Spinal Injections

Indications

Epidural pain injections are useful in the patient with a wide variety of disorders that could generate back pain, including extruded and bulging disks; foraminal and lateral recess stenoses with nerve compression from facet osteoarthritis, capsular hypertrophy, synovial cyst formation, and scar production; spondylolysis with spondylolisthesis; and congenital and/or acquired central canal stenosis. In addition to these mechanical abnormalities, chemical irritation of the nerves can occur with full-thickness annular tears of the disk and extrusion of the nucleus pulposus. The patient will usually present with complaints of low back or radicular pain. A combination of clinical examination and advanced imaging will help define the etiology and better localize the area for treatment.

The zygapophyseal joints, also known as articulating facets, are true diarthrodial synovial joints located in the posterior column of the spine. They function by stabilizing the spine while allowing movement. Each facet joint consists of a superior and an inferior articulating facet whose apposing surfaces are lined with articular cartilage.

Sensory innervation of these joints arises from the medial branch of the dorsal ramus at that level and the one below. It is this dual innervation that makes the diagnosis of primary facet joint pain more difficult than with other diarthrodial synovial joints.

Primary sources of pain include loss of cartilage leading to subchondral marrow edema and laxity of the joint capsule resulting in tendon strains, ligamentous sprains, or acceleration of osteoarthritis. The facet joints can also cause secondary pain from nerve entrapment by hypertrophied osteophytes and ligamenta flava or by synovial cyst formation extending into the spinal canal. The nerve compression may occur as the nerve exits the neural foramen at that level, or compression can occur as the descending nerve travels through the lateral recess. These secondary sources of pain stimulate radicular components, which can confuse the diagnosis.

Facet joint pain is classically described as pain in the low back that increases with hyperextension. This pain may radiate into the buttock and posterior and lateral thigh, can be associated with paralumbar tenderness, but is rarely related to neurologic deficits or pain below the knee. Facet joint pathology is not correlated with central back pain. The pain is worse with rest and improves with repetitive motion. These signs, however, are seen with other disorders of the back and may not be reliable in distinguishing facet joint pain from pain originating from disks, muscles, or ligaments.

Contraindications

Contraindications are few and similar to those used for surgery. These include coagulopathies and active infections. Relative contraindications cover mass lesions lying in the desired needle path and allergies to the agents used in the procedure.

Patients taking blood-thinning medications, with coagulopathies, or with low platelet counts should have their blood-thinning medications discontinued at least 5 days before the procedure or have their coagulopathy or platelet levels corrected before any spine injection procedure is performed. The potential for epidural hemorrhage requiring surgical decompression is increased in these sets of patients and may be life-threatening in some already debilitated patients. The risk of stopping medication must be weighed against the potential benefit.

Active infections are a contraindication to spine injections because the corticosteroids that are often used have the potential to significantly suppress the immune system and exacerbate any preexisting infection. Waiting until the course of antibiotics has been completed is prudent. Those individuals on long-term antibiotics for conditions, such as chronic urinary tract infections, may be treated if they are not currently suffering from an active infection.

If a mass lesion, such as a myelomeningocele or tumor, is present along the only possible entry path, the procedure should be canceled because dural puncture or remote seeding of tumor could result.

Confirmation of an appropriate needle location needs to be accomplished with contrast material when performing spine injections. Patients with allergies to contrast agents may be considered for epidural corticosteroid injection using premedication with oral corticosteroids and the possible addition of diphenhydramine. Alternatives are the traditional anesthesiologist's technique of injection guided by loss of syringe resistance or a CT-guided procedure using gadolinium. C-arm fluoroscopy cannot adequately visualize full-strength gadolinium in the epidural space, especially in the obese patient.

Injections in the areas of previous surgery provide an extra challenge. In those who have undergone posterior decompressions, the dura often adheres to the ligamentum flavum and scar tissue, obliterating the dorsal epidural space at that level. Even transforaminal injections at that level can be tricky, especially if foraminotomies or facetectomies have been performed. In these cases, choosing a different location in the spine that has not been explored surgically would be the best approach for an interlaminar technique.

Facet joint injections in areas of prior surgery can also be challenging. In those undergoing posterior decom­pressions, foraminotomies, or partial facetectomies, the anatomy of the facet joint and inferior recess may be altered with loss of an osseous end point during needle placement. Multiple fluoroscopic checks for depth confirmation are indicated to prevent inadvertent entry into the spinal canal and thecal sac.

Caution is also needed in patients who have facet synovial cysts because these cysts may expand on injection and result in an acute compression of the descending nerve root lying within the lateral recess. This is accompanied by excruciating pain and difficulty in moving the extremity and may require surgical decompression.

Equipment

A C-arm fluoroscopic unit is preferred for performing the injection and checking for vascular filling. The rotation ability of the unit allows for views of the injection in a wide range of projections. Anesthesia of the subcutaneous tissues may be performed with a preservative-free solution of 4.5 mL lidocaine 1%, buffered with 0.5 mL of 8.4% preservative-free sodium bicarbonate. A 22-gauge spinal needle is advanced into the epidural space or the facet joint, and nonionic contrast agent confirms appropriate positioning of the needle tip. An epidural corticosteroid injection can be performed with the injection of 1 to 2 mL (40 mg/mL) of corticosteroid, such as methylprednisolone acetate and 10 mL of a flush solution composed of 6 mL of preservative-free 0.25% bupivacaine and 4 mL of preservative-free 0.9% sodium chloride. A diagnostic nerve root block (using the transforaminal technique) requires less volume so that only the nerve root targeted receives the medication. This can be accomplished with an injection of 1 mL of corticosteroid combined with 1 mL of 0.5% bupivacaine. A facet joint injection consists of 0.5 mL of corticosteroid mixed with 0.5 mL of 0.5% bupivacaine.

Technique

Approaches

Epidural injections will be covered first in this section, followed by a discussion of facet joint injections. Four main approaches to epidural corticosteroid injections are in use. These are the traditional, the interlaminar, the transforaminal, and the caudal techniques.

The interlaminar technique is one of the two most popular. Access immediately caudal to the spinolaminar junction allows quick entry into the dorsal epidural space where epidural fat tends to be most abundant. The ease of access and confidence provided by the combination of contrast injection and C-arm fluoroscopy, coupled with the paucity of complications in experienced hands, has helped this entry point achieve wide acceptance.

Transforaminal epidural corticosteroid injections, also known as nerve blocks, are the newest and, in some practices, most popular of the techniques. Because this type of injection precisely targets the symptomatic region, it is believed that the corticosteroid is more focally directed and concentrated in the area of inflammation. Some studies have even suggested that longer pain relief can be gained with a transforaminal injection when compared with the interlaminar injection.

Unfortunately, transforaminal epidural pain injections have been associated with episodes of spinal cord infarction. These events may result from inadvertent vascular cannulation of the arteria radicularia magna (the artery of Adamkiewicz) or other arteries supplying the spinal cord. The artery of Adamkiewicz is a major supplier of blood flow to the lower third of the spinal cord through the anterior spinal artery, and its obstruction can lead to spinal cord infarction and permanent paraplegia, which is a devastating outcome for a minimally invasive procedure. As a consequence, some centers limit their usage of transforaminal injections and, when they are performed, meticulous examination for any vascular flow is essential. Case reports support that lack of a bloody aspirate is insufficient proof that vascular cannulation has not occurred, indicating that contrast agent administration under fluoroscopy is essential. Even if vessel occlusion does not occur, the injection of corticosteroids into a patent blood vessel would allow the drug to flow systemically away from the intended site of injection. Intuitively, the medication works best when deposited locally.

Patients who have undergone extensive back surgery, or in whom other epidural approaches have failed, may require a transforaminal injection to gain closer proximity to the area of interest. C-arm fluoroscopy and contrast agents are needed for this technique and can be helpful in identifying unintentional vessel entry. Some centers use CT for guidance with transforaminal corticosteroid injections, but an argument can be made that identification of small vessel cannulation may be missed without real-time fluoroscopy.

The caudal injection is particularly useful if there is extensive posterior spinal fusion or severe spinal stenosis and a paucity of epidural fat. The caudal approach has fallen from favor because of its distal access that lies between the buttocks. Careful cleansing of this region is necessary, and meticulous removal of any solution that may have contacted the perineum is required because this area is very susceptible to chemical irritation by some cleansing agents. The caudal injection can be technically difficult in the patient with extreme kyphosis of the sacrum or patients with septa covering the sacral hiatus. Placing a curve on the needle is important to navigate a path to the S3 level.

The traditional method is commonly used by anesthesiologists and uses loss of resistance with a glass syringe as the indicator for entry into the epidural space. Many, including anesthesiologists, have migrated away from this technique due to the discomfort associated with blind attempts to locate an entry point into the spinal canal. Contrast agents and fluoroscopy may assist in identifying undesirable entry into nonepidural spaces, such as the subdural space or the subligamentous space ( eFig. 121-1 ) deep to the ligamentum flavum. This advantage is lost with the traditional technique.