Sciatica

Sciatica is a medical condition characterised by pain going down the leg from the lower back.[1] This pain may go down the back, outside, or front of the leg.[3] Onset is often sudden following activities like heavy lifting, though gradual onset may also occur.[5] Typically, symptoms are only on one side of the body.[3] Certain causes, however, may result in pain on both sides.[3] Lower back pain is sometimes but not always present.[3] Weakness or numbness may occur in various parts of the affected leg and foot.[3]

About 90% of the time sciatica is due to a spinal disc herniation pressing on one of the lumbar or sacral nerve roots.[4] Other problems that may result in sciatica include spondylolisthesis, spinal stenosis, piriformis syndrome, pelvic tumors, and compression by a baby's head during pregnancy.[3] The straight-leg-raising test is often helpful in diagnosis.[3] The test is positive if, when the leg is raised while a person is lying on their back, pain shoots below the knee.[3] In most cases medical imaging is not needed.[2] Exceptions to this are when bowel or bladder function is affected, there is significant loss of feeling or weakness, symptoms are long standing, or there is a concern of a tumor or infection.[2] Conditions that may present similarly are diseases of the hip and early herpes zoster before the rash appears.[3]
Depending on how it is defined, 2% to 40% of people have sciatica at some point in time.[4] It is most common during people's 40s and 50s, and men are more frequently affected than women.[3][2]
Pathophysiology
Sciatica is generally caused by the compression of lumbar nerves L4, or L5 or sacral nerves S1, S2, or S3, or by compression of the sciatic nerve itself. 
This can occur as a result of a spinal disk bulge or spinal disc herniation (a herniated intervertebral disc), or from roughening, enlarging, or misalignment (spondylolisthesis) of the vertebrae, or as a result of degenerated discs that can reduce the diameter of the lateral foramen (natural hole) through which nerve roots exit the spine.

​​1. "Sciatica". Archived from the original on 7 January 2017. Retrieved 2 July 2015.
2. Institute for Quality and Efficiency in Health Care (October 9, 2014). "Slipped disk: Overview". Archived from the original on 8 September 2017. Retrieved 2 July 2015.
3. Ropper, AH; Zafonte, RD (26 March 2015). "Sciatica". The New England Journal of Medicine. 372 (13): 1240–8. doi:10.1056/NEJMra1410151. PMID 25806916.