Analysis of muscle fiber conduction velocity in thyrotoxic periodic paralysis

Abstract

Introduction Thyrotoxic periodic paralysis (TPP) is a disease characterized by reversible episodes of paralysis with hypokalemia under hyperthyroidism. The clinical pictures of TPP are similar with hypokalemic periodic paralysis (HOPP), which is caused by malfunctions of ion channels in muscle membranes thus showing delayed muscle fiber conduction velocity (MFCV). However, the specific mechanism for TPP is largely unknown. We investigated whether or not TPP has a dysfunction at the level of muscle membrane as HOPP by measuring MFCV. Patients and methods Thirteen men with TPP and fifteen age-matched healthy controls were included in this study. We assessed clinical characteristics and performed serial neurophysiologic tests including nerve conduction studies, prolonged exercise tests (PET) and MFCV. Results In TPP, MFCVs were not delayed, while MFCVs seemed to be faster till one year after attack. In conditions with positive PET, MFCVs were faster compared with conditions with negative PET. However, hypokalemia or hyperthyroidism seemed to cause no significant effects on MFCVs. MFCV values were negatively correlated with potassium level, days from attack, and some values of sensory nerve conduction studies. Conclusion Although clinical manifestations are similar with HOPPs, TPP seems to have different mechanisms not delaying MFCV but facilitating MFCV. Our study implicates that TPP does not seem to have malfunctions of ion channels associated with delaying muscle fiber conduction and may be related to some factors facilitating muscle fiber conduction such as nerve excitability. Further studies are needed to support our preliminary results.