Selective Transforaminal Nerve Block - General Principles

Selective Transforaminal Nerve Block: General Principles

Anatomy

Neural Foramen

The neural or intervertebral foramina of the typical cervical, thoracic and lumbar spine are formed by the verterbal pedicles inferiorly and superiorly, the vertebral body and disc anteriorly, and the zygapophysial (facet) joint posteriorly (Figure 1).  Each pedicle demonstrates a notched inferior and superior surface, which when apposed to the pedicle above and below, creates a rounded appearance to the borders of the foramen.  The neural foramina enclose a partly fat-filled compartment which is continuous with the epidural space of the spine and contains nerve roots, the sinuvertebral nerve, blood vessels and lymphatics.

Several pertinent morphologic features of the typical cervical vertebra (C3-C7) are worth mentioning.  Compared to their thoracic and lumbar counterparts, the cervical vertebra exhibit raised superolateral lips known as uncinate processes (Figure 2).  These articulate with the margins of the vertebral body above, forming the uncovertebral joint or "joint of Luschka."  Degenerative changes involving these joints may contribute to narrowing of the neural foramen and associated radicular symptoms.  The transverse processes of C3 to C6 and occasionally C7 also demonstrate bilateral foramen transversarium, which reflect rounded perforations through which the vertebral artery ascends into the skull (Figure 3).  The anterior location of the vertebral artery relative to the neural foramen is important to bear in mind when performing transforaminal blocks in this location.

The neural foramina of the sacrum are distinct from those in the remainder of the spine.  Four paired sacral foramina are found at the margins of the ossified intervertebral discs where one sacral vertebral body has fused with an adjacent one (Figure 4). The foramina run as a continuous osseous tunnels in the sagittal plane.  The anterior (pelvic) foramina are typically larger than the posterior (dorsal) foramina and house the ventral and dorsal primary rami of the sacral nerves, respectively (see below).

Spinal Cord and Nerves

As illustrated in Figures 5 and 6, multiple rootlets emerge from the spinal cord both dorsally and ventrally, which unite to form the dorsal (sensory) and ventral (motor) nerve roots.  The cell bodies of ventral root axons reside within the spinal cord in the ventral gray horn, while the cell bodies of the dorsal root axons are found outside the spinal cord within the dorsal root ganglion in the neural foramen.  Just distal to ganglion, the ventral and dorsal nerve roots unite to form a single, "mixed" motor and sensory spinal nerve, which divides almost immediately into the ventral and dorsal rami.  The spinal nerve receives sympathetic fibers from the gray ramus communicans (Figure 6).

The dorsal and ventral nerve roots and spinal ganglia are enveloped by regular evaginations of the dural sac known as dural sleeves (Figure 5).  In the neural foramen, as the ventral and dorsal nerve roots merge to form the spinal nerve, the dural sleeve becomes the epineurium (Figure 7). The epineurium is in turn surrounded by an epiradicular membrane.  The latter is an extension of the anterior and posterior epidural membranes, which have attachments to the dural sac and posterior longitudinal ligament.  The epiradicular membrane thus envelops the exiting nerve and epineurium and defines the boundaries of the epiradicular space.  This space is an extension of the epidural space and serves as the ideal target for steroid and anesthetic deposition in selective nerve blocks (see "Safe Triangle" below). 

The sinuvertebral or recurrent meningeal nerve (SVN) is also found near the neural foramen. In the lumbar spine the SVN is formed by the union of somatic and autonomic roots from the ventral ramus and gray ramus communicans respectively.  It then courses medially to enter the ipsilateral neural foramen (Figure 8) and subsequently divides into both ascending and descending branches.  These communicate with contralateral sinuvertebral branches as well ascending and descending branches of the adjacent ipsilateral sinuvertebral nerves.  The composition and distribution of the SVN is analogous though not identical in the cervical spine. 

The sinuvertebral nerve innervates various structures in the epidural space including the dural nerve root sleeves, posterior disc annulus, posterior longitudinal ligament and the ventral dural sac.  The distribution and intercommunication among branches of the sinuvertebral nerves result in varying patterns of pain which are often poorly localized.

The "Safe Triangle"

The safe triangle is a space within anterior-superior third of the neural foramen bounded by the pedicle superiorly, the exiting nerve inferomedially and the lateral margin of the neural foramen laterally (Figure 9).  Placement of the needle in the center of the safe triangle allows for injection into the epiradicular space while minimizing the risk of dural puncture.  Ideally the needle tip is directed to the 6 o'clock position of the pedicle on the AP view (effectively avoiding the thecal sac) and in the anterior aspect of the foramen on the lateral view (approximating the nerve-disc interface where pathology is most common). 

Rationale and Clinical Indications

Diagnostic Information

Selective transforaminal nerve blocks (SNB) are frequently used as a diagnostic tool in patients with radicular pain.  In cases where clinical examination, imaging and other diagnostic modalities such as EMG are discordant or do not clearly implicate a specific nerve root as the source of pain, the selective injection of anesthetic and steroid can help elucidate the cause for a patient's symptoms and guide selection of an appropriate level for surgery, if needed.  Findings that may implicate a specific nerve include provocation of the patient's typical pain with injection, immediate pain relief from infiltration with local anesthetic, and extended relief of symptoms from corticosteroid injection.  If SNBs are performed in patients with focal back or neck pain without radicular symptoms, the diagnostic value of the injection may be limited.  Resolution of symptoms in this setting may be related to effects of the injection on the sinuvertebral nerve which innervates various midline structures described above at multiple levels.

Therapeutic Effect

Selective transforaminal nerve blocks employing corticosteroids are also requested for therapeutic purposes. The cause of radicular pain may or may not involve findings of mechanical nerve impingement.  It is well known that degenerative changes of the spine, including disc herniations, facet or uncovertebral joint hypertrophy, and osteophyte formation may lead to referred pain from central or foraminal stenosis and direct nerve compression.  Herniated discs, however, are also known to release prostaglandin E, phospholipase A2 and other mediators of pain and inflammation from the nucleus pulposus into the epidural space. This can result in intraneural edema, pain and alterations in nerve conduction without associated nerve compression. In either case, SNBs may be of therapeutic value by utilizing the potent anti-inflammatory properties of steroids to reduce nerve root irritation and inflammation.

Non-Selective Epidural Injections vs. Selective Nerve Blocks

Some clinicians prefer non-selective epidural steroid injections (NSESIs) over selective nerve blocks (SNB) for patients who have multiple potential pain generating lesions of the lumbar spine. However, disadvantages of NSESIs include the lack of diagnostic yield, higher risk of intrathecal steroid administration (particularly in post-laminectomy patients) as well as the deposition of steroid in the posterior epidural space instead of the dural-disc interface where nerve impingement and inflammation is more likely to occur.  Larger volumes of injected fluid and occasionally higher doses of steroid are also needed with NSESIs to assure spread of adequate steroid and anesthetic to an affected nerve root.  In some cases,  significant neural foraminal narrowing from a lateral disc protrusion or other cause limits the spread of steroid despite larger injection volumes and results in a suboptimal therapeutic response.  Selective transforaminal nerve root blocks may be of greater benefit in such instances and may be requested after a failed NSESI.

In the cervical spine selective nerve blocks are frequently preferred over non-selective epidural injections for radicular symptoms.  This is commonly due to the small dimensions of the posterior cervical epidural space, the risk of more serious complications from intrathecal injection, and the relative paucity of studies examining the therapeutic efficacy of cervical NSESIs.

Clinical Scenario

Patient Symptoms

Patients referred for selective blocks typically present with neck or lower back pain associated with radicular symptoms.

MR Imaging Findings

Imaging findings are highly variable in this patient population.  MRI may demonstrate neural foraminal stenosis from various causes including disc bulge or herniation, hypertrophic facet or uncovertebral joint degenerative changes and spodylolisthesis.  In other instances, imaging may reveal no abnormality at levels implicated by findings on physical exam.

Contraindications

Absolute Contraindications:

• Poorly controlled bleeding diathesis or anticoagulation
• Local cellulitis or pilonidal cyst near anticipated needle entry site
• Systemic infection (bacteremia)
• Uncontrolled diabetes mellitus

Consent Process

Potential Complications and Adverse Reactions:

• Bleeding
• Infection
• Contrast reaction
• Temporary leg weakness
• Temporary increase in pain
• Intrathecal needle placement
  - dural puncture more likely with nonselective epidural injections
• Intrathecal steroid/anesthetic injection
  - self-limited saddle anesthesia and lower extremity weakness
  - spinal headache
  - arachnoiditis
• "Steroid flush"
  - facial flushing related to systemic absorption of intraarticular steroid
• Hyperglycemia in diabetics
• HPA axis suppression
  - typically self-limited

Documents

• Patient information sheet
• Consent form