Lumbar intervertebral discs, facet joints, sacroiliac joint, ligaments, fascia, muscles, and nerve root dura have been shown to be capable of transmitting pain in the lumbar spine with resulting symptoms of low back pain and lower extremity pain. When the source of the pain needs to be known, but cannot be determined by other means (history, physical examination, imaging), either the source or its nerve supply can be determined by performing diagnostic blocks of the suspected structure. Diagnostic blocks are defined as the administration of local anesthetic in order to relieve pain temporarily for obtaining diagnostic information. They are performed to determine the pathophysiology of clinical pain, the site of nociception and the pathway of afferent neural signals.

Diagnostic blocks in the management of low-back pain include:
Facet joint blocks
Provocation discography
Sacroiliac joint blocks
Selective nerve root blocks

Facet joint blocks: Facet joints are capable of causing pain and they have a nerve supply. They have been shown to be a source of pain in patients using diagnostic techniques of known reliability and validity. Facet joints have been implicated as the source of pain in 15%-45% of patients with chronic low-back pain. Conventional clinical and radiologic techniques are unreliable in diagnosing low-back pain. Various patterns of referred pain may be similar for different structures. Most maneuvers used in physical examination are likely to stress several structures simultaneously. Facet joints are blocked by either injection into the joint or by blocking the nerves that innervate the joint. Medial branch blocks are preferred because they are easy to perform, age related joint changes do not affect the procedure, and there is no risk of over-distension. The accuracy of facet joint nerve block is strong in the diagnosis of lumbar facet joint pain.

Figure 1: ?ntra-articular facet block (oblique view)

Figure 2: Medial branch block (oblique view)

Provocation discography: Discs are innervated and can be a source of pain that has pathomorphologic correlates. Prevelance of pain due to internal disc disruption is 39%. (primary discogenic pain 26%) Discography is used to characterize the architecture of the intervertebral disc and to determine if it is a source of pain. It is an invasive diagnostic test that should only be applied to patients with suspected etiology. During the procedure the sterile needle is placed into the center of the intervertabral disc. Radiopaque contrast is injected to provoke pain. Disc morphology is assessed under radiology. Evoked pain is clinically assessed in relation to baseline pain. The evidence for lumbar discography is strong for discogenic pain provided that discography is performed based on the history, physical examination, imaging data and analysis of other precision diagnostic techniques.

Figure 3: The spread of the radiopaque contrast after the needle placement in the center of the intervertebral disc during discography (A-P view and lateral view)

Sacroiliac joint blocks: Sacroiliac joints are innervated and shown to be a source of low-back pain and referred pain in the lower extremity. There are no definite historical, physical, or radiological features to provide accurate diagnosis of sacroiliac joint pain. It has prevalence; 10%-30% with single block, 10%-26.6% with double block. Sacroiliac joints can be anesthetized with intraarticular injection of local anesthetic. Extravasation of local anesthetic out of the joint can lead to false-positive responses. Faulty needle placement, intravascular injection, inability of local anesthetic to reach the painful portion of the joint can cause false-negative responses. The evidence for the accuracy of sacroiliac joint diagnostic injections is moderate for the diagnosis of sacroiliac joint pain.

Figure 4: The spread of the radiopaque contrast after intra-articular needle placement in the sacroiliac joint (A-P view)

Diagnostic selective nerve root blocks: They are performed in patients when history, examination, imaging, and other precision diagnostic injections and electrophysiologic testing do not identify the pain generator. They can differentiate asymptomatic – symptomatic nerve compression. They are performed in multilevel pathology to help identify the pain generator. They are also used when location of symptoms seems to conflict with abnormalities identified with imaging studies. There is limited evidence on the effectiveness of selective nerve root injections as a diagnostic tool for spinal pain. Their role needs to be further clarified by additional research and consensus, but the available literature is supportive of selective nerve root injections as a diagnostic test for equivocal radicular pain. The evidence is moderate for selective nerve root blocks in the preoperative evaluation of patients with negative or inconclusive imaging studies and clinical findings of nerve root irritation. The positive predictive value of diagnostic selective nerve root blocks is low, but they have a useful negative predictive value.

Figure 5: The spread of the radiopaque contrast in the selective nerve root block (A-P view and lateral view

In conclusion, the diagnostic blocks applied in the precision diagnosis of chronic low back pain include lumbar facet joint nerve blocks, lumbar provocation discography, sacroiliac joint blocks, and selective nerve root blocks. Diagnostic blocks are subject to false-positive responses. In order to be valid, they must be target-specific and must be controlled. After diagnostic local anesthetic blocks, the source of pain may be permanently blocked by appropriate invasive techniques including radiofrequency (RF) thermoneurolysis utilizing conventional RF (heat destroys nerves and blocks pain transmission) or pulsed RF (heat never exceeds 42o C preventing damage to the tissues and the nerves).

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