P.18.5 The value of short exercise and short exercise with cooling tests in the diagnosis of myotonic dystrophies (DM1 and DM2)

Abstract

Despite clinical and genetic similarities, myotonic dystrophy type 1 (DM1) and type 2 (DM2) are distinct disorders requiring different diagnostic and management strategies. Although the standard quantitative EMG is useful in diagnosis of myotonic syndromes, it does not differentiate between DM1 and DM2. Other electrophysiological methods such as the short exercise test (SET) and the short exercise test with cooling (SETC), which seem to be more sensitive, have recently been recommended in DM1/DM2 differentiating diagnosis. The aim of our study was to analyze the results of SET/SETC in myotonic dystrophies and to estimate their usefulness in differentiating between DM1 and DM2. Material and method: 60 patients with genetically proven myotonic dystrophy: 32 patients with DM1, (mean age 35.8 ± 12.7yrs) and 28 patients with DM2 (mean age 44.5 ± 12.5 yrs). For every patient a short exercise test (SET) and short exercise test with cooling (SETC) were performed. Results: In DM1 with SET as well as with SETC a significant decline of compound motor action potential (CMAP) amplitude immediately after effort was observed (mean value CMAP amplitude decline was about 20%). In DM2 there was no marked change of CMAP amplitude with either SET or SETC. Conclusions: Electrophysiological tests such as the short exercise test and short exercise test with cooling, may serve as useful tools for differentiating between DM1 and DM2 and in clinical practice as a guide for molecular testing. In contrast to DM2, in DM1 a marked decline of CMAP amplitude was observed in SET as well as in SETC. The mechanism of these opposites remains unclear. Hypothetically the different pattern of response to SET/SETC in DM1 and DM2 could be explained by multifactorial disabilities of muscle ion channels of different intensity in these two diseases.