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:
            - 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 weight bearing and who are taking calcium supplements and vitamin D;
                  - untreated hypercalcemia in these patients leads to renal stones, mental changes, ect;

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

---

The orthopaedic management of hypophosphatemic rickets.
      B Ferris et al.   J. Pediatric Orthopaedics.   Vol 11. 1991. 367-373.

Primary hypophosphatemic rickets: Effect of oral phosphate and vitamin D on growth and surgical treatment.
      GA Evans et al.   JBJS. Vol 62-A. 1980. p 1130-1138.

---