Neuromuscular irritability with tetany, Chvostek and Trousseau signs
Mnemonic
“CATS go numb”:
- C - Convulsions
- A - Arrhythmias, with a prolonged QT
- T - Tetany
- S - Spasms and Stridor
- numb - perioral and peripheral paraesthesia, the earliest symptom
Two named signs:
- Chvostek’s sign - tapping over the facial nerve anterior to the ear causes facial twitching. Sensitive but present in up to 10 per cent of normal people
- Trousseau’s sign - inflating a blood pressure cuff above systolic for 3 minutes causes carpopedal spasm. More specific
“Trousseau is the trustworthy one.”
Always check magnesium: hypomagnesaemia blocks PTH release and causes resistance to it, so hypocalcaemia will not correct until the magnesium does.
Causes divide by PTH: a low PTH means hypoparathyroidism, usually post-surgical; a high PTH means vitamin D deficiency, chronic kidney disease, pseudohypoparathyroidism or acute pancreatitis.
Expansion
Features, all from increased neuromuscular excitability:
- Perioral and peripheral paraesthesiae
- Carpopedal spasm and tetany
- Trousseau sign: carpal spasm after 3 minutes of cuff inflation above systolic
- Chvostek sign: facial twitch on tapping over the facial nerve
- Prolonged QT interval, and in severe cases seizures, laryngospasm and bronchospasm
- Chronic: cataracts, basal ganglia calcification, papilloedema
Causes with low parathyroid hormone
- Post-parathyroidectomy or thyroidectomy, autoimmune hypoparathyroidism, DiGeorge syndrome, severe hypomagnesaemia (which impairs both secretion and action)
Causes with high parathyroid hormone (secondary response)
- Vitamin D deficiency, chronic kidney disease, pseudohypoparathyroidism, hyperphosphataemia from tumour lysis or rhabdomyolysis, acute pancreatitis, citrate in massive transfusion
Two practical points: always check magnesium, since hypocalcaemia will not correct until it is replaced; and alkalosis lowers ionised calcium without changing total calcium, which is why hyperventilation causes tetany.