Cellular Pathology and Apoptosis in Experimental and Human Acute and Chronic Compressive Myelopathy ROWENA ELIZABETH ANNE NEWCOMBE M.B.B.S. B.Med Sci. (Hons.) Discipline of Pathology, School of Medical Sciences University of Adelaide June 2010 A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy CHAPTER 1 INTRODUCTION 1 The term “compressive myelopathy” describes a spectrum of spinal cord injury secondary to compressive forces of varying magnitude and duration. The compressive forces may act over a short period of time, continuously, intermittently or in varied combination and depending on their magnitude may produce a spectrum varying from mild to severe injury. In humans, spinal cord compression may be due to various causes including sudden fracture/dislocation and subluxation of the vertebral column, chronic spondylosis, disc herniation and various neoplasms involving the vertebral column and spinal canal. Neoplasms may impinge on the spinal cord and arise from extramedullary or intramedullary sites. Intramedullary expansion producing a type of internal compression can be due to masses created by neoplasms or fluid such as the cystic cavitation seen in syringomyelia. Acute compression involves an immediate compression of the spinal cord from lesions such as direct trauma. Chronic compression may develop over weeks to months or years from conditions such as cervical spondylosis which may involve osteophytosis or hypertrophy of the adjacent ligamentum flavum. Compressive myelopathies include the pathological changes from direct mechanical compression at one or multiple levels and changes in the cord extending multiple segments above and below the site of compression. Evidence over the past decade suggests that apoptotic cell death in neurons and glia, in particular of oligodendrocytes, may play an important role in the pathophysiology and functional outcome of human chronic compressive myelopathy. The temporal and spatial dimensions of secondary injury involving apoptosis are not yet fully understood. Apoptosis may be reversible depending on progression through the molecular cascade, and an improved understanding of this process in acute and chronic compressive myelopathy may thus be important. Such knowledge is critical to the development of effective therapeutic intervention strategies designed to limit cellular damage within the spinal cord. In this thesis major attention is focused on the spinal cord histopathological and apoptotic changes following compression rather than the lesions producing the compression, as few studies on chronic compressive myelopathy have addressed this. Compressive damage to the spinal cord involves not simply a mechanical distortion, but a complex series of molecular events which evolve at varying time intervals after the initial insult. Depending on the site and nature of the inciting injury, neural, glial, axonal, dendritic and vascular 2 elements may be affected and the pathological changes may vary in time and distance from the direct site of injury. This study aimed to better define the cellular pathology and molecular events, specifically apoptosis, in experimental and human acute and chronic spinal cord injury. Importantly, both spatial and temporal patterns of apoptosis are analysed. The functional alterations, histopathological changes and immunohistochemical markers of neural injury in a rodent model of chronic compression were compared with a rodent model of acute spinal cord injury and human acute and chronic compression. 3 1.1 Human Chronic Compressive Myelopathy 1.1.1 History Giovanni Morgagni (1682-1771) was the first to describe paraplegia secondary to intraspinal bony growths and compression of the spinal cord. Cervical spondylotic myelopathy (CSM) was recognised as an important disease process during 1952 by Brain, Northfield and Wilkinson, who suggested that compression of the spinal cord vasculature and ischaemia may contribute to the pathophysiology of CSM. In 1970, Walshe described an ‘anaemic and oedematous’ cord, with resultant neuronal and myelin sheath damage. There is sparse literature on the initial recognition of neoplastic tumours as a cause of myelopathy. Harvey Cushing carried out research into tumours of the spinal cord and the development of surgical techniques for excision of intradural spinal tumours during the early 20th Century (Cohen-Gadol et al., 2005). The early recognition of neoplastic spinal cord compression is crucial in the prevention of neurological damage and functional loss. The term “syringomyelia” is derived, in part, from the Greek word syrinx (Συριγξ) meaning tube or pipe and refers to a fluid-filled, elongated division within the spinal cord parenchyma. In 1764 Dominonco Cotugno described the cerebrospinal fluid surrounding the spinal cord and noted that this fluid connected with intracerebral fluid. This was followed by further description of the CSF by Albrecht Von Haller in 1766. In 1862, William Withey Gull described the clinical manifestations of syringomyelia in his study of a ‘case of progressive atrophy of the muscles of the hands: enlargement of the ventricle of the cord in the cervical region, with atrophy of the gray matter’. 1.1.2 Definitions Three major mechanisms of chronic compressive myelopathy are addressed in this study: a) Spondylotic myelopathy. b) Neoplastic myelopathy. c) Syringomyelia. 4 Cervical spondylotic myelopathy is defined here as a chronic, degenerative condition affecting the cervical spine involving bone, ligament or soft tissue causing decreased spinal canal diameter and spinal cord compression which is clinically apparent and radiologically verifiable. An example of chronic compressive myelopathy is shown (Figure 1.1). Cervical spondylosis has also been termed osteoarthritis deformans, degenerative arthritis, hypertrophic arthritis, osteophytosis, spondylosis deformans, marginal spondylosis, and cervical spondylitis (Epstein, 1976). Cervical spondylosis has previously been defined as degeneration of the components of the spine leading to alterations in neighbouring supportive structures with narrowing of the spinal canal or intervertebral foramen (Cailliet, 1984, Committee on Continuing Education in Neurosurgery, Dunsker (Ed.) 1981). Spinal tumours may be primary or metastatic and classified according to tissue of origin, benign or malignant, high or low grade, intracapsular or extracapsular. A proposed surgical classification of tumours is the Weinstein-Boriani-Biagini system of staging which indicates intra- or extra-osseous extension (Hart et al., 1997). Neoplastic compression of the spinal cord tissue may result from lesions removed from the cord or from infiltrative lesions. Three major forms of syringomyelia are recognised; communicating, non-communicating and extracannalicular. Communicating describes a fluid-filled cavity in which there is structural communication between the central canal and the fourth ventricle. This is absent in non-communicating syringomyelia. In extracanalicular or primary parenchymal lesions there is no communication with either the fourth ventricle or central canal. In a review article Milhorat (1997) identifies two exceptions to these criteria, in cases of cyst formation within a malignancy of the spinal cord, and in atrophic cyst formation in myelomalacia. Hydromyelia is a dilatation of the central canal but may be classified alongside syringomyelia. Syringobulbia is used to describe extension of the fluid-filled cavity into the brainstem. 1.1.3 Aetiology and Classification Spondylotic myelopathy involving the cervical spine most commonly afflicts males of older age, within the sixth or seventh decades (Durrant and True, 2002) and is the most frequent cause of spinal cord dysfunction in older adults (Orr and Zdeblick, 1999). A 5 combination of sensory and motor deficits may occur, often leading to a loss of dexterity, upper limb weakness and gait difficulties. Cervical spondylosis is characterised by degenerative changes involving bone and connective tissue within joints, ligaments, or the intervertebral disc. Myelopathy may develop anteriorly from degeneration and protrusion of the intervertebral disc or, posteriorly, from degeneration of vertebral joints with osteophytic growths and connective tissue changes. In the latter, there is often ossification of the posterior longitudinal ligament or thickening and dynamic buckling of the ligamentum flavum (Young, 2000). Narrowing of the lateral recesses or intervertebral foramina leads to compression of the nerve roots. In a minority of cases, such narrowing may result in spinal cord compression and myelopathy and it is most often present in the cervical region. Multiple levels, commonly three (Hayashi et al., 1988), are usually involved in CSM and further stenosis is found in the lumbar spine in approximately 5 percent of patients (Epstein et al., 1984). There may be focal involvement of spinal cord tracts. The radiculomedullary artery may be compressed in spondylotic myelopathy and the symptoms vary according to the extent of vascular insufficiency. Repeated direct load onto the head has been shown to increase the rate of cervical spondylosis (Jumah and Nyame, 1994). Degenerative changes have been demonstrated in rugby footballers, who during a match may experience great pressure to their cervical spine (Newcombe, 1994). Flexion and extension movements cause transient canal stenosis, particularly if pre-existing stenosis is present, and these can lead to shear-type forces