Controls

Single, diagnostic blocks have unacceptably high false-positive rates. When these rates have been measured, they amount to between 25 and 41 percent.102,103,104,105 These high false-positive rates compromise the validity of any response. Without performing control blocks, the operator cannot tell if a positive response is true or false.

In practice, two types of control are available.106 Pharmacologic controls require using different agents on the same target on separate occasions. Anatomic controls require applying the test to a different structure.

The ultimate pharmacologic control is a placebo block. However, this requires a sequence of three blocks.106 On the first occasion, a local anesthetic must be used in order to establish, prima facie, that blocking the target nerve or structure does indeed relieve the patient's pain. Unless this is established, there is no point performing control blocks on a structure that is not the source of pain. The second block must be either a local anesthetic or normal saline, allocated randomly and in a double-blind fashion. The second block cannot routinely be the placebo, for an insightful patient would know that the second block is always the "dummy." Chance must be maintained. On the third occasion, the agent administered is the one not administered on the second occasion. Under these conditions, a positive response would be relief of pain on each occasion that a local anesthetic agent was used and no relief when normal saline was used.

There are no ethical objections to such a process, provided that the patient provides informed consent to undergo the sequence of tests. What is often prohibitive, however, is the number of procedures required.

A practical alternative is comparative local anesthetic blocks.106 The patient undergoes the same block on separate occasions, but different local anesthetic agents are used. A concordant response is one in which the duration of relief is concordant with the expected duration of action of the agent used. The patient reports short-lasting relief when a short-acting agent is used (e.g. lidocaine) and long-lasting relief when a long-acting agent is used (e.g. bupivacaine). A discordant response is one in which the patient reports complete relief of pain following each block, but the duration of response is discordant with the expected duration of the agent used. Typically, this amounts to a prolonged response to lidocaine.

Validation studies, using placebo controls as the reference standard, have shown that concordant responses have only a 14 percent chance of being false-positive.94 Reciprocally, that means an 86 percent chance of being true-positive. Discordant responses have a 35 percent chance of being false-positive, but a 65 percent chance of being true-positive. Whether an operator should accept concordant or discordant responses as constituting a positive response depends on the circumstances. If 65 percent diagnostic confidence is enough for practical purposes, a discordant response becomes acceptable. If greater diagnostic confidence is required, say before a destructive therapy based on the diagnosis, a concordant response may be preferable.94

For intraarticular blocks, pharmacologic controls cannot be used, because the normal duration of action of local anesthetic agents within joints is not known. Under these conditions, anatomic controls can be used. Under single-blind conditions, the operator targets an adjacent structure that is believed not to be the source of pain. Anatomic controls, however, are valid only if the target and control structures are both small and indis-tinguishably close to one another. If it is obvious to the patient that a different structure is being tested, the purpose of the control is defeated.

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