Back pain down left leg

by Nathan Wei, MD, FACP, FACR

Nathan Wei is a board-certified rheumatologist and author of the Second Opinion Arthritis Treatment Kit. It's available exclusively at this website... not available in stores.

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The most common cause of pain that starts in the low back and runs down the leg is sciatica. The term “sciatica” refers to pain that radiates along the sciatic nerve and is typically felt in the low back or buttock, down the back of the leg and possibly to the foot. Sciatica is a common form of pain caused by compression of the spinal nerves, and the leg pain is often worse than the back pain.

Sciatica is a symptom and not a diagnosis. It refers to pinching of the sciatic nerve. Sciatica can result 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 buttock 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 (vertebrae that don’t line up properly), spinal stenosis (narrowing of the bony canal that houses the spinal cord), or arthritis in the spine.

Most cases of sciatica are caused by a simple irritation of the nerve and will get better with time and conservative care. However, some sciatica symptoms 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 physician. If the nerve is compressed and the pain and symptoms are severe, surgery may be needed.

• 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 is a surgical emergency.


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 openings between vertebrae).

The sciatic nerve is actually a combination of nerves. It is formed on each 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 buttock and become one large nerve - the sciatic nerve or sciatic trunk. This nerve travels then down the back of each leg, branching out to innervate 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 to the outside 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. When the sciatic nerve is impaired it can lead to muscle weakness in the leg and/or, numbness or tingling.

The nerve roots that exit the spine to form the sciatic nerve are extremely sensitive, and the inner portion of the disc that may herniate contains proteins that cause inflammation. These proteins can irritate the nerve causing sciatica.

The sciatica symptoms one feels (sciatic nerve pain, numbness, tingling, weakness) tend to be different depending on where the pressure on the nerve is located.

Sciatica from L4 nerve root (usually the L3-L4 level)

The patient may have reduced knee-jerk reflex. Symptoms of sciatica stemming from this level of the lower back may include: pain and/or numbness to the medial lower leg and foot; weakness may include the inability to bring the foot upwards (heel walk).


Sciatica from L5 nerve root (usually the L4-L5 level)

The patient may have weakness in extension of the big toe and potentially in the ankle (called foot drop). Symptoms of sciatica originating at this level of the lower back may include: pain and/or numbness to the top of the foot, particularly in the web between the great toe (big toe) and the second toe.

Sciatica from S1 nerve root (the L5-S1 level)

The patient may have reduced ankle-jerk reflex. Symptoms of sciatica originating at this level of the spine may include: pain and/or numbness to the lateral or outer foot; weakness that results in difficulty raising the heel off the ground or walking on the tiptoes.

Pressure on the sacral nerve roots from sacroiliac joint dysfunction

Symptoms may include: sciatic pain or numbness that is often described as a deep ache, inside the leg more so than a linear, well-defined geographic area of pain/numbness found in true sciatica. When severe, nerve root damage can lead to loss of bladder and bowel function.

Pressure on the sciatic nerve from the piriformis muscle

This condition is called piriformis syndrome. Symptoms of piriformis syndrome may include: a sciatic pain and/or numbness in the leg, usually more intense above the knee, which usually starts in the rear rather than the low back, often sparing the low back of symptoms or signs. Piriformis syndrome can mimic the signs and symptoms of sciatica pain from a disc herniation and is part of the differential diagnosis of possible causes of sciatica.

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