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Sciatica symptoms and sciatica nerve anatomy

Sciatica is actually a symptom and not a diagnosis. The term literally means that a patient has pain down the leg from compression on the sciatic nerve. The diagnosis is what is causing the compression (such as a disc herniation).

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 buttock 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 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 buttocks 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 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 buttocks) 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.

Sciatica symptoms and sciatic nerve anatomy
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 or extrude contains proteins that are inflammatory and can easily irritate the nerve. Therefore, if some of the inner portion of the disc (the nucleus) comes too close to the nerve, the nerve may be irritated and become inflamed, causing sciatic pain – or sciatica.

The sciatica symptoms one feels (pain, numbness, tingling, weakness) tend to be different depending on where the pressure on the nerve occurs and the patient’s pain and symptoms can usually be traced to where the injured/irritated nerve originates in the lower back.

L4 nerve root (usually the L3-L4 level)
The patient may have reduced knee-jerk reflex. Symptoms 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).

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 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.

S1 nerve root (the L5-S1 level)
The patient may have reduced ankle-jerk reflex. Symptoms 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: a sciatica-like 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.

Pressure on the sciatic nerve can also come from the piriformis muscle, which can tighten and irritate the sciatic nerve (called Piriformis syndrome).
Symptoms may include: pain and/or numbness in the leg, usually more intense above the knee, that usually starts in the buttock rather than the low back, often sparing the low back of symptoms or signs. Piriformis syndrome can mimic the signs and symptoms of a disc herniation and is part of the differential diagnosis of possible causes of sciatica.

To clarify terminology, the term sciatica is often used to indicate any form of pain that radiates into the leg. If the sciatic nerve is pinched and the pain in the leg is from the nerve (radicular pain) then this is correct. If the pain is referred to the leg from a joint (referred pain) then using the term sciatica is incorrect. Referred pain from arthritis or other joint problems is actually more common than sciatica.

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