Excerpt from Pathophysiology of Chronic Back Pain

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Chronic low back pain (LBP) is the most expensive benign condition in industrialized countries and the most common cause of activity limitation in persons younger than 45 years. It is defined as pain that persists longer than 12 weeks and is often attributed to degenerative or traumatic conditions of the spine. Fibrositis, inflammatory spondyloarthropathy, and metabolic bone conditions are also cited as causes. Although acute LBP has a favorable prognosis, the effect of chronic LBP and its related disability on society is tremendous. Unlike acute LBP, chronic LBP serves no biologic purpose. However, it is a disorder that evolves in a complex milieu influenced by endogenous and exogenous factors, and it alters the individual's productivity to an extent beyond what the initiating pathologic dysfunction would have.

Background

Approximately 80% of Americans experience LBP during their lifetime. An estimated 15-20% develop protracted pain, and approximately 2-8% have chronic pain. Every year, 3-4% of the population is temporarily disabled, and 1% of the working-age population is disabled totally and permanently because of LBP. LBP is second only to the common cold as a cause of lost work time; it is the fifth most frequent cause for hospitalization and the third most common reason to undergo a surgical procedure. Productivity losses from chronic LBP approach $28 billion annually in the United States.

LBP is defined as chronic after 3 months because most normal connective tissues heal within 6-12 weeks unless pathoanatomic instability persists. A slowed rate of tissue repair in the relatively avascular intervertebral disk may impair the resolution of chronic LBP.

Traumatic or degenerative conditions of the spine are the most common causes of chronic LBP. Although disk protrusion and herniation have been popularized as causes of LBP and sciatica, asymptomatic disk herniations on CT and MRI are common. Furthermore, the relationship between the extent of disk protrusion and the degree of clinical symptoms is not clear. A strictly mechanical or pathoanatomic explanation for LBP and sciatica has proved inadequate; therefore, the role of biochemical and inflammatory factors remains under investigation. In fact, this failure of the pathologic model to predict back pain often leads to an ironic predicament for the patient with LBP.

If diagnostic studies are unrevealing of a structural cause, physicians and patients alike question whether the pain has a psychologic, rather than physical, cause. Physical and nonphysical factors, interwoven in a complex fashion, influence the transition from acute to chronic LBP. The identification of all contributing physical and nonphysical factors enables the treating physician to enact a comprehensive approach with the best likelihood for success.

Epidemiology

The estimated yearly prevalence of LBP is 5-20% in the United States and 25-45% in Europe. About 2% of American workers have compensable back injuries each year, ie, a staggering 500,000 cases of work-related spinal injuries. LBP accounts for 19% of all Workers' Compensation claims in the United States. According to the Bureau of Labor and Statistics, metal workers generated 76% of all claims of back strain and/or sprains in the United States. Jobs that require manual-handling activities accounted for more than half of all back pain reports. Injuries to the back were highest among truck drivers, operators of heavy equipment, and construction workers.

An estimated 4.1 million Americans had symptoms of an intervertebral disk disorder between 1985 and 1988, with an annual prevalence of about 2% in men and 1.5% in women. A study of 295 Finnish concrete workers aged 15-64 years revealed that 42% of men, and as many as 60% of those aged 45 years or older, reported having sciatica. When interviewed approximately 5 years later, the lifetime prevalence had increased from 42% to 59%.

Sciatica due to lumbar intervertebral disk herniations usually resolves with conservative treatment. However, it leads to surgery more often than does back pain alone. In a published review of more than 15,000 disk operations, the most common surgical level was L4-5 (49.8%), followed by L5-S1 (46.9%); only 3.4% were performed at levels higher than these. Surgical treatment for lumbar diskogenic syndromes is most common in the United States, where the estimated rate is at least 40% higher than that of in other countries and more than 5 times higher than rates in Scotland and England.

In the United States, the estimated cost of LBP in 1980 was $85 million dollars. Between 1971 and 1981, the number of Americans disabled by LBP grew at 14 times the rate of population growth. Resultant disability in Western culture has reached endemic proportions, with enormous socioeconomic consequences.

LBP is most prevalent in industrialized societies. Genetic factors that predispose persons of specific ethnicity or race to this disorder have not been clearly identified with respect to mechanical, diskogenic, or degenerative causes. Men and women are affected equally, but in those older than 60 year, women report low-back symptoms more often than men.

The incidence of LBP peaks in middle age and declines in old age, when degenerative changes of the spine are universal. Sciatica usually occurs in patients during the fourth and fifth decades of life; the average age of patients who undergo lumbar diskectomy is 42 years.

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