The treatment of pain induced by nerve root or intrinsic peripheral nerve disease is a challenge for the neurologist and includes several techniques that are generally administered by an anesthesiologist. One usually resorts first to one of the antiepileptic drugs discussed earlier and listed in Table 7-3. The next simplest treatments are topical; if the pain is regional and has a predominantly burning quality, capsaicin cream can be applied locally, care being taken to avoid contact with the eyes and mouth. The irritative effect of this chemical, which releases substance P, seems in some cases to mute the pain. There has also been limited success with several “eutectic” mixtures of local anesthetic (EMLA) creams or the simpler lidocaine gel with ketorolac, gabapentin, and other medications; these are applied directly to the affected area, usually the feet, in the morning and evening. Concoctions such as topical ketamine mixed in soy lecithin to produce a gel with drug concentration of 5 mg/mL, have been reportedly useful in treating post herpetic neuralgia according to Quan and associates in a small randomized trial and similarly, aspirin compounded with cold cream or, if available, chloroform. These preparations may provide some relief in postherpetic neuralgia and painful peripheral neuropathies.
Several types of spinal injections, including epidural, root, and facet blocks, have long been used for the treatment of spinal pain. Injections of epidural corticosteroids or mixtures of analgesics and steroids have been helpful in selected cases of lumbar or thoracic nerve root pain, and occasionally in painful peripheral neuropathy, but precise criteria for the use of this measure are not established. Several randomized trials have failed to support a long term beneficial effect of these treatments but a number of our patients have been helped, if only for several days or weeks (see Chap. 10 for a more complete discussion of these approaches). Nerve root blocks with lidocaine or with longer-acting local anesthetics are sometimes helpful in establishing the precise source of radicular pain. Their main therapeutic use in our experience has been for thoracic radiculitis from shingles, chest wall pain after thoracotomy, and diabetic radiculopathy. Similar local injections are used in the treatment of occipital neuralgia. Injection of analgesic compounds into and around facet joints and the extension of this procedure, radiofrequency ablation of the small nerves that innervate the joint, are as controversial as epidural injections, with most studies failing to find a consistent benefit. Despite these drawbacks, we have found both of these approaches useful when pain can be traced specifically to a derangement of these spinal joints, as discussed in Chap. 10.
The intravenous infusion of lidocaine may have a brief beneficial effect on many types of pain, including neuropathic varieties, localized headaches, trigeminal neuralgia, and other facial pains; it is said to be useful in predicting the response to longer-acting agents such as mexiletine, its oral analogue, although this relationship has been erratic in our experience. Mexiletine is given in an initial dose of 150 mg per day and slowly increased to a maximum of 300 mg three times daily; it should be used very cautiously in patients with heart block and has fallen very much out of favor in many centers, partly due to cardiac conduction abnormalities and rare instances of torsade de pointe arrhythmia during and after administration.