Hereditary Spastic Paraplegia

Pathophysiology

The basic underlying defect or defects in HSP remain unknown. However, the disorder is known to be associated with deterioration or degenerative changes of certain spinal cord tracts. Consisting of bundles of myelinated nerve fibers (axons), spinal cord tracts serve as pathways that convey messages to and from the brain. They include descending spinal tracts that carry motor impulses from the brain down the spinal cord as well as ascending tracts that transmit sensory impulses up the spinal cord to the brain. HSP is associated with degeneration that is most severe at the ends of the longest nerve fibers in the central nervous system. These include certain corticospinal (i.e., descending) tracts that serve to convey motor impulses to the legs and ascending tracts (gracilis tract) that convey certain sensory information from the lower limbs. In some cases, there may be less marked degenerative changes of corticospinal tracts that convey motor impulses to the arms and ascending tracts (cuneatus tract) that convey sensory information from the upper limbs; as a result, most patients do not experience associated symptoms of the arms and hands.