Osteoporosis

Chapter 698 Osteoporosis

Osteoporosis is relatively uncommon in children even though it is the most common bone disorder in adults. Bone volume is diminished and the incidence of fractures is greatly increased in this condition. In contrast to osteomalacia, which shows undermineralization and normal bone volume, histologic sections of bone in all forms of osteoporosis reveal a normal degree of mineralization but a reduction in the volume of bone, especially trabecular bone (vertebral bone). In osteoporosis, by definition, there is also a reduced amount of bone tissue (termed osteopenia), which is associated with atraumatic (pathologic) fractures. Osteoporosis in children may be primary or secondary (Table 698-1). The primary osteoporoses can be divided into heritable disorders of connective tissue, including osteogenesis imperfecta, Bruck syndrome, osteoporosis-pseudoglioma syndrome, Ehlers-Danlos syndrome, Marfan syndrome, homocystinuria, and idiopathic juvenile osteoporosis. Secondary forms of osteoporosis include various neuromuscular disorders, chronic illness, endocrine disorders, and drug-induced and inborn errors of metabolism, including lysinuric protein intolerance and Gaucher disease.

When no obvious primary or secondary cause can be detected, idiopathic juvenile osteoporosis should be considered, especially if the following clinical features are evident: onset before puberty, long-bone and lower back pain, vertebral fractures, long-bone and metatarsal fractures, a washed-out appearance of the spine and appendicular skeleton, and improvement after puberty. Trabecular bones such as the spine and metatarsals are particularly affected by atraumatic fractures.

In general, blood values of minerals, vitamin D metabolites, alkaline phosphatase, and parathyroid hormone are normal. Evaluation of bone mineral content and bone density by dual-energy x-ray absorptiometry or, less often, quantitative CT shows markedly reduced values. Several modes of therapy (including oral calcium supplements, calcitriol, bisphosphonates, and calcitonin) have been used with some success in individual conditions, but the effect of these treatments is difficult to gauge because spontaneous recovery occurs after the onset of puberty in >75% of cases.

Osteoporosis-pseudoglioma is an autosomal recessive disorder manifested by variable age at onset, low bone mass, fractures in childhood, and abnormal eye development, and the defective gene has been mapped to chromosome 11q12-13. The mutation is a loss of function in the gene for low-density lipoprotein receptor-related protein 5. Interestingly, gain-of-function mutations result in a gene product that increases bone density.

The life cycle implications of either significant demineralization or osteoporosis in childhood need to be stressed. Events in childhood influence peak bone mass, and late adolescence is a period of rapid bone mineral accretion. Peak bone mass is achieved by 20-35 yr of age (depending on the bone measured), and the contribution during childhood is considerable. A number of measures have been shown to influence bone mass: vitamin D (400-800 IU daily), calcium intake (≥1,200 mg/day in adolescents), and weight-bearing exercise throughout childhood. Weight-bearing exercise enhances bone formation and reduces bone resorption. Other factors that can prevent acquisition of peak bone mass include use of alcohol and tobacco. Excellent sources of dietary calcium include mainly dairy products but also bony fish, green vegetables, and calcium-supplemented drinks (e.g., orange juice). Yogurt and cheeses can be used in lactase-deficient children. Because it appears that adult-onset osteoporosis is the result of a number of genetic factors, thus forming a complex trait interaction, specific interventions during childhood to influence bone mass are not available.

The treatment of secondary osteoporosis is best achieved by treating the underlying disorder when feasible. Hypogonadism should be treated with hormone replacement therapy, especially in thin athletic women. Calcium intake should be increased to 1,500-2,000 mg/day. In glucocorticoid-induced osteoporosis, an emphasis on the lowest possible dose to prevent disease activity (inflammatory bowel disease) with alternate-day or topical therapy and the use of inhaled glucocorticoids in asthma is essential. Special diets for inborn errors of metabolism are also appropriate. Celiac disease may be overrepresented in adults with osteoporosis and should be screened for and treated appropriately (Chapter 330.2). Treatment with bisphosphonates that inhibit bone resorption in certain secondary (glucocorticoid-induced) and adult-onset osteoporosis has been successful. Bisphosphonate therapy is also beneficial in osteogenesis imperfecta and cerebral palsy.