ANNALS OF CLINICAL AND LABORATORY SCIENCE, Vol. 5, No. 4 Copyright ® 1975, Institute for Clinical Science

Clinical and Laboratory Considerations in Metabolic Bone Disease

LYNWOOD H. SMITH, M.D. AND B. LAWRENCE RIGGS, M.D.

Mayo Clinic and Mfiyo Foundation Rochester, MN 55901

ABSTRACT An overview of the common types of metabolic bone disease is described. When the disease is present in pure form, diagnosis is not difficult. When mixed disease is present, as may be the case, the pathophysiology involved must be clearly under­ stood for accurate diagnosis and treatment.

Introduction opausal or senile osteoporosis, a disorder of unknown etiology, is the commonest form of There are many metabolic disorders that bone disease in the Western hemisphere. affect human bones; but, fortunately, the This disorder may simply represent an exag­ ways in which bones can respond are limited geration of the normal loss of bone that oc­ so that certain generalizations are valid for a curs with aging. It is estimated that the total group of diseases causing a characteristic bone loss between youth and old age is metabolic abnormality in the bone. The about 35 percent in women and somewhat common pathologic responses to metabolic less in men. The loss of bone that has oc­ bone disease include osteoporosis, os­ curred in some patients with osteoporosis is teomalacia, Paget’s disease, osteitis fibrosa not significantly different from that in age- cystica and renal osteodystrophy. These are matched normals without osteoporosis. not mutually exclusive, and it is not uncom­ In osteoporosis there is a greater propor­ mon to find more than one abnormality in tional loss of trabecular than of cortical the same patient. bone.3 This, in part, explains the clinical fea­ In this review, consideration will be given tures of osteoporosis, which are related to to the clinical and laboratory findings in os­ the rather characteristic fractures that occur. teoporosis, osteomalacia, Paget’s disease, The principal fractures include compression and the bone disease of hyperparathy­ fractures of the vertebral bodies, Colles’ roidism. Following this, the multiple abnor­ fractures and fractures of the proximal malities that may occur in renal osteodys­ femur, all sites that contain large amounts of trophy will be reviewed. This will allow trabecular bone. At times, these fractures oc­ consideration of the interactions between cur with minimal trauma in weakened bones. the various etiologic factors and the Laboratory tests of calcium and bone me­ resulting pathologic changes in the bones. tabolism in osteoporosis give normal results, and the diagnosis is made from the clinical Osteoporosis history, the roentgenographic findings and Osteoporosis can be defined as a decreased the absence of medical diseases known to be volume of bone tissue relative to the volume associated with bone loss. Abnormal labora­ of anatomic bone.1,311 Primary postmen­ tory findings in the presence of osteoporosis 252 METABOLIC BONE DISEASE 2 5 3

TABLE I owing in part to bone pain and in part to a Secondary Osteoporosis proximal myopathy that may develop. Latent or manifest tetany may be present. Immobilization In patients with osteomalacia, the serum Cushing's syndrome (endogenous or exogenous) calcium-phosphorus product is low, al­ Primary hyperparathyroidism Hyperthyroidism though either individual value may be in the Acromegaly Postgastrectomy normal range. The serum alkaline phos­ Heparin phatase value is always increased in active Multiple myeloma and myeloproliferative diseases Osteogenesis imperfecta tarda osteomalacia, the exception being in hypophosphatasia where the enzyme is deficient. The conditions associated with os­ teomalacia are listed in table II. immediately raises suspicion of a secondary cause. Secondary osteoporosis occurs with a Paget’s Disease number of conditions (table I). The Paget’s disease of the bone is a disorder of recognition of the abnormality is based on unknown etiology characterized by a phase identification of the primary cause of the os­ of rapid bone destruction followed by disor­ teoporosis. ganized new bone formation that distorts and deforms the affected parts of the Osteomalacia skeleton.7 In areas of disease in the bone, Osteomalacia is characterized by impaired there is a marked increase in vascularity and mineralization of new bone as it is laid blood flow. Paget’s disease usually appears down.6 Chemically, there is decreased ash after age 40 years and its incidence increases content of bone; histologically, there is in­ with age. Although it can occur in any bone, creased uncalcified osteoid or cartilage. The it is most common in the vertebrae, skull and classic form of this abnormality is seen in vi­ pelvis. tamin D deficiency (rickets in children and Paget’s disease is often symptomatic, and the most common symptom is dull, aching osteomalacia in adults). Patients with osteomalacia have pain over pain in the involved bone. Symptoms also the long bones. When the osteomalacia is may occur as a result of degenerative severe, these bones will fracture spon­ changes in the hips and other weight-bearing taneously. A waddling gait is characteristic, joints owing to involvement of adjacent bone. In severe cases, there may be enlarge­ ment of the skull, thoracic kyphosis and TABLE II bowing of the long bones. The commonest Conditions Associated with Osteomalacia complication of Paget’s disease is fracture of the affected bone. Immobilization of Vitamin D deficiency patients with significant Paget’s disease may Gastrointestinal malabsorption result in severe hypercalcemia and massive Renal tubular syndromes Renal tubular acidosis hypercalciuria. Because of the increased Fanconi syndrome Vitamin D-resistant rickets vascularity and development of functional Chronic renal disease arteriovenous fistulae in bone, there may be Urinary diversion (ureterosigmoidostomy) Hypophosphatasia increased cardiac output and, rarely, high- Anticonvulsant therapy output heart failure. About 1 percent of Pseudohypovitaminosis D (25-hydroxycholecaliciferol 1-hydroxylase patients with Paget’s disease undergo sar­ deficiency comatous changes in the lesions, and 6 per­ Hypophosphatémie osteomalacia associated with neoplasia cent of all osteogenic sarcomas occur in patients with Paget’s disease.5'9 When severe 2 5 4 SMITH AND RIGGS involvement of the skull and vertebrae is cium, phosphorus and hydroxyproline values present, there may be symptoms owing to all are usually increased. compression of the cranial nerves or spinal cord. In Paget’s disease, the serum and urinary Renal Osteodystrophy calcium and phosphorus values are normal To this point the various common forms of unless the patient is immobilized. Then metabolic bone disease have been presented serum and urinary calcium values may be as distinct entities, but clinically there may significantly increased. Alkaline phosphatase be overlap complicating the picture. No­ and urinary hydroxyproline values are where is this more evident than in the characteristically high in proportion to the patient with advanced renal failure with os­ extent and activity of the disease. teodystrophy.4 To understand the problem, the biochemical disturbances that occur as renal failure develops must be reviewed Primary Hyperparathyroidism (figure 1). With the onset of azotemia, phosphate Primary hyperparathyroidism was initially retention occurs with increase in serum recognized as a rare disorder of bone called phosphorus and decrease in serum calcium “ osteitis fibrosa cystica.” Later, the relation­ values. At the same time, with loss of func­ ship to urolithiasis and nephrocalcinosis be­ tional renal mass there is a decreased conver­ came apparent. Today, the most common sion of vitamin D to its active metabolite, form of primary hyperparathyroidism may 1,25-dihydrotachysterol (this conversion be asymptomatic without bone or renal occurs in the kidney).8 This results in complications. The incidence of roent- decreased absorption of calcium from the genographically detectable bone disease in gut, with a further tendency toward a lower our series of primary hyperparathyroidism serum calcium concentration. The hypo­ was 24 percent, but this technique is rela­ calcemia stimulates release of parathy­ tively insensitive.10 When microradiographic roid hormone (secondary hyperparathyroid­ studies of bone biopsy specimens were done ism) which begins to appear when the on patients without primary hyper­ clearance of inulin reaches 30 to 40 ml parathyroidism with roentgenographic bone per min.2 If this stimulation is prolonged, disease, abnormalities were uniformly pres­ massive parathyroid hyperplasia may occur ent.12 Characteristic roentgenographic with overfunction and, at times, hyper­ changes include subperiosteal resorption calcemia. These metabolic changes can re- (usually seen best in the hands), diffuse in­ crease in bone trabeculation, decreased radiodensity of bone, chondrocalcinosis and Phosphate Parathyroid------»Bone resorption bone cysts (brown tumors). The bone disease of hyperparathyroidism is usually asymptomatic. When the bone disease is severe, bone pains, pathologic frac­ tures and skeletal deformities may be pres­ ent. At present, extensive skeletal deformity is distinctly rare. In primary hyperparathyroidism with osteitis fibrosa cystica, the serum calcium, al­ kaline phosphatase and immunoreactive Figure 1. Schematic presentation of disturbances in calcium metabolism that may occur in patients with parathyroid hormone values are high and the chronic renal failure. 1,25-DHT = 1,25-dihydroxy vi­ serum phosphorus value is low. Urinary cal­ tamin D. METABOLIC BONE DISEASE 2 5 5

TABLE III

Metabolic Bone Disease

Alkaline Condition Calcium Phosphorus Phosphatase Calcium phosphorus Hydroxyproline

Osteoporosis Normal Normal Normal Normal Normal Normal Osteomalacia Low Low High Normal High High Paget's disease Normal Normal Very high Normal Normal Very high Primary hyperpara­ High Low High High High High thyroidism with osteitis fibrosa cystica Renal osteodystrophy Low, Very high Very high normal or high

*These usually are low, but their meaning is difficult to assess owing to the decreased glomerular filtration rate and filtered load. suit in severe changes of osteitis fibrosa nephrotic component of the chronic renal cystica in the bones. disease. Serum phosphorus and alkaline At the same time, systemic acidosis may phosphatase values usually are very high. develop with renal failure. In this setting, Urine studies are not helpful because of the bone buffers hydrogen ion with release of decreased glomerular filtration rate. calcium and demineralization of the bones. The laboratory findings in the common There is also a buildup in the serum of com­ forms of metabolic bone disease can be sum­ pounds that provide an increased binding of marized as outlined in table III. When the calcium.13 This causes a decreased avail­ disorder is secondary to a primary disease, as ability of calcium for mineralization. Finally, with secondary osteoporosis, then the the decreased absorption of calcium from specific laboratory tests for that disease are the gut secondary to the abnormalities of vi­ needed. tamin D metabolism results in a decreased availability of calcium for mineralization. References These three metabolic disturbances (aci­ dosis, increased binding and abnormal vi­ 1. ALBRIGHT, F., SMITH, P. H., and RICH­ tamin D metabolism) result in osteomalacia. ARDSON, A. M.: Postmenopausal osteoporosis: its clinical features. J.A.M. A. 116:2465-2474, 1941. Histologically, both forms of metabolic 2. ARNAUD, C. D„ WILSON, D. M., and SMITH, bone disease usually are present in renal L. H .: Primary hyperparathyroidism, renal lithiasis osteodystrophy. Chemically and roent- and the measurement of parathyroid hormone in serum by radioimmunoassay. International Sympo­ genographically, osteitis fibrosa usually pre­ sium on Renal Stone Research, Madrid, 1972: Uri­ dominates. Depending on the more prom­ nary Calculi: Recent Advances in Aetiology, Stone inent change, bone symptoms will mimic the Structure, and Treatment. Cifuentes Delatte, L., Rapado, A., and Hodgkinson, A., eds., S. Karger, primary disorder. Basel, pp. 346-353, 1973. In renal osteodystrophy, the serum cal­ 3. BARZEL, U. S.: Osteoporosis. Grune & Stratton, cium concentration can be low, normal or New York, 1970. 4. KLEEMAN, C. R. Divalent ion metabolism and os­ high, depending on the degree of para­ teodystrophy in chronic renal failure. Arch. Intern. thyroid hyperplasia. The total serum cal­ Med. 124:261-321; 389-454; 519-577; 649-683, cium concentration may be low in spite of 1969. 5. MCKENNA, R. J., SCHWINN, C. P., SOONG, normal ionized calcium value if the serum K. Y., et al: Osteogenic sarcoma arising in Paget’s albumin concentration is low as a result of a disease. Cancer 17:42-66, 1964. 256 SMITH AND RIGGS

6. NORDIN, B. E. C.: The osteomalacias. Metabolic et al: primary hyperparathyroidism: a prospective Bone and Stone Disease. Williams & Wilkins Com­ clinical study. Amer. J. Med. 50:670-678, 1971. pany, Baltimore, pp. 53-95, 1973. 11. RIGGS, B. L., JOWSEY, J., KELLY, P. J., et al: 7. NORDIN, B. E. C .: Paget’s disease. Metabolic Treatment for postmenopausal and senile osteopo­ Bone and Stone Disease. Williams & Wilkins Com­ rosis. Med. Clin. North Amer. 56:989-997, 1972. pany, Baltimore, pp. 170-181, 1973. 12. RIGGS, B. L„ KELLY, P. J., JOWSEY, J., et al: Skeletal alterations in hyperparathyroidism: de­ 8. NORMAN, A. W.: 1,25-dihydroxyvitamin D3: A termination of bone formation, resorption and mor­ kidney-produced steroid hormone essential to cal­ phologic changes by microradiography. J. Clin. cium homeostasis. Amer. J. Med. 57:21-27,1974. Endocrinol. Metab. 25:777-783, 1965. 9. PRICE, C. H. G.: The incidence of osteogenic sar­ 13. YENDT, E. R., CONNOR, T. B„ and HOWARD, coma in south-west England and its relationship to J. E .: In vitro calcification of rachitic rat cartilage in Paget’s disease of bone. J. Bone Joint Surg. 44:366- normal and pathological human sera with some ob­ 376, 1962. servations on the pathogenesis of renal rickets. Bull. 10. PURNELL, D. C., SMITH, L. H., SCHOLZ, D. A., Johns Hopkins Hosp. 96:1-19, 1955.