A 19-year-old woman presented with generalized weakness of four hours. She did not have any recent illness or chest or abdominal pain, but she said she was feeling tired and weak.
She noted that she hadn't been eating well recently, but did not have nausea, vomiting, constipation, or diarrhea. She had a blood pressure of 120/80 mm Hg, pulse of 80 bpm, respiratory rate of 18 bpm, and temperature of 36.7°C.
What was causing her symptoms?
Find the diagnosis and case discussion on the next page.
Diagnosis: Hypokalemic Periodic Paralysis
Hypokalemic periodic paralysis is a neuromuscular disorder where there is a defect in the ion channels in the muscle. Hypokalemia is the most common cause of periodic paralysis, but hypokalemic periodic paralysis is a rare disorder; its prevalence is estimated at one in 100,000. (Adv Genet. 2008;63:3.)
The two most common causes of periodic paralysis are autosomal dominant inheritance or thyrotoxicosis. In autosomal dominant inheritance, a mutation is often present in the gene coding for the dihydropyridine-sensitive calcium channel in the skeletal muscle, CACNA1S. (Cell. 1994;77:863; http://bit.ly/2Ciqfew.) Another less frequent genetic mutation in this disorder occurs in the skeletal sodium channel, SCN4A. (Brain. 2001;124[Pt 6]:1091.) Mutations in these genes alter the calcium and sodium channels, reducing the ability of the skeletal muscles to contract.
These patients often have attacks that occur suddenly. The weakness does not extend to the respiratory muscles. Patients often first start experiencing these attacks in late childhood or as teenagers, but the weakness is temporary, lasting minutes to days. Exercise, stress, and high-carbohydrate meals can trigger episodes. All of these can increase insulin and epinephrine, causing potassium to move into cells and serum potassium levels to lower. (Kidney Int. 1996;49:9; http://bit.ly/2WKaS8a.)
Patients are usually found to have weakness that affects proximal muscles more than distal muscles. The legs tend to be more affected than the arms. Serum potassium levels can be anywhere from 1.5 to 2.4 mEq/L, but the average level tends to be closer to 2.4 mEq/L. (Neurology. 2004;63:164.)
The initial workup should include checking electrolytes, thyroid function tests, and an ECG, which can show signs of hypokalemia, including depression of the ST segment, decreased amplitude of the T wave, and a U wave. (Image.) It is also recommended to check the arterial blood gas to look for acidosis and other electrolytes. Periodic paralysis should not have metabolic acidosis or alkalosis. (QJM. 2001;94:133.) Further workup could include an electromyogram, provocative testing, and genetic testing.
Treatment includes potassium supplementation, which should be given incrementally, and levels should be monitored closely. It is important that potassium not be given in any liquid that contains carbohydrate because it could increase insulin. (Arch Neurol. 1983;40:539; http://bit.ly/2r9jb1x.) One should also be careful about not giving too much and causing rebound hyperkalemia, which could also cause weakness. Periodic paralysis attacks can be reduced by using acetazolamide, dichlorphenamide, or even spironolactone. (Brain. 2006;129[Pt 1]:8.)
Our patient's potassium was 2.5 mEq/L. She was admitted for slow repletion and scheduled for genetic testing.
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Dr. Kaplanis an assistant professor of emergency medicine at the University of Colorado School of Medicine in Aurora. Read her past columns athttp://bit.ly/EMN-QuickConsult.