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What is slipped disk?
The disks are pads of tissue situated between each of the vertebrae that make up the spinal column. Each disk consists of a tough, fibrous outer ring called the annulus fibrous and a softer, jellylike inner layer called the nucleus pulpous. The function of the disk is to act as both a strong connection between the vertebrae and a cushion to absorb weight on the spinal column.
A slipped disk does not really slip; the tough outer fibrous ring (annulus) cracks open and the softer inner layer protrudes (prolapse) through the crack, like toothpaste coming out through a crack in a toothpaste tube. For this reason doctors prefer to speak of a disk prolapse rather than a slipped disk.
The nucleus of a disk is softest and most jellylike during childhood. Over the years the nucleus gradually dries out so that by middle age it has a consistency similar to crabmeat. As someone gets older, the nucleus becomes even firmer. In elderly people the disk is mainly a section of scar tissue; this accounts for the fact that old people lose height.
Slipped disk occurs less frequently as people get older; it is a disorder affecting young adults and people in early middle age. Disk protrusion occurs where the outer layer of the disk is weakest; that is, just in front of the nerve roots, which emerge from the spinal cord at each vertebral level. There is very little free space within the spinal canal, and the protruding disk material presses on the nerve root at that level and causes the painful symptoms of a slipped disk.
The area of the spine most likely to be affected is the lowermost part of the back. Here the greatest strains occur, and it is not surprising that most disks that fail are at this level. However, it is possible for disks to prolapse at any level along the length of the spinal column – in the back or the neck.
Causes of slipped disk
- Intervertebral joint degeneration
- Severe strain or trauma
Symptoms of slipped disk
When a prolapsing disk presses on a nerve root, symptoms occur both in the back and in the area that the nerve root supplies. For example, a slipped disk in the lower back can cause pain in the legs.
Symptoms in the back can include severe backache. Often the sufferer will not be able to localize the pain with any accuracy. He or she may also develop painful spasms in the muscles that lie along each side of the spine, particularly in the early stages.
The patient will feel more pain when moving about and some relief when lying flat. Coughing or sneezing can cause the prolapsing disk material to bulge out suddenly, causing a sharp pain in the back or legs. In addition there may be a curvature of the spineâ€”the patient unconsciously leans away from the side of the disk prolapse to try to relieve the pressure from the nerve root that is involved.
If the pressure on the nerve root is not too severe, the nerve will continue to work but will be painful. The brain cannot tell that the painful pressure is coming from the area of the disk, but instead interprets the information as pain originating in the nerve end. In a lower back disk protrusion the sciatic nerve can be irritated, and the individual may feel pain in the thigh, calf, ankle, or foot. This pain can shoot down a leg and is then called sciatica.
More severe pressure on the nerve root may cause the nerve to stop functioning altogether. Areas of skin that the nerve supplies will become numb, so that a light touch or even a pinprick cannot be felt, Muscles supplied by the nerve will become weak or even completely paralyzed. Reflexes such as the knee jerk reflex may disappear.
If only one nerve root is involved this is not too serious, because each nerve supplies only a small area of skin, or a limited number of muscles. If the nerves to the bladder or genitals are affected, however, their function can be permanently lost. Urgent medical attention is needed to relieve the pressure on these nerves.
Diagnosis of the slipped disk
Obtaining a careful patient history is vital because the events that intensify disk pain are diagnostically significant. The straight leg raising test and its variants are perhaps the best tests for slipped disk, but may still be negative.
For the straight leg raising test, the patient lies in a supine position while the examiner places one hand on the patient’s ilium, to stabilize the pelvis, and the other hand under the ankle, and then slowly raises the patient’s leg. The test is positive only if the patient complains of posterior leg (sciatic) pain, not back pain.
Video for straight leg raising test:
In Lasegue test, the patient lies flat while the thigh and knee are flexed to a 90 degree angle. Resistance and pain as well as loss of ankle or knee jerk reflex indicate spinal root compression.
Video for Lasegue test:
X-rays of the spine are essential to rule out other abnormalities but may not diagnose slipped disk because marked disk prolapse can be present despite a normal X-ray.
A thorough check of the patient’s peripheral vascular statusâ€”including posterior tibial and dorsalis pedis pulses and skin temperature of limbsâ€”helps rule out ischemic disease, another cause of leg pain or numbness.
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