Scoliosis is an abnormal curvature of the spine. There are several types of scoliosis: idiopathic, congenital, neuromuscular, and degenerative. Scoliosis most commonly affects the thoracic and thoraco-lumbar spine. Residual curves may be present in the cervical spine; the body prefers to have the visual apparatus (the eyes) horizontal to the viewing field. An upper thoracic curve or a curve that is severe enough may affect the lower cervical spine, in that respect the upper cervical spine will compensate to keep the head level and the eyes at a horizontal viewing field.