Variants of impaired parathyroid hormone sensitivity
Classical pseudohypoparathyroidism (PHP) is characterized by resistance of both the kidneys and bones to parathyroid hormone. However, other forms of the disease also exist:
- Pseudohypohyperparathyroidism (Pseudohypohyperparathyroidism). This is a very rare variant in which unresponsiveness to the hormone is exhibited only by the kidneys, whereas bone tissue retains a normal response. On histological and radiological levels, this pathology strongly resembles renal osteodystrophy.
- Pseudo-pseudohypoparathyroidism (Pseudo-Pseudohypoparathyroidism). In this form, patients have dysplastic and phenotypic features completely identical to classical PHP, but parathyroid gland pathologies and biochemical abnormalities are entirely absent.
Severe clinical cases may be associated with the GNAS1 mutation. In such situations, massive soft tissue calcification is observed, leading to pain, focal skin necrosis, and severe restriction of mobility. On MRI, these patients may exhibit multifocal spinal canal stenosis leading to acute myelopathy and hemiparesis.
Imaging features of pseudohypoparathyroidism
Radiological anomalies may be completely absent at birth and manifest only over time.
Late manifestations include:
- Premature closure of the epiphyseal growth plates (premature epiphyseal fusion).
- Thickening of the cranial vault (calvarial thickening).
- Formation of bone exostoses.
- Calcification of the basal ganglia (brain CT is the most sensitive method for detecting them).
- Calcification and ossification of soft tissues (has a plaque-like distribution in the subcutaneous tissue, best visualized on radiographs; periarticular distribution in the hands and feet is rare).
The most important marker is shortening of the metacarpal bones (predominantly the 4th and 5th digits). This is accompanied by the so-called positive metacarpal sign: if a tangent line is drawn through the heads of the 4th and 5th metacarpal bones, it will intersect the 3rd metacarpal bone (normally this does not happen).
Mineralization disorders: Rickets and Osteomalacia
In addition to impaired sensitivity to parathyroid hormone, bone tissue problems can arise due to defects in bone matrix mineralization. Normally, this process depends on the levels of 1,25-dihydroxyvitamin D, calcium, phosphorus, alkaline phosphatase, and the maintenance of normal pH. A deficiency of any of these elements or systemic acidosis leads to pathology, the essence of which is a reduction in the mineral-to-osteoid ratio.
Rickets and osteomalacia are the same pathological process, with names depending on the patient's age:
- Rickets (in children). Affects the immature, growing skeleton. Anomalies are localized predominantly at the ends of the bones, in the zones of endochondral ossification, forming a classic radiological picture.
- Osteomalacia (in adults). Affects the mature skeleton after the completion of endochondral ossification. Looser zones (Looser zones) serve as a specific (pathognomonic) radiological sign of defective osteoid mineralization.
Vitamin D metabolism
Vitamin D is a key factor in mineralization. It circulates as prohormones that require two stages of hydroxylation to convert into the active form.
- Vitamin D2 is formed by the irradiation of ergosterol (found in yeast and mushrooms) and is used in medications and supplements.
- Vitamin D3 is synthesized in the deep layers of the skin from 7-dehydrocholesterol under the influence of ultraviolet light.
Stages of hydroxylation:
- In the liver (at the 25th position), 25(OH)D2 and 25(OH)D3 are formed. The latter is the predominant form circulating in the blood bound to a carrier protein.
- In the kidneys (at the 1st position), the active hormone form—1,25(OH)2D—is formed.
Vitamin D deficiency can develop due to nutritional causes (lack in the diet, sun deprivation) or due to malabsorption syndromes, as the vitamin is fat-soluble.
How to remember the metacarpal sign: "A line through the fourth and fifth cuts the third in half — this is a sign of PHP for us."
Class and exam questions
What is the difference between pseudo-pseudohypoparathyroidism and pseudohypoparathyroidism?
Externally (phenotypically) and radiologically, patients look identical, but in pseudo-pseudohypoparathyroidism, biochemical abnormalities and parathyroid gland pathologies are completely absent.
What diseases require differential diagnosis for metacarpal shortening?
This sign is not specific to pseudohypoparathyroidism alone. It is also found in Beckwith-Wiedemann syndrome, basal cell nevus syndrome, multiple epiphyseal dysplasia, juvenile idiopathic arthritis, and sickle cell anemia with infarctions.
How does rickets differ from osteomalacia?
This is the same process of defective mineralization. Rickets develops in the immature, growing skeleton (affecting endochondral ossification zones), while osteomalacia develops in the mature skeleton (manifesting as Looser zones).
Where does vitamin D activation occur?
Activation occurs in two stages: the first hydroxylation stage is carried out in the liver, and the second (formation of the active form) in the kidneys.
What else to study on this topic
- Pathophysiology of target tissue resistance to parathyroid hormone in GNAS1 mutations
- Differential diagnosis of metacarpal and metatarsal shortening
- Radiological criteria for Looser zones in osteomalacia
- Clinical presentation and consequences of severe systemic acidosis for bone tissue
- Malabsorption syndromes leading to fat-soluble vitamin deficiency
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