If you look at someone’s back, you’ll see that the spine runs straight down the middle. When a person has scoliosis, their backbone curves to the side.

In as many as 80% of cases, doctors don’t find the exact reason for a curved spine. Scoliosis without a known cause is what doctors call “idiopathic.”

Some kinds of scoliosis do have clear causes. Doctors divide those curves into two types — structural and nonstructural.

In nonstructural scoliosis, the spine works normally, but looks curved. Why does this happen? There are a number of reasons, such as one leg’s being longer than the other, muscle spasms, and inflammations like appendicitis. When these problems are treated, this type of scoliosis often goes away.

In structural scoliosis, the curve of the spine is rigid and can’t be reversed.

Causes include:

Cerebral palsy
Muscular dystrophy
Birth defects
Infections
Tumors
Genetic conditions like Marfan syndrome and Down syndrome

Congenital scoliosis begins as a baby’s back develops before birth. Problems with the tiny bones in the back, called vertebrae, can cause the spine to curve. The vertebrae may be incomplete or fail to divide properly. Doctors may detect this condition when the child is born. Or, they may not find it until the teen years.

Family history and genetics can also be risk factors for idiopathic scoliosis. If you or one of your children has this condition, make sure your other kids are screened regularly.

Scoliosis shows up most often during growth spurts, usually when kids are between 10 and 15 years old. About the same number of boys and girls are diagnosed with minor idiopathic scoliosis. But curves in girls are 10 times more likely to get worse and may need to be treated.

Scoliosis diagnosed during the teen years can continue into adulthood. The greater the angle of the spine curve, the more likely it is to increase over time. If you had scoliosis in the past, have your doctor check your back regularly.

Degenerative scoliosis affects adults. It usually develops in the lower back as the disks and joints of the spine begin to wear out as you age.