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Evolution of spinal cord surgery for pain.

The evolution of spinal cord procedures for pain closely followed the evolving understanding of the anatomy and workings of the pain pathways. Clinical observations often led to laboratory studies to define the pain pathways. More knowledge led to more and improved pain procedures. Postoperative observations led to better definition of pain perception. In addition, empirically derived pain procedures sometimes led to increased knowledge of the anatomy and physiology of pain perception.12 Spinal cord procedures for pain initially involved interruption of pain pathways and, many years later, the stimulation of the spinal cord in an attempt to inhibit pain transmission. Let us begin by reviewing the historical concept of pain pathways involving the spinal cord (Fig. 2.1). This classical illustration from 1968 by Struppler54 demonstrates the division of larger fibers, transmitting touch and proprioception, from small C-fibers that transmit pain through peripheral nerves to the spinal cord via the dorsal roots. After synapsing in the dorsal root entry zone, the next order neurons decussate in the anterior commissure to ascend in the contralateral anterolateral quadrant of the spinal cord as the lateral spinothalamic tract. After ascending to the brainstem, the tract sends many fibers to the ventroposterior lateral nucleus of the thalamus, which is associated with perception of sensation, including “pain.” The majority of fibers, however, course medially into the reticular formation, medial and intralaminar thalamus, and limbic areas, perhaps where “suffering” is presumably perceived. This illustration shows an often neglected, but poorly demonstrated, pain pathway, the spinoreticular and reticulospinal multisynaptic system that presumably is involved with visceral pain. Not illustrated is Willis’ dorsal column visceral pain pathway, which was not discovered until some years after this illustration appeared. Even before the primary pain pathway was fully appreciated, there was an attempt to control pain by section of peripheral nerves in the 18 century. As one might imagine, except when the pain was produced by injury to the individual nerve, results were not satisfactory. It was demonstrated experimentally by Bell5 in 1811 and by Magendie30 in 1822 that incoming sensory information was carried by the dorsal nerve root, and that the ventral root was dedicated to outgoing motor function. However, it was not until 1899 that Abbe1 performed the first rhizotomy or posterior root section in the United States, which was the first spinal surgery for pain. Because it was necessary to perform rhizotomy at a number of levels to treat somatic pain, however, it was rapidly replaced when cordotomy was developed. The identification of the primary pain pathway as the lateral spinothalamic tract was made after clinical observation of patients who had lost pain sensation after injury to the anterolateral quadrant of the spinal cord. In 1871, Müller36 observed a patient who had hemisection of the spinal cord with bilateral dorsal column involvement caused by a stab wound, and noted contralateral hemianalgesia, along with bilateral loss of touch sensation. In 1878, Gowers20 examined a patient who had a discreet injury to the anterolateral quadrant of the cervical spinal cord, caused by a bone spicule driven into the spinal cord after a gunshot wound, and noted discreet contralateral analgesia. This led Spiller52 to define the spinothalamic pathway in experimental animals in 1905. He noted that the pain sensation entered the dorsal columns supplying the injured area of the body, ascended several levels, and synapsed with the second order neurons, which cross in the anterior commissure to the contralateral spinothalamic tract and ascended to the brainstem. This led Spiller and Martin53 to perform the first surgical anterolateral cordotomy in 1912. The cordotomy procedure was refined by Frazier11 in 1920. Over the course of the following 51 years, it became the most commonly used and most often successful operation for pain. Throughout that time, numerous technical advances were incorporated to make the resultant analgesia more predictable and to decrease the risk of impaired function. It was noted, however, that chronic pain most often eventually recurred, despite interruption of the known pain pathway, although cancer pain was more often permanently alleviated.15 Study of patients with section of the spinothalamic tract led to significant physiological observations. One particularly interesting finding was made in 1943. It is commonly observed that the level of analgesia begins three to five segments below the level of incision into the spinal cord. Hyndman24 also sectioned Lissauer’s tract, which lies at the dorsal root entry zone, at the same level as the lateral spinothalamic Copyright © 2006 by Lippincott Williams & Wilkins 0148-703/06/5301-0011