Medial Branch Blocks

Cervical medial branch blocks can be used to test if a zygapophysial joint is the source of a patient's neck pain. They involve anesthetizing, under fluoroscopic control, the small nerves that innervate the target joint, each with no more than 0.3 mL of local anesthetic89 (Figure 36.5).

Cervical medial branch joint blocks have face-validity, in that they selectively anesthetize the target nerves, and do not anesthetize any nearby structures that realistically might be the source of pain.90 Single diagnostic blocks,

Figure 36.5 A lateral fluoroscopy view of a needle in place on the articular pillar of C5 in preparation for a C5 medial branch block.

however, are not valid. They carry a false-positive rate of some 27 percent.91 Controls are, therefore, required in each and every patient. When performed under controlled conditions, cervical medial branch blocks have proven construct validity.92

Two types of control are available. Foremost, placebo controls can be used. However, this requires three blocks to be performed on separate occasions.89 The first block must be with a local anesthetic in order to establish, prima facie, that the joint is painful. The second block cannot summarily be a local anesthetic agent, for a mischievous patient would know that they were expected to respond. Rather, in order to maintain chance, the second agent must be randomized as either a local anesthetic or a placebo. For the third block, the reciprocal agent should be used. A valid response would be relief of pain on each occasion that a local anesthetic was used, but no relief when placebo was administered. However, although they are stringent, placebo-controlled blocks are not practical in most clinical circumstances.

An alternative are comparative diagnostic blocks.89,92 Blocks are performed on separate occasions using different local anesthetic agents. A valid response is one in which the patient obtains a duration of relief concordant with the expected duration of action of the agent administered, i.e. long-lasting relief when a long-acting agent is used, and short-lasting relief when a short-acting agent is used. Controlled studies have shown that diagnostic decisions based on this paradigm are robust.93

Epidemiologic studies, using double-blind, controlled, diagnostic blocks, have shown that zygapophysial joint pain is the single most common basis of chronic neck pain, both after whiplash and in heterogeneous samples. In patients with a history of whiplash, prevalence figures (with 95 percent confidence intervals) of 54 percent (40-68 percent)94 and 60 percent (46-73 percent)95 have been reported. In patients with headache after whiplash, the prevalence of C2-3 zygapophysial joint pain was 53 percent (37-68 percent).96 Amongst drivers involved in high-speed collisions, the prevalence was as high as 74 percent (65-83 percent).97 In patients with neck pain not restricted to those with whiplash, the prevalence of cervical zygapophysial joint pain has been at least 36 percent (27-45 percent) in a rehabilitation practice,98 and 60 percent (50-70 percent) in a pain clinic.99

Of all the possible diagnostic tests that might be applied to a patient with neck pain, cervical medial branch blocks are the only validated test. Of all the possible causes of chronic neck pain, zygapophysial joint pain is the only proven entity and is the most common cause of neck pain after whiplash.

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