A 7-year-old girl presents to the emergency department with her parents immediately after having a seizure at home. Her past medical history is significant for short stature and unspecified cognitive impairment. She is otherwise healthy and has not had any recent illnesses. Physical examination is significant for a round face and contraction of her facial muscles when the angle of her jaw is tapped. Electrocardiogram shows a long QT interval, and initial laboratory tests are significant for low calcium, normal vitamin D, normal chloride, and elevated parathyroid hormone (PTH).
Which of the following additional features is likely to be present in this individual?
D) Shortened metacarpals
This patient is presenting with symptomatic hypocalcemia and likely has pseudohypoparathyroidism, a group of disorders defined by parathyroid hormone (PTH) resistance. Specifically, this patient likely has pseudohypoparathyroidism type 1A (Albright hereditary osteodystrophy), which occurs due to GNAS1 mutation resulting in low cAMP response to PTH. Laboratory tests in these patients reveal elevated PTH, hypocalcemia, and hyperphosphatemia. Physical examination reveals round facies, short stature, short metacarpals, and developmental delay, all of which are present in this patient.
Answer choice A: Absent thymic shadow on chest x-ray, is incorrect. This finding is present in patients with DiGeorge syndrome, an immunodeficiency resulting in thymic aplasia. Although DiGeorge syndrome can result in hypocalcemia secondary to parathyroid aplasia, this child’s otherwise healthy past medical history makes this diagnosis less likely.
Answer choice B: Bowing of the lower extremities, is incorrect. This finding is present in rickets, which results from defective cartilage mineralization in growth plates. In children, this is often due to nutritional deficiency resulting in low vitamin D and/or calcium. This patient’s vitamin D level is normal.
Answer choice C: Elevated lipase, is incorrect. This lab finding is present in acute pancreatitis. Although acute pancreatitis can cause hypocalcemia, it most often results from gallstones, alcohol consumption, or hypertriglyceridemia and presents with epigastric pain. This patient lacks the characteristic risk factors for acute pancreatitis to be causing her hypocalcemia.
Answer choice E: Urine pH > 5.5, is incorrect. Alkalinization of the urine can occur in distal (type 1) renal tubular acidosis, in which H+ ion excretion in the distal convoluted tubule is impaired. Although type 1 RTA can result in hypercalciuria and hypocalcemia, it is accompanied by hypokalemia and hyperchloremia. This patient’s chloride levels are normal, making this diagnosis less likely.
Key Learning Point
Pseudohypoparathyroidism is a hereditary cause of hypocalcemia and is characterized by concomitant short stature, developmental delay, and shortened metacarpals.
