types of vitamin D rickets

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Types of Vitamin D Rickets

Rickets caused by vitamin D-related disturbances is broadly divided into three main categories:

1. Vitamin D Deficiency Rickets (Nutritional Rickets)

Cause: Deficient dietary intake of vitamin D or inadequate sunlight exposure → low serum vitamin D levels.

Features:

Rachitic bony changes can be florid: widening of costochondral junctions (rachitic rosary), cupping and fraying of metaphyses
Responds rapidly to dietary vitamin D supplementation
Bowing is less prominent than in other types
Congenital rickets is a rare subset, occurring at birth due to maternal vitamin D deficiency; risk is compounded in breast-fed infants of deficient mothers

Treatment: Nutritional supplementation (2,000–5,000 IU/day × 6–12 weeks)

2. Vitamin D-Dependent Rickets (VDDR) — "Pseudo-Vitamin D Deficiency Rickets"

These are autosomal recessive conditions where vitamin D intake and serum levels may be normal, but errors occur in activation or receptor interaction.

Type 1 (VDDR-I) — 1α-Hydroxylase Deficiency

Defect: Deficiency of renal 1α-hydroxylase (CYP27B1), impairing the second hydroxylation step (25-OH-D → 1,25-OH₂-D)
Serum 1,25(OH)₂D is inappropriately low
Presents in children <2 years with severe bony changes, hypocalcaemic tetany, and seizures
Treatment: Calcitriol (0.25–0.5 mcg/day)

Type 2 (VDDR-II) — Vitamin D Receptor (VDR) Mutation (Hereditary Vitamin D-Resistant Rickets, HVDRR)

Defect: Mutations in the VDR gene → defective interaction with target end-organ receptors
Serum 1,25(OH)₂D levels are very high (receptor resistance)
Presents in infants <1 year; characteristically associated with alopecia and poor dentition
Treatment: High-dose calcitriol (partially effective in those with residual receptor function); calcium and phosphate infusions can correct rickets in some patients; requirements often decrease after puberty

3. Vitamin D-Resistant (Hypophosphataemic) Rickets

Mechanism: Impaired proximal renal tubular reabsorption of phosphate → increased renal phosphate (and calcium) excretion. Serum vitamin D levels are normal or elevated.

Subtypes:

Subtype	Genetics	Gene Involved
X-linked hypophosphataemic rickets (XLH)	X-linked dominant	PHEX gene (most common)
Autosomal dominant hypophosphataemic rickets (ADHR)	AD	FGF23 gene
Autosomal recessive hypophosphataemic rickets (ARHR)	AR	Rarer
Hereditary hypophosphataemic rickets with hypercalciuria (HHRH)	AR	SLC34A3
Fanconi syndrome-associated	Various	Defective glucose + phosphate resorption

Clinical features of untreated disease: Severe bowing deformities, short stature, dental abnormalities.

4. Other Related Causes

Renal Osteodystrophy: Chronic renal failure → failure of the second hydroxylation step within renal glomeruli → secondary hyperparathyroidism, raised phosphate, low/normal calcium. Radiologically shows features of both rickets and hyperparathyroidism (osteosclerosis, subperiosteal bone resorption).
Tumour-induced (Oncogenic) Rickets: Tumours secrete a phosphaturic substance (FGF23), causing phosphaturia and elevated alkaline phosphatase with normal serum calcium.
Malabsorption-related: Conditions causing fat-soluble vitamin malabsorption (treated with very high-dose supplements, 10,000–300,000 IU/day).

Summary Table

Type	Key Defect	25-OH-D	1,25-(OH)₂D	Phosphate	Inheritance
Nutritional	Low intake/sun	↓	↓	↓	—
VDDR Type 1	1α-hydroxylase deficiency	Normal	↓↓	↓	AR
VDDR Type 2 (HVDRR)	VDR mutation	Normal	↑↑	↓	AR
X-linked hypophosphataemic	PHEX mutation → ↑FGF23	Normal	Low-normal	↓↓	X-linked dominant
Renal osteodystrophy	Renal failure	Normal/↓	↓	↑	Acquired

— Grainger & Allison's Diagnostic Radiology, p. 1911 — Katzung's Basic and Clinical Pharmacology, 16th Ed. — The Harriet Lane Handbook, 23rd Ed.

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