Biostatistics Self-Assessment Questions
A study was undertaken to evaluate the use of computed tomography (CT) in the diagnosis of lumbar disk herniation. Eighty patients with lumbar disk herniation confirmed by surgery were evaluated with CT, as were 50 patients without herniation. The CT results were positive in 56 of the patients with herniation and in 10 of the patients without herniation. The likelihood ratio is
Answer / Explanation
The answer is B.
Lumbar Disk Herniation
Symptomatic disk herniations are seen in all age groups but have their peak in patients between 35 and 45 years of age. Although smoking is a general risk factor for disk degeneration and herniation, occupational risk factors include sedentary work and motor vehicle driving. Sciatica, characterized by pain radiating down the leg in a dermatomal distribution, is the most common symptom and found in 40% of patients with disk herniation. Approximately 50% of patients recover within 1 month, and 96% function normally by 6 months. The rate of surgical treatment in the United States is three times higher than that in Sweden.
A disk herniation is usually preceded by degenerative changes inside the disk. Circumferential tears in the annulus progress to radial tears, and these in turn frequently cause internal disruption or frank herniation. Two pathologic patterns can be distinguished. In a contained disk protrusion, the annulus fibers are intact. In a noncontained disk herniation, the annulus is completely disrupted. Disk material can be subligamentous or sequestered as a free fragment. The pain accompanying disk herniation may be caused by direct pressure on the nerve root or may be induced by breakdown products from a degenerated nucleus pulposus or by an autoimmune reaction. Disk material is a direct source of chemically irritative substances such as phospholipase A2, prostaglandin E2, substance P, and lactic acid. Biochemical studies in operated disk fragments demonstrate an advanced aging process. The hydration of the disk changes from 90% during childhood to 70% by the sixth decade, and the ability of proteoglycans to aggregate decreases with advancing age.
Symptoms and Signs
The typical sciatica is commonly preceded by back pain for a period of days or weeks. This scenario suggests that a compression of nerve fibers in the outer layers of the annulus preceded the rupture of the disk material into the spinal canal and the advent of leg pain. A complete physical examination is necessary. Although the dominating symptom is pain, patients often present with scoliosis or a sciatic list. The mobility of the lumbar spine is diminished more in flexion than in extension. Coughing, sneezing, or a voluntary Valsalva maneuver commonly aggravates the radiating pain. Prolonged sitting also accentuates the pain.
In more than 90% of cases, lumbar disk herniations are localized at L4-L5 and L5-S1. Paracentral disk herniations typically affect the traversing nerve root at the affected level, whereas lateral and foraminal herniations affect the exiting nerve root at the affected level. Compression of the L4 nerve root, which leads to pain and numbness in the L4 dermatome, can occur in a central disk herniation at L3-L4 or in a lateral herniation at L4-L5. When the L4 nerve root is affected, there may be weakness of the quadriceps muscle, and the patella tendon reflex may be depressed or absent. Central or paracentral disk herniations at L4-L5 usually compromise the L5 nerve root, where they may cause numbness in the L5 dermatome and weakness of the foot and toe dorsiflexors. A disk herniation at L5-S1 usually compromises the S1 nerve root, causing numbness or pain in the S1 dermatome, weak plantarflexion of the foot, loss of the Achilles tendon reflex, or tingling in the nerve distribution.
The straight leg–raising test should be performed. The Lasègue sign (pain when the affected leg is elevated) is positive in 98% of patients with lumbar disk herniation, and the cross-Lasègue sign (pain radiating to the affected leg when the contralateral leg is elevated) is positive in 20%. This test is less accurate in older (more than 60 years) patients and in patients with chronic lumbar disk herniation. For lesions involving the L3 or L4 nerve root, the femoral nerve stretch test should be applied. The radicular pain is reproduced when the knee is flexed while the hip is slightly extended.
Radicular pain is typical and should be distinguished from referred pain, which commonly radiates from the lower back into the posterior thigh and ends at the knee level. The posterior spinal elements are frequently a source of this pain. Anterior thigh pain may indicate a retroperitoneal process, such as renal disease or a tumor of the uterus or bladder. Hip disorders, including trochanteric bursitis and coxarthrosis, must be ruled out. The presence of incontinence, perianal numbness, and bilateral leg pain associated with numbness suggests a cauda equina syndrome and requires immediate surgical attention. A primary tumor or metastatic disease involving the spine can present with radiculopathy, and symptoms and signs such as pain at night, a previous history of cancer, and loss of weight should raise the suspicion of the examiner.
In cases of lumbar disk herniation, the goal of treatment is to return the patient to normal activities as quickly as possible. Unnecessary surgery should be avoided. In determining the proper treatment plan, a knowledge of the natural history of lumbar disk herniation is important. In a prospective study of 280 patients with lumbar disk herniations, Weber compared the outcome of a group treated conservatively with the outcome of a group treated with diskectomy. Although better results were seen in the surgically treated patients at 1-year follow-up, the groups showed nearly equal results in terms of function 4 and 10 years later. The study demonstrated a slight tendency to a more favorable outcome with surgery.
Two days of bed rest followed by a good physical therapy program will often lead to significant alleviation of symptoms within 2 or 3 weeks. Analgesics and nonsteroidal medication may also be included in the regimen. Chiropractic adjustments should be avoided in patients with documented disk herniation. Although the role of epidural corticosteroids is unclear, they seem to be successful in decreasing the acute sciatic pain.
Approximately 10% of patients with lumbar disk herniation ultimately require surgery. Surgery is recommended if the sciatica is severe and disabling and tension signs are positive, if symptoms persist without improvement for longer than 1 month, or if findings on clinical examination and in diagnostic tests are consistent with nerve root compromise.
When a standard diskectomy is used, the overall success rate is 85%, and 95% of the patients with successful surgery return to work. Microdiskectomy minimizes the dissection and has an equally high success rate. Newer techniques using muscle-splitting approaches and small tubular retractors require even less soft-tissue violation than microdiskectomy. With this technique, only removal of the extruded part of the disk or of the free fragment is necessary. Postoperative discomfort is minimized and speed of recovery is maximized. Risks of surgery include dural tear, wrong-level exploration, hemorrhage, infection, and nerve deficit.
In cases of contained disk protrusion, percutaneous automated diskectomy or chemonucleolysis may be considered. Each of these approaches has a success rate of approximately 75%. When percutaneous diskectomy is used, a cannula is placed into the disk space under fluoroscopic control, a cutting instrument is fitted inside the cannula, and disk material is then cut and suctioned at the same time. Insertion of an optical device through an extra portal makes direct visualization of the disk possible. Although a multicenter analysis of percutaneous diskectomy showed that only 55% of patients returned to work following treatment, the success rate appears to be higher in the centers with the greatest experience.
Chemonucleolysis of herniated disks is used extensively in Europe.
Chymopapain is injected into the nucleus of the contained herniated disk, and it degrades the nucleus pulposus enzymatically but leaves the annulus intact. This procedure fell into disfavor in the United States after a series of deaths occurred secondary to anaphylaxis. Other complications associated with the procedure include transverse myelitis, diskitis, seizures, and subarachnoid hemorrhage. Many of the previous bad results have been linked with poor patient selection or technical error.
Because experience with laser diskectomy is limited, this extradural approach must still be viewed as experimental. Thus far, its success rate is slightly lower than that of percutaneous diskectomy.