Our patient’s PLP had disappeared with the beginning symptoms of a cauda equina syndrome caused by meningioma and gradually reactivated after surgical removal of the tumor. We thought that the suppression of the PLP was caused by the mass-induced ischemic and mechanical damage to the cauda equina and that its reactivation was caused by electrophysiological improvement after removal of the mass. During cauda equina compression, nerve fibers have differing susceptibilities both to ischemia and mechanical trauma. The small myelinated and unmyelinated fibers (e.g., Aδ and C-fibers) that subserve pain sensation and parasympathetic function are much less resilient to mechanical compression than the larger fibers concerned with motor power, light touch, and joint position sense. Reversibility is dependent on relief of both the mechanical pressure obstructing axoplasmic flow and the resolution of ischemia/venous congestion (4).
The mechanism of mechanical damage is used clinically for the treatment of trigeminal neuralgia in the balloon compression technique. Balloon compression of the trigeminal nerve has been used effectively to treat trigeminal neuralgia for two decades. The compression injures large myelinated fibers, removing the trigger to the presumed ephaptic transmission of pain. Because small unmyelinated fibers, which mediate the corneal reflex, are selectively preserved, the compression technique may be an advantage in the treatment of ophthalmic division nerve pain (5).
Some studies demonstrated that evoked potentials in the medulla spinalis suppressed by cervical medullary compression recovered promptly after surgical decompression (6). Brihaye (7) reported a case in which painless phantom phenomenon extinguished itself progressively, paralleling the aggravation of cervical radiculopathy because of a herniated disk and reappeared on slow recovery from radiculopathy 6 months after removal of the herniated disk. Iida et al. (8) reported another similar case, in which PLP was relieved and remained asymptomatic for 15 years reactivated after cervical spinal decompression.
Conversely, secondary phantom limb problems, reactivated after an asymptomatic interval after the amputation, have been reported after intervertebral disk hernia, spinal cord injury, and vertebral neoplasia (9–12). Pithwa and Rajasekaran (13) reported an interesting case in which secondary PLP developed as a result of a cervical epidural tubercular abscess and was relieved after treatment with antituberculous therapy.
This phenomenon may be dependent on whether the individual nerve fibers are partly or completely affected and whether the cauda equina or medulla spinalis is incompletely or completely compressed. Partial loss of cauda equina or medulla spinalis functions therefore may cause little or no disability (14). For reactivating PLP just after decompression, another possible explanation may be that mechanical stimuli or inflammatory responses resulting from the surgical stress to the spinal cord resulted in functional changes in the afferent pathways of the amputated limb. But, in our case and Brihaye’s (7) case PLP was reactivated 3 and 6 months, respectively, after decompression. Therefore, this mechanism is excluded in our case.
Regional anesthesia with local anesthetics provides a reversible and transient deafferentation model in humans and may be hypothesized as sharing fundamentally similar mechanisms. Many cases of reactivation of PLP in amputees have been reported during neuraxial, plexus, and peripheral nerve blocks (15–17).These examples support the theory, elaborated by Melzack (18), that this is attributable to a decrease in the tonic inhibitory influence exerted by the brainstem reticular formation, permitting pain. However, Tessler and Kleiman (19) prospectively studied 23 spinal anesthetics in 17 patients with previous lower limb amputation. Only one patient developed clinically significant PLP. The difference may depend on whether conduction of the individual nerve fibers, plexus, or spinal cord ceased completely or partially.
Martin et al. (15) presented a case in which lumbar plexus block resulted in apparent unmasking of PLP previously experienced in the distribution of the sciatic nerve, and PLP was subsequently alleviated by sciatic blockade. They suggested that afferent input from intact axons in neighboring peripheral nerves (which share the same spinal cord segments) may exert an influence in the tonic inhibition of phantom pain generated peripherally by injured axons.
Paqueron et al. (20) described a late-onset PLP during a continuous analgesic popliteal nerve block with small-concentration local anesthetic after foot surgery and its alleviation and recurrence when stopping and resuming the local anesthetic infusion. They suggested that small-concentration local anesthetics block the small- diameter fibers (Aδ and C-fibers,), and a partial deafferentation of these small sensory fibers might lead to PLP.
In summary, we present a case in which the PLP in the amputated leg disappeared during cauda equina compression by meningioma and reactivated after surgical decompression. This case suggests that although incomplete compression or block with local anesthetics may result in reactivation of PLP that had once been completely relieved and remained asymptomatic for years, complete compression or blockade of nerves, a nerve plexus, cauda equina, or medullary cord may result in suppression of PLP, and decompression or recovery of the block may cause reactivation. In conclusion, the cited literature and the present case highlight the relative importance of peripheral and central mechanisms in PLP.
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© 2005 International Anesthesia Research Society
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