Hyperparathyroidism
The disease is associated with elevated levels of parathyroid hormone (PTH) and alkaline phosphatase. High PTH excessively stimulates osteoclasts, triggering active bone resorption.
Classification:
- Primary: Direct excess of PTH (e.g., due to an adenoma). Causes hypercalcemia and hypophosphatemia.
- Secondary: Compensatory response to hypocalcemia (a classic example is rickets).
- Tertiary: Loss of calcium sensitivity by the parathyroid glands, commonly seen in patients on hemodialysis.
Radiographic pattern of resorption: Radiologically, areas of bone destruction have a typical localization. Subperiosteal resorption (radial margins of the phalanges), subchondral (sacroiliac joints, pubic symphysis, clavicles), subligamentous (at the Achilles tendon insertion site), and trabecular («salt and pepper» skull sign) resorption are distinguished.
Additionally, brown tumors, chondrocalcinosis, osteosclerosis, and rugger-jersey spine may be found.
Hypoparathyroidism and resistance syndromes
Decreased PTH levels lead to hypocalcemia and hyperphosphatemia. On radiographs, this may manifest as osteosclerosis, thickening of the cranial vault and the alveolar lamina dura, as well as basal ganglia calcification.
Of particular interest are genetic disorders associated with mutations in the GNAS gene:
- Pseudohypoparathyroidism (PHP type 1a): Inherited maternally. Target organs fail to respond to PTH. Manifests with obesity, round facies, muscle tetany, and brachydactyly (shortening of the IV and V digits).
- Pseudopseudohypoparathyroidism (PPHP): Paternal mutation. The skeletal phenotype is identical to PHP (the same brachydactyly due to premature fusion of cone-shaped epiphyses), but blood calcium and phosphate levels are completely normal.
- PHP type 1b: Isolated PTH resistance without skeletal abnormalities.
Hypothyroidism
Associated with thyroxine deficiency or resistance to it (e.g., due to a thyroid hormone receptor alpha mutation). Congenital hypothyroidism is included in routine neonatal screening because, if left untreated, it causes severe developmental delay.
Radiological findings in pediatrics:
- Marked skeletal maturation delay.
- Delayed appearance and fragmentation of secondary ossification centers.
- Multiple Wormian bones in the skull.
- Increased risk of slipped capital femoral epiphysis.
- Shortening of long tubular bones and dense metaphyseal bands.
Skeletal manifestations of hemoglobinopathies
Although sickle cell anemia and thalassemia are blood disorders, they cause massive bone marrow remodeling due to compensatory extramedullary hematopoiesis and thromboembolic infarctions.
Sickle cell anemia: Frequent bone infarctions lead to dactylitis in children, avascular necrosis (AVN) of the epiphyses, and deformation of vertebral endplates (H-shaped or «fish» vertebrae). Osteomyelitis easily develops against the background of infarctions (in 70% of cases the causative agent is Salmonella).
Thalassemia: Chronic anemia stimulates expansion of the diploic space of the skull with thinning of the outer table. Trabeculae align perpendicularly to the tables, creating the classic hair-on-end sign. Overgrowth of facial bones is also characteristic.
Briefly: answers to topical questions
Which specific bones and joints are most frequently affected by avascular necrosis in sickle cell anemia?
- Femoral head (caput femoris) is a frequent localization of avascular necrosis, leading to articular head collapse and epiphyseal sclerosis.
- Humeral head (caput humeri) is a common site of epiphyseal involvement in ischemic changes; the process is often bilateral.
What precisely does the «rugger-jersey spine» sign represent on a radiograph?
- Rugger-jersey spine sign (rugger-jersey spine) is a radiographic pattern of increased bone density predominantly in the axial skeleton.
- Endplates (lamina terminalis) become dense within the vertebral bodies, while a band of normal or reduced density is preserved in the center.
- Alternating bands — the alternation of normal and sclerotic bands creates a characteristic striped pattern.
Resorption zones in hyperparathyroidism can be remembered using the 4 S's: Subperiosteal (phalanges), Subchondral (joints), Subligamentous (tendons), Spongy/Trabecular (skull).
Class and exam questions
What is the main difference between PHP and PPHP in laboratory tests?
In pseudohypoparathyroidism (PHP), hypocalcemia is observed due to PTH resistance. In pseudopseudohypoparathyroidism (PPHP), calcium and phosphate levels remain normal.
How to differentiate bone infarction from osteomyelitis in sickle cell anemia?
Both conditions look similar, but the presence of fluid collection (pus) or cortical disruption on radiographs definitively points to infection (osteomyelitis).
Why does the «hair-on-end» skull sign occur in thalassemia?
This is the result of compensatory expansion of the diploic space due to increased erythropoiesis; the bone trabeculae orient themselves perpendicularly to the tables of the cranial vault.
What else to study on this topic
- Pathogenesis and radiological differential diagnosis of brown tumors
- Features of bone destruction in neonatal hyperparathyroidism
- Mechanism of rugger-jersey spine formation
- Albright hereditary osteodystrophy: clinical and radiological syndrome
- Epigenetic imprinting of the GNAS gene
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