Redefining the clinical phenotypes of non-dystrophic myotonic syndromes

Objective: To redefine phenotypical characteristics for both chloride (CICh) and sodium channelopathies (NaCh) in non-dystrophic myotonic syndromes (NDM).
Methods: In a cross-sectional, nationwide study, standardised interviews and clinical bedside tests were performed in 62 genetically confirmed NDM patients, 32 CICh and 30 NaCh.
Results: Standardised interviews revealed that CICh reported a higher frequency of muscle weakness (75 vs 36.7%; p<0.01), the warm-up phenomenon (100 vs 46.7%; p<0.001), and difficulties in standing up quickly (90.6 vs 50.0%; p<0.001), running (90.6% vs 66.7; p<0.05) and climbing stairs (90.6 vs 63.3%; p=0.01). Patients with NaCh reported an earlier onset (4.4 vs 9.6 years; p<0.001), and higher frequencies of paradoxical (50.0 vs 0%; p<0.001) and painful myotonia (56.7 vs 28.1%; p<0.05). Standardised clinical bedside tests showed a higher incidence and longer relaxation times of myotonia in the leg muscles for CICh (100 vs 60%; mean duration of chair tests 12.5 vs 6.3 s; p<0.001), and in eyelid muscles for NaCh (96.7 vs 46.9%; mean relaxation time of 19.2 vs 4.3 s; p<0.001). Transient paresis was only observed in CICh (62.5%) and paradoxical myotonia only in NaCh (30.0%). Multivariate logistic regression analyses allowed clinical guidelines to be proposed for genetic testing.
Conclusion: This study redefined the phenotypical characteristics of NDM in both CICh and NaCh. The clinical guidelines proposed may help clinicians working in outpatient clinics to perform a focused genetic analysis of either CLCN1 or SCN4A.