Diseases And Conditions
Lumbar disc herniation
The human spine is composed of 7 cervical, or neck, vertebrae, 12 thoracic, or upper back vertebrae and 5 lumbar, or low back vertebrae. The vertebrae are the bones that make up the spine. Between the bones are the intervertebral discs, or just discs for short. The discs have an important function in stabilizing the spine. The discs allow rotation, flexion and extension of the spine without undue stress. The discs also absorb much of the axial load to the spine. Axial load refers to the compression felt by the spine during everyday activities like sitting, walking or standing. Certain activities, like jumping, exercising or carrying heavy objects increase the compression felt by the spine.
Discs are named based on whether they are cervical (C), thoracic (T) or lumbar (L) and the two vertebral levels that they lie between. For example, the disc between the 4th lumbar vertebra and the 5th lumbar vertebra is called L4/5.
The disc is composed of two layers, an outer fibrous layer (annulus fibrosis) and an inner gelatinous layer (nucleus pulposus). An injury to the disc will likely tear the outer fibrous layer, called an annular tear, causing pain across the region of the disc. If the tear to the outer fibrous layer is large enough, the inner gelatinous layer may pass through the tear. This is a condition called disc herniation. When the herniated disc pushes or irritates the passing nerves of the spine, there is nerve pain. There are various degrees of herniation. In the mildest form, a partial tear of the outer layer causes the disc to bulge. If the bulge involves less than half of the disc circumference it’s called a broad based herniation. If the herniation involves less than 25% of the disc circumference, it’s called a focal herniation. Focal herniations are called protrusions, if they have a broad base of attachment to the disc, or extrusions if the size of displaced disc material is larger than the attachment to the disc. Sometimes the herniated disc will completely detach from the parent disc. This is called sequestration.
Lumbar (or low back) intervertebral disc herniation is very common, typically occurring at L4/5 and L5/S1 (the 4th and 5th lumber intervertebral discs). When there is injury or tearing of the disc, the pain is axial, or felt along the lower back. This pain may be felt across the back, extending to both sides. This is usually a deep and dull pain. Sometimes this pain is only felt on one side of the back. Occasionally the pain may refer to the hip, thigh or buttocks.
When a disc herniation occurs in the lower back, the herniated disc material may extend backwards into the spinal canal, the bony canal where the spinal nerves travel. The disc may also herniate to the back and side, pinching a nerve as it exits the spinal canal. The nerve compression usually causes burning pain. This nerve pain is called radiculopathy. If worse, the herniated disc may cause weakness to the muscles of the legs and feet. Occasionally a large disc herniation may extend directly posteriorly. This may actually push against the multitude of nerves traveling down the spinal canal. These nerves make up the cauda equina. Compression of these nerves can cause a cauda equine syndrome, characterized by weakness, loss of sensation and bowel or bladder incontinence. This is a medical emergency.
The diagnosis of lumbar or low back disc herniation is usually made by MRI. In cases where an MRI cannot be performed, usually in patients with a pacemaker, surgical implants or severe claustrophobia, a procedure called a myelogram can be performed. This involves injecting dye into the spine via a spinal tap. In cases where there has been previous spine surgery with screws and plates MRI may be difficult to interpret. Myelogram may be helpful in these cases as well.
