Jacksonville Medicine, October, 1998
Duval County Medical Society, Jacksonville, FL 32204
Acute pain plays an important protective role in many diseases of the body and helps direct the appropriate therapy to the primary problem. However, when the pain persists and becomes chronic it can have severe detrimental effects. With chronic pain, patients can become depressed, addicted to medication, and develop chronic pain behavior.1 Secondary gain, such as disability compensation and increased attention from family and friends, may only add to the problem.
Treatment of intractable pain presents special challenges to neurosurgery. This article will describe two methods we use to treat this problem _ spinal cord stimulation and intraspinal delivery of morphine by way of an implantable programmable drug delivery system. Spinal cord stimulation (also called dorsal column stimulation) is best suited for back and lower extremity pain related to adhesive arachnoiditis,2 failed back syndrome,3 causalgia,4 phantom limb and stump pain,5 and ischemic pain.6 Intrathecal administration of morphine is not only useful for all of the above conditions, but is also useful in intractable pain from cancer.7 Consideration is usually given for the use of spinal cord stimulation first. If that is not satisfactory, then morphine administration is tried. Spinal cord stimulation has been found to be most useful for leg pain, but does help back pain in some instances. When back pain is primarily the presenting complaint, morphine administration is usually much more effective.8
Spinal cord stimulation is thought to relieve pain through the "gate control theory" of Melzack and Wall, which suggests that stimulating large diameter nerve fibers inhibit small diameter nerve fiber input to the brain.9 In other words, applying a direct physical or electrical stimulus to the spinal cord should, in effect, electrically block pain signals from traveling to the patient's brain following painful stimuli at lower levels along the spinal cord. Shealy and coworkers10 first utilized this concept in 1967, proposing to place stimulating electrodes over the dorsal columns of the spinal cord. Since then, improvements in hardware and patient selection have improved results with this procedure.
The technique for spinal cord stimulation utilizes a percutaneous electrode for the initial testing of patients. This temporary electrode is used to determine whether patients will receive pain relief. The temporary electrode is implanted under local anesthesia and requires only a short stay visit. Only local anesthesia is used so that the patient is able to verbally respond when questioned about the location of the stimulus and its intensity. Patients are placed in the prone position, and under fluoroscopic control a 15 gauge Touhy needle is inserted into the lumbar area of the L2-L3 interspace. After determining that the needle is in the epidural space, the electrode is inserted through this needle and is guided fluoroscopically to the T8 or T9 area of the spinal cord. The electrode may be placed to the center or either side of the cord to produce electrical stimuli in the area of the predominate pain. The electrode is connected with sterile cables to a 9 volt battery-powered external testing power source. The parameters of current, frequency, and pulse width are varied so that the patient perceives a pleasant tingling parasthesia into the area of pain.
Once appropriate localization of the stimulation is achieved, the electrode is sutured in place and externalized through the skin. The patient is tested for several days at home to determine if the stimulation is in the appropriate area and whether the pain relief is adequate. If the patient receives no relief, the electrode is removed again through short-stay. But, if the patient is satisfied with the pain control, the electrode is permanently implanted and connected to a permanent extension lead which is connected to a spinal pacemaker. This pulse generator power source (ITREL III®, Medtronic, Inc., Minneapolis, Minnesota) is usually placed in a subcutaneous pocket in the left upper quadrant of the abdomen which provides several control parameters such as voltage, pulse width, rate or frequency, and an on/off cycling effect. At times, the percutaneous electrode does not give satisfactory broad enough stimulation and this percutaneous electrode is removed and replaced with a flat electrode measuring approximately 0.5 x 3 inches. Unlike the percutaneous electrode, this flat electrode (RESUME ELECTRODE®, Medtronic, Inc., Minneapolis, Minnesota) is larger and provides better surface area for stimulation with less chance of movement or breakage. This electrode is inserted through a mini-laminotomy under local anesthesia with test stimulation for accurate placement. Once the stimulation is localized, the RESUME ELECTRODE® leads are then tunneled subcutaneously and connected to the spinal pacemaker.
Patients are able to control the ITREL III® with a hand held programmer which can change a few modalities such as increase or decrease the intensity of stimulation by controlling the voltage and the rate of stimulation. However, more discrete and accurate changes in stimulation parameters may be programmed externally through the patient's skin by way of a radiotelemetry console programmer controlled by the physician. Patients with predominately lower extremity pain usually achieve the best results from spinal cord stimulation. Approximately 60-65% of patients with pain in the low back and legs will receive at least some relief of pain with this technique. It is apparent that spinal cord stimulation is not a panacea, but can be an effective approach, and usually a better alternative, than ablative surgery to help alleviate chronic debilitating benign pain in some patients. Complications are rare, but include the potential for spinal cord injury and infection.
Intraspinal morphine, a more recently introduced pain relieving procedure, has been found to be both safe and efficacious, but is a more aggressive technique to use for pain relief. It is usually tried after spinal cord stimulation is deemed not to be effective.11 Its effectiveness comes because it fills the opiate receptors in the cord, thus blocking the pathway from pain receptors in the periphery. Again, the pain that is most easily relieved is pain confined to the back and lower extremities. A test dose of spinal morphine is utilized to ensure adequate response prior to implanting the system. Each patients is tested with at least two lumbar subarachnoid injections of 0.5 to 2 mg of morphine, and at times are also injected with a placebo on a subsequent visit. The patients are observed for several hours for any untoward side effects in response to the test dose. Respiratory depression has not been observed with any of the test doses, however, nausea, vomiting, pruritis, and urinary retention may occur. After determining that the patient has received satisfactory pain relief, the patient is admitted to the hospital for placement of a programmable drug delivery system (SYNCHROMED PUMP®, Medtronic, Inc., Minneapolis, Minnesota) which consists of a pump connected to a catheter leading to the subarachnoid space.
The patient is normally placed in a lateral position under general anesthesia, and a silicone catheter is inserted by way of a 14 gauge Touhy needle into the lumbar subarachnoid space. The catheter is threaded cephalad so that the tip of the catheter is at approximately the T10-T11 level. At this point, the catheter is secured to the deep fascia and is tunneled subcutaneously around the patient's side to the abdominal area and connected to the pump. The pump is placed in a subcutaneous pocket no greater than 0.5 inch deep to the skin surface to facilitate percutaneous refilling the pump's 18cc reservoir. The pump is programmable by way of an external radiotelemetry programmer. The amount of morphine may be varied with the patient's needs by varying the concentration of morphine in the reservoir and by reprogramming the pump. The time between refills varies from 4 to 12 weeks. Usually, a patient's pain can be relieved with 0.5 to 5 mg of morphine per 24 hours. Since the pump is programmable, it can be customized to the patient's analgesic requirements. For instance, the pump provides for continuous delivery but also has a bolus delay mode where a specific increased amount of drug may be given at timed intervals for patients experiencing more pain at specific times of the day. The pump is extremely accurate in its delivery and is not subject to changes in body temperature or atmospheric pressures. Intraspinal analgesia can give patients a much better quality of life, reducing or eliminating heavy oral narcotics and/or freeing them from external IVs or pumps which sometimes requires hospitalization.
Disappointing results have been noted in some cases with morphine intrathecal administered by the pump. Why some patients will obtain excellent results with the test dose, yet after a few weeks of treatment with the pump permanently implanted and infusing morphine will cease to get relief, even with increased dosage, is a mystery. Tolerance obviously develops to the drug and some patients get absolutely no relief after varying periods of time. Furthermore, unexpected and unusual side effects have been occasionally noted, including pedal edema and excessive perspiration. Morphine has not been found to be a completely satisfactory drug for all patients, and the search for other drugs continues. Dilaudid® and clonidine have been found to be satisfactory alternative drugs in some cases. Marcaine® has also been mixed with all of the above drugs with varying success. Other new experimental drugs are being tested and hopefully the ideal drug will be developed which provides excellent analgesia, with minimal build-up of tolerance and other side effects.
The are other modalities of pain treatment, such as deep brain stimulation and the ablative type procedures, but the most useful and least harmful to the patient remains spinal cord stimulation and implantable drug infusion pump.