One common cause that can lead to significant hyperpigmentation of the skin on the posterior side of the spine with a darkened appearance is occult spina bifida. Patients with recessive spina bifida will have localized hyperpigmentation of the skin on the spine, with darkening and localized hair growth. And a relatively obvious depression will be produced locally, distinguishing it from the rest of the skin on the spinal surface. This condition requires only a local radiograph to determine if it is due to occult spina bifida. The darkening of the skin due to spina bifida is usually limited. Most of the relatively widespread manifestations of darkening and hyperpigmentation of the skin in the spine are due to excessive local physiotherapy. Excessive local application of physical therapy can cause localized skin hyperpigmentation. Once the above two conditions are ruled out as causing the symptoms of darkening of the spine, you should promptly visit the orthopedic department of a regular hospital.