A rare condition, usually of the upper part of the spinal cord. A central cavity forms, surrounded by a kind of scar tissue. The cause is unknown. The condition begins between the ages of 20 and 45, and causes either no serious disability or disability which does not become serious until a late stage. Various nerve fibres can be interrupted as the cavity forms and this leads to reduced sensitivity to pain and temperature in the hands and arms. Later on, lesions in the skin and joints arise because of the interruption of nerves in the autonomic nervous system. Finally, slack paralysis occurs, with atrophy (muscle wastage) in the arms and spastic paralysis with numbness in the legs. The cavity may extend upwards, so that sensation and strength in the face and throat may be lost.