Sciatica and the Sciatic Nerve

Sciatica commonly refers to pain that radiates along the sciatic nerve and is typically felt in the rear, down the back of the leg and possibly to the foot. Sciatica is one of the most common forms of pain caused by compression of the spinal nerves, and the leg pain often feels much worse than the back pain.

Sciatica is actually a symptom and not a diagnosis. The term literally means that a patient has pain down the leg resulting from compression of the sciatic nerve. The diagnosis is what is causing the compression (e.g. herniated disc).

The sciatic nerve is the largest single nerve in the human body; it runs from each side of the lower spine through deep in the rear and back of the thigh, and all the way down to foot, connecting the spinal cord with the leg and foot muscles.

Sciatica pain can occur when the nerve roots in the lower spine are irritated or compressed. Most often, sciatica pain is caused when the L5 or S1 nerve root in the lower spine is irritated by a herniated disc. When this happens, pain radiates into the rear and back of the thigh and calf, and occasionally may extend down to the foot. Numbness, tingling, and/or a burning or prickling sensation are also common symptoms.

Degenerative disc disease may also irritate the nerve root and cause sciatica, while conditions that mimic sciatica include piriformis syndrome and sacroiliac joint dysfunction. Sciatica may also be felt if the nerve is actually mechanically compressed, such as from spondylolisthesis, spinal stenosis, or arthritis in the spine.

Most cases of sciatica are caused by a simple irritation to the nerve and will get better with time and conservative care. However, some sciatica symptoms may indicate a potentially serious injury to the sciatic nerve:

• If weakness or numbness is present, the nerve may be damaged and it is important to seek attention from a health care professional. If the nerve is compressed and the pain and symptoms are severe, surgery may be warranted.

• If there is bowel or bladder incontinence (inability to control the bowel or bladder) and/or progressive weakness or loss of sensation in the legs, the condition may be serious and immediate medical attention should be sought.

Sciatic nerve anatomy

The sciatic nerve is the largest and longest single nerve in the human body, about as big around as a thumb at its largest point. The nerve originates in the lower spine as nerve roots exit the spinal cord (through the bones in the spine), and extends all the way down the back of the leg to the toes.

The sciatic nerve is actually a combination of nerves. It is formed on the right and left hand side of the lower spine by the combination of the fourth and fifth lumbar nerves and the first three nerves in the sacral spine. Each nerve exits the spine between two vertebral segments and is named for the segment above it.

• The nerve that exits between lumbar segment 4 and lumbar segment 5 (L4 and L5) is called the L4 nerve root, and the nerve that exits between the L5 and Sacral segment 1 is called the L5.

• The nerves that emerge from the sacral foramen are called the S1, S2 and S3 nerves.

The five nerves group together on the front surface of the piriformis muscle (in the rear) and become one large nerve - the sciatic nerve. This nerve travels then down the back of each leg, branching out to innervate specific regions of the leg and foot.

In the lower thigh/above the back of the knee, the sciatic nerve divides into two nerves, the tibial and peroneal nerves, which innervate different parts of the lower leg:

• The peroneal nerves travel laterally (sideways) along the outer aspect of the knee to the upper foot.

• The tibial nerves continue to travel downward towards the feet and innervate the heel and sole of the foot.

The sciatic nerve supplies sensation and strength to the leg as well as the reflexes of the leg. It connects the spinal cord with the outside of the thigh, the hamstring muscles in the back of the thighs, and muscles in the lower leg and feet. As such, when the sciatic nerve is impaired it can lead to muscle weakness in the leg and/or, numbness or tingling.