Pseudohypoparathyroidism type 1B - a rare cause of tetany: case report.

Pseudohypoparathyroidism (PHP) is a rare group of disorders characterised by end-organ resistance to the parathyroid hormone (PTH). A 16-year-old boy presented with a 2-year history of involuntary dystonic movements involving mainly the left hand, initially after writing and later during physical exercise. Serum calcium was 1.37 mmol/L (2.20-2.69), phosphate 2.1 mmol/L (0.8-1.45) and PTH 302 ng/L (12-88). CT scan of the head demonstrated multiple subcortical and diffuse basal ganglia calcifications. Genetic analysis confirmed a methylation defect in the GNAS cluster on chromosome 20q13.32 which established the diagnosis. Treatment with calcitriol and calcium carbonate led to complete remission of symptoms. Causes of hypocalcaemia should be considered in evaluating patients with movement disorders. The diagnosis of PHP-1B is challenging but the overall prognosis is excellent.