Muscle membrane excitability after exercise in thyrotoxic periodic paralysis and thyrotoxicosis without periodic paralysis

Abstract

We evaluated whether the paralytic attacks in thyrotoxic periodic paralysis (TPP) are primarily due to the abnormal excitability of the muscle membrane caused by a preexisting latent abnormality or to the effects of thyroid hormone. The prolonged exercise (PE) test was used to evaluate muscle membrane excitability in 21 patients with TPP and 11 patients with thyrotoxicosis without paralytic attacks (Tw/oPP) in the hyperthyroid state. The PE tests were compared between the hyperthyroid and euthyroid states in five of the TPP and three of the Tw/oPP patients. Compared to 20 healthy subjects, a significant increase in compound muscle action potential (CMAP) amplitudes immediately after exercise and a significant time-dependent gradual decline in CMAP amplitudes starting from 20 min after exercise were observed in the TPP patients. A significant decline in CMAP amplitudes was also observed in the Tw/oPP patients but only at 50 min after exercise. All of the TPP and Tw/oPP patients had a tendency to improve in the euthyroid state; the PE tests remained abnormal only in the TPP patients. Paralytic attacks in TPP patients are due primarily to a preexisting latent abnormal excitability of the muscle membrane, possibly genetic in origin.

Twitch and M‐wave potentiation induced by intermittent maximal voluntary quadriceps contractions: Differences between direct quadriceps and femoral nerve stimulation

“…Thus, by applying electrical stimulation over the quadriceps muscle belly, M‐wave changes would reflect more closely the alterations in peripheral properties of the neuromuscular system as a “smaller portion” of the peripheral nerve is involved in this stimulation modality compared with nerve stimulation. For this reason, use of direct quadriceps stimulation in some routine neurophysiological tests seems particularly convenient when investigating disorders that affect membrane excitability, such as muscle channelopathies, myotonic dystrophy, and thyrotoxic periodic paralysis …”

Channelopathies of Skeletal Muscle Excitability

“…These studies preceded the discovery of Kir2.6 mutations in TPP, and so the status of Kir2.6 was not known. No published reports are available on V rest of fibers in ATS, however, the late decremental response in the CMAP exercise test confirms an abnormal susceptibility to loss of fiber excitability for both TPP and ATS (6, 246). The depolarization of V rest during an attack, in turn, is attributed to loss of function changes observed for Kir2.1 and Kir 2.6.…”

“…The clinical signs of hyperthyroidism may be mild in TPP, and attacks of severe weakness may occur without overt thyrotoxicosis(162). The CMAP exercise test shows a late decline in amplitude, consistent with a post-exercise loss of fiber excitability (6). When patients become euthryoid, the attacks of weakness stop, and the CMAP exercise test normalizes (108).…”

Clinical and neurophysiological variability in Andersen‐Tawil syndrome

“…Long exercise tests and repeated short exercise tests were performed according to the Fournier method . On the long exercise test, an abnormal result was defined as a compound muscle action potential amplitude after exercise that was less than 80% of baseline . On the repeated short exercise test, an abnormal result was defined as a postexercise increase or decrease in the amplitude of compound muscle action potential .…”