Nerve Function Tests for Evaluating Low Back Problems

Topic Overview

The nerves that carry messages to and from your legs come from your low back. By checking your muscle strength, your reflexes, and your sensation (feeling), your doctor can tell whether there is pressure on a nerve root coming from your spinal column. He or she can often also tell which nerve root is involved.

Muscle strength

Muscle strength tests can detect true muscle weakness, which is one sign of pressure on a nerve root. (Sometimes leg weakness is actually due to pain, not pressure on a nerve.) Most people who have herniated discs that cause symptoms also have some nerve root compression. Nerve root compression usually originates in the lumbosacral region.

Specific muscles receive impulses from specific nerves, so finding out which muscles are weak shows your doctor where nerve roots are being compressed.

Muscle strength tests include:

• Hip flexion. You sit on the edge of the exam table with your knees bent and feet hanging down. Then you lift your thigh up off the table while your doctor pushes down on your leg near your knee. (This test can also be done while you are lying on your back.) If your painful leg is weaker than the other leg, you may have nerve root compression at the higher part of your low back, in the area of the last thoracic and the first, second, and third lumbar vertebrae (T12, L1, L2, L3 region).
• Knee extension. While in the sitting position, you straighten out your knee while your doctor pushes down on your leg near your ankle. If your painful leg is weaker than the other leg, you may have nerve root compression at the second, third, or fourth lumbar vertebrae (L2, L3, or L4 region).
• Ankle dorsiflexion. While you are in the sitting position, your doctor pushes down on your feet while you try to pull your ankles upward. If there is weakness in one leg, the ankle will give way to the downward pressure. This is a sign of possible nerve root compression at the level of the fifth lumbar vertebra (L4 or L5 region).
• Great toe extension. While you are in the sitting position, your doctor pushes down on your big toes while you try to extend them (bend them back toward you). If there is weakness in one leg, its big toe will give way to the pressure. This is a sign of possible nerve root compression at the level of the fifth lumbar vertebra (L5 region).
• Plantar flexion power. You stand and rise up on your toes on both feet and then on each foot separately. Toe raises are difficult, if not impossible, to do if a particular nerve region is compressed. This is a sign of possible nerve root compression at the level of the first sacral vertebra (S1 region).

Sensory testing

Just as your muscles receive signals through certain nerves, other nerves carry signals back to your spinal cord from specific sections of your skin and other tissues. Testing your sense of feeling helps your doctor find out what nerve root may be compressed.

Your sense of feeling may be tested in several ways. Your doctor will probably ask you to close your eyes during this testing, because it's easy to imagine the feeling if you can see the test being done. Testing may include touching your skin lightly with a cotton ball or pricking your skin lightly with a pin.

Areas that send messages through particular nerves

Area of skin	Nerve level
The front of your thigh	L1, L2, L3, L4
The inside of your lower leg, from the knee to the inner ankle and arch	L4
The top of your foot and toes	L5
The outside of your ankle and foot	S1

Reflexes

Tendons attach the muscles to the bones. Reflexes are little movements of the muscle when the tendon is tapped. A reflex can be decreased or absent if there is a problem with the nerve supply. To test your reflexes, your doctor will use a rubber hammer to tap firmly on the tendon. If certain reflexes are decreased or absent, it will show what nerve might be compressed. Not all nerve roots have a reflex associated with them.

• Patellar tendon reflex. You sit on the exam table with your knee bent and your foot hanging down, not touching the floor. Your doctor will use a rubber hammer to tap firmly on the tendon just below your kneecap. In a normal test, your knee will extend and lift your foot a little. A decreased or absent reflex may mean that there is compression in the L2, L3, or L4 region.
• Achilles tendon reflex. You sit on a table with your knees bent and feet hanging down, or you may be asked to lie down on your stomach with your legs straight and your feet off the edge of the exam table. Your doctor will use a rubber hammer to tap firmly on the Achilles tendon, which connects the muscle at the back of your calf to your heel bone. In a normal test, your foot will move as though you were going to point your toes. A decreased or absent reflex may mean that there is compression in the S1 region.