What are the differential diagnosis of Vitamin D Dependant rickets
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"vitamin D dependent rickets" differential diagnosis
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Differential Diagnosis of Vitamin D-Dependent Rickets (VDDR)
Vitamin D-Dependent Rickets (VDDR) refers to two distinct autosomal recessive disorders where vitamin D intake is normal but there is either a defect in its activation (Type I) or its receptor function (Type II). Both must be carefully distinguished from a wide array of other rachitic conditions that share clinical and radiographic features.
Understanding VDDR First
| Feature | VDDR Type I (VDDR-1 / Pseudovitamin D Deficiency) | VDDR Type II (HVDRR) |
|---|---|---|
| Defect | Mutation in CYP27B1 (renal 1alpha-hydroxylase deficiency) | Mutation in VDR gene - end-organ resistance |
| Age of onset | Typically < 2 years | Typically < 1 year |
| Serum 25(OH)D | Normal | Normal or elevated |
| Serum 1,25(OH)2D | Low (inappropriately low for Ca level) | Very high (extremely elevated) |
| Serum Ca | Low | Low |
| Serum PO4 | Low | Low |
| PTH | High (secondary HPT) | High |
| ALP | Elevated | Elevated |
| Key distinguishing feature | Responds to calcitriol replacement | Alopecia; complete resistance to all vitamin D forms |
- Brenner and Rector's The Kidney, p. 811
- Katzung's Basic and Clinical Pharmacology, p. 1228
Differential Diagnoses
1. Nutritional Vitamin D Deficiency Rickets
- Most common cause of rickets worldwide
- Due to inadequate dietary vitamin D or insufficient sunlight exposure
- Serum 25(OH)D is low (this is the key distinguishing test - normal in VDDR)
- Florid rachitic changes on X-ray (cupping/fraying of metaphyses, rachitic rosary)
- Responds rapidly to dietary vitamin D supplementation
- Bowing is less prominent than in VDDR
- Grainger & Allison's Diagnostic Radiology, p. 1911
2. Vitamin D-Resistant (Hypophosphataemic) Rickets
A group of phosphate-wasting conditions. The serum vitamin D levels may be normal or even elevated, unlike VDDR.
a. X-Linked Hypophosphataemic Rickets (XLH) - most common hereditary rickets
- Dominant mutation in the PHEX gene - impairs renal proximal tubular reabsorption of phosphate
- FGF-23 is markedly elevated
- Serum Ca is normal; phosphate is very low; 1,25(OH)2D is inappropriately normal/low
- Associated with bowing deformities, short stature, dental abnormalities
- Does NOT respond to vitamin D alone; requires phosphate supplementation + calcitriol
b. Autosomal Dominant/Recessive Hypophosphataemic Rickets
- Rarer variants; similar biochemistry to XLH
c. Hypophosphataemic Rickets with Hypercalciuria (HHRH)
-
Autosomal recessive; primary renal phosphate leak
-
Unlike XLH, 1,25(OH)2D is appropriately elevated, resulting in hypercalciuria
-
Grainger & Allison's Diagnostic Radiology, p. 1911
-
Henry's Clinical Diagnosis and Management, p. 3337
3. Renal Osteodystrophy
- Chronic renal failure causes failure of the second hydroxylation step in renal glomeruli - mimics VDDR type I biochemically
- But the context is chronic kidney disease with elevated serum phosphate (hyperphosphatemia), uremia, and features of secondary hyperparathyroidism on imaging (osteosclerosis, subperiosteal resorption, acro-osteolysis)
- In VDDR, renal function is preserved
- Grainger & Allison's Diagnostic Radiology, p. 1911
4. Fanconi Syndrome
- Generalized proximal renal tubular dysfunction causing phosphaturia, glucosuria, aminoaciduria, and uricosuria
- Can be primary (hereditary) or secondary to:
- Wilson's disease
- Cystinosis
- Galactosemia
- Lowe syndrome (oculocerebrorenal syndrome)
- Heavy metal poisoning (lead, cadmium)
- Drug-induced (ifosfamide, tenofovir)
- Presents as rickets with phosphaturia, glycosuria, and aminoaciduria
- Distinguished from VDDR by the multi-analyte tubular loss pattern in urine
5. Tumour-Induced Osteomalacia (Oncogenic Rickets/Osteomalacia)
- Mesenchymal tumours (typically hemangiopericytoma, phosphaturic mesenchymal tumour) secrete FGF-23
- Causes severe hypophosphataemia due to renal phosphate wasting
- Urine phosphate and ALP elevated; serum calcium normal
- Distinguished by its acquired nature in adults/older children; resolves on tumour removal
- Grainger & Allison's Diagnostic Radiology, p. 1911
6. Nutritional Calcium Deficiency Rickets
- Common in Sub-Saharan Africa where vitamin D levels may be adequate but calcium intake is very low
- 25(OH)D is normal; 1,25(OH)2D is elevated (compensatory); PTH is elevated
- Clinical rickets with normal 25(OH)D; must be distinguished from VDDR by dietary history and calcium supplementation response
7. Hypoparathyroidism
- Low PTH leads to hypocalcaemia, but unlike VDDR, serum phosphate is high (hyperphosphataemia), not low
- No rachitic changes on X-ray typically
- PTH level is the key differentiating lab value
8. Malabsorption-Related Rickets
- Coeliac disease, Crohn's disease, cystic fibrosis, short bowel syndrome
- Causes poor vitamin D and/or calcium absorption
- Serum 25(OH)D is LOW (unlike VDDR)
- Distinguished by GI history, malabsorption workup, and response to high-dose oral or IM vitamin D
9. Hepatic Rickets (Hepatic Osteodystrophy)
- Severe liver disease impairs 25-hydroxylation of vitamin D (first activation step in liver)
- 25(OH)D is low; clinical context of liver disease
- Differs from VDDR where 25-hydroxylation is intact
10. Drug-Induced Rickets / Anticonvulsant Osteomalacia
- Phenytoin, phenobarbital induce hepatic CYP3A4, accelerating vitamin D catabolism
- Results in low 25(OH)D
- History of anticonvulsant use is key
11. Neonatal Hypoparathyroidism / DiGeorge Syndrome
- Presents with hypocalcaemia in the neonatal period (seizures, tetany)
- PTH absent or low; phosphate high
- No rachitic bony changes on X-ray; differentiated by PTH and chromosomal microarray (22q11 deletion)
Key Biochemical Differentiating Table
| Condition | Ca | PO4 | 25(OH)D | 1,25(OH)2D | PTH | Urine PO4 |
|---|---|---|---|---|---|---|
| VDDR Type I | Low | Low | Normal | Low | High | Low |
| VDDR Type II | Low | Low | Normal | Very High | High | Low |
| Nutritional VitD deficiency | Low | Low | Low | Low-Normal | High | Low |
| XLH (Vitamin D resistant) | Normal | Very Low | Normal | Low-Normal | Normal | High |
| Renal osteodystrophy | Low | High | Normal | Low | High | Variable |
| Fanconi syndrome | Low | Low | Normal | Low | High | High + glycosuria + aminoaciduria |
| Hypoparathyroidism | Low | High | Normal | Low | Low | Low |
| Tumour rickets | Normal | Low | Normal | Low | Normal | High |
- Henry's Clinical Diagnosis and Management by Laboratory Methods (Table 16.1), p. 3349
- Brenner and Rector's The Kidney, p. 811
- Katzung's Basic and Clinical Pharmacology, p. 1228
Clinical Clues at a Glance
- Alopecia in a rachitic child + extremely elevated 1,25(OH)2D + resistance to all vitamin D forms = VDDR Type II
- Normal 25(OH)D + Low 1,25(OH)2D in rachitic child = VDDR Type I until proven otherwise
- Glucosuria + aminoaciduria + phosphaturia = Fanconi syndrome
- Hyperphosphatemia with rickets = renal osteodystrophy or hypoparathyroidism
- Very low phosphate + normal Ca + normal 25(OH)D = phosphate-wasting disorder (XLH, tumour rickets)
- Low 25(OH)D = nutritional deficiency, malabsorption, hepatic disease, or drug-induced
- Acquired phosphate wasting in an adult = think tumour-induced osteomalacia first
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