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Hypophosphatemic Vitamin D-Resistant Rickets

- See: Rickets:

- Discussion:
    - this is the most frequently encountered form of rickets and consists of a genetic or acquired fault in the handling of phosphate in the proximal tubule;
    - patholophysiology:
           - decreased reabsorpion of phosphate by the renal tubule (causing hypophosphatemia) (otherwise the renal function is normal, ie BUN and Cr are normal);
           - decreased absorption of calcium and phosphorous from the GI tract;
    - genetics:
           - hypophosphatemic Vitamin D-resistant rickets is one of the few disorders inherited as a sex-linked dominant trait;
           - as in other sex-linked dominant disease, the degree of expressivity varies;
    - diff dx:
           - renal osteodystrophy 
           - metaphyseal chondrodysplasia
           - hypophosphatasia:
                   - in contrast to vitamin D resistant rickets, hypophosphatasia shows a reduction in serum alk phos;

- Clinical Presentation:
    - classic picture is short stature, bowing of lower limbs (esp at knees causing genu varum) & rachitic changes in the long bones;
    - ht at initial dx is usually <10 % & always< 25th
    - coxa vara is also common in untreated patients;
    - in some affected patients, the disorder is manifested only by a low serum phosphorus;
           - in others there is also widening of epiphyseal plates and bowing of the legs;

- Radiographs:    

- Lab Data:
    - serum phosphorus is low (see hypo PO4), serum Ca is usually normal (or low normal), and serum alk phos is elevated when turnover of bone is increased;
           - patients w/ hypophosphatemic ricket have low concentrations of inorganic phosphorous, secondary to abnormal reabsorption of phosphate;
    - serum BUN and Cr are normal (which distinguishes this from renal osteodystrophy);

- Treatment:
    - administration of high doses of vitamin D (by itself) will have no beneficial effect;
    - instead, it is more appropriate to manage these patients with neutral phosphate orally and 1,25-dihydroxyvitamin D;
          - if oral phosphate supplements are given alone, secondary hyperparathyroidism may result;
          - organic phosphate should be given every 4 hours along w/ supportive vit D therapy;
    - surgical considerations:
          - it is important to avoid "recumbency hypercalcemia" which is common in postoperative patients who are non wt bearing and who are taking Ca supp and vit D;
                 - untreated hypercalcemia in these patients leads to renal stones, mental changes, etc;

- Complications:
    - hypercalcemia and secondary extraskeletal calcification may occur w/ overly aggressive therapy

The orthopaedic management of hypophosphatemic rickets.   

Primary hypophosphatemic rickets: Effect of oral phosphate and vitamin D on growth and surgical treatment.