Interlaminar Epidural Injection

Overview

Epidural injection of local anesthetics has long been used to produce surgical anesthesia for operative procedures on the trunk, abdomen, and lower extremities. Continuous infusion of local anesthetic and opioid combinations through indwelling epidural catheters are now frequently used to provide analgesia for several days after major surgery. The most common use of epidural injection in the pain clinic is to place steroids into the epidural space, where they spread to bathe the spinal nerves to either side of midline. The rationale for injecting steroids is that they suppress inflammation involving the nerve and adjacent soft tissues. This inflammation is thought to be the cause for acute radicular pain. A needle can be advanced directly into the epidural space between adjacent vertebral laminae using a posterior approach to the spinal canal near midline. This is the most common approach to the epidural space and has been termed the “interlaminar approach.” Identification of the epidural space requires familiarity with the loss-of-resistance (LOR) technique, which is described in detail later in this section. Using this technique, the epidural space is identified by the sudden decrease or LOR to injection that occurs as a needle passes from the interspinous ligament between adjacent spinous processes through the ligamentum flavum and into the epidural space. Although anesthesiologists have used epidural anesthesia and analgesia for many years, more recently, pain practitioners have been moving toward use of the transforaminal route for placing steroids near an inflamed nerve root. The rationale for using the transforaminal route of injection, rather than the interlaminar route, is that the injectate is delivered directly to the target nerve in the area of inflammation. This ensures that the medication reaches the target area in maximum concentration at the site of the suspected pathology. There have been few studies directly comparing the interlaminar and transforaminal routes for epidural injection of steroids, thus it is unclear if the safety or efficacy of either of the two techniques is superior.

Anatomy

Bony Anatomy of the Vertebrae and Spinal Column

The structure of the vertebrae is distinct in the cervical, thoracic, and lumbar regions (Fig. 5-1). A typical vertebra consists of a spinous process that joins at its most anterior extent with the laminae, which extend anterolaterally to each side of midline. The epidural space lies anterior to the laminae and is bordered laterally by the pedicles and anteriorly by the vertebral body. Access to the epidural space is through the space between adjacent laminae (the “interlaminar” space). The superior and inferior articular processes of the facet joints lie posterolaterally at the junction between the laminae and the pedicles and provide the posterolateral articulating surfaces between adjacent vertebrae. The sacral hiatus is the area where the fifth sacral vertebra (S5) lacks a spinous process and laminae posteriorly (Fig. 5-2). The two sacral cornua lie on either side of the sacral hiatus and cephalad to the coccyx.

Individual vertebral components can affect epidural block technique. The spinous processes vary in their degree of angulation at the various vertebral levels (Fig. 5-3). In the cervical and lumbar regions, the spinous process attaches to the lamina nearly horizontally, thus facilitating a midline perpendicular approach to the neuraxis. Conversely, the midthoracic spinous processes (T5 to T9) are steeply angled in a cephalad to caudad direction to such an extent that the paramedian approach gives more direct access to the epidural space and is easier to carry out than the midline approach. High (T1 to T4) and low (T10 to T12) thoracic spinous processes are intermediate in their orientation and are thus amenable to either a steeply angled midline or a paramedian approach. The laminae become more vertically oriented as one progresses caudally; therefore, “walking off” the lamina is associated with progressively deeper needle placement from superior to inferior in the thoracic region. This also accounts for the shallower depth on entering the epidural space in the lumbar region.

Figure 5-1. Anatomy of the cervical, thoracic, and lumbar vertebrae.

Surface landmarks can assist in identifying the approximate vertebral interspace (Table 5-1). In most humans, the C7 spinous process (the vertebrae prominens) is the most noticeable midline structure at the posterior neck base. A line drawn between the inferior angles of the scapulae lies approximately at the level of the T7 spinous process, while a line drawn between the iliac crests crosses the tip of the L4 spinous process or the L4/L5 interspace. The spinal cord generally terminates at about the L2 level (Fig. 5-4), and the dural sac ends at S2 (the level of the posterior-superior iliac spines). The tip of an equilateral triangle drawn between the posterior-superior iliac spines and directed caudally overlies the sacral cornua and sacral hiatus.

Figure 5-2. Anatomy of the posterior sacrum and coccyx.

Anatomy of the Epidural Space

Just as the bony structure of the vertebrae varies from the cervical to the sacral levels, so does the anatomy of the epidural space. The epidural space extends from the foramen magnum to the sacrococcygeal ligament. It is filled with epidural fat, a robust venous plexus, and loose areolar tissue. The dimensions of the epidural space are less in the thoracic and cervical space as compared with the lumbar region. The ligamentum flavum is a structure of variable thickness and completeness that defines the posterolateral soft-tissue boundaries of the epidural space. Because the leather-like consistency of the ligamentum flavum resists active expulsion of fluid from a syringe, loss of this resistance is valuable in signaling entry into the epidural space as a needle is advanced. The ligament’s structure is steep and tent-like, with the peak of the tent’s roof in the midline and most posterior and the substance of the ligamentum flavum extending in an anterolateral direction to both sides of midline forming the eaves of the tent’s roof. The lateral aspect of the ligamentum flavum may be as much as 1 cm more anterior than at the midline, thus entry into the epidural space will occur at a significantly deeper level when the needle strays laterally from midline.

In the cervical and thoracic epidural spaces, the ligamentum flavum often does not fuse in the midline, which can become problematic when using the LOR technique. When the dense ligamentum flavum is absent in the midline, it is possible to enter the epidural space without ever sensing significant resistance to injection. The ligamentum flavum is thickest at the lumbar and thoracic levels and thinnest at the cervical level. Its thickness also diminishes at the cephalad aspect of each interlaminar space and as the ligamentum flavum tapers off laterally. The epidural space itself progressively narrows in anterior-posterior (AP) dimension from the lumbar level (5 to 6 mm) to the thoracic level (3 to 5 mm) and is narrowest between the C3 and C6 vertebral levels (2 mm). Because the spinal cord typically terminates at the L2 vertebral level, unintentional needle puncture of the dura below this level encounters the free-floating filum terminale and individual nerves of the cauda equina, rather than the spinal cord (Fig. 5-4). The posterior epidural space is narrow at cervical and thoracic levels, thus when the epidural space is entered, the needle tip lies in close proximity to the spinal cord.

Figure 5-3. Anatomy of the vertebral column. In most humans, the most prominent spinous process at the base of the neck is the spinous processes of C7 (the vertebrae prominens). Note that the angle of the spinous processes changes dramatically from cervical to lumbar levels, with the steepest cephalad to caudad angle in the midthoracic region. The approximate plane of needle entry for interlaminar epidural injection is shown for cervical, thoracic, and lumbar levels.

Table 5-1 Correlation between Surface Landmarks and Vertebral Levels

Surface Landmark	Approximate Vertebral Level
Most prominent spinous process at base of neck (vertebra prominens)	C7 spinous process
Inferior angle of scapula	T7 spinous process
Superior margin of iliac crest	L4 spinous process or L4/L5 interspace
Posterior-superior iliac spine	S2, termination of the dural sac
Inferior tip of equilateral triangle between posterior-superior iliac spines	Sacral hiatus

Above C7 to T1 and at intermittent areas along the posterior spinal canal, the epidural space is best described as a potential space that is easily dilated by injection of anesthetic solutions. Distribution of solutions within the epidural space is not uniform, especially as distance from the injection site increases. Rather, solutions spread along various routes as determined by small, low-resistance channels that exist between the epidural fat and veins. Nevertheless, solutions flow preferentially along the dural sheaths of the spinal nerves because the only significant barrier to epidural flow appears to be the posterior longitudinal ligament. Thus fluid injected within the epidural space will tend to exit the spinal canal through the adjacent intervertebral foramina. In patients who have undergone previous spinal surgery, scarring of the posterior epidural space is common, and the flow of injected solutions is less predictable. Cases have been reported where fluid injected into the epidural space forms an area of loculation that can tent the dura anteriorly and lead to compression of the neural elements, thus it is essential to avoid injecting fluid into the epidural space under high pressure.

Figure 5-4. Appearance of the conus medullaris and cauda equina on magnetic resonance imaging (MRI). T2-weighted MRI of the lumbosacral spine (without contrast) in a patient with a small central disc protrusion at L4/5 and a prominent central disc protrusion at L5/S1. A: Axial image at the level of termination of the conus medullaris (inferior endplate of the L1 vertebral body), demonstrating the inferior most extent of the conus and the nerves of the cauda equina (dashed line illustrates the position of the corresponding sagittal image in (B)). B: Axial image at the level of termination of the conus medullaris, demonstrating the inferior most extent of the conus and the nerves of the cauda equina (dashed line illustrates the position of the corresponding axial image in (A)).

Loss-of-resistance Technique

Anesthesiologists learn to identify the epidural space “blindly” without the help of fluoroscopic guidance. This is accomplished using the LOR technique. Even when radiographic guidance is available, the LOR technique is still needed to identify the epidural space. Image guidance can help direct the needle toward the midline, between laminae, but neither AP nor lateral images can identify the precise location of the epidural space. The LOR technique is identical in the cervical, thoracic, and lumbar regions. After the skin and subcutaneous tissue have been anesthetized with a small volume of local anesthetic, an epidural needle is seated in the interspinous ligament, advancing 2 to 3 cm from the skin’s surface (the most common type of needle is the 18- or 20-gauge Tuohy needle; see Fig. 1-4). A syringe containing air or saline is then attached to the needle. Many practitioners prefer a 10-mL syringe containing 1 to 3 mL of preservative-free, isotonic saline and a small (∽0.5 mL) bubble of air (Fig. 5-5A). The needle shaft is then grasped by the thumb and index finger of the nondominant hand and advanced 1 to 2 mm at a time, while the first three fingers of the dominant hand are used to place gentle, steady or intermittent pressure on the plunger of the syringe to test for resistance to injection as the needle is advanced toward the epidural space. The small bubble in the syringe is more compressible than the saline and serves to visually reinforce the degree of resistance felt with each push on the syringe’s plunger. As the needle tip traverses the ligamentum flavum and enters the dorsal epidural space, there is a discreet LOR to injection, and the saline exits the syringe to the epidural space. Also, because there is very low resistance to injection, the air bubble is no longer compressed (Fig. 5-5B).

Patient Selection

The most common indication for epidural injection in the pain clinic is to place corticosteroid adjacent to an inflamed nerve root that is causing radicular symptoms. Nerve root inflammation may stem from an acutely herniated intervertebral disc causing nerve root irritation or other causes of nerve root impingement such as isolated foraminal stenosis due to spondylitic spurring of the bony margins of the foramen. Epidural steroid injection is also used to treat symptoms of neurogenic claudication associated with spinal stenosis (stenosis of the central spinal canal). There are no scientific guidelines or any body of scientific literature to help choose between the interlaminar route and the transforaminal route for epidural injection of steroids, and each has unique complications. The spread of injectate during interlaminar injection, particularly when volumes of 5 mL or more are used, will often extend to both sides of midline and bathe the spinal nerves at the interspace of injection and at several adjacent interspaces. Thus, in those patients who present with bilateral radicular symptoms due to a midline disc herniation or neurogenic claudication in both legs due to central canal stenosis, it seems logical (if yet unproven) that interlaminar injection would be more likely to get the steroid solution to the target sites of nerve irritation.

Level of Evidence

Quality of Evidence and Grading of Recommendation

Grade of Recommendation/ Description	Benefit vs Risk and Burdens	Methodological Quality of Supporting Evidence	Implications
RECOMMENDATION: Interlaminar epidural steroid injections may be used as part of a multimodal treatment regimen to provide pain relief in selected patients with radicular pain or radiculopathy.
1B/strong recommendation, moderate quality evidence	Benefits clearly outweigh risks and burden	For short-term relief of radicular pain (up to 6 weeks): I: RCTs with important limitations (inconsistent results, methodological flaws	Strong recommendation, can apply to most patients in most circumstances without reservation

There has been an exponential rise in the use of epidural injection of steroids during the past decade in the United States, while the prevalence of acute pain associated with various spinal disorders has changed little. Several organizations have closely examined the scientific literature and made evidence-based guidelines regarding the use of this treatment. The available randomized controlled trials examining the efficacy of epidural injection of steroids are limited to use in the treatment of radicular pain associated with acute lumbar intervertebral disc herniations. The American Academy of Neurology Technology Assessment Committee published an analysis in 2007, concluding, “(a) epidural steroid injections may result in some improvement in radicular lumbosacral pain when assessed between 2 and 6 weeks following the injection, compared to control treatments. The average magnitude of effect is small and generalizability of the observation is limited by the small number of studies, highly selected patient populations, few techniques and doses, and variable comparison treatments; (b) in general, epidural steroid injection for radicular lumbosacral pain does not impact average impairment of function, need for surgery, or provide long-term pain relief beyond 3 months. Their routine use for these indications is not recommended; (c) there is insufficient evidence to make any recommendation for the use of epidural steroid injections to treat radicular cervical pain.”

Figure 5-5. The LOR technique for identification of the epidural space using an interlaminar approach. A: The needle is seated in the interspinous ligament, and a syringe containing 1 to 3 mL of preservative-free saline and a small (∽0.5 mL) air bubble is attached to the needle. The shaft of the needle is grasped firmly with the nondominant hand and intermittent or continuous light pressure is applied to the syringe plunger with the dominant hand. (Cont.)

The American Pain Society Low Back Pain Guideline Panel published a report in 2009, concluding, “In patients with persistent radiculopathy due to herniated lumbar disc, it is recommended that clinicians discuss risks and benefits of epidural steroid injection as an option (weak recommendation, moderate-quality evidence). It is recommended that shared decision making regarding epidural steroid injection include a specific discussion about inconsistent evidence showing moderate short-term benefits, and lack of longterm benefits. There is insufficient evidence to adequately
evaluate benefits and harms of epidural steroid injection for spinal stenosis.”

Figure 5-5. (Continued) B: As the needle passes through the ligamentum flavum and into the epidural space, there is a sudden decrease or “loss” of resistance to injection. The air bubble in the syringe expands, and the saline in the syringe flows into the epidural space. Note the close proximity of the posterior surface of the dural sac; advancing the needle just a few additional millimeters will result in dural puncture and intrathecal location of the needle tip.

Most recently, the American Society of Anesthesiologists Task Force on Chronic Pain Management published A 2010 Practice Guideline, offering the following recommendations: “Epidural steroid injections with or without local anesthetics may be used as part of a multimodal treatment regimen to provide pain relief in selected patients with radicular pain or radiculopathy. Shared decision making regarding epidural steroid injections should include a specific
discussion of potential complications, particularly with regard to the transforaminal approach. Transforaminal epidural injections should be performed with appropriate image guidance to confirm correct needle position and spread of contrast before injecting a therapeutic substance; image guidance may be considered for interlaminar epidural injections.”

Figure 5-6. Position for interlaminar cervical epidural injection. The patient is placed prone with a small headrest under the forehead to allow for air flow between the table and the patient’s nose and mouth. The C-arm is angled 15 to 20 degrees caudally from the axial plane.

There are numerous randomized controlled trials of varying methodologic quality that show more rapid resolution of acute radicular pain associated with acute lumbar disc herniation following epidural injection of steroids, and it is in this group that the evidence of efficacy is strongest. The use of this treatment for radicular pain associated with acute disc herniations occurring at the cervical and thoracic levels is common and most experts find this to be a reasonable extrapolation from the existing scientific evidence. The use of epidural injection of steroids to treat acute radicular pain associated with foraminal stenosis or neurogenic claudication associated with stenosis of the central spinal canal has been less well studied but remains common, again as an extrapolation from their usefulness in those with acute disc herniations. The use of epidural injection of steroids for the treatment of nonradicular spinal pain lacks scientific validation.

Figure 5-7. Position and angle of needle entry for cervical interlaminar epidural injection. An 18- or 20-gauge Tuohy needle is advanced in the midline with 15 to 20 degrees of cranial angulation from the axial plane parallel to the spinous processes.

Cervical Epidural Injection

Positioning

The patient lies prone, facing the table, with a small headrest under the forehead to allow for air flow between the table and the patient’s nose and mouth (Fig. 5-6). The C-arm is rotated 15 to 20 degrees caudally from the axial plane without any oblique angulation. This allows for good visualization of the interlaminar space and needle advancement between adjacent spinous processes (Figs. 5-7 and 5-8)

Block Technique

The skin and subcutaneous tissues overlying the interspace where the block is to be carried out are anesthetized with 1 to 2 mL of 1% lidocaine. The cervical interspaces with the largest interlaminar distance are typically found at C6/C7 and C7/T1. Because of the ease of entry, many practitioners will place the needle via one of these larger interspaces, regardless of the level of pathology, and rely on the flow of steroid in the epidural space to reach the level of pathology. The same technique can be carried out at all cervical

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