Cause. Prolapse of a disc is often precipitated by injury. Spontaneous age-degeneration of the disc is probably an important predisposing factor.
Pathology. The discs between L.5 and S.I and between L.4 and L.5 are those most often affected. Part of the gelatinous nucleus pulposus protrudes through a rent in the annulus fibrosus in its weakest part, which is postero-lateral or sometimes the torn annulus itself protrudes back ward If it is small, the protrusion bulges the pain-sensitive posterior longitudinal ligament, causing pain in the back. If it is large, the protrusion herniated through the posterior ligament and may impinge upon an issuing cause sciatic pain. The nerve affected is that which leaves the spinal canal at the interspace next below the site of the disc lesion. Thus the first spinal nerve is impinged upon by a prolapse between L.5 and S.I, the fifth lumbar nerve by a prolapse between L.4 and L.5, and so on. Natural healing is by shrinkage and fibrosis of the extruded disc material; not by its reposition within the disc
Secondary effect. Progressive degeneration of the disc leads, after months or years, to osteoarthritis with ultimate involvement of the posterior intervertebral (facet) joints as well as the central (body-to-body) joints.
Clinical features. In a typical case of disc prolapse at the L.4-5 or L.5- S.l level tile clinical picture is clearly defined. The patient is aged between 18 and 60. A few hours or days after jarring or straining the back he is seized, while twisting, stooping or coughing, with agonising pain in the lumbar region. He is unable to move. The acute pain gradually lessens in severity, but after a few days a radiating pain is felt in one or other buttock and down the back or side of the thigh to the calf and foot. Tingling or numbness is felt in the calf or foot. The pain is aggravated by coughing or sneezing.
On examination ion the patient with a fully developed acute attack stands either with a lumbar scoliosis (sciatic scoliosis) or with the normal anterior lumbar curve obliterated, Forward flexion is greatly restricted, as also may be extension. Lateral flexion, on the other hand, is usually free and painless—certainly to one side if not to both. Straight leg raising is restricted on the affected side. Careful tests may reveal slight muscle wasting or weakness in the distribution of the affected nerve, and the corresponding tendon jerk (knee jerk in L.3-L.4 lesions; ankle Jerk in L.5-S.I lesions) is impaired or absent.
Variations. Atypical cases are common. Thus a definite history of injury or strain is often lacking. The pain may begin gradually rather than suddenly. The symptoms may be confined to the back and never radiate to the lower limb (acute lumbago). On the other hand, the pain is sometimes felt predominantly in the limb and is scarcely perceptible in the back. The the severity of the pain varies greatly from cage to case, and its exact distributiondepends upon the level of the disc prolapse, for instance, in the relatively common cases of high lumbar or mid-lumbar prolapse the pain radiates towards the groin and the front of the thigh rather than to the back of the thigh and leg. In severe case in which the prolapse is almost central there may be Pressure upon the cauda equina, with consequent loss of bladder sensibility and retention of urine.
Radiographic features. In a case of acute prolapsed disc plain radiographs do not show any abnormality, and the purpose of radiography is mainly exclude other causes of back pain and sciatica. It is only when a disc has been deranged for many months or years that appreciable narrowing of the disc space and spurring of the foint margins (denoting secondary osteoarthritis are observed. Myelography or preferably, radiography may outline the disc protusion, thus indicating its level. This investigation, however, it superseded eventually by magnetic resonance imaging, which can show the intervcrtebral disc substance and the nerve roots. Discography and ascending spinal venography have an occasional place.
Investigations, Lumbar puncture reveals either normal cerebro-spinal fluid or, commonly, a slight increase of protein content. Correlation of pathology with clinical features. The initial injury or strain marks the time when the annulus fibrosus is torn or damaged. The nucleus pulposus is very gelatinous and an interval elapses before it becomes extruded. Bulging of the extruded material beneath the posterior longitudinal ligament corresponds to the stage of acute back pain. Herniation through the ligament with impingement against the adjacent nerve is responsible for the radiating limb pain.
Diagnosis. Prolapsed intervertebral disc must be differentiated from other causes of pain in the back or leg. The conditions with which it is most likely to be confused are: tuberculosis of the spine or sacro-iliac joints; intraspinal tumour; tumour of the spine or pelvis: spondylolisthesis; ankylosing spondylitis; osteoarthritis of the spine; arthritis of the hip; and occlusion of the aorta or of the iliac or femoral artery, with consequent
ischaemic pain in the proximal limb muscles on exercise. A dramatically sudden onset is always suggestive of a mechanical derangement and especially of a prolapsed disc, whereas pain that increases relentlessly without intermission suggests a progressive lesion, inflammatory or neoplastic. Although the clinical features are often highly suggestive,definitive diagnosis rests upon radiculography or magnetic resonance imaging.