Tethered cord syndrome can be of a congenital (primary) origin or acquired (secondary or developmental).
Various congenital anomalies, particularly spina bifida, are often associated with congenital tethered cord syndrome. Spina bifida is a birth defect due to incomplete closure of the posterior spinal cord and bony vertebral arch (lamina). Many cases with this anomaly leave a portion of the spinal cord protruded through the spinal canal, typically forming a myelomeningocele. Such birth defects, if located in the tail (caudal) end of the spinal cord, can cause tethered cord syndrome. In others, where the anomalous structure is attached to the wide area of the spinal cord, signs and symptoms reflect local effects on the spinal cord, and not stretched-induced dysfunction (tethered cord syndrome).
Types of spina bifida associated with tethered cord syndrome include an abnormal connection of inelastic tissue to the caudal spinal cord, dermal sinus tract, which extends from the intraspinal connective tissue to the skin (dermal sinus tract), a split spinal cord (diastematomyelia), and a benign fatty mass or tumor (lipoma) continuous to the spinal cord. The other fatty anomaly is a lipomyelomeningocele, in which a lipoma extrudes from the spinal canal underneath the lining of the spinal cord (meninges), but covered by normal skin.
In many individuals, tethered cord syndrome is caused mechanically by an inelastic often-thickened filum terminale. This structure, which is composed of glial tissue (the supportive structure of nerve cells) and covered by pia mater, is a delicate strand of fibrous tissue, bridging the spinal cord tip and the sacrum (the tailbone). Due to its high viscoelasticity, the filum allows movement of the spinal cord. If abnormal fibrous tissue grows into the filum and replaces glial tissue, the filum loses its elasticity and abnormally fixes (tethers) the spinal cord, and becomes the mechanical cause of tethered cord syndrome. The inelastic filum is commonly thickened in children, but found less frequently in adolescents and adults.
Genetic factors are involved in development of anomalous caudal spine and spinal cord, e.g. myelomeningocele, and in some cases of lipomyelomeningocele. Since tethered cord syndrome is a physiological disorder and develops only when it is abnormally stretched, it cannot be connected to genetic factors, unless the congenital susceptibility of spinal cord to oxidative metabolic impairment is proven.
Secondary causes of tethered cord syndrome include tumors, infection or the development of scar tissue (fibrosis) connected to the spinal cord. Tethered cord syndrome may develop as a complication of spinal surgery. Trauma to the spine results in a band of scar formation attached to the spinal cord and can cause tethered cord syndrome. However, some researchers believe that trauma alone is not enough to cause the disorder. They propose that tethering and abnormal tension were already present before the trauma, which worsened the condition.
Some researchers have speculated that some cases of tethered cord syndrome that occur due to anomalies that can cause stretching of the spinal cord may have a genetic basis or that some individuals are genetically predisposed to developing the disorder in these specific cases. Although genetic factors are found in patients with myelominingocele, more research is necessary to determine the exact role that genetic factors play in the development of stretch-causing anomalies.
Pathophysiologically, neuronal dysfunction in tethered cord syndrome results partly from inability for the spinal cord neurons to utilize oxygen, that is, the impaired oxidative metabolism, partly due to lack of oxygen supply (ischemic effect), and partly to ion channel dysfunction directly related neuronal membrane stretching. The spinal cord consists of a long bundle of neuronal fibers (axons) and the interneurons that connect sensory and motor fibers within the cord. During gestation, the spinal cord is continuous to the brain and runs in the spinal canal to the tailbone area. In general, the spinal cord is protected from external insult by two mechanisms; 1) encased in the spinal column, that is, a rigid structure, 2) floating free in the spinal fluid space of the spinal canal. In addition, the spinal cord is continuous to the filum terminale, which is extremely extensible because of its high viscoelasticity. If the spinal cord is tethered at its caudal end, and if the spinal cord is unable to grow as fast as the vertebral column in childhood, the spinal cord is stretched beyond its physiological tolerance. In turn, this causes various metabolic abnormalities in the spinal cord and, ultimately, the various neurological symptoms of this disorder.
Normally, the spinal cord ascends in the spinal canal as the spinal column starts to grow faster than the spinal cord at 9th weeks of gestation. Consequently, the spinal cord is pulled upwards due to this growth difference. By three months of age, the tip of the spinal cord reaches the normal level between T12 and L2 vertebrae. An elastic, extremely extensible filum allows for the ascension of the less elastic spinal cord. If the filum becomes inelastic in an embryo, then the spinal cord tip is anchored and ceases to ascend. Compensatory to the stretching force, the lower (lumbosacral) spinal cord naturally grows more than seen in normal subjects, and becomes elongated. Associated with tethered cord syndrome, the elongated cord is often noted in children, but less often in adults.
In most cases, the abnormal tension of the spinal cord increases over time, but disturbing symptoms often develop quickly during a few weeks. Certain activities such as flexing or extending the lower spinal column can put additional tension on the spinal cord and often worsen tethered cord syndrome. Participation in physical activities such as strenuous sports and ballet dancing with high kicks can worsen the signs and symptoms. Special physical features such as abnormal curvature of the spine (scoliosis and exaggerated lordosis) are the potential for symptomatic acceleration. It should be warned that slight flexion of the lower (lumbosacral) spine always aggravates back pain by spinal cord stretching.