Hypokalemic paralysis is characterized by symmetric muscle weakness due to low serum potassium levels, which is completely disappeared when the potassium levels become normalized. It can be classified into primary (familial) or secondary depending on the etiology. Primary hypokalemic paralysis, which is called hypokalemic periodic paralysis (HPP), is mostly caused by a gene mutation that codes calcium or sodium channel in skeletal muscle. It is quite rare, with an estimated prevalence of 1 in 100,000. Secondary hypokalemic paralysis can be caused by potassium ion loss through the gastrointestinal or renal system. It can also be developed by an intracellular shift of potassium ion by pharmacologic causes such as insulin and beta-adrenergic agonists [1-3].

Although hypokalemic paralysis can be suspected with clinical features, electromyography (EMG) can be used to help to establish the diagnosis. During the attack of paralysis, compound muscle action potential (CMAP) amplitude declines and increased small polyphasic motor unit action potentials (MUAPs) with decreased interference pattern can be observed on the needle EMG. However, the routine electrodiagnostic examination can be normal between the attacks. Therefore, many provocative tests have been proposed. Among them, the long exercise test (LET) has been regarded as the most useful and sensitive diagnostic test for HPP in the inter-attack state and is recommended by muscle channelopathies guidelines [4-6].

In this article, we reported two cases of hypokalemic paralyses. The first case is a secondary hypokalemic paralysis in diabetic ketoacidosis, and the second case is the HPP. The EMG study was performed during the paralytic attack in the first case, while it was performed between attacks in the second one. Therefore, each case presented distinct electrodiagnostic findings. We reported diagnostic approaches to hypokalemic paralysis according to symptoms and intended to emphasize the timely diagnosis of EMG study and utility of the LET.

Hypokalemic paralysis has heterogeneous etiologies with a final common clinical symptom which presents as an acute systematic weakness. An electrodiagnostic study is a valuable diagnostic tool and should be conducted to differentiate other possible causes of weakness [7]. In this article, we described two cases of hypokalemic paralysis; one is secondary hypokalemic paralysis, and the other is the HPP, a hereditary type. This case report is meaningful in that it provided a comprehensive review of hypokalemic paralysis that occurred after insulin treatment for diabetic ketoacidosis.

There are several reports about the mechanisms of muscle weakness due to hypokalemia. One of them is paradoxical depolarization. When hypokalemia is occurred, paradoxical depolarization is triggered and the resting membrane potential of muscle fiber is sustained at -60 mV, resulting in sodium channel inactivation. Another is hyperpolarization. Hypokalemia results in the hyperpolarization of the cell membrane potential, which leads to depolarization block of the muscle fiber membrane. The blockage of membrane depolarization decreases muscle fiber excitability, resulting in reducing CMAP amplitude and decreased insertional activity in EMG study. Also, increased insertional activities and spontaneous activities may be observed in the early attack of paralysis [8,9]. In the first case, most of the CMAP amplitudes were very small, and decreased insertional activities were observed. Especially, there were no MUAPs on the deltoid muscle. The prolongation of motor and sensory latencies and absent or prolonged F wave and H reflex was considered to be due to the early phase of diabetic polyneuropathy. This electrodiagnostic result was compatible with the prior reports of hypokalemic paralysis on its maximal period of paralytic attack [2].

In HPP, which has muscle channelopathies, underlying muscle fiber membrane instability exists. The LET reveals a progressive loss of muscle fiber excitability in the asymptomatic phase of the patient with HPP. It is known that there is an abnormal increment of CMAP during the exercise period, followed by a prolonged gradual decrement during the post-exercise phase [5]. This increment and decrement of CMAP may appear in the normal population. However, based on recent research, 40% amplitude or 50% area decrement with the peak-to-nadir method is suggested as optimal cutoff points for diagnosis [10]. In the second case, the CMAP area decreased to 57.5%, which corresponds to the suggested cutoff values. The LET takes considerable time and has an obstacle that is accompanied by discomfort due to repetitive stimulation to the same nerve. Therefore, the examiner should set stimulation intensity to the just supramaximal to minimize the patient’s discomfort from stimuli. Since the genetic testing for HPP may require several weeks for results and the cost of analysis is prohibitively high, the importance of LET is once again highlighted.

The limitation of our report is that the completeness of the data has been compromised. First, we could not perform a follow-up EMG study due to the patient’s refusal. Also, in the second case, we did not conduct peroneal nerve motor conduction study, and ulnar and superficial peroneal nerve sensory conduction studies. Considering that the patient’s weakness was fully recovered at the time of the examination and the rest of the nerve conduction studies were within normal limits, the above conduction studies were thought to be less diagnostic but to increase the patient’s discomfort. Another limitation was that we could not perform the genetic testing to exclude or confirm HPP in both cases. Although it is a very rare circumstance, in the first case, the possibility that underlying HPP may exacerbate hypokalemia due to insulin treatment cannot be completely excluded. Early diagnosis and management of hypokalemic paralysis is important since it can prevent life-threatening complications. However, periodic symptoms may resolve when patients visit a hospital, and the serum potassium level may be normal, which makes the diagnosis difficult. This case report highlights that the EMG study is a useful and sensitive diagnostic test for hypokalemic paralysis. A proper combination of routine EMG study and LET should be performed in patients suspected of hypokalemic paralysis to evaluate the disease state and exclude other possible causes of paralysis.