Back pain down left leg
by Nathan Wei, MD, FACP, FACR
Nathan Wei is a nationally known 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 sciatic nerve is the largest and longest nerve in the body. It has its origins in the lower spine and is actually a combination of nerves. It is formed by the confluence of the fourth and fifth lumbar nerves and the first three nerves in the sacral spine. These nerve roots unite to form a single trunk.
The term “sciatica” refers to pain that radiates along the distribution of the sciatic nerve and is typically felt in the low back/buttock, down the back of the leg and often into the foot. Sciatica is caused by compression of the spinal nerve roots, most commonly the fourth and fifth lumbar and first sacral. Often, the leg pain is worse than the back pain. Numbness, tingling, and/or a burning sensation are also common symptoms.
Sciatica is a symptom. The cause of sciatica may be disc herniation, degenerative disc disease, degenerative arthritis, compression by a swollen muscle such as the pyriformis, spinal stenosis, spondylolisthesis, and malignancy.
Most cases of sciatica will get better with time and conservative management. However, some sciatica symptoms indicate a potentially serious problem:
• If weakness or numbness is present, the nerve may have been damaged. If the nerve is compressed to the point where pain is unbearable, surgery may be indicated.
• 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 considered a surgical emergency.
In the lower posterior (back) 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 nerve travels laterally to the outside along the outer part of the knee to the upper foot.
• The tibial nerve continue to travel downward towards the foot and innervates the heel and sole of the foot.
Sciatica from L4 nerve root compression may cause a lost knee reflex. Symptoms of sciatica at this level include pain and/or numbness to the medial lower leg and foot. Weakness leads to inability to dorsiflex the foot (heel walk).
Sciatica from L5 nerve root compression causes weakness in big toe extension and can lead to foot drop. Symptoms of sciatica originating at this level of the lower back may include pain and/or numbness in the top of the foot.
Sciatica from S1 nerve root leads to pain and/or numbness in the lateral or outer foot. The patient will have difficulty walking on their toes.
Pressure on the sacral nerve roots from sacroiliac joint leads to symptoms such as deep aching pain and if severe, nerve damage can lead to loss of bladder and bowel function.
Pressure on the sciatic nerve from the piriformis muscle is called piriformis syndrome. Symptoms of piriformis syndrome may include sciatic pain and/or numbness in the leg above the knee. The pain starts in the buttock. Piriformis syndrome can mimic the signs and symptoms of sciatica pain from a disc herniation.
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