osteoporosis of disuse
osteoporosis
[os″te-o-po-ro´sis]Postmenopausal, estrogen-deficient osteoporosis is the most common type; more than half the women in the United States 50 years of age or older are likely to have radiologically detectable evidence of abnormally decreased bone mass (osteopenia) in the spine, and in more than a third, major orthopedic problems related to osteoporosis will eventually occur. Most fractures sustained by women over the age of 50 are secondary to osteoporosis. Women at risk for this disorder can reduce that risk by maintaining adequate calcium levels with dietary calcium or calcium supplements (see diet-related bone loss below), and taking estrogen in the perimenopausal period when indicated. Estrogen replacement therapy is especially recommended for women whose ovaries were removed before age 50.
Age-related osteoporosis is a type that occurs in both men and women and is caused by bone loss that normally accompanies aging.
Diet-related bone loss is caused by chronic dietary deficiencies in calcium and protein, as well as deficiency in vitamin C, which is an essential cofactor in collagen metabolism. Intestinal absorption of calcium becomes less efficient with age; hence older persons need more rather than less dietary calcium to maintain a positive calcium balance. Although dairy products are the primary source of dietary calcium, supplementary calcium is needed by some women. Healthy premenopausal women over the age of 30 may need as much as 1000 mg of calcium a day, which is the amount supplied by a quart of milk. However, for pregnant women and those over the age of 50, the recommended daily intake increases to more than 1500 mg. Lactating women need 2000 mg of calcium daily to prevent untimely catabolism of bone. The vitamin D metabolite 1,25-dihydroxycholecalciferol is the active hormone that helps maintain normal serum calcium and phosphate levels. Because of inadequate exposure to sunlight, decreased intestinal absorption of vitamin D, and limited intake of milk, elderly persons often are vitamin D–deficient. Vitamin D is a component of multivitamins, and health care providers often recommend supplemental multivitamins for the elderly.
Disuse osteoporosis is related to the response of bone mass change to mechanical stress. Net bone mass does not change throughout much of adult life; however, living bone is never metabolically at rest and constantly remodels and reappropriates its mineral stores along lines of mechanical stress. Without weight-bearing stress, bone mass diminishes. As much as 30 to 40 per cent of initial bone mass may be lost after six months of total immobilization, as in paraplegia and quadriplegia due to spinal cord injury. Movement alone is not sufficient to prevent osteoporosis. There must be weight-bearing activity and the use of antigravity muscles to maintain healthy bones.
Heritable osteoporosis includes at least four types of congenital diseases grouped under the term osteogenesis imperfecta. Symptoms of varying severity that are characteristic of these disorders include skeletal fragility, multiple pathologic fractures, generalized osteoporosis, and scoliosis. All of the diseases included under osteogenesis imperfecta are thought to be associated with defective bone matrix formation.
Endocrine-mediated bone loss can produce osteoporosis because numerous endocrine hormones affect skeletal remodeling and hence skeletal mass. Examples of endocrine disorders that can produce associated osteopenia include hypogonadism, hyperthyroidism, hyperparathyroidism, and hyperadrenalism or chronic glucocorticoid hormone excess.
Disease-related bone loss can occur with almost any kind of chronic disease that is associated with malnutrition and disuse. Osteopenia is also a common complication of most tumors of the bone marrow. Leukemia, lymphoma, and the extremely rare mast cell tumor also may be associated with osteoporosis.
Idiopathic osteoporosis, in both the adult and juvenile form, is extremely uncommon. Drug-induced bone loss may be associated with long-term use of heparin for anticoagulation therapy or with the administration of methotrexate, which has both cytotoxic and calciuric effects.
During the intervals between compression fractures, the patient may be symptom-free, but kyphosis, decrease in height, and appearance of a “dowager's hump” are reliable indicators of the early progress of the disease. Two other associated effects of vertebral compression are the result of a decrease in the size of the thoracic and abdominal cavities. The patient experiences diminished activity tolerance as a result of disease-related postural changes and often reports early satiety and a bloated feeling after eating only a small amount of food.
Radiographs of the thoracic and lumbar spine show a visible loss of bone density. In general, as much as 30 to 50 per cent of the bone mass must be lost before the decrease can be seen on x-ray. Bone density measurement can help in evaluation of this disease and prediction of the likelihood of fracture.
Estrogen replacement therapy is often prescribed for women at menopause. Because it increases the risk for breast and gynecologic malignancies, careful assessment of these patients is necessary. Pharmacologic agents approved by the Food and Drug Association for osteoporosis treatment or treatment include the bisphosphonatesalendronate and risedronate; calcitonin, therapy" >estrogen replacement therapy, and modulators" >selective estrogen receptor modulators such as raloxifene.
Information on osteoporosis can be obtained by writing the National Osteoporosis Foundation, 1232 22nd St. NW, Washington, DC 20037-1292 or consulting their web site at http://www.nof.org. They have also published clinical guidelines for the prevention and treatment of osteoporosis on their web site.
