Familial hypophosphatemic rickets is usually inherited as an X-linked dominant trait X-Linked Dominant Genetic disorders determined by a single gene (Mendelian disorders) are easiest to analyze and the most well understood. If expression of a trait requires only one copy of a gene (one allele)... read more ; other familial patterns occur but are rarer (1 General reference Hypophosphatemic rickets is a genetic disorder characterized by hypophosphatemia, defective intestinal absorption of calcium, and rickets or osteomalacia unresponsive to vitamin D. It is usually... read more ).
Sporadic acquired cases sometimes are caused by benign mesenchymal tumors that produce a humoral factor that decreases proximal renal tubular resorption of phosphate (tumor-induced osteomalacia).
Pathophysiology of Hypophosphatemic Rickets
The observed abnormality is decreased proximal renal tubular resorption of phosphate, resulting in renal phosphate wasting and hypophosphatemia Hypophosphatemia Hypophosphatemia is a serum phosphate concentration < 2.5 mg/dL (0.81 mmol/L). Causes include alcohol use disorder, burns, starvation, and diuretic use. Clinical features include muscle weakness... read more . This defect is due to circulating factors called phosphatonins. The principle phosphatonin in hereditary hypophosphatemic rickets is fibroblast growth factor-23 (FGF-23). Decreased intestinal calcium and phosphate absorption also occurs. Deficient bone mineralization is due to low phosphate levels and osteoblast dysfunction rather than to the low calcium and elevated parathyroid hormone (PTH) levels as in calcipenic rickets ( see Vitamin D Deficiency and Dependency Vitamin D Deficiency and Dependency Inadequate exposure to sunlight predisposes to vitamin D deficiency. Deficiency impairs bone mineralization, causing rickets in children and osteomalacia in adults and possibly contributing... read more ). Because 1,25-dihydroxyvitamin D3 levels are normal to slightly low, a defect in conversion is presumed; hypophosphatemia would normally cause elevated 1,25-dihydroxyvitamin D3 levels.
There are several forms of hypophosphatemic rickets (see table Forms of Hereditary Hypophosphatemic Rickets Forms of Hereditary Hypophosphatemic Rickets* ). A form of hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is known to occur due to mutations in the proximal tubule type 2c sodium-phosphate cotransporter (NaPi2c). Defective phosphate transport and hypophosphatemia in this case result in appropriately increased 1,25-dihydroxyvitamin D3 levels, thus leading to hypercalciuria.
Symptoms and Signs of Hypophosphatemic Rickets
The disease manifests as a spectrum of abnormalities, from hypophosphatemia alone to growth retardation and short stature to severe rickets or osteomalacia. Children usually present after they begin walking, with bowing of the legs and other bone deformities, pseudofractures (ie, x-ray findings in osteomalacia that may represent areas of prior stress fractures that have been replaced by inadequately mineralized osteoid vs areas of bony erosions), bone pain, and short stature. Bony outgrowth at muscle attachments may limit motion.
Rickets of the spine or pelvis, dental enamel defects, and tetany that occur in dietary vitamin D deficiency are rarely present in hypophosphatemic rickets.
Patients with HHRH may present with nephrolithiasis and/or nephrocalcinosis.
Diagnosis of Hypophosphatemic Rickets
Serum levels of calcium, phosphate, alkaline phosphatase, 1,25-dihydroxyvitamin D3, parathyroid hormone (PTH), FGF-23, and creatinine
Urinary phosphate and creatinine levels (for calculation of the tubular reabsorption of phosphate)
Often genetic testing
Serum phosphate levels are depressed, but urinary phosphate excretion is large. Serum calcium and PTH are normal, and alkaline phosphatase often is elevated. Hypophosphatemia-induced stimulation of calcitriol production does not occur. Typically, calcidiol levels are normal, whereas calcitriol levels are normal to low.
In calcipenic rickets, hypocalcemia is present, hypophosphatemia is mild or absent, and urinary phosphate is not elevated.
Bone x-rays are typically done.
The different forms of hypophosphatemic rickets are diagnosed based on a combination of family history, clinical presentation, laboratory tests (blood and urine), and imaging tests (see table Forms of Hereditary Hypophosphatemic Rickets Forms of Hereditary Hypophosphatemic Rickets* ). Genetic testing using specific gene panels or whole exome sequencing, often in consultation with a genetic specialist, is helpful to confirm the diagnosis.
Because forms with elevated serum levels of FGF-23 are worsened by iron deficiency Iron Deficiency Iron (Fe) is a component of hemoglobin, myoglobin, and many enzymes in the body. Heme iron is contained mainly in animal products. It is absorbed much better than nonheme iron (eg, in plants... read more , complete blood count and iron tests are indicated in patients with those forms.
Family members of patients with hypophosphatemic rickets may be carriers or potentially affected. Full siblings of patients who have an autosomal recessive disorder have a 25% chance of having the disorder. Boys born to a mother with a pathogenic PHEX variant have a 50% chance of having X-linked hypophosphatemia (XLH), and all children born to a parent affected by autosomal dominant hypophosphatemic rickets (ADHR) have a 50% chance of having ADHR.
Prenatal and preimplantation genetic screening can be offered to families where a member is known to have hypophosphatemic rickets.
Children of people with a known family history of XLH or ADHR and siblings of people with autosomal recessive forms of hypophosphatemic rickets should be evaluated for prior fractures as well as for features of poor growth, bone deformities, and rickets (which can be confirmed by x-ray studies). Serum calcium and phosphate levels also should be measured. Further tests can include measurement of vitamin D, intact PTH, and alkaline phosphatase levels. Urine tests, including random measurement of calcium, phosphate, and creatinine levels or a 24-hour urine collection, also can be done (see table Forms of Hereditary Hypophosphatemic Rickets Forms of Hereditary Hypophosphatemic Rickets* ).
Treatment of Hypophosphatemic Rickets
Oral phosphate and calcitriol
Burosumab for X-linked hypophosphatemia
Treatment of hypophosphatemic rickets consists of neutral phosphate solution or tablets. Starting dose in children is 10 mg/kg (based on elemental phosphorus) 4 times a day. Phosphate supplementation lowers ionized calcium concentrations and further inhibits calcitriol conversion, leading to secondary hyperparathyroidism and exacerbating urinary phosphate wasting. Therefore, oral vitamin D is given as calcitriol, initially 5 to 10 ng/kg 2 times a day. This, however, is not the case with HHRH or HHN (hypophosphatemia, hypercalcemia, and nephrocalcinosis), where 1,25-dihydroxyvitamin D3 levels are elevated and dosing with calcitriol can be detrimental.
Phosphate dose may need to be increased to achieve bone growth or relieve bone pain. Diarrhea may limit oral phosphate dosage. Increase in plasma phosphate and decrease in alkaline phosphatase concentrations, healing of rickets, and improvement of growth rate occur. Hypercalcemia, hypercalciuria, and nephrocalcinosis with reduced renal function may complicate treatment. Patients undergoing treatment need frequent follow-up evaluations.
Burosumab is an anti–FGF-23 monoclonal antibody that has become the treatment of choice for X-linked hypophosphatemia (XLH) and has replaced the conventional therapy described above (1 Treatment reference Hypophosphatemic rickets is a genetic disorder characterized by hypophosphatemia, defective intestinal absorption of calcium, and rickets or osteomalacia unresponsive to vitamin D. It is usually... read more ). Dosing in children < 10 kg is started at 1 mg/kg (rounded to nearest 1 mg) subcutaneously every 2 weeks. For children 6 months to < 18 years and > 10 kg, starting dose is 0.8 mg/kg (rounded to the nearest 10 mg) subcutaneously every 2 weeks. For adults ≥ 18 years, starting dose is 1 mg/kg (rounded to the nearest 10 mg) subcutaneously every 4 weeks. The dose may be titrated upwards according to the manufacturer’s instructions to a maximum of 2 mg/kg or 90 mg as needed to normalize serum phosphate.
Iron deficiency Iron Deficiency Iron (Fe) is a component of hemoglobin, myoglobin, and many enzymes in the body. Heme iron is contained mainly in animal products. It is absorbed much better than nonheme iron (eg, in plants... read more upregulates expression of bone FGF-23 and can exacerbate conditions with high FGF-23 levels/impaired FGF cleavage. Therefore, repletion of iron Treatment Iron deficiency is the most common cause of anemia and usually results from blood loss; malabsorption, such as with celiac disease, is a much less common cause. Symptoms are usually nonspecific... read more is essential for patients with iron deficiency in the setting of high FGF-23 hypophosphatemic conditions.
Adults with oncogenic rickets may dramatically improve once the mesenchymal tumor that causes the disorder is removed. Otherwise, oncogenic rickets is treated with calcitriol 5 to 10 ng/kg orally 2 times a day and elemental phosphorus 250 mg to 1 g orally 3 or 4 times a day.
1. Imel EA, Glorieux FH, Whyte MP, et al: Burosumab versus conventional therapy in children with X-linked hypophosphataemia: A randomised, active-controlled, open-label, phase 3 trial. Lancet 393(10189):2416–2427, 2019. doi: 10.1016/S0140-6736(19)30654-3. Clarification and additional information. Lancet 394(10193):120, 2019. doi: 10.1016/S0140-6736(19)31426-6
Decreased renal resorption of phosphate results in renal phosphate wasting and hypophosphatemia.
There is deficient bone mineralization due to low phosphate levels and osteoblast dysfunction.
Children have growth retardation, bone pain and deformities (eg, leg bowing), and short stature.
Patients with hypophosphatemic rickets with hypercalciuria (HHRH) may present with nephrolithiasis and/or nephrocalcinosis.
Diagnose by finding low serum phosphate levels, elevated urinary phosphate, and normal serum calcium and parathyroid hormone.
Treat with oral phosphate supplements and, except for HHRH, vitamin D (given as calcitriol).
Use burosumab for X-linked hypophosphatemia.
The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
Manufacturer’s instructions: Dosing, administration, and storage information for burosumab