This week, we are sharing another USMLE® Step 1-style practice question to test your knowledge of medical topics. Today’s case focuses on a 52-year-old man with severe lower-extremity cramping. The cramping appeared after recent thyroid surgery. Can you figure it out?
A 52-year-old man comes to the emergency room because of severe lower-extremity cramping. The symptoms started three days ago. Past medical history is notable for hypertension, and he recently underwent surgery for a “thyroid problem.” He currently takes propranolol. Temperature is 37.4°C (99.3°F), pulse is 80/min, respirations are 18/min and blood pressure is 141/78 mmHg. While the blood pressure is being measured in the left arm, spasms are noted in the patient’s left hand. Cardiac, pulmonary, and abdominal exams are noncontributory. Blood tests and an electrocardiogram (ECG) are ordered. Which of the following findings will most likely be observed on the patient’s ECG?
A. Flattened T waves
B. QT-interval prolongation
C. Positive deflection at the J point
D. Peaked T waves
E. ST segment depression
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The correct answer to today’s USMLE® Step 1 Question is…
B. QT-interval prolongation
Before we get to the Main Explanation, let’s look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!
Incorrect answer explanations
The incorrect answers to today’s USMLE® Step 1 Question are…
A. Flattened T waves
Incorrect: Flattened T waves are observed in patients with hypokalemia. Hypokalemia can also present with muscle spasms and cramping. However, the patient’s history of recent thyroid surgery is concerning for hypocalcemia, since the operation may damage or remove the parathyroid glands. Hypocalcemia does not typically present with flattened T waves.
C. Positive deflection at the J point
Incorrect: An Osborn wave (a positive deflection at the J point) may be observed in patients with hypercalcemia. Hypercalcemia can present with bone pain, constipation, kidney stones, and altered mental status. However, the patient’s history of recent thyroid surgery is more concerning for hypocalcemia.
D. Peaked T waves
Incorrect: Peaked T waves are observed in patients with hyperkalemia. The condition can present with arrhythmias and muscle weakness. However, it would be atypical for hyperkalemia to present with muscle spasms, and there are no conditions in the patient’s history (e.g., kidney disease, hypoaldosteronism, etc.) that would increase the risk of hyperkalemia.
E. ST segment depression
Incorrect: ST segment depression can be seen in conditions such as subendocardial ischemia, digoxin toxicity or hypokalemia. However, these conditions would not present with muscle spasms and cramps as observed in this patient.
Main Explanation
This patient with severe lower extremity cramping after a recent thyroid surgery is most likely having hypocalcemia secondary to iatrogenic hypoparathyroidism. It is the most common cause of hypoparathyroidism, as thyroid surgery often leads to the damage and/or removal of the parathyroid glands. Another common cause is autoimmune destruction of the parathyroid glands.
Hypoparathyroidism causes hypocalcemia and hyperphosphatemia. These electrolyte derangements make neurons more excitable, which can trigger tetany and muscle spasms. Other findings include paresthesias, the Chvostek sign, which is when facial muscles twitch after the facial nerve is lightly tapped below the zygomatic process, and the Trousseau sign, which is when tightening of a blood pressure cuff causes muscle spasm in the wrist and hand (as seen in this patient). If the hypocalcemia and hyperphosphatemia are severe, life-threatening complications like seizures and cardiac arrhythmias can occur.
Diagnosis is based on blood tests for parathyroid hormone, calcium, vitamin D, albumin, phosphorus, and magnesium. Also, an electrocardiogram might have changes such as a prolonged QT (see EKG below), prolonged ST segment, and arrhythmias, like torsade de pointes and atrial fibrillation.
EKG with prolonged QT interval
Image reproduced from Wikimedia Commons
Major Takeaway
Iatrogenic hypoparathyroidism is the most common cause of hypoparathyroidism. The condition causes hypocalcemia and presents with symptoms including muscle spasms, cramps, seizures, and arrhythmias. ECG may show prolonged QT, prolonged ST segment, and arrhythmias, like torsade de pointes and atrial fibrillation.
References
Goyal, A., Singh, S. (2019) Hypocalcemia. StatPearls [Internet]. Web Address: https://www.statpearls.com/kb/viewarticle/23258.
Mannstadt, M., Bilezikian, J.P., Thakker, R.V., Hannan, F.M., Clarke, B.L., Rejnmark, L., Mitchell, D.M., Vokes, T.J., Winer, K.K., Shoback, D.M. (2017) Hypoparathyroidism. Nature Reviews Disease Primer. 3, 17055. Doi: 10.1038/nrdp.2017.55.
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The United States Medical Licensing Examination (USMLE®) is a joint program of the Federation of State Medical Boards (FSMB®) and National Board of Medical Examiners (NBME®). Osmosis is not affiliated with NBME nor FSMB.
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