Selective Dorsal Rhizotomy (SDR) is a neurosurgical procedure that reduces spasticity by selectively cutting abnormal sensory nerve rootlets in the spinal cord. SDR only reduces spasticity, not other types of high tone. It is the only irreversible tone-reducing treatment.

The following is an explanation of each term:
➡ Selective: Only certain abnormal nerve rootlets are cut.
Think of nerve roots and rootlets like the roots of a tree which further sub-divide into rootlets.
➡ Dorsal: Dorsal refers to the sensory nerve rootlets. The dorsal nerve rootlets, i.e., the sensory nerve rootlets, are cut. (The sensory nerve rootlets are termed dorsal because they are located toward the back of the body. The motor nerve rootlets are termed ventral because they are toward the front.)
➡ Rhizotomy: The word rhizo means root and the word otomy means to cut into.
Putting it all together, selective dorsal rhizotomy means that certain abnormal, dorsal nerve rootlets are cut.