border of the first lumbar vertebral body. The conus medullaris is attached to the coccyx by means of a neural-fibrous band called the filum terminale, which is surrounded by the nerves of the lower lumbar and sacral roots, known as the cauda equina. Within the bony vertebral column are three membranes: the pia mater, the arachnoid mater, and the dura mater. The pia mater is a highly vascular membrane that closely invests the spinal cord. The arachnoid mater is a delicate, nonvascular membrane closely adherent to the third and outermost layer, the dura. The subarachnoid space, located between the pia mater and arachnoid mater, contains (1) cerebrospinal fluid (CSF); (2) spinal nerves; (3) a trabecular network between the two membranes; (4) blood vessels that supply the spinal cord; and (5) lateral extensions of the pia mater—the dentate ligaments. The dura mater is a membrane, composed of collagen, that encapsulates the spinal cord, the deeper meningeal layers, and the subarachnoid space. This layer forms a connective tissue sheath along the vertical axis of the central nervous system (CNS) that is contiguous with connective tissue covering the lateral extension of spinal nerve roots as they exit the intervertebral foramina. The interface between the dural and arachnoid layers has been described as a potential space capable of expansion subsequent to mechanical trauma. Unintentional injection of local anesthetic into this subdural space may explain some cases of failed spinal anesthesia. It may also explain the rare, slow-to-develop cases of high spinal anesthesia after the unintentional subdural injection of larger volumes of local anesthetic intended for epidural administration. Although the spinal cord ends at the level of the bodies of L1 and L2 in most people, the subarachnoid space and cauda equina continue to the S2 level ( Fig. 12.1 ).

Distal neuraxial anatomy. In pregnant women, the spinal cord usually ends at the lower border of the first lumbar vertebral body. The subarachnoid space continues to the second sacral vertebral level.

The Epidural Space

The epidural space is located external to the sac of the dura mater and contains loose connective tissue, adipose tissue, lymphatics, spinal nerve roots, and the internal vertebral venous plexus (Batson’s plexus) ( Fig. 12.2 ). This space is bound by the posterior longitudinal ligament anteriorly, the ligamentum flavum and the periosteum of the lamina posteriorly, the pedicles of the vertebrae, and the intervertebral foramina with their contents laterally. The epidural space is closed at the foramen magnum, where the spinal dura attaches to the dura of the cranium and at the sacral hiatus by the sacrococcygeal ligament. Frequently, anatomic references will illustrate neuraxial anatomy by way of sagittal and/or transverse cross-section. This may result in the erroneous impression that the epidural space is a continuous columnar entity that envelops the dural sac at all points about its perimeter. Investigations using cryomicrotome sections and three-dimensional reconstruction of radiologic data verify that the epidural space is in fact discontinuous along the vertical and lateral axes of the spinal canal. It varies in anteroposterior thickness according to dermatomal distribution, being widest at the level of lumbar vertebrae and thinnest in the cervical region. Epiduroscopy and epidurography suggest the presence of a dorsal median connective tissue band in some individuals. Anatomic dissection and computed tomographic epidurography have also suggested the presence of epidural space septa. This band (or these septa) may provide an explanation for unilateral or incomplete epidural anesthesia. However, some investigators have suggested that the dorsal median band is an artifact of epidural space distention or an anatomic manifestation of the previously unappreciated epidural space segmentation.

(A) Sagittal section of the epidural space demonstrates that the contents of the epidural space depend on the level of the section. (B) Three-dimensional drawing of the epidural space shows the discontinuity of the epidural contents. However, this potential space can be dilated by the injection of fluid into the epidural space.

(Redrawn from Stevens RA. Neuraxial blocks. In Brown DL, ed. Regional Anesthesia and Analgesia. Philadelphia: WB Saunders; 1976:323).

The Vertebral Column and Ligaments

The ligamentum flavum lies posterior to the epidural space ( Fig. 12.3 ). The lamina, the spinous processes of the vertebral bodies, and the interspinous ligaments lie posterior to the ligamentum flavum. Posterior to these structures are the supraspinous ligament (which extends from the external occipital protuberance to the coccyx), subcutaneous tissue, and skin. Historically, some have described the ligamentum flavum as a single ligament. In actuality, however, it is composed of two curvilinear ligaments that join in the middle and form an acute angle with a ventral opening. Much like the epidural space, the ligamentum flavum is not uniform from skull to sacrum; indeed, it is not uniform even within a single intervertebral space. The thickness of the ligamentum flavum varies with vertebral level, body mass index, race, and age, as does the distance between the skin and epidural space. In a mixed-race adult population, the mean (SD) distance from the skin to the epidural space was 5.4 cm (1.1). The distance was greater among black parturients and white parturients than among Asian parturients at any given body mass index ( Table 12.1 ).

Central neuraxial anatomy. Note the variable thickness of the ligamentum flavum, which is greatest in the midline and decreases as it fans out laterally.

(Illustration by Naveen Nathan, MD, Northwestern University Feinberg School of Medicine, Chicago, IL.)

Monitoring

The American Society of Anesthesiologists (ASA) has published guidelines for administration of neuraxial anesthesia in obstetric patients (see Appendix A ). Among other things, these recommendations address (1) the required presence of qualified anesthesia and obstetric care providers; (2) immediate availability of resuscitation medication and equipment ( Box 12.1 ); (3) mandatory preprocedural intravenous access; and (4) employment and documentation of maternal vital signs and fetal heart rate (FHR) monitoring.

During the initiation of neuraxial analgesia for labor, all patients are monitored with an automatic blood pressure cuff and a pulse oximeter to facilitate continuous assessment of the maternal heart rate and oxygenation. Maternal blood pressure is measured every 1 to 2 minutes after the administration of the test and therapeutic doses of local anesthetic for approximately 15 to 20 minutes, or until the mother is hemodynamically stable. Subsequently (during maintenance of neuraxial analgesia), maternal blood pressure is measured every 15 to 30 minutes or more frequently if hypotension ensues. Continuous pulse oximetry during maintenance analgesia is used in selected patients (e.g., patients with obstructive sleep apnea or cardiovascular disease). Rarely, invasive hemodynamic monitoring is necessary. The sensory level of analgesia and the intensity of motor block are assessed after the administration of the test and therapeutic doses of local anesthetic and at regular intervals thereafter.

The ASA Task Force on Obstetric Anesthesia has made the following recommendation regarding FHR monitoring during performance of neuraxial anesthesia procedures:

Fetal heart rate patterns should be monitored by a qualified individual before and after administration of neuraxial analgesia for labor. Continuous electronic recording of the fetal heart rate patterns may not be necessary in every clinical setting and may not be possible during placement of a neuraxial catheter.

The anesthesia provider cannot predict when hypotension will occur during the administration of neuraxial anesthesia. In addition, there is concern that intrathecal administration of an opioid is associated with a higher incidence of nonreassuring FHR patterns than other neuraxial techniques (see Chapter 23 ). Thus, we believe that continuous electronic FHR monitoring should be performed both during (if possible) and after the administration of neuraxial analgesia in all laboring women. In some cases, the mother’s position or maternal obesity precludes the use of an external Doppler device to monitor the FHR. In such cases (especially when there is concern regarding fetal well-being), it is helpful for the obstetric provider to place a fetal scalp electrode to monitor the FHR.

Informed Consent, Patient-Procedure Verification, and Partner’s Presence

Before initiation of neuraxial anesthesia, a preprocedural verification process (i.e., “time-out”) is performed in compliance with national patient safety recommendations. The participation of the patient, the anesthesia care provider, and a third party such as a member of the nursing staff may lead to the discovery of concerns that should be addressed before the initiation of neuraxial anesthesia. The risk associated with neuraxial procedures relates to (1) the physical instrumentation of the spinal axis and (2) physiologic changes associated with medication administration via this anatomic route. Contraindications to needle or catheter placement include patient refusal or inability to cooperate, ongoing bleeding diathesis, infection either at the site of intended intervention or untreated systemic blood-borne illness, and increased intracranial pressure predisposing to cerebral herniation. Contraindications to injecting local anesthetics via the epidural or spinal route include severe hypovolemia and allergy to local anesthetics. It is axiomatic that patient refusal represents an absolute contraindication to an elective procedure. A thorough preoperative assessment of current fetal well-being, maternal volume status, intrapartum systemic opioid use, antibiotic administration for ongoing chorioamnionitis or other infectious process, and a brief reiteration of known maternal disease states, including allergies, will readily identify most of the major concerns that would render neuraxial anesthesia potentially hazardous. The anesthesia provider should weigh the risks and benefits of neuraxial anesthesia for each patient.

Informed consent should include a frank discussion about anesthetic procedures and risks (see Chapter 32 ). Surveys of postpartum women have demonstrated that most parturients want to know the possible complications of epidural analgesia, even those that are rare. It is best to relay this information before the onset of labor (e.g., during antenatal classes), or early in the intrapartum period, although doing so is not always feasible. Some anesthesia providers fear that distressed, desperate, or sedated parturients may not understand the discussion of anesthetic procedures. A 2017 survey study of 206 postpartum women found no difference in recall of risks (discussed early after admission for labor) between women with and without pain at the time of the consent discussion. The preanesthetic evaluation allows the physician to communicate a sense of concern and to demonstrate a commitment to the patient’s care. Most laboring women understand the need for informed consent, and they appreciate the opportunity to participate in decisions about their care. Adequacy of consent can be demonstrated not only by documentation of information provided to the patient but also by the lack of patient objection to a procedure and the cooperation provided by the patient during the procedure.

Management of the pregnant patient occurs in a unique clinical care environment in which the presence of the patient’s spouse or family members must be addressed. Most often, the dictates of local institutions will establish whether a partner’s presence during neuraxial labor analgesia is acceptable. Intuition may suggest that a partner who remains present during the conduct of neuraxial analgesia may help alleviate the patient’s ongoing anxiety regarding the procedure. Conversely, the partner may be so apprehensive or disruptive that the partner’s presence becomes counterproductive to the care of the patient. Orbach-Zinger et al. randomized 84 nulliparous women to either presence or absence of their partner during labor epidural catheter placement. Interestingly, patient and partner anxiety, as measured by a validated anxiety questionnaire, were less when partners were absent during the procedure.

Patient Positioning

Pregnant women have an exaggerated lumbar lordosis, and it is more difficult for them to flex the lumbar spine. However, most pregnant women are young, and youth usually allows sufficient flexibility to facilitate the insertion of a needle into the epidural or subarachnoid space. Whether the block is initiated in the lateral or sitting position is a matter of provider and patient preference. Notable advantages of the lateral position include (1) orthostatic hypotension is less likely and (2) the position often facilitates continuous FHR monitoring during placement of the epidural catheter. Vincent and Chestnut performed a study in which they observed that neither the sitting nor the lateral position was consistently superior with regard to patient comfort. The sitting position is likely associated with a higher incidence of orthostatic hypotension and syncope. However, the sitting position is preferred—and may be required—in obese parturients, in whom identification of the midline is usually significantly easier in the sitting position. Further, morbidly obese women may experience hypoxemia when placed in the lateral decubitus position.

One study demonstrated a greater reduction in maternal cardiac output with maximal lumbar flexion in the lateral decubitus position than in the sitting position during identification of the epidural space in laboring women. The researchers speculated that maximal lumbar flexion in the lateral decubitus position results in concealed aortocaval compression. They recommended that “the tight fetal curl position be avoided,” especially when the patient assumes the lateral decubitus position.

Aortocaval compression must be avoided to the extent possible (see Chapter 2 ). If maternal hydration is inadequate and if aortocaval compression is not avoided, the onset of anesthesia-induced sympathetic blockade may result in decreased venous return, cardiac output, and uteroplacental perfusion.

Maternal position during placement of the epidural catheter does not seem to affect the incidence of unintentional dural puncture. However, adoption of the lateral recumbent head-down position for epidural catheter placement may reduce the incidence of unintentional epidural venous puncture.

When spinal or epidural anesthesia is performed with the patient in a lateral position, the patient’s back should lie at, and parallel to, the edge of the bed, for at least two reasons. First, the edge is the firmest section of the mattress. If the patient lies away from the edge of the bed, the patient’s weight will depress the mattress, and the anesthesia provider must work in a “downhill” direction. Second, this position allows anesthesia providers to keep their elbows flexed, facilitating control of fine hand and wrist muscle movements. The plane of the entire back should be perpendicular to the mattress. When asked to flex the lower back, patients typically roll the top shoulder forward, an action that rotates the spine (which is undesirable) but does not flex the lower back.

Similarly, patients positioned sitting should have their feet supported by a stool with the backs of their knees against the edge of the bed, a maneuver that helps position the patient’s back closer to the anesthesia provider. The shoulders should be relaxed symmetrically over the hips and buttocks. Beds in obstetric units often break at the foot, and the split in the mattress encourages the patient’s seat to slope downhill if she is straddling the mattress split; this position will cause spine rotation and may make the procedure more difficult.

When spinal anesthesia is performed, the patient’s posture relative to the baricity of the anesthetic solution should be considered, because it influences the extent and latency of blockade, and the incidence of hypotension. The incidence, timing, and extent of hypotension in the period immediately after initiation of the block depend on the type of block (e.g., spinal, epidural, or combined spinal-epidural [CSE]), drug characteristics (e.g., baricity, concentration, dose), patient position during the procedure, and patient position in the period following the procedure. For example, when spinal anesthesia is initiated with a hyperbaric solution for instrumental vaginal delivery, it often makes sense for the patient to be sitting to ensure the rapid onset of sacral anesthesia. Conversely, spinal anesthesia for cervical cerclage can be initiated with the patient in the steep lateral Trendelenburg position with a hypobaric anesthetic solution.

Posture has less influence on the spread of epidural anesthesia. During epidural anesthesia, a unilateral block more likely results from the malposition of the catheter (or perhaps an anatomic barrier within the epidural space) than from patient position, particularly after a bolus injection. Norris and Dewan observed that gravity did not augment the spread of anesthesia in patients receiving epidural anesthesia for cesarean delivery, and they concluded that posture does not need to be manipulated to ensure adequate bilateral epidural anesthesia. In at least two studies, it was noted that the use of the sitting position is not necessary for the development of good sacral anesthesia when large volumes of epidural local anesthetic are given for cesarean delivery. However, Reid and Thorburn observed that use of the sitting position appeared to delay the spread of anesthesia to the midthoracic dermatomes. In comparison with the bolus administration of epidural local anesthetic, the extent of blockade may be more gravity dependent when the anesthetic is administered as a continuous infusion over a prolonged period.

Some anesthesiologists contend that maternal position after epidural catheter placement affects the efficacy of epidural analgesia, although this is a matter of some dispute. Beilin et al. observed that the placement of the laboring woman in the supine position with a 30-degree leftward tilt was associated with better epidural analgesia than maintenance of the left lateral decubitus position. In contrast, Preston et al. observed no difference in analgesia and a significantly higher incidence of fetal bradycardia with the supine wedged position than with the full lateral position.

Caudal anesthesia is used infrequently in modern obstetric anesthesia practice. However, there remain some circumstances in which a caudal technique is useful and/or advantageous. It is a good choice for the second stage of labor in selected patients in whom the lumbar epidural approach is hazardous or contraindicated (e.g., fusion or instrumentation of the lumbar spine). In most cases, caudal anesthesia can be successfully performed with the patient in a lateral decubitus position.

Aseptic Technique

In the past decade, there has been growing appreciation for the importance of sterile technique and the gravity of infectious complications related to neuraxial anesthesia. The incidence of epidural abscess and spinal meningitis is generally so low that many of the available recommendations are based on evidence from other domains of infection control (e.g., surgical wound site and central venous catheter–related infection). Nonetheless, death and devastating neurologic compromise resulting from neuraxial infection can occur.

Infection of the epidural space tends to result in the formation of an abscess, most commonly formed by Staphylococcus aureus found in the epidermis of either the patient or the anesthesia provider. In contrast, meningitis associated with neuraxial procedures is most commonly caused by Streptococcus viridans. Viridans species of streptococcus may reside in the oronasopharyngeal tract of providers or patients or in the patient’s vagina. Potential routes of infection include the (1) epidural catheter track, (2) bloodstream, (3) equipment, and (4) injectate contamination. A more in-depth discussion of neuraxial infection is found in Chapter 31 .

Guidelines describing aseptic technique for regional and neuraxial anesthetic procedures have been published by professional anesthesiology organizations and the U.S. Centers for Disease Control and Prevention (CDC). The following recommendations deserve emphasis:

• 1.
  

  Given that the oropharyngeal and skin flora of the anesthesia provider are implicated in many cases of neuraxial infection, the provider should don a surgical face mask and hat before initiation of spinal/epidural anesthesia. Microbial sampling in laminar-flow operating theaters has shown a 22-fold increase in bacterial counts when a face mask and hat are not worn.

  
  
• 2.
  

  Washing hands with an alcohol-based antiseptic solution is recommended because this has been shown to be superior to antimicrobial soap. Jewelry (e.g., rings, watches) should be removed before and sterile gloves worn after hand cleansing.

  
  
• 3.
  

  The patient’s skin should be decontaminated, preferably with a chlorhexidine-in-alcohol solution. Evidence suggests that chlorhexidine has superior bacteriocidal and bacteriostatic efficacy compared with povidone-iodine. If chlorhexidine is not available, then povidone-iodine with alcohol, rather than povidone-iodine alone, is preferred. Of importance, the anesthesia provider is encouraged to exercise patience in allowing the antiseptic to dry, because a major mechanism of antisepsis is the desiccating action of alcohol.

Spinal Anesthesia

The first equipment decision involves determining whether to perform a single-shot or continuous technique. Traditionally, a large-bore epidural needle and catheter are used for continuous spinal anesthesia, because the U.S. Food and Drug Administration rescinded approval for the use of small-bore microcatheters in 1992. Therefore, the risk for post–dural puncture headache is significant. Continuous spinal analgesia or anesthesia using an “epidural” catheter sited in the subarachnoid space is useful after unintentional dural puncture with an epidural needle. In the morbidly obese patient, it may be easier to manipulate and advance a rigid epidural needle than a more flexible spinal needle; thus, the technique is useful for establishing continuous analgesia or anesthesia in this patient population, particularly when the need for anesthesia is urgent. A 23-gauge, FDA-approved, spinal catheter is now available in the United States. The catheter is inserted using a “catheter-over-needle” technique. Initial observational study found that this catheter may have clinical utility ; however, further study is required to characterize ease of use and complications. Several commercial spinal catheters are available in Europe, but at the current time, a single-shot technique is preferred for spinal anesthesia for most obstetric patients.

The primary equipment choice for spinal anesthesia concerns the type and size of the spinal needle. Cutting-bevel needles (e.g., Quincke) are rarely used in contemporary obstetric anesthesia practice because of the unacceptably high incidence of post–dural puncture headache associated with their use. Instead, noncutting (“pencil-point”) needles (e.g., Whitacre, Sprotte, Gertie Marx) are used almost exclusively ( Fig. 12.6 ). Pencil-point needles cause more trauma to the dura than occurs with cutting-bevel needles. This likely results in a more intense inflammatory response. Presumably, the inflammation results in more rapid closure of the dural defect.

Spinal needle assortment often used in parturients. The Sprotte and Whitacre needles have cone-shaped bevels, whereas the Quincke and Atraucan needles have a cutting bevel. (Other sizes are available in some of these needle designs.)

Needle size must also be determined. Larger needles offer a greater fidelity of tactile feedback as the anesthesia provider traverses tissue planes of variable impedance when performing spinal anesthesia. Furthermore, larger needles are more likely to withstand the high resistance encountered when contacting bone without bending or shearing. In general, the “ease-of-use” advantages associated with larger needles must be balanced against a lower incidence of post–dural puncture headache with smaller needles. Most anesthesia providers use 25- or 27-gauge noncutting needles for routine spinal anesthesia in obstetric patients. However, anesthesia providers should make individual decisions based on their own skills, the practice setting, and the patient. The urgency of the procedure may also influence the choice of needle size. For example, a 27-gauge needle might be chosen for spinal anesthesia for an elective procedure, and a larger (e.g., 22-gauge) needle might be chosen when the subarachnoid space must be entered quickly because of severe fetal compromise.

With a small-gauge needle (i.e., 24-gauge or smaller), use of an introducer needle is preferable. The introducer needle engages the interspinous ligament and more accurately guides the trajectory of the smaller spinal needle than is possible with use of a small-gauge spinal needle alone. The introducer needle also aids with skin puncture; it is often difficult to puncture the skin with noncutting needles.

Either the midline or the paramedian approach can be used to enter the subarachnoid space. The midline approach requires the patient to reduce her lumbar lordosis to allow access to the subarachnoid space between adjacent spinous processes (usually L3 to L4, sometimes L4 to L5 or L2 to L3). The interspinous space may be identified with one (usually the thumb or index finger) or two fingers (usually the index and middle fingers) of the anesthesia provider’s nondominant hand. The single finger “slides” along the skin in the midline from cephalad to caudad until it “settles” into an interspinous space. The two fingers identify the interspinous space by palpating the caudad border of the more cephalad spine. The fingers identify the midline by rolling in a medial-to-lateral direction ( Fig. 12.7 ).

The midline approach for spinal needle insertion requires accurate identification of a lumbar interspinous space. The palpating fingers are rolled in a medial-to-lateral and cephalad-to-caudad direction; an introducer is then inserted through the interspinous space almost perpendicular to the lumbar spinous process. Once the introducer is seated in the interspinous ligament, the spinal needle is inserted; the needle is stabilized in a tripod fashion during insertion (much like a dart being thrown).

Next, the anesthesia provider injects local anesthetic intradermally and subcutaneously. The introducer needle is inserted into the substance of the interspinous ligament. It is helpful if the introducer needle is embedded in the interspinous ligament; therefore, obese patients may require a longer introducer needle. The introducer needle should lie in the sagittal midline plane. It is then grasped and steadied with the fingers of the nondominant hand while the dominant hand holds the spinal needle like a dart. The fifth finger may be used as a tripod against the patient’s back to prevent patient movement from causing unintentional needle insertion to a level deeper than intended, and to “brake” the needle. As the needle passes through the ligamentum flavum and the dura, characteristic changes in resistance are noted. A “pop” is often perceived as the needle tip traverses the ligamentum flavum. A subsequent and more pronounced pop is perceived as the needle tip exits the dura-arachnoid. The stylet is removed, and CSF should appear in the needle hub. If CSF does not appear, the stylet is replaced, and the needle is advanced a few millimeters and again checked for CSF flow. This process continues until either bone is encountered or CSF returns through the needle. If neither occurs, the needle and introducer are withdrawn and the process is repeated.

Although with time and practice the tactile feedback produced by advancing a needle through tissues of variable resistance will become utterly familiar to the anesthesia provider, the novice may be unsure of the anatomic position of the needle tip, especially if unexpected resistance (i.e., contact with bone) or an unexpected and premature “pop sensation” is encountered during needle advancement. A stepwise problem-solving approach is reasonable. First, the anesthesia provider should reconfirm that (1) the patient has normal anatomy (i.e., not scoliotic); (2) she is acceptably positioned without rotation of the spine (often recognized by asymmetric shoulder position); and (3) the chosen point of needle insertion is the true midline plane. If these assertions are true, and the needle tip encounters bone, it is highly likely that the osseous structure is either the inferior or superior spinous process. One of two maneuvers may overcome this barrier. After slight withdrawal of the needle, simple angulation in a cephalad or caudad direction may redirect the needle trajectory sufficiently to achieve access to the central neuraxial canal. One must appreciate the “toughness” of the interspinous ligament. Even a 17-gauge epidural needle can be bent if the angle is changed without some prior retraction of the needle. Furthermore, if a spinal needle/introducer complex is used, care must be exercised that angulation of the spinal needle does not occur without first withdrawing it into the lumen of the introducer. Thereafter, the entire spinal needle/introducer complex is angulated before the spinal needle is re-advanced. Angulation of the spinal needle without first withdrawing it into the introducer creates a fulcrum at the junction of the introducer tip where the spinal needle emerges and can potentially damage or even shear the delicate spinal needle ( Fig. 12.8 ).

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