Rickets is a disease of growing bones where defective mineralization occurs in both bone and cartilage of epiphyseal growth plate. The condition is associated with biochemical abnormalities, bone deformities, developmental delays, impaired growth and sometimes even seizures (in the late course of disease).
Rickets is a potentially devastating condition, as affected children can experience delays in learning to walk, fractures and crippling deformities. Furthermore, this disease dramatically increases the risk of pneumonia, which is a condition that contributes significantly to childhood mortality in developing regions of the world.
Pathogenesis and types of rickets
Vitamin D has a major impact on skeletal growth and health. Any disturbance in its production, absorption, or metabolism is paramount in the development of rickets. Insufficient concentrations of vitamin D concentrations result in decreased intestinal absorption of calcium.
Even a slight decrease in serum calcium concentrations that can ensue stimulates the secretion of parathyroid hormone, which in turn mobilizes calcium and phosphorus from bones in order to restore serum concentrations to the normal levels. When mineral deficiency occurs at the growth plate, growth slows and bone age is retarded.
Nutritional rickets results either from insufficient sunlight exposure or inadequate intake of dietary vitamin D, calcium, or phosphorus. This type of disease continues to be problematic among infants in many communities, namely among those who are exclusively breast-fed.
Type I of vitamin D–dependent rickets is a result of a defective gene that codes for the production of renal 25(OH)D3–1-α-hydroxylase. On the other hand, type II of vitamin D–dependent rickets represents a rare autosomal disorder caused by mutations in the vitamin D receptor.
Rickets refractory to vitamin D treatment are usually caused by a heritable form known as vitamin D–resistant rickets or familial hypophosphatemic rickets. Furthermore, various medical conditions (such as malignancy, renal osteodystrophy and malabsorption syndromes) and various medications can cause rickets.
Impaired growth and bone pain or tenderness in arms, legs, pelvis and spine are commonly observed signs and symptoms of rickets. Bone fractures are also increased in patients with this disease. Dental deformities include delayed formation of teeth, defects in the structure of teeth, holes in the enamel, and increased incidence of dental caries.
Characteristic skeletal deformities are bowed legs or knock knees (genu varum and genu valgum, respectively), asymmetrical or odd-shaped skull, costochondral swelling (also known as rachitic rosary), as well as cranial, pelvic and spinal deformities (with scoliosis or kyphosis in the latter group).
A semicoronal impression (known as Harrison's groove) is sometimes seen over the abdomen at the level of the insertion of the diaphragm, which results from pulling on the weakened ribs by the diaphragm during inspiration. The sternum may project anteriorly and appear prominent (pectus carinatum or pigeon breast).
Clinical presentation of rickets is similar around the world, but the age of presentation and the risk of hypocalcemic symptoms (such as tetany or uncontrolled muscle spasms) can vary significantly depending on the age of presentation and the relative importance of vitamin D deficiency (when compared to calcium) in different populations.
In areas where vitamin D deficiency is more commonly observed, rickets typically presents during the first year of life, often with clinically-significant hypocalcemia. On the other hand, calcium deficiency accounts for much of the prevalent nutritional rickets in Africa, thus the disease usually presents from the second year of life (with infrequently observed hypocalcemic tetany).