Scoliosis is characterized by an abnormal, sideways curvature of the spine, departing from the normal front-to-back alignment. It is a descriptive term for a complex spinal deformity involving a curve of 10 degrees or more, as measured on an X-ray. The various ways this curvature manifests and develops have led to distinct classification systems. These systems help medical professionals diagnose the specific form of the condition and determine the best course of action.
Primary Classification: Categorizing by Cause
The most fundamental way to categorize scoliosis is by its etiology, or underlying cause. This classification system divides the condition into three main types, though the large majority of cases fall into the category where the cause remains unknown. This primary grouping helps focus the initial diagnostic investigation and informs the long-term prognosis.
Idiopathic Scoliosis is the most common form, accounting for approximately 80% of all diagnosed cases. The term “idiopathic” signifies that no specific cause, such as an underlying disease or injury, has been identified for the spinal deformation. While the exact trigger is unknown, current understanding suggests a multifactorial origin involving genetic factors and hormonal influences during periods of rapid growth.
Congenital Scoliosis is present at birth and results from a malformation of the vertebrae during embryonic development. This typically occurs when vertebrae do not form completely (formation failure) or fail to separate properly (segmentation failure). Because these defects cause the spine to grow unevenly, an abnormal curve develops.
The third major type is Neuromuscular Scoliosis, caused by conditions affecting the muscles or nerves that support the spine. This type often develops in individuals with conditions like cerebral palsy, muscular dystrophy, or spina bifida. The resulting inability of the muscles to maintain spinal stability leads to the progressive development of a spinal curve.
The Most Common Type: Subdivisions of Idiopathic Scoliosis
Because Idiopathic Scoliosis is the most prevalent form, it is further divided into subcategories based on the patient’s age when the curve is first detected. This age of onset correlates with the amount of remaining growth and the overall risk of curve progression. The earliest form is Infantile Idiopathic Scoliosis, diagnosed in children from birth up to three years of age.
Juvenile Idiopathic Scoliosis is diagnosed between the ages of four and nine years. This period often sees slower curve progression compared to other childhood groups, but the patient still has significant growth remaining. The subsequent category, Adolescent Idiopathic Scoliosis (AIS), is the most common subtype overall, with onset occurring between the ages of 10 and 18. AIS is typically diagnosed during the rapid growth spurt of puberty, when the curve is most likely to worsen.
The final subdivision is Adult Scoliosis, which refers to a curve that is either the progression of an existing, untreated curve from childhood or a new curve developed after skeletal maturity. This adult-onset form is often degenerative, resulting from age-related wear and tear on the discs and facet joints of the spine.
Understanding Curve Nature: Structural Versus Non-Structural
A different classification system focuses on the physical nature of the curve itself: whether it is fixed or flexible. Structural Scoliosis describes a fixed, three-dimensional curve where the vertebrae have rotated in addition to curving sideways. This is a true spinal deformity because the curve does not disappear when the patient bends forward or lies down. The structural type is irreversible and encompasses Idiopathic, Congenital, and Neuromuscular scoliosis.
Conversely, Non-Structural Scoliosis, also called functional scoliosis, is a flexible, two-dimensional curve that involves no vertebral rotation. This type corrects itself when the underlying cause is addressed or when the patient changes position, such as lying flat. Non-structural curves are often temporary, caused by external factors like a leg length discrepancy or a muscle spasm. The distinction between structural and non-structural guides the treatment approach toward either correcting spinal mechanics or resolving the temporary external factor.