Chemistry

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Vitamin D avitaminoses

Rickets ("English disease", Glisson's disease)

The most common form of rickets results from a vitamin D deficiency. There are also some vitamin D-resistant forms of rickets. A vitamin D deficiency in adults leads to osteomalacia.

Rickets is a disturbed mineralization of the bones due to an impairment of the calcium or phosphate metabolism. The X-ray shows a bone deformation typical of this disease: "bowlegs".

A deficiency in vitamin D leads to disorders of the calcium and phosphate metabolism. Initially, there is insufficient calcium absorption from the intestine. The resulting hypocalcemia (decrease in the calcium level in the blood) induces secondary hyperparathyroidism (compensatory overfunction of the parathyroid glands → release of parathyroid hormone). Parathyroid hormone decalcifies bones, releases calcium from the skeleton and reduces the reabsorption of phosphate in the renal tubules.

The onset of rickets initially leads to restlessness and nervousness, head sweat and hair loss on the back of the head. Muscle hypotonia appear with slackening of the abdominal wall (so-called "frog belly"), ammoniacal urine odor and cramps. Due to the lack of calcification of the bones, they are deformed by body weight and muscle tension. There are characteristic bone deformations, which initially show up in a softening of the skull bone (so-called "craniotabes"), followed by a flattening of the head (Caput quadratum). Later on, disturbances in cartilage degradation and the accumulation of non-calcified osteoid lead to swellings at the bone-cartilage boundaries: "rachitic rosary" on the ribs, "double humps" (Marfan's sign) on wrists and ankles, "pearl-string fingers" and enlargement of the lower thoracic opening ("Bell Thorax"). There is delayed tooth eruption with damage to the enamel and, when the skeleton is stressed, pelvic and long bone deformations. Common complications are rachitic tetany and broken bones ("greenwood fractures").

The diagnosis is usually made by x-ray examinations and determination of the calcium and phosphate concentration in the serum. As therapy, high doses of vitamin D are given (orally, except in the case of absorption disorders), possibly combined with UV radiation. In addition, bone deformations must already have occurred can be corrected with plaster casts and physiotherapy.

Vitamin D-resistant rickets

The most common cause is a genetic disorder of the phosphate metabolism, chronic phosphate diabetes. Here, the tubular phosphate reabsorption is reduced, combined with a vitamin D regulatory disorder. Since this disease usually only takes effect after the first year of life, the extremities are particularly affected.

In hyperphosphatemic renal rickets, phosphate excretion is reduced and the serum phosphate level increases. This hyperphosphataemia leads to the compensatory release of parathyroid hormone and thus induces bone decalcification.

Resorption disorders of calcium or phosphate are known as renal rickets. In addition to chronic phosphate diabetes, they can also occur as a result of other metabolic disorders, e.g. in cystinosis or tyrosinosis (type I).

Osteomalacia

If there is a vitamin D deficiency in adults, hypovitaminosis is known as osteomalacia. The cause is usually a disruption of vitamin D absorption, but osteomalacia can also occur in chronic liver and kidney diseases.

The disease initially manifests itself in diffuse skeletal complaints, followed by painful bone deformations. The x-ray shows a reduced density and contour blurring, especially in the statically unloaded tubular bones of the upper extremities. Looser conversion zones often appear, visible as clear lightening stripes in the X-ray image. Fractures ("fatigue fractures") often occur in these areas.

The therapy depends on the cause: oral or parenteral administration of vitamin D, calcium or phosphate, possibly also of vitamin D metabolites.


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