†Department of Orthopaedics and Rehabilitation, University of Miami School of Medicine, P.O. Box 016960, Miami, Florida 33101. E-mail address for Dr. Brown: email@example.com.
Phantom limb pain, a well known phenomenon, was described originally by Ambroise Paré in the seventeenth century. Recent investigators have described the sensation in patients who have had amputation of a limb2,5,7,8.
We are aware of at least one report of a patient with a transfemoral amputation who had a herniated disc between the fifth lumbar and first sacral vertebrae that caused pain in the stump6. Although pain in a stump most often results from the formation of a neuroma6, the patient in that report became symptom-free after a laminectomy and excision of the herniated disc. In an earlier report, two patients who had had an amputation of a limb were found to have a herniated lumbar disc that caused the perception of radicular pain distal to the level of the amputation4. After excision of the lumbar disc, the radicular pain was relieved in both patients.
We report on a patient who had phantom pain in the distribution of the sciatic nerve three years after a segment of that nerve had been removed in the course of an excision of an intermediate-grade liposarcoma of the posterior aspect of the right thigh. Removal of the intervertebral disc between the fourth and fifth lumbar vertebrae relieved the pain.
A sixty-two-year-old woman was found to have an intermediate-grade liposarcoma in the posterior aspect of the right thigh in 1984. She was managed with one course of neoadjuvant chemotherapy, which included intra-arterial administration of cisplatin (164 milligrams) and intravenous administration of doxorubicin (Adriamycin) (three doses of forty-one milligrams), before excision of the tumor. During the operation, a fourteen-centimeter segment of the sciatic nerve was found to be infiltrated with tumor. A segment of the sciatic nerve was completely resected three centimeters proximal and distal to the tumor mass, which was located adjacent to the ischial tuberosity and was loosely attached to the piriformis, the obturator internus, and the inferior and superior gemelli muscles. The operation was followed by radiation therapy. Because of the curvilinear operative scar, a combination of anterior and posterior opposed fields was used. The dose was calculated at a depth of 6.75 centimeters from the surface, and the total dose was 6240 centigray. The subsequent course of recovery was uneventful even though the patient had lost sensation in the distribution of the fifth lumbar and first sacral nerve roots and had no detectable function of the extensor communis, extensor hallucis, peroneal, posterior tibial, gastrocnemius, soleus, or flexor hallucis longus muscles. Function of the hamstrings was fair, and that of the iliopsoas and quadriceps was normal.
In 1987, pain developed in the right lower extremity after a fall. The pain was perceived to be located in the distribution of the sciatic nerve distal to the level of the resection. Because of the persistent pain, the patient had difficulty standing and walking. The pain was described as an aching sensation located in the right buttock and radiating to the lateral aspect of the thigh, calf, and ankle. The muscle function and the sensory deficit in the lower extremity remained identical to that found immediately after the resection of the tumor. The results of the straight-leg-raising test were negative, and there was no restriction of back motion.
The posterior aspect of the thigh was tender in the region where the proximal nerve stump was buried in scar tissue. Thus, a neuroma resulting from the complete transection of the nerve was thought to be the cause of the pain in the stump. Exploration of the stump of the sciatic nerve revealed no evidence of recurrent tumor, but a traumatic neuroma was found and excised. After this procedure, the symptoms worsened. A magnetic resonance imaging scan revealed a herniated disc between the fourth and fifth lumbar vertebrae and lateral-recess stenosis (Figs. 1 and 2). There was flattening of the thecal sac, narrowing of the right lateral recess, and bilateral hypertrophy of the facet. After excision of the herniated disc, the pain completely resolved. At a follow-up examination two years later, the condition of the patient was unchanged. She had had no recurrence of sciatic pain in the lower extremity.
Loss of a segment of a peripheral nerve leads to disruption of retrograde and antegrade nerve conduction and axonal transport. Degeneration of a nerve occurs in both directions from the site of injury, and a neuroma may form at the site of injury. A sensation distal to the site of a nerve injury, a so-called phantom sensation, may be unpleasant, is often painful, and can result in marked physical and psychological morbidity3. Our patient perceived pain in the distribution of the sciatic nerve even though the sciatic nerve was not intact.
Stimulating the free endings of a normal sensory nerve leads to a retrograde impulse, which in turn may eventually be perceived at the level of the cerebral cortex as pain. Compressing a preganglionic or postganglionic nerve root initially results in loss of sensation or muscle weakness in the distribution of that nerve root. Inflammation of a nerve root generates ectopic signals that may be perceived at the level of the cerebral cortex as pain in the distribution of the nerve root distal to the site of the ectopic stimulus—that is, classic sciatic pain is perceived as the result of compression and irritation of a nerve root by a herniated lumbar disc1,10.
The phantom sciatica in our patient did not resolve until the herniated disc was resected. The herniated disc had compressed the nerve root and the dorsal root ganglion. The impulses from compression traveling backward along the conduction pathway were perceived as an ectopic form of impulse generation. Such ectopic signals are abnormal discharges in afferent fibers and have been reported in the cells of dorsal root ganglia of frogs11. Ectopic stimulation of the nerve root initiated by compression at these sites was then transmitted through the dorsal root ganglion and preganglionic fibers to the spinal cord and the central nervous system, where they were perceived as pain along the course of the sciatic nerve distal to the site of the resection of the sciatic nerve. Pain was perceived in the corresponding distribution of the sciatic nerve even though a segment of that nerve had been lost. The pain occurred as a result of a misperception of the ectopic signal as interpreted by the brain.
An understanding and awareness of the pathophysiology of the perception of pain by means of an ectopic stimulus is important for an accurate differential diagnosis of pain in a limb. It is important to consider spinal causes of radicular pain when evaluating phantom limb pain in a patient who has had an amputation or a resection or injury of the sciatic nerve.
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.
Investigation performed at the Department of Orthopaedics and Rehabilitation, University of Miami School of Medicine, Miami
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